Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

97 files changed, 14534 insertions, 0 deletions

diff --git a/tools/fuzzing/common/FuzzingMutate.cpp b/tools/fuzzing/common/FuzzingMutate.cpp
new file mode 100644
index 0000000000..bb5e930125
--- /dev/null
+++ b/tools/fuzzing/common/FuzzingMutate.cpp
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FuzzingMutate.h"
+#include "FuzzingTraits.h"
+
+namespace mozilla {
+namespace fuzzing {
+
+/**
+ * Randomly mutates a byte inside |aData| by using bit manipulation.
+ */
+/* static */
+void FuzzingMutate::ChangeBit(uint8_t* aData, size_t aLength) {
+  size_t offset = RandomIntegerRange<size_t>(0, aLength);
+  aData[offset] ^= (1 << FuzzingTraits::Random(9));
+}
+
+/**
+ * Randomly replaces a byte inside |aData| with one in the range of [0, 255].
+ */
+/* static */
+void FuzzingMutate::ChangeByte(uint8_t* aData, size_t aLength) {
+  size_t offset = RandomIntegerRange<size_t>(0, aLength);
+  aData[offset] = RandomInteger<unsigned char>();
+}
+
+}  // namespace fuzzing
+}  // namespace mozilla
diff --git a/tools/fuzzing/common/FuzzingMutate.h b/tools/fuzzing/common/FuzzingMutate.h
new file mode 100644
index 0000000000..f24f557669
--- /dev/null
+++ b/tools/fuzzing/common/FuzzingMutate.h
@@ -0,0 +1,24 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_fuzzing_FuzzingMutate_h
+#define mozilla_fuzzing_FuzzingMutate_h
+
+#include <random>
+
+namespace mozilla {
+namespace fuzzing {
+
+class FuzzingMutate {
+ public:
+  static void ChangeBit(uint8_t* aData, size_t aLength);
+  static void ChangeByte(uint8_t* aData, size_t aLength);
+};
+
+}  // namespace fuzzing
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/common/FuzzingTraits.cpp b/tools/fuzzing/common/FuzzingTraits.cpp
new file mode 100644
index 0000000000..9e6ba3ac1d
--- /dev/null
+++ b/tools/fuzzing/common/FuzzingTraits.cpp
@@ -0,0 +1,41 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <prinrval.h>
+#include <thread>
+#include <mutex>
+#include "FuzzingTraits.h"
+
+namespace mozilla {
+namespace fuzzing {
+
+/* static */
+unsigned int FuzzingTraits::Random(unsigned int aMax) {
+  MOZ_ASSERT(aMax > 0, "aMax needs to be bigger than 0");
+  std::uniform_int_distribution<unsigned int> d(0, aMax);
+  return d(Rng());
+}
+
+/* static */
+bool FuzzingTraits::Sometimes(unsigned int aProbability) {
+  return FuzzingTraits::Random(aProbability) == 0;
+}
+
+/* static */
+size_t FuzzingTraits::Frequency(const size_t aSize, const uint64_t aFactor) {
+  return RandomIntegerRange<size_t>(0, ceil(float(aSize) / aFactor)) + 1;
+}
+
+/* static */
+std::mt19937_64& FuzzingTraits::Rng() {
+  static std::mt19937_64 rng;
+  static std::once_flag flag;
+  std::call_once(flag, [&] { rng.seed(PR_IntervalNow()); });
+  return rng;
+}
+
+}  // namespace fuzzing
+}  // namespace mozilla
diff --git a/tools/fuzzing/common/FuzzingTraits.h b/tools/fuzzing/common/FuzzingTraits.h
new file mode 100644
index 0000000000..b8c9d76ab7
--- /dev/null
+++ b/tools/fuzzing/common/FuzzingTraits.h
@@ -0,0 +1,117 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_fuzzing_FuzzingTraits_h
+#define mozilla_fuzzing_FuzzingTraits_h
+
+#include "mozilla/Assertions.h"
+#include <cmath>
+#include <random>
+#include <type_traits>
+
+namespace mozilla {
+namespace fuzzing {
+
+class FuzzingTraits {
+ public:
+  static unsigned int Random(unsigned int aMax);
+  static bool Sometimes(unsigned int aProbability);
+  /**
+   * Frequency() defines how many mutations of a kind shall be applied to a
+   * target buffer by using a user definable factor. The higher the factor,
+   * the less mutations are being made.
+   */
+  static size_t Frequency(const size_t aSize, const uint64_t aFactor);
+
+  static std::mt19937_64& Rng();
+};
+
+/**
+ * RandomNumericLimit returns either the min or max limit of an arithmetic
+ * data type.
+ */
+template <typename T>
+T RandomNumericLimit() {
+  static_assert(std::is_arithmetic_v<T> == true,
+                "T must be an arithmetic type");
+  return FuzzingTraits::Sometimes(2) ? std::numeric_limits<T>::min()
+                                     : std::numeric_limits<T>::max();
+}
+
+/**
+ * RandomInteger generates negative and positive integers in 2**n increments.
+ */
+template <typename T>
+T RandomInteger() {
+  static_assert(std::is_integral_v<T> == true, "T must be an integral type");
+  double r =
+      static_cast<double>(FuzzingTraits::Random((sizeof(T) * CHAR_BIT) + 1));
+  T x = static_cast<T>(pow(2.0, r)) - 1;
+  if (std::numeric_limits<T>::is_signed && FuzzingTraits::Sometimes(2)) {
+    return (x * -1) - 1;
+  }
+  return x;
+}
+
+/**
+ * RandomIntegerRange returns a random integral within a [min, max] range.
+ */
+template <typename T>
+T RandomIntegerRange(T min, T max) {
+  static_assert(std::is_integral_v<T> == true, "T must be an integral type");
+  MOZ_ASSERT(min < max);
+  std::uniform_int_distribution<T> d(min, max);
+  return d(FuzzingTraits::Rng());
+}
+/**
+ * uniform_int_distribution is undefined for char/uchar. Need to handle them
+ * separately.
+ */
+template <>
+inline unsigned char RandomIntegerRange(unsigned char min, unsigned char max) {
+  MOZ_ASSERT(min < max);
+  std::uniform_int_distribution<unsigned short> d(min, max);
+  return static_cast<unsigned char>(d(FuzzingTraits::Rng()));
+}
+template <>
+inline char RandomIntegerRange(char min, char max) {
+  MOZ_ASSERT(min < max);
+  std::uniform_int_distribution<short> d(min, max);
+  return static_cast<char>(d(FuzzingTraits::Rng()));
+}
+
+/**
+ * RandomFloatingPointRange returns a random floating-point number within a
+ * [min, max] range.
+ */
+template <typename T>
+T RandomFloatingPointRange(T min, T max) {
+  static_assert(std::is_floating_point_v<T> == true,
+                "T must be a floating point type");
+  MOZ_ASSERT(min < max);
+  std::uniform_real_distribution<T> d(
+      min, std::nextafter(max, std::numeric_limits<T>::max()));
+  return d(FuzzingTraits::Rng());
+}
+
+/**
+ * RandomFloatingPoint returns a random floating-point number in 2**n
+ * increments.
+ */
+template <typename T>
+T RandomFloatingPoint() {
+  static_assert(std::is_floating_point_v<T> == true,
+                "T must be a floating point type");
+  int radix = RandomIntegerRange<int>(std::numeric_limits<T>::min_exponent,
+                                      std::numeric_limits<T>::max_exponent);
+  T x = static_cast<T>(pow(2.0, static_cast<double>(radix)));
+  return x * RandomFloatingPointRange<T>(-1.0, 1.0);
+}
+
+}  // namespace fuzzing
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/common/moz.build b/tools/fuzzing/common/moz.build
new file mode 100644
index 0000000000..afa19eddb4
--- /dev/null
+++ b/tools/fuzzing/common/moz.build
@@ -0,0 +1,11 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+SOURCES += ["FuzzingMutate.cpp", "FuzzingTraits.cpp"]
+
+EXPORTS += ["FuzzingMutate.h", "FuzzingTraits.h"]
+
+FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/docs/fuzzing_interface.rst b/tools/fuzzing/docs/fuzzing_interface.rst
new file mode 100644
index 0000000000..2b45abf35b
--- /dev/null
+++ b/tools/fuzzing/docs/fuzzing_interface.rst
@@ -0,0 +1,499 @@
+Fuzzing Interface
+=================
+
+The fuzzing interface is glue code living in mozilla-central in order to
+make it easier for developers and security researchers to test C/C++
+code with either `libFuzzer <https://llvm.org/docs/LibFuzzer.html>`__ or
+`afl-fuzz <http://lcamtuf.coredump.cx/afl/>`__.
+
+These fuzzing tools, are based on *compile-time instrumentation* to measure
+things like branch coverage and more advanced heuristics per fuzzing test.
+Doing so allows these tools to progress through code with little to no custom
+logic/knowledge implemented in the fuzzer itself. Usually, the only thing
+these tools need is a code "shim" that provides the entry point for the fuzzer
+to the code to be tested. We call this additional code a *fuzzing target* and
+the rest of this manual describes how to implement and work with these targets.
+
+As for the tools used with these targets, we currently recommend the use of
+libFuzzer over afl-fuzz, as the latter is no longer maintained while libFuzzer
+is being actively developed. Furthermore, libFuzzer has some advanced
+instrumentation features (e.g. value profiling to deal with complicated
+comparisons in code), making it overall more effective.
+
+What can be tested?
+~~~~~~~~~~~~~~~~~~~
+
+The interface can be used to test all C/C++ code that either ends up in
+``libxul`` (more precisely, the gtest version of ``libxul``) **or** is
+part of the JS engine.
+
+Note that this is not the right testing approach for testing the full
+browser as a whole. It is rather meant for component-based testing
+(especially as some components cannot be easily separated out of the
+full build).
+
+.. note::
+
+   **Note:** If you are working on the JS engine (trying to reproduce a
+   bug or seeking to develop a new fuzzing target), then please also read
+   the :ref:`JS Engine Specifics Section <JS Engine Specifics>` at the end
+   of this documentation, as the JS engine offers additional options for
+   implementing and running fuzzing targets.
+
+
+Reproducing bugs for existing fuzzing targets
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you are working on a bug that involves an existing fuzzing interface target,
+you have two options for reproducing the issue:
+
+
+Using existing builds
+^^^^^^^^^^^^^^^^^^^^^
+
+We have several fuzzing builds in CI that you can simply download. We recommend
+using ``fuzzfetch`` for this purpose, as it makes downloading and unpacking
+these builds much easier.
+
+You can install ``fuzzfetch`` from
+`Github <https://github.com/MozillaSecurity/fuzzfetch>`__ or
+`via pip <https://pypi.org/project/fuzzfetch/>`__.
+
+Afterwards, you can run
+
+::
+
+   $ python -m fuzzfetch -a --fuzzing --gtest -n firefox-fuzzing
+
+to fetch the latest optimized build. Alternatively, we offer non-ASan debug builds
+which you can download using
+
+::
+
+   $ python -m fuzzfetch -d --fuzzing --gtest -n firefox-fuzzing
+
+In both commands, ``firefox-fuzzing`` indicates the name of the directory that
+will be created for the download.
+
+Afterwards, you can reproduce the bug using 
+
+::
+
+   $ FUZZER=TargetName firefox-fuzzing/firefox test.bin
+
+assuming that ``TargetName`` is the name of the fuzzing target specified in the
+bug you are working on and ``test.bin`` is the attached testcase.
+
+.. note::
+
+   **Note:** You should not export the ``FUZZER`` variable permanently
+   in your shell, especially if you plan to do local builds. If the ``FUZZER``
+   variable is exported, it will affect the build process.
+
+If the CI builds don't meet your requirements and you need a local build instead,
+you can follow the steps below to create one:
+
+Local build requirements and flags
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+You will need a Linux environment with a recent Clang. Using the Clang downloaded
+by ``./mach bootstrap`` or a newer version is recommended.
+
+The only build flag required to enable the fuzzing targets is ``--enable-fuzzing``,
+so adding
+
+::
+
+  ac_add_options --enable-fuzzing
+
+to your ``.mozconfig`` is already sufficient for producing a fuzzing build.
+However, for improved crash handling capabilities and to detect additional errors,
+it is strongly recommended to combine libFuzzer with :ref:`AddressSanitizer <Address Sanitizer>`
+by adding
+
+::
+
+  ac_add_options --enable-address-sanitizer
+
+at least for optimized builds and bugs requiring ASan to reproduce at all
+(e.g. you are working on a bug where ASan reports a memory safety violation
+of some sort).
+
+Once your build is complete, you **must** additionally run
+
+::
+
+  $ ./mach gtest dontruntests
+
+to force the gtest libxul to be built.
+
+.. note::
+
+   **Note:** If you modify any code, please ensure that you run **both** build
+   commands to ensure that the gtest libxul is also rebuilt. It is a common mistake
+   to only run ``./mach build`` and miss the second command.
+
+Once these steps are complete, you can reproduce the bug locally using the same
+steps as described above for the downloaded builds.
+
+
+Developing new fuzzing targets
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Developing a new fuzzing target using the fuzzing interface only requires a few steps.
+
+
+Determine if the fuzzing interface is the right tool
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The fuzzing interface is not suitable for every kind of testing. In particular
+if your testing requires the full browser to be running, then you might want to
+look into other testing methods.
+
+The interface uses the ``ScopedXPCOM`` implementation to provide an environment
+in which XPCOM is available and initialized. You can initialize further subsystems
+that you might require, but you are responsible yourself for any kind of
+initialization steps.
+
+There is (in theory) no limit as to how far you can take browser initialization.
+However, the more subsystems are involved, the more problems might occur due to
+non-determinism and loss of performance.
+
+If you are unsure if the fuzzing interface is the right approach for you or you
+require help in evaluating what could be done for your particular task, please
+don't hestitate to :ref:`contact us <Fuzzing#contact-us>`.
+
+
+Develop the fuzzing code
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Where to put your fuzzing code
+''''''''''''''''''''''''''''''
+
+The code using the fuzzing interface usually lives in a separate directory
+called ``fuzztest`` that is on the same level as gtests. If your component
+has no gtests, then a subdirectory either in tests or in your main directory
+will work. If such a directory does not exist yet in your component, then you
+need to create one with a suitable ``moz.build``. See  `the transport target
+for an example <https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/moz.build>`__
+
+In order to include the new subdirectory into the build process, you will
+also have to modify the toplevel ``moz.build`` file accordingly. For this
+purpose, you should add your directory to ``TEST_DIRS`` only if ``FUZZING_INTERFACES``
+is set. See again `the transport target for an example
+<https://searchfox.org/mozilla-central/rev/de7676288a78b70d2b9927c79493adbf294faad5/media/mtransport/moz.build#18-24>`__.
+
+How your code should look like
+''''''''''''''''''''''''''''''
+
+In order to define your fuzzing target ``MyTarget``, you only need to implement 2 functions:
+
+1. A one-time initialization function.
+
+   At startup, the fuzzing interface calls this function **once**, so this can
+   be used to perform one-time operations like initializing subsystems or parsing
+   extra fuzzing options.
+   
+   This function is the equivalent of the `LLVMFuzzerInitialize <https://llvm.org/docs/LibFuzzer.html#startup-initialization>`__
+   function and has the same signature. However, with our fuzzing interface,
+   it won't be resolved by its name, so it can be defined ``static`` and called
+   whatever you prefer. Note that the function should always ``return 0`` and
+   can (except for the return), remain empty.
+
+   For the sake of this documentation, we assume that you have ``static int FuzzingInitMyTarget(int* argc, char*** argv);``
+
+2. The fuzzing iteration function.
+
+   This is where the actual fuzzing happens, and this function is the equivalent
+   of `LLVMFuzzerTestOneInput <https://llvm.org/docs/LibFuzzer.html#fuzz-target>`__.
+   Again, the difference to the fuzzing interface is that the function won't be
+   resolved by its name. In addition, we offer two different possible signatures
+   for this function, either
+
+   ``static int FuzzingRunMyTarget(const uint8_t* data, size_t size);``
+
+   or
+
+   ``static int FuzzingRunMyTarget(nsCOMPtr<nsIInputStream> inputStream);``
+
+   The latter is just a wrapper around the first one for implementations that
+   usually work with streams. No matter which of the two signatures you choose
+   to work with, the only thing you need to implement inside the function
+   is the use of the provided data with your target implementation. This can
+   mean to simply feed the data to your target, using the data to drive operations
+   on the target API, or a mix of both.
+
+   While doing so, you should avoid altering global state in a permanent way,
+   using additional sources of data/randomness or having code run beyond the
+   lifetime of the iteration function (e.g. on another thread), for one simple
+   reason: Coverage-guided fuzzing tools depend on the **deterministic** nature
+   of the iteration function. If the same input to this function does not lead
+   to the same execution when run twice (e.g. because the resulting state depends
+   on multiple successive calls or because of additional external influences),
+   then the tool will not be able to reproduce its fuzzing progress and perform
+   badly. Dealing with this restriction can be challenging e.g. when dealing
+   with asynchronous targets that run multi-threaded, but can usually be managed
+   by synchronizing execution on all threads at the end of the iteration function.
+   For implementations accumulating global state, it might be necessary to
+   (re)initialize this global state in each iteration, rather than doing it once
+   in the initialization function, even if this costs additional performance.
+
+   Note that unlike the vanilla libFuzzer approach, you are allowed to ``return 1``
+   in this function to indicate that an input is "bad". Doing so will cause
+   libFuzzer to discard the input, no matter if it generated new coverage or not.
+   This is particularly useful if you have means to internally detect and catch
+   bad testcase behavior such as timeouts/excessive resource usage etc. to avoid
+   these tests to end up in your corpus.
+
+
+Once you have implemented the two functions, the only thing remaining is to
+register them with the fuzzing interface. For this purpose, we offer two
+macros, depending on which iteration function signature you used. If you
+sticked to the classic signature using buffer and size, you can simply use
+
+::
+
+  #include "FuzzingInterface.h"
+
+  // Your includes and code
+
+  MOZ_FUZZING_INTERFACE_RAW(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);
+
+where ``MyTarget`` is the name of the target and will be used later to decide
+at runtime which target should be used.
+
+If instead you went for the streaming interface, you need a different include,
+but the macro invocation is quite similar:
+
+::
+
+  #include "FuzzingInterfaceStream.h"
+
+  // Your includes and code
+
+  MOZ_FUZZING_INTERFACE_STREAM(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);
+
+For a live example, see also the `implementation of the STUN fuzzing target
+<https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/stun_parser_libfuzz.cpp>`__.
+
+Add instrumentation to the code being tested
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+libFuzzer requires that the code you are trying to test is instrumented
+with special compiler flags. Fortunately, adding these on a per-directory basis
+can be done just by including the following directive in each ``moz.build``
+file that builds code under test:
+
+::
+
+  # Add libFuzzer configuration directives
+  include('/tools/fuzzing/libfuzzer-config.mozbuild')
+
+
+The include already does the appropriate configuration checks to be only
+active in fuzzing builds, so you don't have to guard this in any way.
+
+.. note::
+
+   **Note:** This include modifies `CFLAGS` and `CXXFLAGS` accordingly
+   but this only works for source files defined in this particular
+   directory. The flags are **not** propagated to subdirectories automatically
+   and you have to ensure that each directory that builds source files
+   for your target has the include added to its ``moz.build`` file.
+
+By keeping the instrumentation limited to the parts that are actually being
+tested using this tool, you not only increase the performance but also potentially
+reduce the amount of noise that libFuzzer sees.
+
+
+Build your code
+^^^^^^^^^^^^^^^
+
+See the :ref:`Build instructions above <Local build requirements and flags>` for instructions
+how to modify your ``.mozconfig`` to create the appropriate build.
+
+
+Running your code and building a corpus
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+You need to set the following environment variable to enable running the
+fuzzing code inside Firefox instead of the regular browser.
+
+-  ``FUZZER=name``
+
+Where ``name`` is the name of your fuzzing module that you specified
+when calling the ``MOZ_FUZZING_INTERFACE_RAW`` macro. For the example
+above, this would be ``MyTarget`` or ``StunParser`` for the live example.
+
+Now when you invoke the firefox binary in your build directory with the
+``-help=1`` parameter, you should see the regular libFuzzer help. On
+Linux for example:
+
+::
+
+   $ FUZZER=StunParser obj-asan/dist/bin/firefox -help=1
+
+You should see an output similar to this:
+
+::
+
+   Running Fuzzer tests...
+   Usage:
+
+   To run fuzzing pass 0 or more directories.
+   obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]
+
+   To run individual tests without fuzzing pass 1 or more files:
+   obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]
+
+   Flags: (strictly in form -flag=value)
+    verbosity                      1       Verbosity level.
+    seed                           0       Random seed. If 0, seed is generated.
+    runs                           -1      Number of individual test runs (-1 for infinite runs).
+    max_len                        0       Maximum length of the test input. If 0, libFuzzer tries to guess a good value based on the corpus and reports it.
+   ...
+
+
+Reproducing a Crash
+'''''''''''''''''''
+
+In order to reproduce a crash from a given test file, simply put the
+file as the only argument on the command line, e.g.
+
+::
+
+   $ FUZZER=StunParser obj-asan/dist/bin/firefox test.bin
+
+This should reproduce the given problem.
+
+
+FuzzManager and libFuzzer
+'''''''''''''''''''''''''
+
+Our FuzzManager project comes with a harness for running libFuzzer with
+an optional connection to a FuzzManager server instance. Note that this
+connection is not mandatory, even without a server you can make use of
+the local harness.
+
+You can find the harness
+`here <https://github.com/MozillaSecurity/FuzzManager/tree/master/misc/afl-libfuzzer>`__.
+
+An example invocation for the harness to use with StunParser could look
+like this:
+
+::
+
+   FUZZER=StunParser python /path/to/afl-libfuzzer-daemon.py --fuzzmanager \
+       --stats libfuzzer-stunparser.stats --libfuzzer-auto-reduce-min 500 --libfuzzer-auto-reduce 30 \
+       --tool libfuzzer-stunparser --libfuzzer --libfuzzer-instances 6 obj-asan/dist/bin/firefox \
+       -max_len=256 -use_value_profile=1 -rss_limit_mb=3000 corpus-stunparser
+
+What this does is
+
+-  run libFuzzer on the ``StunParser`` target with 6 parallel instances
+   using the corpus in the ``corpus-stunparser`` directory (with the
+   specified libFuzzer options such as ``-max_len`` and
+   ``-use_value_profile``)
+-  automatically reduce the corpus and restart if it grew by 30% (and
+   has at least 500 files)
+-  use FuzzManager (need a local ``.fuzzmanagerconf`` and a
+   ``firefox.fuzzmanagerconf`` binary configuration as described in the
+   FuzzManager manual) and submit crashes as ``libfuzzer-stunparser``
+   tool
+-  write statistics to the ``libfuzzer-stunparser.stats`` file
+
+
+JS Engine Specifics
+~~~~~~~~~~~~~~~~~~~
+
+The fuzzing interface can also be used for testing the JS engine, in fact there
+are two separate options to implement and run fuzzing targets:
+
+Implementing in C++
+^^^^^^^^^^^^^^^^^^^
+
+Similar to the fuzzing interface in Firefox, you can implement your target in
+entirely C++ with very similar interfaces compared to what was described before.
+
+There are a few minor differences though:
+
+1. All of the fuzzing targets live in `js/src/fuzz-tests`.
+
+2. All of the code is linked into a separate binary called `fuzz-tests`,
+   similar to how all JSAPI tests end up in `jsapi-tests`. In order for this
+   binary to be built, you must build a JS shell with ``--enable-fuzzing``
+   **and** ``--enable-tests``. Again, this can and should be combined with
+   AddressSanitizer for maximum effectiveness. This also means that there is no
+   need to (re)build gtests when dealing with a JS fuzzing target and using
+   a shell as part of a full browser build.
+
+3. The harness around the JS implementation already provides you with an
+   initialized ``JSContext`` and global object. You can access these in
+   your target by declaring
+
+   ``extern JS::PersistentRootedObject gGlobal;``
+
+   and
+
+   ``extern JSContext* gCx;``
+
+   but there is no obligation for you to use these.
+
+For a live example, see also the `implementation of the StructuredCloneReader target
+<https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/testStructuredCloneReader.cpp>`__.
+
+
+Implementing in JS
+^^^^^^^^^^^^^^^^^^
+
+In addition to the C++ targets, you can also implement targets in JavaScript
+using the JavaScript Runtime (JSRT) fuzzing approach. Using this approach is
+not only much simpler (since you don't need to know anything about the
+JSAPI or engine internals), but it also gives you full access to everything
+defined in the JS shell, including handy functions such as ``timeout()``.
+
+Of course, this approach also comes with disadvantages: Calling into JS and
+performing the fuzzing operations there costs performance. Also, there is more
+chance for causing global side-effects or non-determinism compared to a
+fairly isolated C++ target.
+
+As a rule of thumb, you should implement the target in JS if
+
+* you don't know C++ and/or how to use the JSAPI (after all, a JS fuzzing target is better than none),
+* your target is expected to have lots of hangs/timeouts (you can catch these internally),
+* or your target is not isolated enough for a C++ target and/or you need specific JS shell functions.
+
+
+There is an `example target <https://searchfox.org/mozilla-central/source/js/src/shell/jsrtfuzzing/jsrtfuzzing-example.js>`__
+in-tree that shows roughly how to implement such a fuzzing target.
+
+To run such a target, you must run the ``js`` (shell) binary instead of the
+``fuzz-tests`` binary and point the ``FUZZER`` variable to the file containing
+your fuzzing target, e.g.
+
+::
+
+   $ FUZZER=/path/to/jsrtfuzzing-example.js obj-asan/dist/bin/js --fuzzing-safe --no-threads -- <libFuzzer options here>
+
+More elaborate targets can be found in `js/src/fuzz-tests/ <https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/>`__.
+
+Troubleshooting
+~~~~~~~~~~~~~~~
+
+
+Fuzzing Interface: Error: No testing callback found
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This error means that the fuzzing callback with the name you specified
+using the ``FUZZER`` environment variable could not be found. Reasons
+for are typically either a misspelled name or that your code wasn't
+built (check your ``moz.build`` file and build log).
+
+
+``mach build`` doesn't seem to update my fuzzing code
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Keep in mind you always need to run both the ``mach build`` and
+``mach gtest dontruntests`` commands in order to update your fuzzing
+code. The latter rebuilds the gtest version of ``libxul``, containing
+your code.
diff --git a/tools/fuzzing/docs/index.rst b/tools/fuzzing/docs/index.rst
new file mode 100644
index 0000000000..4be57b3be5
--- /dev/null
+++ b/tools/fuzzing/docs/index.rst
@@ -0,0 +1,434 @@
+Fuzzing
+=======
+
+.. toctree::
+  :maxdepth: 1
+  :hidden:
+  :glob:
+  :reversed:
+
+  *
+
+This section focuses on explaining the software testing technique called
+“Fuzzing” or “Fuzz Testing” and its application to the Mozilla codebase.
+The overall goal is to educate developers about the capabilities and
+usefulness of fuzzing and also allow them to write their own fuzzing
+targets. Note that not all fuzzing tools used at Mozilla are open
+source. Some tools are for internal use only because they can easily
+find critical security vulnerabilities.
+
+What is Fuzzing?
+----------------
+
+Fuzzing (or Fuzz Testing) is a technique to randomly use a program or
+parts of it with the goal to uncover bugs. Random usage can have a wide
+variety of forms, a few common ones are
+
+-  random input data (e.g. file formats, network data, source code, etc.)
+
+-  random API usage
+
+-  random UI interaction
+
+with the first two being the most practical methods used in the field.
+Of course, these methods are not entirely separate, combinations are
+possible. Fuzzing is a great way to find quality issues, some of them
+being also security issues.
+
+Random input data
+~~~~~~~~~~~~~~~~~
+
+This is probably the most obvious fuzzing method: You have code that
+processes data and you provide it with random or mutated data, hoping
+that it will uncover bugs in your implementation. Examples are media
+formats like JPEG or H.264, but basically anything that involves
+processing a “blob” of data can be a valuable target. Countless security
+vulnerabilities in a variety of libraries and programs have been found
+using this method (the AFLFuzz
+`bug-o-rama <http://lcamtuf.coredump.cx/afl/#bugs>`__ gives a good
+impression).
+
+Common tools for this task are e.g.
+`libFuzzer <https://llvm.org/docs/LibFuzzer.html>`__ and
+`AFLFuzz <http://lcamtuf.coredump.cx/afl/>`__, but also specialized
+tools with custom logic like
+`LangFuzz <https://www.usenix.org/system/files/conference/usenixsecurity12/sec12-final73.pdf>`__
+and `Avalanche <https://github.com/MozillaSecurity/avalanche>`__.
+
+Random API Usage
+~~~~~~~~~~~~~~~~
+
+Randomly testing APIs is especially helpful with parts of software that
+expose a well-defined interface (see also :ref:`Well-defined
+behavior and Safety`). If this interface is additionally exposed to
+untrusted parties/content, then this is a strong sign that random API
+testing would be worthwhile here, also for security reasons. APIs can be
+anything from C++ layer code to APIs offered in the browser.
+
+A good example for a fuzzing target here is the DOM (Document Object
+Model) and various other browser APIs. The browser exposes a variety of
+different APIs for working with documents, media, communication,
+storage, etc. with a growing complexity. Each of these APIs has
+potential bugs that can be uncovered with fuzzing. At Mozilla, we
+currently use domino (internal tool) for this purpose.
+
+Random UI Interaction
+~~~~~~~~~~~~~~~~~~~~~
+
+A third way to test programs and in particular user interfaces is by
+directly interacting with the UI in a random way, typically in
+combination with other actions the program has to perform. Imagine for
+example an automated browser that surfs through the web and randomly
+performs actions such as scrolling, zooming and clicking links. The nice
+thing about this approach is that you likely find many issues that the
+end-user also experiences. However, this approach typically suffers from
+bad reproducibility (see also :ref:`Reproducibility`) and is therefore
+often of limited use.
+
+An example for a fuzzing tool using this technique is `Android
+Monkey <https://developer.android.com/studio/test/monkey>`__. At
+Mozilla however, we currently don’t make much use of this approach.
+
+Why Fuzzing Helps You
+---------------------
+
+Understanding the value of fuzzing for you as a developer and software
+quality in general is important to justify the support this testing
+method might need from you. When your component is fuzzed for the first
+time there are two common things you will be confronted with:
+
+**Bug reports that don’t seem real bugs or not important:** Fuzzers
+find all sorts of bugs in various corners of your component, even
+obscure ones. This automatically leads to a larger number of bugs that
+either don’t seem to be bugs (see also the :ref:`Well-defined behavior and
+safety` section below) or that don’t seem to be important bugs.
+
+Fixing these bugs is still important for the fuzzers because ignoring them
+in fuzzing costs resources (performance, human resources) and might even
+prevent the fuzzer from hitting other bugs. For example certain fuzzing tools
+like libFuzzer run in-process and have to restart on every crash, involving a
+costly re-read of the fuzzing samples.
+
+Also, as some of our code evolves quickly, a corner case might become a
+hot code path in a few months.
+
+**New steps to reproduce:** Fuzzing tools are very likely to exercise
+your component using different methods than an average end-user. A
+common technique is modify existing parts of a program or write entirely
+new code to yield a fuzzing "target". This target is specifically
+designed to work with the fuzzing tools in use. Reproducing the reported
+bugs might require you to learn these new steps to reproduce, including
+building/acquiring that target and having the right environment.
+
+Both of these issues might seem like a waste of time in some cases,
+however, realizing that both steps are a one-time investment for a
+constant stream of valuable bug reports is paramount here. Helping your
+security engineers to overcome these issues will ensure that future
+regressions in your code can be detected at an earlier stage and in a
+form that is more easily actionable. Especially if you are dealing with
+regressions in your code already, fuzzing has the potential to make your
+job as a developer easier.
+
+One of the best examples at Mozilla is the JavaScript engine. The JS
+team has put great quite some effort into getting fuzzing started and
+supporting our work. Here’s what Jan de Mooij, a senior platform
+engineer for the JavaScript engine, has to say about it:
+
+*“Bugs in the engine can cause mysterious browser crashes and bugs that
+are incredibly hard to track down. Fortunately, we don't have to deal
+with these time consuming browser issues very often: usually the fuzzers
+find a reliable shell test long before the bug makes it into a release.
+Fuzzing is invaluable to us and I cannot imagine working on this project
+without it.”*
+
+Levels of Fuzzing in Firefox/Gecko
+----------------------------------
+
+Applying fuzzing to e.g. Firefox happens at different "levels", similar
+to the different types of automated tests we have:
+
+Full Browser Fuzzing
+~~~~~~~~~~~~~~~~~~~~
+
+The most obvious method of testing would be to test the full browser and
+doing so is required for certain features like the DOM and other APIs.
+The advantage here is that we have all the features of the browser
+available and testing happens closely to what we actually ship. The
+downside here though is that browser testing is by far the slowest of
+all testing methods. In addition, it has the most amount of
+non-determinism involved (resulting e.g. in intermittent testcases).
+Browser fuzzing at Mozilla is largely done with the `Grizzly
+framework <https://blog.mozilla.org/security/2019/07/10/grizzly/>`__
+(`meta bug <https://bugzilla.mozilla.org/show_bug.cgi?id=grizzly>`__)
+and one of the most successful fuzzers is the Domino tool (`meta
+bug <https://bugzilla.mozilla.org/show_bug.cgi?id=domino>`__).
+
+Summarizing, full browser fuzzing is the right technique to investigate
+if your feature really requires it. Consider using other methods (see
+below) if your code can be exercised in this way.
+
+The Fuzzing Interface
+~~~~~~~~~~~~~~~~~~~~~
+
+**Fuzzing Interface**
+
+The fuzzing interface is glue code living in mozilla-central in order to make it
+easier for developers and security researchers to test C/C++ code with either libFuzzer or afl-fuzz.
+
+This interface offers a gtest (C++ unit test) level component based
+fuzzing approach and is suitable for anything that could also be
+tested/exercised using a gtest. This method is by far the fastest, but
+usually limited to testing isolated components that can be instantiated
+on this level. Utilizing this method requires you to write a fuzzing
+target similar to writing a gtest. This target will automatically be
+usable with libFuzzer and AFLFuzz. We offer a :ref:`comprehensive manual <Fuzzing Interface>`
+that describes how to write and utilize your own target.
+
+A simple example here is the `SDP parser
+target <https://searchfox.org/mozilla-central/rev/efdf9bb55789ea782ae3a431bda6be74a87b041e/media/webrtc/signaling/fuzztest/sdp_parser_libfuzz.cpp#30>`__,
+which tests the SipccSdpParser in our codebase.
+
+Shell-based Fuzzing
+~~~~~~~~~~~~~~~~~~~
+
+Some of our fuzzing, e.g. JS Engine testing, happens in a separate shell
+program. For JS, this is the JS shell also used for most of the JS tests
+and development. In theory, xpcshell could also be used for testing but
+so far, there has not been a use case for this (most things that can be
+reached through xpcshell can also be tested on the gtest level).
+
+Identifying the right level of fuzzing is the first step towards
+continuous fuzz testing of your code.
+
+Code/Process Requirements for Fuzzing
+-------------------------------------
+
+In this section, we are going to discuss how code should be written in
+order to yield optimal results with fuzzing.
+
+Defect Oracles
+~~~~~~~~~~~~~~
+
+Fuzzing is only effective if you are able to know when a problem has
+been found. Crashes are typically problems if the unit being tested is
+safe for fuzzing (see Well-defined behavior and Safety). But there are
+many more problems that you would want to find, correctness issues,
+corruptions that don’t necessarily crash etc. For this, you need an
+*oracle* that tells you something is wrong.
+
+The simplest defect oracle is the assertion (ex: ``MOZ_ASSERT``).
+Assertions are a very powerful instrument because they can be used to
+determine if your program is performing correctly, even if the bug would
+not lead to any sort of crash. They can encode arbitrarily complex
+information about what is considered correct, information that might
+otherwise only exist in the developers’ minds.
+
+External tools like the sanitizers (AddressSanitizer aka ASan,
+ThreadSanitizer aka TSan, MemorySanitizer aka MSan and
+UndefinedBehaviorSanitizer - UBSan) can also serve as oracles for
+sometimes severe issues that would not necessarily crash. Making sure
+that these tools can be used on your code is highly useful.
+
+Examples for bugs found with sanitizers are `bug
+1419608 <https://bugzilla.mozilla.org/show_bug.cgi?id=1419608>`__,
+`bug 1580288 <https://bugzilla.mozilla.org/show_bug.cgi?id=1580288>`__
+and `bug 922603 <https://bugzilla.mozilla.org/show_bug.cgi?id=922603>`__,
+but since we started using sanitizers, we have found over 1000 bugs with
+these tools.
+
+Another defect oracle can be a reference implementation. Comparing
+program behavior (typically output) between two programs or two modes of
+the same program that should produce the same outputs can find complex
+correctness issues. This method is often called differential testing.
+
+One example where this is regularly used to find issues is the Mozilla
+JavaScript engine: Running random programs with and without JIT
+compilation enabled finds lots of problems with the JIT implementation.
+One example for such a bug is `Bug
+1404636 <https://bugzilla.mozilla.org/show_bug.cgi?id=1404636>`__.
+
+Component Decoupling
+~~~~~~~~~~~~~~~~~~~~
+
+Being able to test components in isolation can be an advantage for
+fuzzing (both for performance and reproducibility). Clear boundaries
+between different components and documentation that explains the
+contracts usually help with this goal. Sometimes it might be useful to
+mock a certain component that the target component is interacting with
+and that is much harder if the components are tightly coupled and their
+contracts unclear. Of course, this does not mean that one should only
+test components in isolation. Sometimes, testing the interaction between
+them is even desirable and does not hurt performance at all.
+
+Avoiding external I/O
+~~~~~~~~~~~~~~~~~~~~~
+
+External I/O like network or file interactions are bad for performance
+and can introduce additional non-determinism. Providing interfaces to
+process data directly from memory instead is usually much more helpful.
+
+Well-defined Behavior and Safety
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This requirement mostly ties in where defect oracles ended and is one of
+the most important problems seen in the wild nowadays with fuzzing. If a
+part of your program’s behavior is unspecified, then this potentially
+leads to bad times if the behavior is considered a defect by fuzzing.
+For example, if your code has crashes that are not considered bugs, then
+your code might be unsuitable for fuzzing. Your component should be
+fuzzing safe, meaning that any defect oracle (e.g. assertion or crash)
+triggered by the fuzzer is considered a bug. This important aspect is
+often neglected. Be aware that any false positives cause both
+performance degradation and additional manual work for your fuzzing
+team. The Mozilla JS developers for example have implemented this
+concept in a “--fuzzing-safe” switch which disables harmful functions.
+Sometimes, crashes cannot be avoided for handling certain error
+conditions. In such situations, it is important to mark these crashes in
+a way the fuzzer can recognize and distinguish them from undesired
+crashes. However, keep in mind that crashes in general can be disruptive
+to the fuzzing process. Performance is an important aspect of fuzzing
+and frequent crashes can severely degrade performance.
+
+Reproducibility
+~~~~~~~~~~~~~~~
+
+Being able to reproduce issues found with fuzzing is necessary for
+several reasons: First, you as the developer probably want a test that
+reproduces the issue so you can debug it better. Our feedback from most
+developers is that traces without a reproducible test can help to find a
+problem, but it makes the whole process very complicated. Some of these
+non-reproducible bugs never get fixed. Second, having a reproducible
+test also helps the triage process by allowing an automated bisection to
+find the responsible developer. Last but not least, the test can be
+added to a test suite, used for automated verification of fixes and even
+serve as a basis for more fuzzing.
+
+Adding functionality to the program that improve reproducibility is
+therefore a good idea in case non-reproducible issues are found. Some
+examples are shown in the next section.
+
+While many problems with reproducibility are specific for the project
+you are working on, there is one source of these problems that many
+programs have in common: Threading. While some bugs only occur in the
+first place due to concurrency, some other bugs would be perfectly
+reproducible without threads, but are intermittent and hard to with
+threading enabled. If the bug is indeed caused by a data race, then
+tools like ThreadSanitizer will help and we are currently working on
+making ThreadSanitizer usable on Firefox. For bugs that are not caused
+by threading, it sometimes makes sense to be able to disable threading
+or limit the amount of worker threads involved.
+
+Supporting Code
+~~~~~~~~~~~~~~~
+
+Some possibilities of what support implementations for fuzzing can do
+have already been named in the previous sections: Additional defect
+oracles and functionality to improve reproducibility and safety. In
+fact, many features added specifically for fuzzing fit into one of these
+categories. However, there’s room for more: Often, there are ways to
+make it easier for fuzzers to exercise complex and hard to reach parts
+of your code. For example, if a certain optimization feature is only
+turned on under very specific conditions (that are not a requirement for
+the optimization), then it makes sense to add a functionality to force
+it on. Then, a fuzzer can hit the optimization code much more
+frequently, increasing the chance to find issues. Some examples from
+Firefox and SpiderMonkey:
+
+- The `FuzzingFunctions <https://searchfox.org/mozilla-central/rev/efdf9bb55789ea782ae3a431bda6be74a87b041e/dom/webidl/FuzzingFunctions.webidl#15>`__
+  interface in the browser allows fuzzing tools to perform GC/CC, tune various
+  settings related to garbage collection or enable features like accessibility
+  mode. Being able to force a garbage collection at a specific time helped
+  identifying lots of problems in the past.
+
+- The --ion-eager and --baseline-eager flags for the JS shell force JIT
+  compilation at various stages, rather than using the builtin
+  heuristic to enable it only for hot functions.
+
+- The --no-threads flag disables all threading (if possible) in the JS shell.
+  This makes some bugs reproduce deterministically that would otherwise be
+  intermittent and harder to find. However, some bugs that only occur with
+  threading can’t be found with this option enabled.
+
+Another important feature that must be turned off for fuzzing is
+checksums. Many file formats use checksums to validate a file before
+processing it. If a checksum feature is still enabled, fuzzers are
+likely never going to produce valid files. The same often holds for
+cryptographic signatures. Being able to turn off the validation of these
+features as part of a fuzzing switch is extremely helpful.
+
+An example for such a checksum can be found in the
+`FlacDemuxer <https://searchfox.org/mozilla-central/rev/efdf9bb55789ea782ae3a431bda6be74a87b041e/dom/media/flac/FlacDemuxer.cpp#494>`__.
+
+Test Samples
+~~~~~~~~~~~~
+
+Some fuzzing strategies make use of existing data that is mutated to
+produce the new random data. In fact, mutation-based strategies are
+typically superior to others if the original samples are of good quality
+because the originals carry a lot of semantics that the fuzzer does not
+have to know about or implement. However, success here really stands and
+falls with the quality of the samples. If the originals don’t cover
+certain parts of the implementation, then the fuzzer will also have to
+do more work to get there.
+
+
+Fuzz Blockers
+~~~~~~~~~~~~~
+
+Fuzz blockers are issues that prevent fuzzers from being as
+effective as possible. Depending on the fuzzer and its scope a fuzz blocker
+in one area (or component) can impede performance in other areas and in
+some cases block the fuzzer all together. Some examples are:
+
+- Frequent crashes - These can block code paths and waste compute
+  resources due to the need to relaunch the fuzzing target and handle
+  the results (regardless of whether it is ignored or reported). This can also
+  include assertions that are mostly benign in many cases are but easily
+  triggered by fuzzers.
+
+- Frequent hangs / timeouts - This includes any issue that slows down
+  or blocks execution of the fuzzer or the target.
+
+- Hard to bucket - This includes crashes such as stack overflows or any issue
+  that crashes in an inconsistent location. This also includes issues that
+  corrupt logs/debugger output or provide a broken/invalid crash report.
+
+- Broken builds - This is fairly straightforward, without up-to-date builds
+  fuzzers are unable to run or verify fixes. 
+
+- Missing instrumentation - In some cases tools such as ASan are used as
+  defect oracles and are required by the fuzzing tools to allow for proper
+  automation. In other cases incomplete instrumentation can give a false sense
+  of stability or make investigating issues much more time consuming. Although
+  this is not necessarily blocking the fuzzers it should be prioritized
+  appropriately.
+
+Since these types of crashes harm the overall fuzzing progress, it is important
+for them to be addressed in a timely manner. Even if the bug itself might seem
+trivial and low priority for the product, it can still have devastating effects
+on fuzzing and hence prevent finding other critical issues. 
+
+Issues in Bugzilla are marked as fuzz blockers by adding “[fuzzblocker]”
+to the “Whiteboard” field. A list of open issues marked as fuzz blockers
+can be found on `Bugzilla <https://bugzilla.mozilla.org/buglist.cgi?cmdtype=dorem&remaction=run&namedcmd=fuzzblockers&sharer_id=486634>`__.
+
+
+Documentation
+~~~~~~~~~~~~~
+
+It is important for the fuzzing team to know how your software, tests
+and designs work. Even obvious tasks, like how a test program is
+supposed to be invoked, which options are safe, etc. might be hard to
+figure out for the person doing the testing, just as you are reading
+this manual right now to find out what is important in fuzzing.
+
+Contact Us
+~~~~~~~~~~
+
+The fuzzing team can be reached at
+`fuzzing@mozilla.com <mailto:fuzzing@mozilla.com>`__ or
+`on Matrix <https://chat.mozilla.org/#/room/#fuzzing:mozilla.org>`__
+and will be happy to help you with any questions about fuzzing
+you might have. We can help you find the right method of fuzzing for
+your feature, collaborate on the implementation and provide the
+infrastructure to run it and process the results accordingly.
diff --git a/tools/fuzzing/faulty/Faulty.cpp b/tools/fuzzing/faulty/Faulty.cpp
new file mode 100644
index 0000000000..526ec50c86
--- /dev/null
+++ b/tools/fuzzing/faulty/Faulty.cpp
@@ -0,0 +1,786 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <cerrno>
+#include <climits>
+#include <cmath>
+#include <fstream>
+#include <mutex>
+#include <prinrval.h>
+#include <type_traits>
+#ifdef _WINDOWS
+#  include <process.h>
+#  define getpid _getpid
+#else
+#  include <unistd.h>
+#endif
+#include "base/string_util.h"
+#include "FuzzingMutate.h"
+#include "FuzzingTraits.h"
+#include "chrome/common/ipc_channel.h"
+#include "chrome/common/ipc_message.h"
+#include "chrome/common/file_descriptor_set_posix.h"
+#include "mozilla/ipc/Faulty.h"
+#include "nsComponentManagerUtils.h"
+#include "nsNetCID.h"
+#include "nsIFile.h"
+#include "nsIFileStreams.h"
+#include "nsILineInputStream.h"
+#include "nsIRunnable.h"
+#include "nsThreadUtils.h"
+#include "nsLocalFile.h"
+#include "nsNetCID.h"
+#include "nsPrintfCString.h"
+#include "nsTArray.h"
+#include "nsXULAppAPI.h"
+#include "prenv.h"
+
+#ifdef IsLoggingEnabled
+// This is defined in the Windows SDK urlmon.h
+#  undef IsLoggingEnabled
+#endif
+
+namespace mozilla {
+namespace ipc {
+
+using namespace mozilla::fuzzing;
+
+/**
+ * FuzzIntegralType mutates an incercepted integral type of a pickled message.
+ */
+template <typename T>
+void FuzzIntegralType(T* v, bool largeValues) {
+  static_assert(std::is_integral_v<T> == true, "T must be an integral type");
+  switch (FuzzingTraits::Random(6)) {
+    case 0:
+      if (largeValues) {
+        (*v) = RandomInteger<T>();
+        break;
+      }
+      [[fallthrough]];
+    case 1:
+      if (largeValues) {
+        (*v) = RandomNumericLimit<T>();
+        break;
+      }
+      [[fallthrough]];
+    case 2:
+      if (largeValues) {
+        (*v) = RandomIntegerRange<T>(std::numeric_limits<T>::min(),
+                                     std::numeric_limits<T>::max());
+        break;
+      }
+      [[fallthrough]];
+    default:
+      switch (FuzzingTraits::Random(2)) {
+        case 0:
+          // Prevent underflow
+          if (*v != std::numeric_limits<T>::min()) {
+            (*v)--;
+            break;
+          }
+          [[fallthrough]];
+        case 1:
+          // Prevent overflow
+          if (*v != std::numeric_limits<T>::max()) {
+            (*v)++;
+            break;
+          }
+      }
+  }
+}
+
+/**
+ * FuzzFloatingPointType mutates an incercepted floating-point type of a
+ * pickled message.
+ */
+template <typename T>
+void FuzzFloatingPointType(T* v, bool largeValues) {
+  static_assert(std::is_floating_point_v<T> == true,
+                "T must be a floating point type");
+  switch (FuzzingTraits::Random(6)) {
+    case 0:
+      if (largeValues) {
+        (*v) = RandomNumericLimit<T>();
+        break;
+      }
+      [[fallthrough]];
+    case 1:
+      if (largeValues) {
+        (*v) = RandomFloatingPointRange<T>(std::numeric_limits<T>::min(),
+                                           std::numeric_limits<T>::max());
+        break;
+      }
+      [[fallthrough]];
+    default:
+      (*v) = RandomFloatingPoint<T>();
+  }
+}
+
+/**
+ * FuzzStringType mutates an incercepted string type of a pickled message.
+ */
+template <typename T>
+void FuzzStringType(T& v, const T& literal1, const T& literal2) {
+  switch (FuzzingTraits::Random(5)) {
+    case 4:
+      v = v + v;
+      [[fallthrough]];
+    case 3:
+      v = v + v;
+      [[fallthrough]];
+    case 2:
+      v = v + v;
+      break;
+    case 1:
+      v += literal1;
+      break;
+    case 0:
+      v = literal2;
+      break;
+  }
+}
+
+Faulty::Faulty()
+    // Mutate messages as a blob.
+    : mFuzzMessages(!!PR_GetEnv("FAULTY_MESSAGES"))
+      // Enables the strategy for fuzzing pipes.
+      ,
+      mFuzzPipes(!!PR_GetEnv("FAULTY_PIPE"))
+      // Enables the strategy for fuzzing pickled messages.
+      ,
+      mFuzzPickle(!!PR_GetEnv("FAULTY_PICKLE"))
+      // Uses very large values while fuzzing pickled messages.
+      // This may cause a high amount of malloc_abort() / NS_ABORT_OOM crashes.
+      ,
+      mUseLargeValues(!!PR_GetEnv("FAULTY_LARGE_VALUES"))
+      // Use the provided blacklist as whitelist.
+      ,
+      mUseAsWhitelist(!!PR_GetEnv("FAULTY_AS_WHITELIST"))
+      // Sets up our target process.
+      ,
+      mIsValidProcessType(IsValidProcessType()) {
+  if (mIsValidProcessType) {
+    FAULTY_LOG("Initializing for new process of type '%s' with pid %u.",
+               XRE_GetProcessTypeString(), getpid());
+
+    /* Setup random seed. */
+    const char* userSeed = PR_GetEnv("FAULTY_SEED");
+    unsigned long randomSeed = static_cast<unsigned long>(PR_IntervalNow());
+    if (userSeed) {
+      long n = std::strtol(userSeed, nullptr, 10);
+      if (n != 0) {
+        randomSeed = static_cast<unsigned long>(n);
+      }
+    }
+    FuzzingTraits::Rng().seed(randomSeed);
+
+    /* Setup directory for dumping messages. */
+    mMessagePath = PR_GetEnv("FAULTY_MESSAGE_PATH");
+    if (mMessagePath && *mMessagePath) {
+      if (CreateOutputDirectory(mMessagePath) != NS_OK) {
+        mMessagePath = nullptr;
+      }
+    }
+
+    /* Set IPC messages blacklist. */
+    mBlacklistPath = PR_GetEnv("FAULTY_BLACKLIST");
+    if (mBlacklistPath && *mBlacklistPath) {
+      FAULTY_LOG("* Using message blacklist    = %s", mBlacklistPath);
+    }
+
+    FAULTY_LOG("* Fuzzing strategy: messages = %s",
+               mFuzzMessages ? "enabled" : "disabled");
+    FAULTY_LOG("* Fuzzing strategy: pickle   = %s",
+               mFuzzPickle ? "enabled" : "disabled");
+    FAULTY_LOG("* Fuzzing strategy: pipe     = %s",
+               mFuzzPipes ? "enabled" : "disabled");
+    FAULTY_LOG("* Fuzzing probability        = %u", DefaultProbability());
+    FAULTY_LOG("* Fuzzing mutation factor    = %u", MutationFactor());
+    FAULTY_LOG("* RNG seed                   = %lu", randomSeed);
+
+    sMsgCounter = 0;
+  }
+}
+
+// static
+bool Faulty::IsValidProcessType(void) {
+  bool isValidProcessType;
+  const bool targetChildren = !!PR_GetEnv("FAULTY_CHILDREN");
+  const bool targetParent = !!PR_GetEnv("FAULTY_PARENT");
+  const bool isParent = XRE_IsParentProcess();
+
+  if (targetChildren && !targetParent) {
+    // Fuzz every child process type but not the parent process.
+    isValidProcessType = isParent;
+  } else if (!targetChildren && targetParent && !isParent) {
+    // Fuzz inside any of the above child process only.
+    isValidProcessType = true;
+  } else if (targetChildren && targetParent) {
+    // Fuzz every process type.
+    isValidProcessType = true;
+  } else {
+    // Fuzz no process type at all.
+    isValidProcessType = false;
+  }
+
+  if (!isValidProcessType) {
+    FAULTY_LOG("Disabled for this process of type '%s' with pid %d.",
+               XRE_GetProcessTypeString(), getpid());
+  }
+
+  return isValidProcessType;
+}
+
+// static
+unsigned int Faulty::DefaultProbability() {
+  static std::once_flag flag;
+  static unsigned probability;
+
+  std::call_once(flag, [&] {
+    probability = FAULTY_DEFAULT_PROBABILITY;
+    // Defines the likelihood of fuzzing a message.
+    if (const char* p = PR_GetEnv("FAULTY_PROBABILITY")) {
+      long n = std::strtol(p, nullptr, 10);
+      if (n != 0) {
+        probability = n;
+      }
+    }
+  });
+
+  return probability;
+}
+
+// static
+bool Faulty::IsLoggingEnabled(void) {
+  static bool enabled;
+  static std::once_flag flag;
+  std::call_once(flag, [&] { enabled = !!PR_GetEnv("FAULTY_ENABLE_LOGGING"); });
+  return enabled;
+}
+
+// static
+uint32_t Faulty::MutationFactor() {
+  static uint64_t sPropValue = FAULTY_DEFAULT_MUTATION_FACTOR;
+  static bool sInitialized = false;
+
+  if (sInitialized) {
+    return sPropValue;
+  }
+  sInitialized = true;
+
+  const char* factor = PR_GetEnv("FAULTY_MUTATION_FACTOR");
+  if (factor) {
+    long n = strtol(factor, nullptr, 10);
+    if (n != 0) {
+      sPropValue = n;
+      return sPropValue;
+    }
+  }
+  return sPropValue;
+}
+
+// static
+Faulty& Faulty::instance() {
+  static Faulty faulty;
+  return faulty;
+}
+
+//
+// Strategy: Pipes
+//
+
+void Faulty::MaybeCollectAndClosePipe(int aPipe, unsigned int aProbability) {
+#ifndef _WINDOWS
+  if (!mFuzzPipes) {
+    return;
+  }
+
+  if (aPipe > -1) {
+    FAULTY_LOG("Collecting pipe %d to bucket of pipes (count: %zu)", aPipe,
+               mFds.size());
+    mFds.insert(aPipe);
+  }
+
+  if (mFds.size() > 0 && FuzzingTraits::Sometimes(aProbability)) {
+    std::set<int>::iterator it(mFds.begin());
+    std::advance(it, FuzzingTraits::Random(mFds.size()));
+    FAULTY_LOG("Trying to close collected pipe: %d", *it);
+    errno = 0;
+    while ((close(*it) == -1 && (errno == EINTR))) {
+      ;
+    }
+    FAULTY_LOG("Pipe status after attempt to close: %d", errno);
+    mFds.erase(it);
+  }
+#endif
+}
+
+//
+// Strategy: Pickle
+//
+
+void Faulty::MutateBool(bool* aValue) { *aValue = !(*aValue); }
+
+void Faulty::FuzzBool(bool* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      bool oldValue = *aValue;
+      MutateBool(aValue);
+      FAULTY_LOG("Message field |bool| of value: %d mutated to: %d",
+                 (int)oldValue, (int)*aValue);
+    }
+  }
+}
+
+void Faulty::MutateChar(char* aValue) { FuzzIntegralType<char>(aValue, true); }
+
+void Faulty::FuzzChar(char* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      char oldValue = *aValue;
+      MutateChar(aValue);
+      FAULTY_LOG("Message field |char| of value: %c mutated to: %c", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateUChar(unsigned char* aValue) {
+  FuzzIntegralType<unsigned char>(aValue, true);
+}
+
+void Faulty::FuzzUChar(unsigned char* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      unsigned char oldValue = *aValue;
+      MutateUChar(aValue);
+      FAULTY_LOG("Message field |unsigned char| of value: %u mutated to: %u",
+                 oldValue, *aValue);
+    }
+  }
+}
+
+void Faulty::MutateInt16(int16_t* aValue) {
+  FuzzIntegralType<int16_t>(aValue, true);
+}
+
+void Faulty::FuzzInt16(int16_t* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      int16_t oldValue = *aValue;
+      MutateInt16(aValue);
+      FAULTY_LOG("Message field |int16| of value: %d mutated to: %d", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateUInt16(uint16_t* aValue) {
+  FuzzIntegralType<uint16_t>(aValue, true);
+}
+
+void Faulty::FuzzUInt16(uint16_t* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      uint16_t oldValue = *aValue;
+      MutateUInt16(aValue);
+      FAULTY_LOG("Message field |uint16| of value: %d mutated to: %d", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateInt(int* aValue) {
+  FuzzIntegralType<int>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzInt(int* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      int oldValue = *aValue;
+      MutateInt(aValue);
+      FAULTY_LOG("Message field |int| of value: %d mutated to: %d", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateUInt32(uint32_t* aValue) {
+  FuzzIntegralType<uint32_t>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzUInt32(uint32_t* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      uint32_t oldValue = *aValue;
+      MutateUInt32(aValue);
+      FAULTY_LOG("Message field |uint32| of value: %u mutated to: %u", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateLong(long* aValue) {
+  FuzzIntegralType<long>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzLong(long* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      long oldValue = *aValue;
+      MutateLong(aValue);
+      FAULTY_LOG("Message field |long| of value: %ld mutated to: %ld", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateULong(unsigned long* aValue) {
+  FuzzIntegralType<unsigned long>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzULong(unsigned long* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      unsigned long oldValue = *aValue;
+      MutateULong(aValue);
+      FAULTY_LOG("Message field |unsigned long| of value: %lu mutated to: %lu",
+                 oldValue, *aValue);
+    }
+  }
+}
+
+void Faulty::MutateUInt64(uint64_t* aValue) {
+  FuzzIntegralType<uint64_t>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzUInt64(uint64_t* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      uint64_t oldValue = *aValue;
+      MutateUInt64(aValue);
+      FAULTY_LOG("Message field |uint64| of value: %" PRIu64
+                 " mutated to: %" PRIu64,
+                 oldValue, *aValue);
+    }
+  }
+}
+
+void Faulty::MutateInt64(int64_t* aValue) {
+  FuzzIntegralType<int64_t>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzInt64(int64_t* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      int64_t oldValue = *aValue;
+      MutateInt64(aValue);
+      FAULTY_LOG("Message field |int64| of value: %" PRIu64
+                 " mutated to: %" PRIu64,
+                 oldValue, *aValue);
+    }
+  }
+}
+
+void Faulty::MutateDouble(double* aValue) {
+  FuzzFloatingPointType<double>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzDouble(double* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      double oldValue = *aValue;
+      MutateDouble(aValue);
+      FAULTY_LOG("Message field |double| of value: %f mutated to: %f", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::MutateFloat(float* aValue) {
+  FuzzFloatingPointType<float>(aValue, mUseLargeValues);
+}
+
+void Faulty::FuzzFloat(float* aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      float oldValue = *aValue;
+      MutateFloat(aValue);
+      FAULTY_LOG("Message field |float| of value: %f mutated to: %f", oldValue,
+                 *aValue);
+    }
+  }
+}
+
+void Faulty::FuzzString(std::string& aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      std::string oldValue = aValue;
+      FuzzStringType<std::string>(aValue, "xoferiF", std::string());
+      FAULTY_LOG("Message field |string| of value: %s mutated to: %s",
+                 oldValue.c_str(), aValue.c_str());
+    }
+  }
+}
+
+void Faulty::FuzzWString(std::wstring& aValue, unsigned int aProbability) {
+  if (mIsValidProcessType) {
+    if (mFuzzPickle && FuzzingTraits::Sometimes(aProbability)) {
+      std::wstring oldValue = aValue;
+      FAULTY_LOG("Message field |wstring|");
+      FuzzStringType<std::wstring>(aValue, L"xoferiF", std::wstring());
+    }
+  }
+}
+
+// static
+nsresult Faulty::CreateOutputDirectory(const char* aPathname) {
+  nsCOMPtr<nsIFile> path;
+  bool exists;
+  nsresult rv;
+
+  rv = NS_NewNativeLocalFile(nsDependentCString(aPathname), true,
+                             getter_AddRefs(path));
+
+  rv = path->Exists(&exists);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (!exists) {
+    rv = path->Create(nsIFile::DIRECTORY_TYPE, 0755);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  return NS_OK;
+}
+
+/* static */
+nsresult Faulty::ReadFile(const char* aPathname, nsTArray<nsCString>& aArray) {
+  nsresult rv;
+  nsCOMPtr<nsIFile> file;
+
+  rv = NS_NewLocalFile(NS_ConvertUTF8toUTF16(aPathname), true,
+                       getter_AddRefs(file));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  bool exists = false;
+  rv = file->Exists(&exists);
+  if (NS_WARN_IF(NS_FAILED(rv)) || !exists) {
+    return rv;
+  }
+
+  nsCOMPtr<nsIFileInputStream> fileStream(
+      do_CreateInstance(NS_LOCALFILEINPUTSTREAM_CONTRACTID, &rv));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = fileStream->Init(file, -1, -1, 0);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsCOMPtr<nsILineInputStream> lineStream(do_QueryInterface(fileStream, &rv));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsAutoCString line;
+  bool more = true;
+  do {
+    rv = lineStream->ReadLine(line, &more);
+    if (line.IsEmpty()) {
+      continue;
+    }
+    if (line.CharAt(0) == '#') {
+      /* Ignore comments. */
+      continue;
+    }
+    aArray.AppendElement(line);
+  } while (more);
+
+  return NS_OK;
+}
+
+bool Faulty::IsMessageNameBlacklisted(const char* aMessageName) {
+  static bool sFileLoaded = false;
+  static nsTArray<nsCString> sMessageBlacklist;
+
+  if (!sFileLoaded && mBlacklistPath) {
+    /* Run ReadFile() on the main thread to prevent
+       MOZ_ASSERT(NS_IsMainThread()) in nsStandardURL via nsNetStartup(). */
+    nsCOMPtr<nsIRunnable> r =
+        NS_NewRunnableFunction("Fuzzer::ReadBlacklistOnMainThread", [&]() {
+          if (Faulty::ReadFile(mBlacklistPath, sMessageBlacklist) != NS_OK) {
+            sFileLoaded = false;
+          } else {
+            sFileLoaded = true;
+          }
+        });
+    NS_DispatchToMainThread(r.forget(), NS_DISPATCH_SYNC);
+  }
+
+  if (!sFileLoaded) {
+    return false;
+  }
+
+  if (sMessageBlacklist.Length() == 0) {
+    return false;
+  }
+
+  return sMessageBlacklist.Contains(aMessageName);
+}
+
+// static
+std::vector<uint8_t> Faulty::GetDataFromIPCMessage(IPC::Message* aMsg) {
+  const Pickle::BufferList& buffers = aMsg->Buffers();
+  std::vector<uint8_t> data;
+  data.reserve(buffers.Size());
+
+  Pickle::BufferList::IterImpl i = buffers.Iter();
+  while (!i.Done()) {
+    size_t s = i.RemainingInSegment();
+    data.insert(data.end(), i.Data(), i.Data() + s);
+
+    i.Advance(buffers, s);
+  }
+
+  return data;
+}
+
+// static
+void Faulty::CopyFDs(IPC::Message* aDstMsg, IPC::Message* aSrcMsg) {
+#ifndef _WINDOWS
+  FileDescriptorSet* dstFdSet = aDstMsg->file_descriptor_set();
+  FileDescriptorSet* srcFdSet = aSrcMsg->file_descriptor_set();
+  for (size_t i = 0; i < srcFdSet->size(); i++) {
+    int fd = srcFdSet->GetDescriptorAt(i);
+    dstFdSet->Add(fd);
+  }
+#endif
+}
+
+UniquePtr<IPC::Message> Faulty::MutateIPCMessage(const char* aChannel,
+                                                 UniquePtr<IPC::Message> aMsg,
+                                                 unsigned int aProbability) {
+  if (!mIsValidProcessType || !mFuzzMessages) {
+    return aMsg;
+  }
+
+  sMsgCounter += 1;
+  LogMessage(aChannel, aMsg.get());
+
+  /* Skip immediately if we shall not try to fuzz this message. */
+  if (!FuzzingTraits::Sometimes(aProbability)) {
+    return aMsg;
+  }
+
+  const bool isMessageListed = IsMessageNameBlacklisted(aMsg->name());
+
+  /* Check if this message is blacklisted and shall not get fuzzed. */
+  if (isMessageListed && !mUseAsWhitelist) {
+    FAULTY_LOG("BLACKLISTED: %s", aMsg->name());
+    return aMsg;
+  }
+
+  /* Check if the message is whitelisted. */
+  if (!isMessageListed && mUseAsWhitelist) {
+    /* Silently skip this message. */
+    return aMsg;
+  }
+
+  /* Retrieve BufferLists as data from original message. */
+  std::vector<uint8_t> data(GetDataFromIPCMessage(aMsg.get()));
+
+  /* Check if there is enough data in the message to fuzz. */
+  uint32_t headerSize = aMsg->HeaderSizeFromData(nullptr, nullptr);
+  if (headerSize == data.size()) {
+    FAULTY_LOG("IGNORING: %s", aMsg->name());
+    return aMsg;
+  }
+
+  /* Mutate the message data. */
+  size_t maxMutations = FuzzingTraits::Frequency(data.size(), MutationFactor());
+  FAULTY_LOG("FUZZING (%zu bytes): %s", maxMutations, aMsg->name());
+  while (maxMutations--) {
+    /* Ignore the header data of the message. */
+    uint32_t pos = RandomIntegerRange<uint32_t>(headerSize, data.size() - 1);
+    switch (FuzzingTraits::Random(6)) {
+      case 0:
+        break;
+      case 1:
+        data.at(pos) = RandomIntegerRange<uint8_t>(0, 1);
+        break;
+      case 2:
+        data.at(pos) ^= (1 << FuzzingTraits::Random(9));
+        break;
+      case 3:
+        data.at(pos) = RandomIntegerRange<uint8_t>(254, 255);
+        break;
+      default:
+        data.at(pos) = RandomIntegerRange<uint8_t>(0, 255);
+    }
+  }
+
+  /* Build new message. */
+  auto mutatedMsg = MakeUnique<IPC::Message>(
+      reinterpret_cast<const char*>(data.data()), data.size());
+  CopyFDs(mutatedMsg.get(), aMsg.get());
+
+  /* Dump original message for diff purposes. */
+  DumpMessage(aChannel, aMsg.get(),
+              nsPrintfCString(".%zu.o", sMsgCounter).get());
+  /* Dump mutated message for diff purposes. */
+  DumpMessage(aChannel, mutatedMsg.get(),
+              nsPrintfCString(".%zu.m", sMsgCounter).get());
+
+  return mutatedMsg;
+}
+
+void Faulty::LogMessage(const char* aChannel, IPC::Message* aMsg) {
+  if (!mIsValidProcessType) {
+    return;
+  }
+
+  std::string fileName =
+      nsPrintfCString("message.%u.%zu", getpid(), sMsgCounter).get();
+
+  FAULTY_LOG("Process: %u | Size: %10zu | %-20s | %s => %s",
+             XRE_GetProcessType(), aMsg->Buffers().Size(), fileName.c_str(),
+             aChannel, aMsg->name());
+}
+
+void Faulty::DumpMessage(const char* aChannel, IPC::Message* aMsg,
+                         std::string aAppendix) {
+  if (!mIsValidProcessType || !mMessagePath) {
+    return;
+  }
+
+  std::vector<uint8_t> data(GetDataFromIPCMessage(aMsg));
+  std::string fileName;
+
+  if (!aAppendix.empty()) {
+    fileName = nsPrintfCString("%s/message.%u%s", mMessagePath, getpid(),
+                               aAppendix.c_str())
+                   .get();
+  } else {
+    fileName = nsPrintfCString("%s/%s", mMessagePath, fileName.c_str()).get();
+  }
+
+  std::fstream fp;
+  fp.open(fileName, std::fstream::out | std::fstream::binary);
+  fp.write(reinterpret_cast<const char*>(data.data()), data.size());
+  fp.close();
+}
+
+}  // namespace ipc
+}  // namespace mozilla
diff --git a/tools/fuzzing/faulty/Faulty.h b/tools/fuzzing/faulty/Faulty.h
new file mode 100644
index 0000000000..21ea1276eb
--- /dev/null
+++ b/tools/fuzzing/faulty/Faulty.h
@@ -0,0 +1,134 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ipc_Faulty_h
+#define mozilla_ipc_Faulty_h
+
+#include <set>
+#include <string>
+#include <vector>
+#include "base/string16.h"
+#include "nsDebug.h"
+#include "nsTArray.h"
+#include "mozilla/UniquePtr.h"
+
+#ifdef IsLoggingEnabled
+// This is defined in the Windows SDK urlmon.h
+#  undef IsLoggingEnabled
+#endif
+
+#define FAULTY_DEFAULT_PROBABILITY 1000
+#define FAULTY_DEFAULT_MUTATION_FACTOR 10
+#define FAULTY_LOG(fmt, args...)                                  \
+  if (mozilla::ipc::Faulty::IsLoggingEnabled()) {                 \
+    printf_stderr("[Faulty] (%10u) " fmt "\n", getpid(), ##args); \
+  }
+
+namespace IPC {
+// Needed for blacklisting messages.
+class Message;
+}  // namespace IPC
+
+namespace mozilla {
+namespace ipc {
+
+class Faulty {
+ public:
+  // Used as a default argument for the Fuzz|datatype| methods.
+  static unsigned int DefaultProbability();
+  static bool IsLoggingEnabled(void);
+  static std::vector<uint8_t> GetDataFromIPCMessage(IPC::Message* aMsg);
+  static nsresult CreateOutputDirectory(const char* aPathname);
+  static nsresult ReadFile(const char* aPathname, nsTArray<nsCString>& aArray);
+  static void CopyFDs(IPC::Message* aDstMsg, IPC::Message* aSrcMsg);
+
+  static Faulty& instance();
+
+  // Fuzzing methods for Pickle.
+  void FuzzBool(bool* aValue, unsigned int aProbability = DefaultProbability());
+  void FuzzChar(char* aValue, unsigned int aProbability = DefaultProbability());
+  void FuzzUChar(unsigned char* aValue,
+                 unsigned int aProbability = DefaultProbability());
+  void FuzzInt16(int16_t* aValue,
+                 unsigned int aProbability = DefaultProbability());
+  void FuzzUInt16(uint16_t* aValue,
+                  unsigned int aProbability = DefaultProbability());
+  void FuzzInt(int* aValue, unsigned int aProbability = DefaultProbability());
+  void FuzzUInt32(uint32_t* aValue,
+                  unsigned int aProbability = DefaultProbability());
+  void FuzzLong(long* aValue, unsigned int aProbability = DefaultProbability());
+  void FuzzULong(unsigned long* aValue,
+                 unsigned int aProbability = DefaultProbability());
+  void FuzzInt64(int64_t* aValue,
+                 unsigned int aProbability = DefaultProbability());
+  void FuzzUInt64(uint64_t* aValue,
+                  unsigned int aProbability = DefaultProbability());
+  void FuzzFloat(float* aValue,
+                 unsigned int aProbability = DefaultProbability());
+  void FuzzDouble(double* aValue,
+                  unsigned int aProbability = DefaultProbability());
+  void FuzzString(std::string& aValue,
+                  unsigned int aProbability = DefaultProbability());
+  void FuzzWString(std::wstring& aValue,
+                   unsigned int aProbability = DefaultProbability());
+  void FuzzBytes(void* aData, int aLength,
+                 unsigned int aProbability = DefaultProbability());
+
+  // Fuzzing methods for pipe fuzzing.
+  void MaybeCollectAndClosePipe(
+      int aPipe, unsigned int aProbability = DefaultProbability());
+
+  // Fuzzing methods for message blob fuzzing.
+  void DumpMessage(const char* aChannel, IPC::Message* aMsg,
+                   std::string aAppendix = nullptr);
+  bool IsMessageNameBlacklisted(const char* aMessageName);
+  UniquePtr<IPC::Message> MutateIPCMessage(
+      const char* aChannel, UniquePtr<IPC::Message> aMsg,
+      unsigned int aProbability = DefaultProbability());
+
+  void LogMessage(const char* aChannel, IPC::Message* aMsg);
+
+ private:
+  std::set<int> mFds;
+
+  const bool mFuzzMessages;
+  const bool mFuzzPipes;
+  const bool mFuzzPickle;
+  const bool mUseLargeValues;
+  const bool mUseAsWhitelist;
+  const bool mIsValidProcessType;
+
+  const char* mMessagePath;
+  const char* mBlacklistPath;
+
+  size_t sMsgCounter;
+
+  Faulty();
+  DISALLOW_EVIL_CONSTRUCTORS(Faulty);
+
+  static bool IsValidProcessType(void);
+  static uint32_t MutationFactor();
+
+  // Fuzzing methods for Pickle
+  void MutateBool(bool* aValue);
+  void MutateChar(char* aValue);
+  void MutateUChar(unsigned char* aValue);
+  void MutateInt16(int16_t* aValue);
+  void MutateUInt16(uint16_t* aValue);
+  void MutateInt(int* aValue);
+  void MutateUInt32(uint32_t* aValue);
+  void MutateLong(long* aValue);
+  void MutateULong(unsigned long* aValue);
+  void MutateInt64(int64_t* aValue);
+  void MutateUInt64(uint64_t* aValue);
+  void MutateFloat(float* aValue);
+  void MutateDouble(double* aValue);
+};
+
+}  // namespace ipc
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/faulty/moz.build b/tools/fuzzing/faulty/moz.build
new file mode 100644
index 0000000000..afe1aa7a45
--- /dev/null
+++ b/tools/fuzzing/faulty/moz.build
@@ -0,0 +1,13 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+SOURCES += ["Faulty.cpp"]
+
+EXPORTS.mozilla.ipc += ["Faulty.h"]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/interface/FuzzingInterface.cpp b/tools/fuzzing/interface/FuzzingInterface.cpp
new file mode 100644
index 0000000000..4f82b98161
--- /dev/null
+++ b/tools/fuzzing/interface/FuzzingInterface.cpp
@@ -0,0 +1,29 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Common code for the unified fuzzing interface
+ */
+
+#include <stdlib.h>
+#include "FuzzingInterface.h"
+
+namespace mozilla {
+
+#ifdef JS_STANDALONE
+static bool fuzzing_verbose = !!getenv("MOZ_FUZZ_LOG");
+void fuzzing_log(const char* aFmt, ...) {
+  if (fuzzing_verbose) {
+    va_list ap;
+    va_start(ap, aFmt);
+    vfprintf(stderr, aFmt, ap);
+    va_end(ap);
+  }
+}
+#else
+LazyLogModule gFuzzingLog("nsFuzzing");
+#endif
+
+}  // namespace mozilla
diff --git a/tools/fuzzing/interface/FuzzingInterface.h b/tools/fuzzing/interface/FuzzingInterface.h
new file mode 100644
index 0000000000..ad49ed3c01
--- /dev/null
+++ b/tools/fuzzing/interface/FuzzingInterface.h
@@ -0,0 +1,115 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Interface definitions for the unified fuzzing interface
+ */
+
+#ifndef FuzzingInterface_h__
+#define FuzzingInterface_h__
+
+#include <fstream>
+
+#ifdef LIBFUZZER
+#  include "FuzzerExtFunctions.h"
+#endif
+
+#include "FuzzerRegistry.h"
+#include "mozilla/Assertions.h"
+
+#ifndef JS_STANDALONE
+#  include "mozilla/Logging.h"
+#endif
+
+namespace mozilla {
+
+#ifdef JS_STANDALONE
+void fuzzing_log(const char* aFmt, ...);
+#  define MOZ_LOG_EXPAND_ARGS(...) __VA_ARGS__
+
+#  define FUZZING_LOG(args) fuzzing_log(MOZ_LOG_EXPAND_ARGS args);
+#else
+extern LazyLogModule gFuzzingLog;
+
+#  define FUZZING_LOG(args) \
+    MOZ_LOG(mozilla::gFuzzingLog, mozilla::LogLevel::Verbose, args)
+#endif  // JS_STANDALONE
+
+typedef int (*FuzzingTestFuncRaw)(const uint8_t*, size_t);
+
+#ifdef __AFL_COMPILER
+
+static int afl_interface_raw(const char* testFile,
+                             FuzzingTestFuncRaw testFunc) {
+  char* buf = NULL;
+
+  while (__AFL_LOOP(1000)) {
+    std::ifstream is;
+    is.open(testFile, std::ios::binary);
+    is.seekg(0, std::ios::end);
+    int len = is.tellg();
+    is.seekg(0, std::ios::beg);
+    MOZ_RELEASE_ASSERT(len >= 0);
+    if (!len) {
+      is.close();
+      continue;
+    }
+    buf = (char*)realloc(buf, len);
+    MOZ_RELEASE_ASSERT(buf);
+    is.read(buf, len);
+    is.close();
+    testFunc((uint8_t*)buf, (size_t)len);
+  }
+
+  free(buf);
+
+  return 0;
+}
+
+#  define MOZ_AFL_INTERFACE_COMMON()                                      \
+    char* testFilePtr = getenv("MOZ_FUZZ_TESTFILE");                      \
+    if (!testFilePtr) {                                                   \
+      fprintf(stderr,                                                     \
+              "Must specify testfile in MOZ_FUZZ_TESTFILE environment "   \
+              "variable.\n");                                             \
+      return 1;                                                           \
+    }                                                                     \
+    /* Make a copy of testFilePtr so the testing function can safely call \
+     * getenv                                                             \
+     */                                                                   \
+    std::string testFile(testFilePtr);
+
+#  define MOZ_AFL_INTERFACE_RAW(initFunc, testFunc, moduleName)          \
+    static int afl_fuzz_##moduleName(const uint8_t* data, size_t size) { \
+      MOZ_RELEASE_ASSERT(data == NULL && size == 0);                     \
+      MOZ_AFL_INTERFACE_COMMON();                                        \
+      return ::mozilla::afl_interface_raw(testFile.c_str(), testFunc);   \
+    }                                                                    \
+    static void __attribute__((constructor)) AFLRegister##moduleName() { \
+      ::mozilla::FuzzerRegistry::getInstance().registerModule(           \
+          #moduleName, initFunc, afl_fuzz_##moduleName);                 \
+    }
+#else
+#  define MOZ_AFL_INTERFACE_RAW(initFunc, testFunc, moduleName) /* Nothing */
+#endif  // __AFL_COMPILER
+
+#ifdef LIBFUZZER
+#  define MOZ_LIBFUZZER_INTERFACE_RAW(initFunc, testFunc, moduleName)          \
+    static void __attribute__((constructor)) LibFuzzerRegister##moduleName() { \
+      ::mozilla::FuzzerRegistry::getInstance().registerModule(                 \
+          #moduleName, initFunc, testFunc);                                    \
+    }
+#else
+#  define MOZ_LIBFUZZER_INTERFACE_RAW(initFunc, testFunc, \
+                                      moduleName) /* Nothing */
+#endif
+
+#define MOZ_FUZZING_INTERFACE_RAW(initFunc, testFunc, moduleName) \
+  MOZ_LIBFUZZER_INTERFACE_RAW(initFunc, testFunc, moduleName);    \
+  MOZ_AFL_INTERFACE_RAW(initFunc, testFunc, moduleName);
+
+}  // namespace mozilla
+
+#endif  // FuzzingInterface_h__
diff --git a/tools/fuzzing/interface/FuzzingInterfaceStream.cpp b/tools/fuzzing/interface/FuzzingInterfaceStream.cpp
new file mode 100644
index 0000000000..125783cf11
--- /dev/null
+++ b/tools/fuzzing/interface/FuzzingInterfaceStream.cpp
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Interface implementation for the unified fuzzing interface
+ */
+
+#include "nsIFile.h"
+#include "nsIPrefService.h"
+#include "nsIProperties.h"
+
+#include "FuzzingInterfaceStream.h"
+
+#include "mozilla/Assertions.h"
+
+#ifndef JS_STANDALONE
+#  include "nsNetUtil.h"
+#endif
+
+namespace mozilla {
+
+#ifdef __AFL_COMPILER
+
+void afl_interface_stream(const char* testFile,
+                          FuzzingTestFuncStream testFunc) {
+  nsresult rv;
+  nsCOMPtr<nsIProperties> dirService =
+      do_GetService(NS_DIRECTORY_SERVICE_CONTRACTID);
+  MOZ_RELEASE_ASSERT(dirService != nullptr);
+  nsCOMPtr<nsIFile> file;
+  rv = dirService->Get(NS_OS_CURRENT_WORKING_DIR, NS_GET_IID(nsIFile),
+                       getter_AddRefs(file));
+  MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
+  file->AppendNative(nsDependentCString(testFile));
+  while (__AFL_LOOP(1000)) {
+    nsCOMPtr<nsIInputStream> inputStream;
+    rv = NS_NewLocalFileInputStream(getter_AddRefs(inputStream), file);
+    MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
+    if (!NS_InputStreamIsBuffered(inputStream)) {
+      nsCOMPtr<nsIInputStream> bufStream;
+      rv = NS_NewBufferedInputStream(getter_AddRefs(bufStream),
+                                     inputStream.forget(), 1024);
+      MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
+      inputStream = bufStream;
+    }
+    testFunc(inputStream.forget());
+  }
+}
+
+#endif
+
+}  // namespace mozilla
diff --git a/tools/fuzzing/interface/FuzzingInterfaceStream.h b/tools/fuzzing/interface/FuzzingInterfaceStream.h
new file mode 100644
index 0000000000..eb5637c844
--- /dev/null
+++ b/tools/fuzzing/interface/FuzzingInterfaceStream.h
@@ -0,0 +1,90 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Interface definitions for the unified fuzzing interface with streaming
+ * support
+ */
+
+#ifndef FuzzingInterfaceStream_h__
+#define FuzzingInterfaceStream_h__
+
+#ifdef JS_STANDALONE
+#  error "FuzzingInterfaceStream.h cannot be used in JS standalone builds."
+#endif
+
+#include "gtest/gtest.h"
+#include "nsComponentManagerUtils.h"
+#include "nsCOMPtr.h"
+#include "nsIInputStream.h"
+
+#include "nsDirectoryServiceDefs.h"
+#include "nsStreamUtils.h"
+#include "nsStringStream.h"
+
+#include <fstream>
+
+#include "FuzzingInterface.h"
+
+namespace mozilla {
+
+typedef int (*FuzzingTestFuncStream)(nsCOMPtr<nsIInputStream>);
+
+#ifdef __AFL_COMPILER
+void afl_interface_stream(const char* testFile, FuzzingTestFuncStream testFunc);
+
+#  define MOZ_AFL_INTERFACE_COMMON(initFunc)                              \
+    if (initFunc) initFunc(NULL, NULL);                                   \
+    char* testFilePtr = getenv("MOZ_FUZZ_TESTFILE");                      \
+    if (!testFilePtr) {                                                   \
+      fprintf(stderr,                                                     \
+              "Must specify testfile in MOZ_FUZZ_TESTFILE environment "   \
+              "variable.\n");                                             \
+      return;                                                             \
+    }                                                                     \
+    /* Make a copy of testFilePtr so the testing function can safely call \
+     * getenv                                                             \
+     */                                                                   \
+    std::string testFile(testFilePtr);
+
+#  define MOZ_AFL_INTERFACE_STREAM(initFunc, testFunc, moduleName) \
+    TEST(AFL, moduleName)                                          \
+    {                                                              \
+      MOZ_AFL_INTERFACE_COMMON(initFunc);                          \
+      ::mozilla::afl_interface_stream(testFile.c_str(), testFunc); \
+    }
+#else
+#  define MOZ_AFL_INTERFACE_STREAM(initFunc, testFunc, moduleName) /* Nothing \
+                                                                    */
+#endif
+
+#ifdef LIBFUZZER
+#  define MOZ_LIBFUZZER_INTERFACE_STREAM(initFunc, testFunc, moduleName)       \
+    static int LibFuzzerTest##moduleName(const uint8_t* data, size_t size) {   \
+      if (size > INT32_MAX) return 0;                                          \
+      nsCOMPtr<nsIInputStream> stream;                                         \
+      nsresult rv = NS_NewByteInputStream(getter_AddRefs(stream),              \
+                                          Span((const char*)data, size),       \
+                                          NS_ASSIGNMENT_DEPEND);               \
+      MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));                                    \
+      testFunc(stream.forget());                                               \
+      return 0;                                                                \
+    }                                                                          \
+    static void __attribute__((constructor)) LibFuzzerRegister##moduleName() { \
+      ::mozilla::FuzzerRegistry::getInstance().registerModule(                 \
+          #moduleName, initFunc, LibFuzzerTest##moduleName);                   \
+    }
+#else
+#  define MOZ_LIBFUZZER_INTERFACE_STREAM(initFunc, testFunc, \
+                                         moduleName) /* Nothing */
+#endif
+
+#define MOZ_FUZZING_INTERFACE_STREAM(initFunc, testFunc, moduleName) \
+  MOZ_LIBFUZZER_INTERFACE_STREAM(initFunc, testFunc, moduleName);    \
+  MOZ_AFL_INTERFACE_STREAM(initFunc, testFunc, moduleName);
+
+}  // namespace mozilla
+
+#endif  // FuzzingInterfaceStream_h__
diff --git a/tools/fuzzing/interface/harness/FuzzerRunner.cpp b/tools/fuzzing/interface/harness/FuzzerRunner.cpp
new file mode 100644
index 0000000000..fc2094aa59
--- /dev/null
+++ b/tools/fuzzing/interface/harness/FuzzerRunner.cpp
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * * This Source Code Form is subject to the terms of the Mozilla Public
+ * * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <cstdlib>
+
+#include "FuzzerRunner.h"
+#include "mozilla/Attributes.h"
+#include "prenv.h"
+
+#include "FuzzerTestHarness.h"
+
+namespace mozilla {
+
+// We use a static var 'fuzzerRunner' defined in nsAppRunner.cpp.
+// fuzzerRunner is initialized to nullptr but if this file is linked in,
+// then fuzzerRunner will be set here indicating that
+// we want to call into either LibFuzzer's main or the AFL entrypoint.
+class _InitFuzzer {
+ public:
+  _InitFuzzer() { fuzzerRunner = new FuzzerRunner(); }
+  void InitXPCOM() { mScopedXPCOM = new ScopedXPCOM("Fuzzer"); }
+  void DeinitXPCOM() {
+    if (mScopedXPCOM) delete mScopedXPCOM;
+    mScopedXPCOM = nullptr;
+  }
+
+ private:
+  ScopedXPCOM* mScopedXPCOM;
+} InitLibFuzzer;
+
+static void DeinitXPCOM() { InitLibFuzzer.DeinitXPCOM(); }
+
+int FuzzerRunner::Run(int* argc, char*** argv) {
+  /*
+   * libFuzzer uses exit() calls in several places instead of returning,
+   * so the destructor of ScopedXPCOM is not called in some cases.
+   * For fuzzing, this does not make a difference, but in debug builds
+   * when running a single testcase, this causes an assertion when destroying
+   * global linked lists. For this reason, we allocate ScopedXPCOM on the heap
+   * using the global InitLibFuzzer class, combined with an atexit call to
+   * destroy the ScopedXPCOM instance again.
+   */
+  InitLibFuzzer.InitXPCOM();
+  std::atexit(DeinitXPCOM);
+
+  const char* fuzzerEnv = getenv("FUZZER");
+
+  if (!fuzzerEnv) {
+    fprintf(stderr,
+            "Must specify fuzzing target in FUZZER environment variable\n");
+    exit(1);
+  }
+
+  std::string moduleNameStr(fuzzerEnv);
+  FuzzerFunctions funcs =
+      FuzzerRegistry::getInstance().getModuleFunctions(moduleNameStr);
+  FuzzerInitFunc initFunc = funcs.first;
+  FuzzerTestingFunc testingFunc = funcs.second;
+  if (initFunc) {
+    int ret = initFunc(argc, argv);
+    if (ret) {
+      fprintf(stderr, "Fuzzing Interface: Error: Initialize callback failed\n");
+      exit(1);
+    }
+  }
+
+  if (!testingFunc) {
+    fprintf(stderr, "Fuzzing Interface: Error: No testing callback found\n");
+    exit(1);
+  }
+
+#ifdef LIBFUZZER
+  int ret = mFuzzerDriver(argc, argv, testingFunc);
+#else
+  // For AFL, testingFunc points to the entry function we need.
+  int ret = testingFunc(NULL, 0);
+#endif
+
+  InitLibFuzzer.DeinitXPCOM();
+  return ret;
+}
+
+#ifdef LIBFUZZER
+void FuzzerRunner::setParams(LibFuzzerDriver aDriver) {
+  mFuzzerDriver = aDriver;
+}
+#endif
+
+}  // namespace mozilla
diff --git a/tools/fuzzing/interface/harness/FuzzerRunner.h b/tools/fuzzing/interface/harness/FuzzerRunner.h
new file mode 100644
index 0000000000..6b19e751cd
--- /dev/null
+++ b/tools/fuzzing/interface/harness/FuzzerRunner.h
@@ -0,0 +1,24 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * * This Source Code Form is subject to the terms of the Mozilla Public
+ * * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FuzzerRegistry.h"
+
+namespace mozilla {
+
+class FuzzerRunner {
+ public:
+  int Run(int* argc, char*** argv);
+
+#ifdef LIBFUZZER
+  void setParams(LibFuzzerDriver aDriver);
+
+ private:
+  LibFuzzerDriver mFuzzerDriver;
+#endif
+};
+
+extern FuzzerRunner* fuzzerRunner;
+
+}  // namespace mozilla
diff --git a/tools/fuzzing/interface/harness/FuzzerTestHarness.h b/tools/fuzzing/interface/harness/FuzzerTestHarness.h
new file mode 100644
index 0000000000..1530ac86c5
--- /dev/null
+++ b/tools/fuzzing/interface/harness/FuzzerTestHarness.h
@@ -0,0 +1,260 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Test harness for XPCOM objects, providing a scoped XPCOM initializer,
+ * nsCOMPtr, nsRefPtr, do_CreateInstance, do_GetService, ns(Auto|C|)String,
+ * and stdio.h/stdlib.h.
+ */
+
+#ifndef FuzzerTestHarness_h__
+#define FuzzerTestHarness_h__
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Attributes.h"
+
+#include "prenv.h"
+#include "nsComponentManagerUtils.h"
+#include "nsServiceManagerUtils.h"
+#include "nsCOMPtr.h"
+#include "nsString.h"
+#include "nsAppDirectoryServiceDefs.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIDirectoryService.h"
+#include "nsIFile.h"
+#include "nsIObserverService.h"
+#include "nsIServiceManager.h"
+#include "nsXULAppAPI.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+
+namespace {
+
+static uint32_t gFailCount = 0;
+
+/**
+ * Prints the given failure message and arguments using printf, prepending
+ * "TEST-UNEXPECTED-FAIL " for the benefit of the test harness and
+ * appending "\n" to eliminate having to type it at each call site.
+ */
+MOZ_FORMAT_PRINTF(1, 2) void fail(const char* msg, ...) {
+  va_list ap;
+
+  printf("TEST-UNEXPECTED-FAIL | ");
+
+  va_start(ap, msg);
+  vprintf(msg, ap);
+  va_end(ap);
+
+  putchar('\n');
+  ++gFailCount;
+}
+
+//-----------------------------------------------------------------------------
+
+class ScopedXPCOM final : public nsIDirectoryServiceProvider2 {
+ public:
+  NS_DECL_ISUPPORTS
+
+  explicit ScopedXPCOM(const char* testName,
+                       nsIDirectoryServiceProvider* dirSvcProvider = nullptr)
+      : mDirSvcProvider(dirSvcProvider) {
+    mTestName = testName;
+    printf("Running %s tests...\n", mTestName);
+
+    nsresult rv = NS_InitXPCOM(&mServMgr, nullptr, this);
+    if (NS_FAILED(rv)) {
+      fail("NS_InitXPCOM returned failure code 0x%" PRIx32,
+           static_cast<uint32_t>(rv));
+      mServMgr = nullptr;
+      return;
+    }
+  }
+
+  ~ScopedXPCOM() {
+    // If we created a profile directory, we need to remove it.
+    if (mProfD) {
+      nsCOMPtr<nsIObserverService> os =
+          do_GetService(NS_OBSERVERSERVICE_CONTRACTID);
+      MOZ_ASSERT(os);
+      if (os) {
+        MOZ_ALWAYS_SUCCEEDS(os->NotifyObservers(
+            nullptr, "profile-change-net-teardown", nullptr));
+        MOZ_ALWAYS_SUCCEEDS(
+            os->NotifyObservers(nullptr, "profile-change-teardown", nullptr));
+        MOZ_ALWAYS_SUCCEEDS(
+            os->NotifyObservers(nullptr, "profile-before-change", nullptr));
+        MOZ_ALWAYS_SUCCEEDS(
+            os->NotifyObservers(nullptr, "profile-before-change-qm", nullptr));
+        MOZ_ALWAYS_SUCCEEDS(os->NotifyObservers(
+            nullptr, "profile-before-change-telemetry", nullptr));
+      }
+
+      if (NS_FAILED(mProfD->Remove(true))) {
+        NS_WARNING("Problem removing profile directory");
+      }
+
+      mProfD = nullptr;
+    }
+
+    if (mServMgr) {
+      NS_RELEASE(mServMgr);
+      nsresult rv = NS_ShutdownXPCOM(nullptr);
+      if (NS_FAILED(rv)) {
+        fail("XPCOM shutdown failed with code 0x%" PRIx32,
+             static_cast<uint32_t>(rv));
+        exit(1);
+      }
+    }
+
+    printf("Finished running %s tests.\n", mTestName);
+  }
+
+  already_AddRefed<nsIFile> GetProfileDirectory() {
+    if (mProfD) {
+      nsCOMPtr<nsIFile> copy = mProfD;
+      return copy.forget();
+    }
+
+    // Create a unique temporary folder to use for this test.
+    // Note that runcppunittests.py will run tests with a temp
+    // directory as the cwd, so just put something under that.
+    nsCOMPtr<nsIFile> profD;
+    nsresult rv = NS_GetSpecialDirectory(NS_OS_CURRENT_PROCESS_DIR,
+                                         getter_AddRefs(profD));
+    NS_ENSURE_SUCCESS(rv, nullptr);
+
+    rv = profD->Append(u"cpp-unit-profd"_ns);
+    NS_ENSURE_SUCCESS(rv, nullptr);
+
+    rv = profD->CreateUnique(nsIFile::DIRECTORY_TYPE, 0755);
+    NS_ENSURE_SUCCESS(rv, nullptr);
+
+    mProfD = profD;
+    return profD.forget();
+  }
+
+  already_AddRefed<nsIFile> GetGREDirectory() {
+    if (mGRED) {
+      nsCOMPtr<nsIFile> copy = mGRED;
+      return copy.forget();
+    }
+
+    char* env = PR_GetEnv("MOZ_XRE_DIR");
+    nsCOMPtr<nsIFile> greD;
+    if (env) {
+      NS_NewLocalFile(NS_ConvertUTF8toUTF16(env), false, getter_AddRefs(greD));
+    }
+
+    mGRED = greD;
+    return greD.forget();
+  }
+
+  already_AddRefed<nsIFile> GetGREBinDirectory() {
+    if (mGREBinD) {
+      nsCOMPtr<nsIFile> copy = mGREBinD;
+      return copy.forget();
+    }
+
+    nsCOMPtr<nsIFile> greD = GetGREDirectory();
+    if (!greD) {
+      return greD.forget();
+    }
+    greD->Clone(getter_AddRefs(mGREBinD));
+
+#ifdef XP_MACOSX
+    nsAutoCString leafName;
+    mGREBinD->GetNativeLeafName(leafName);
+    if (leafName.EqualsLiteral("Resources")) {
+      mGREBinD->SetNativeLeafName("MacOS"_ns);
+    }
+#endif
+
+    nsCOMPtr<nsIFile> copy = mGREBinD;
+    return copy.forget();
+  }
+
+  ////////////////////////////////////////////////////////////////////////////
+  //// nsIDirectoryServiceProvider
+
+  NS_IMETHODIMP GetFile(const char* aProperty, bool* _persistent,
+                        nsIFile** _result) override {
+    // If we were supplied a directory service provider, ask it first.
+    if (mDirSvcProvider && NS_SUCCEEDED(mDirSvcProvider->GetFile(
+                               aProperty, _persistent, _result))) {
+      return NS_OK;
+    }
+
+    // Otherwise, the test harness provides some directories automatically.
+    if (0 == strcmp(aProperty, NS_APP_USER_PROFILE_50_DIR) ||
+        0 == strcmp(aProperty, NS_APP_USER_PROFILE_LOCAL_50_DIR) ||
+        0 == strcmp(aProperty, NS_APP_PROFILE_LOCAL_DIR_STARTUP)) {
+      nsCOMPtr<nsIFile> profD = GetProfileDirectory();
+      NS_ENSURE_TRUE(profD, NS_ERROR_FAILURE);
+
+      nsCOMPtr<nsIFile> clone;
+      nsresult rv = profD->Clone(getter_AddRefs(clone));
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      *_persistent = true;
+      clone.forget(_result);
+      return NS_OK;
+    } else if (0 == strcmp(aProperty, NS_GRE_DIR)) {
+      nsCOMPtr<nsIFile> greD = GetGREDirectory();
+      NS_ENSURE_TRUE(greD, NS_ERROR_FAILURE);
+
+      *_persistent = true;
+      greD.forget(_result);
+      return NS_OK;
+    } else if (0 == strcmp(aProperty, NS_GRE_BIN_DIR)) {
+      nsCOMPtr<nsIFile> greBinD = GetGREBinDirectory();
+      NS_ENSURE_TRUE(greBinD, NS_ERROR_FAILURE);
+
+      *_persistent = true;
+      greBinD.forget(_result);
+      return NS_OK;
+    }
+
+    return NS_ERROR_FAILURE;
+  }
+
+  ////////////////////////////////////////////////////////////////////////////
+  //// nsIDirectoryServiceProvider2
+
+  NS_IMETHODIMP GetFiles(const char* aProperty,
+                         nsISimpleEnumerator** _enum) override {
+    // If we were supplied a directory service provider, ask it first.
+    nsCOMPtr<nsIDirectoryServiceProvider2> provider =
+        do_QueryInterface(mDirSvcProvider);
+    if (provider && NS_SUCCEEDED(provider->GetFiles(aProperty, _enum))) {
+      return NS_OK;
+    }
+
+    return NS_ERROR_FAILURE;
+  }
+
+ private:
+  const char* mTestName;
+  nsIServiceManager* mServMgr;
+  nsCOMPtr<nsIDirectoryServiceProvider> mDirSvcProvider;
+  nsCOMPtr<nsIFile> mProfD;
+  nsCOMPtr<nsIFile> mGRED;
+  nsCOMPtr<nsIFile> mGREBinD;
+};
+
+NS_IMPL_QUERY_INTERFACE(ScopedXPCOM, nsIDirectoryServiceProvider,
+                        nsIDirectoryServiceProvider2)
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+ScopedXPCOM::AddRef() { return 2; }
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+ScopedXPCOM::Release() { return 1; }
+
+}  // namespace
+
+#endif  // FuzzerTestHarness_h__
diff --git a/tools/fuzzing/interface/harness/moz.build b/tools/fuzzing/interface/harness/moz.build
new file mode 100644
index 0000000000..0eff84c8aa
--- /dev/null
+++ b/tools/fuzzing/interface/harness/moz.build
@@ -0,0 +1,16 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("fuzzer-runner")
+
+SOURCES += [
+    "FuzzerRunner.cpp",
+]
+EXPORTS += [
+    "FuzzerRunner.h",
+]
+
+FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/interface/moz.build b/tools/fuzzing/interface/moz.build
new file mode 100644
index 0000000000..8a51007174
--- /dev/null
+++ b/tools/fuzzing/interface/moz.build
@@ -0,0 +1,32 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("fuzzer-interface")
+
+EXPORTS += [
+    "FuzzingInterface.h",
+]
+
+SOURCES += [
+    "FuzzingInterface.cpp",
+]
+
+if CONFIG["JS_STANDALONE"]:
+    FINAL_LIBRARY = "js"
+else:
+    EXPORTS += [
+        "FuzzingInterfaceStream.h",
+    ]
+
+    SOURCES += [
+        "FuzzingInterfaceStream.cpp",
+    ]
+
+    DIRS += [
+        "harness",
+    ]
+
+    FINAL_LIBRARY = "xul-gtest"
diff --git a/tools/fuzzing/ipc/ProtocolFuzzer.cpp b/tools/fuzzing/ipc/ProtocolFuzzer.cpp
new file mode 100644
index 0000000000..e2d7aae96d
--- /dev/null
+++ b/tools/fuzzing/ipc/ProtocolFuzzer.cpp
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/CompositorBridgeParent.h"
+
+#include "ProtocolFuzzer.h"
+
+namespace mozilla {
+namespace ipc {
+
+nsTArray<nsCString> LoadIPCMessageBlacklist(const char* aPath) {
+  nsTArray<nsCString> blacklist;
+  if (aPath) {
+    nsresult result = Faulty::ReadFile(aPath, blacklist);
+    MOZ_RELEASE_ASSERT(result == NS_OK);
+  }
+  return blacklist;
+}
+
+mozilla::dom::ContentParent* ProtocolFuzzerHelper::CreateContentParent(
+    const nsACString& aRemoteType) {
+  auto* cp = new mozilla::dom::ContentParent(aRemoteType);
+  // TODO: this duplicates MessageChannel::Open
+  cp->GetIPCChannel()->mWorkerThread = GetCurrentSerialEventTarget();
+  cp->GetIPCChannel()->mMonitor = new RefCountedMonitor();
+  return cp;
+}
+
+void ProtocolFuzzerHelper::CompositorBridgeParentSetup() {
+  mozilla::layers::CompositorBridgeParent::Setup();
+}
+
+}  // namespace ipc
+}  // namespace mozilla
diff --git a/tools/fuzzing/ipc/ProtocolFuzzer.h b/tools/fuzzing/ipc/ProtocolFuzzer.h
new file mode 100644
index 0000000000..f57852038a
--- /dev/null
+++ b/tools/fuzzing/ipc/ProtocolFuzzer.h
@@ -0,0 +1,114 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ipc_ProtocolFuzzer_h
+#define mozilla_ipc_ProtocolFuzzer_h
+
+#include "chrome/common/ipc_message.h"
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/dom/ContentParent.h"
+
+namespace mozilla {
+namespace ipc {
+
+class ProtocolFuzzerHelper {
+ public:
+  static mozilla::dom::ContentParent* CreateContentParent(
+      const nsACString& aRemoteType);
+
+  static void CompositorBridgeParentSetup();
+
+  static void AddShmemToProtocol(IToplevelProtocol* aProtocol,
+                                 Shmem::SharedMemory* aSegment, int32_t aId) {
+    MOZ_ASSERT(!aProtocol->mShmemMap.Contains(aId),
+               "Don't insert with an existing ID");
+    aProtocol->mShmemMap.Put(aId, aSegment);
+  }
+
+  static void RemoveShmemFromProtocol(IToplevelProtocol* aProtocol,
+                                      int32_t aId) {
+    aProtocol->mShmemMap.Remove(aId);
+  }
+};
+
+template <typename T>
+void FuzzProtocol(T* aProtocol, const uint8_t* aData, size_t aSize,
+                  const nsTArray<nsCString>& aIgnoredMessageTypes) {
+  while (true) {
+    uint32_t msg_size =
+        IPC::Message::MessageSize(reinterpret_cast<const char*>(aData),
+                                  reinterpret_cast<const char*>(aData) + aSize);
+    if (msg_size == 0 || msg_size > aSize) {
+      break;
+    }
+    IPC::Message m(reinterpret_cast<const char*>(aData), msg_size);
+    aSize -= msg_size;
+    aData += msg_size;
+
+    // We ignore certain message types
+    if (aIgnoredMessageTypes.Contains(m.name())) {
+      continue;
+    }
+
+    uint8_t num_shmems = 0;
+    if (aSize) {
+      num_shmems = *aData;
+      aData++;
+      aSize--;
+
+      for (uint32_t i = 0; i < num_shmems; i++) {
+        if (aSize < sizeof(uint16_t)) {
+          break;
+        }
+        size_t shmem_size = *reinterpret_cast<const uint16_t*>(aData);
+        aData += sizeof(uint16_t);
+        aSize -= sizeof(uint16_t);
+
+        if (shmem_size > aSize) {
+          break;
+        }
+        RefPtr<Shmem::SharedMemory> segment(
+            Shmem::Alloc(Shmem::PrivateIPDLCaller(), shmem_size,
+                         SharedMemory::TYPE_BASIC, false));
+        if (!segment) {
+          break;
+        }
+
+        Shmem shmem(Shmem::PrivateIPDLCaller(), segment.get(), i + 1);
+        memcpy(shmem.get<uint8_t>(), aData, shmem_size);
+        ProtocolFuzzerHelper::AddShmemToProtocol(
+            aProtocol, segment.forget().take(), i + 1);
+
+        aData += shmem_size;
+        aSize -= shmem_size;
+      }
+    }
+    // TODO: attach |m.header().num_fds| file descriptors to |m|. MVP can be
+    // empty files, next implementation maybe read a length header from |data|
+    // and then that many bytes.
+
+    if (m.is_sync()) {
+      UniquePtr<IPC::Message> reply;
+      aProtocol->OnMessageReceived(m, *getter_Transfers(reply));
+    } else {
+      aProtocol->OnMessageReceived(m);
+    }
+    for (uint32_t i = 0; i < num_shmems; i++) {
+      Shmem::SharedMemory* segment = aProtocol->LookupSharedMemory(i + 1);
+      Shmem::Dealloc(Shmem::PrivateIPDLCaller(), segment);
+      ProtocolFuzzerHelper::RemoveShmemFromProtocol(aProtocol, i + 1);
+    }
+  }
+}
+
+nsTArray<nsCString> LoadIPCMessageBlacklist(const char* aPath);
+
+}  // namespace ipc
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/ipc/moz.build b/tools/fuzzing/ipc/moz.build
new file mode 100644
index 0000000000..fce506a227
--- /dev/null
+++ b/tools/fuzzing/ipc/moz.build
@@ -0,0 +1,27 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("fuzzer-ipc-protocol")
+
+LOCAL_INCLUDES += [
+    "/dom/base",
+    "/dom/ipc",
+]
+
+SOURCES += [
+    "ProtocolFuzzer.cpp",
+]
+
+EXPORTS += [
+    "ProtocolFuzzer.h",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+
+# Add libFuzzer configuration directives
+include("/tools/fuzzing/libfuzzer-config.mozbuild")
diff --git a/tools/fuzzing/libfuzzer-config.mozbuild b/tools/fuzzing/libfuzzer-config.mozbuild
new file mode 100644
index 0000000000..91998e0050
--- /dev/null
+++ b/tools/fuzzing/libfuzzer-config.mozbuild
@@ -0,0 +1,13 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+include('libfuzzer-flags.mozbuild')
+
+if CONFIG['FUZZING']:
+    if CONFIG['LIBFUZZER']:
+        # Add trace-pc coverage for libfuzzer
+        CFLAGS += libfuzzer_flags
+        CXXFLAGS += libfuzzer_flags
diff --git a/tools/fuzzing/libfuzzer-flags.mozbuild b/tools/fuzzing/libfuzzer-flags.mozbuild
new file mode 100644
index 0000000000..97f9c6cf45
--- /dev/null
+++ b/tools/fuzzing/libfuzzer-flags.mozbuild
@@ -0,0 +1,11 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+libfuzzer_flags = []
+
+if CONFIG['LIBFUZZER_FLAGS']:
+    libfuzzer_flags += CONFIG['LIBFUZZER_FLAGS']
+
diff --git a/tools/fuzzing/libfuzzer/FuzzerBuiltins.h b/tools/fuzzing/libfuzzer/FuzzerBuiltins.h
new file mode 100644
index 0000000000..4c0ada8266
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerBuiltins.h
@@ -0,0 +1,35 @@
+//===- FuzzerBuiltins.h - Internal header for builtins ----------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Wrapper functions and marcos around builtin functions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_BUILTINS_H
+#define LLVM_FUZZER_BUILTINS_H
+
+#include "FuzzerPlatform.h"
+
+#if !LIBFUZZER_MSVC
+#include <cstdint>
+
+#define GET_CALLER_PC() __builtin_return_address(0)
+
+namespace fuzzer {
+
+inline uint8_t  Bswap(uint8_t x)  { return x; }
+inline uint16_t Bswap(uint16_t x) { return __builtin_bswap16(x); }
+inline uint32_t Bswap(uint32_t x) { return __builtin_bswap32(x); }
+inline uint64_t Bswap(uint64_t x) { return __builtin_bswap64(x); }
+
+inline uint32_t Clzll(unsigned long long X) { return __builtin_clzll(X); }
+inline uint32_t Clz(unsigned long long X) { return __builtin_clz(X); }
+inline int Popcountll(unsigned long long X) { return __builtin_popcountll(X); }
+
+}  // namespace fuzzer
+
+#endif  // !LIBFUZZER_MSVC
+#endif  // LLVM_FUZZER_BUILTINS_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerBuiltinsMsvc.h b/tools/fuzzing/libfuzzer/FuzzerBuiltinsMsvc.h
new file mode 100644
index 0000000000..c5bec9787d
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerBuiltinsMsvc.h
@@ -0,0 +1,72 @@
+//===- FuzzerBuiltinsMSVC.h - Internal header for builtins ------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Wrapper functions and marcos that use intrinsics instead of builtin functions
+// which cannot be compiled by MSVC.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_BUILTINS_MSVC_H
+#define LLVM_FUZZER_BUILTINS_MSVC_H
+
+#include "FuzzerPlatform.h"
+
+#if LIBFUZZER_MSVC
+#include <intrin.h>
+#include <cstdint>
+#include <cstdlib>
+
+// __builtin_return_address() cannot be compiled with MSVC. Use the equivalent
+// from <intrin.h>
+#define GET_CALLER_PC() _ReturnAddress()
+
+namespace fuzzer {
+
+inline uint8_t  Bswap(uint8_t x)  { return x; }
+// Use alternatives to __builtin functions from <stdlib.h> and <intrin.h> on
+// Windows since the builtins are not supported by MSVC.
+inline uint16_t Bswap(uint16_t x) { return _byteswap_ushort(x); }
+inline uint32_t Bswap(uint32_t x) { return _byteswap_ulong(x); }
+inline uint64_t Bswap(uint64_t x) { return _byteswap_uint64(x); }
+
+// The functions below were mostly copied from
+// compiler-rt/lib/builtins/int_lib.h which defines the __builtin functions used
+// outside of Windows.
+inline uint32_t Clzll(uint64_t X) {
+  unsigned long LeadZeroIdx = 0;
+
+#if !defined(_M_ARM) && !defined(_M_X64)
+  // Scan the high 32 bits.
+  if (_BitScanReverse(&LeadZeroIdx, static_cast<unsigned long>(X >> 32)))
+    return static_cast<int>(63 - (LeadZeroIdx + 32)); // Create a bit offset from the MSB.
+  // Scan the low 32 bits.
+  if (_BitScanReverse(&LeadZeroIdx, static_cast<unsigned long>(X)))
+    return static_cast<int>(63 - LeadZeroIdx);
+
+#else
+  if (_BitScanReverse64(&LeadZeroIdx, X)) return 63 - LeadZeroIdx;
+#endif
+  return 64;
+}
+
+inline uint32_t Clz(uint32_t X) {
+  unsigned long LeadZeroIdx = 0;
+  if (_BitScanReverse(&LeadZeroIdx, X)) return 31 - LeadZeroIdx;
+  return 32;
+}
+
+inline int Popcountll(unsigned long long X) {
+#if !defined(_M_ARM) && !defined(_M_X64)
+  return __popcnt(X) + __popcnt(X >> 32);
+#else
+  return __popcnt64(X);
+#endif
+}
+
+}  // namespace fuzzer
+
+#endif  // LIBFUZER_MSVC
+#endif  // LLVM_FUZZER_BUILTINS_MSVC_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerCommand.h b/tools/fuzzing/libfuzzer/FuzzerCommand.h
new file mode 100644
index 0000000000..87308864af
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerCommand.h
@@ -0,0 +1,178 @@
+//===- FuzzerCommand.h - Interface representing a process -------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// FuzzerCommand represents a command to run in a subprocess.  It allows callers
+// to manage command line arguments and output and error streams.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_COMMAND_H
+#define LLVM_FUZZER_COMMAND_H
+
+#include "FuzzerDefs.h"
+#include "FuzzerIO.h"
+
+#include <algorithm>
+#include <sstream>
+#include <string>
+#include <vector>
+
+namespace fuzzer {
+
+class Command final {
+public:
+  // This command line flag is used to indicate that the remaining command line
+  // is immutable, meaning this flag effectively marks the end of the mutable
+  // argument list.
+  static inline const char *ignoreRemainingArgs() {
+    return "-ignore_remaining_args=1";
+  }
+
+  Command() : CombinedOutAndErr(false) {}
+
+  explicit Command(const Vector<std::string> &ArgsToAdd)
+      : Args(ArgsToAdd), CombinedOutAndErr(false) {}
+
+  explicit Command(const Command &Other)
+      : Args(Other.Args), CombinedOutAndErr(Other.CombinedOutAndErr),
+        OutputFile(Other.OutputFile) {}
+
+  Command &operator=(const Command &Other) {
+    Args = Other.Args;
+    CombinedOutAndErr = Other.CombinedOutAndErr;
+    OutputFile = Other.OutputFile;
+    return *this;
+  }
+
+  ~Command() {}
+
+  // Returns true if the given Arg is present in Args.  Only checks up to
+  // "-ignore_remaining_args=1".
+  bool hasArgument(const std::string &Arg) const {
+    auto i = endMutableArgs();
+    return std::find(Args.begin(), i, Arg) != i;
+  }
+
+  // Gets all of the current command line arguments, **including** those after
+  // "-ignore-remaining-args=1".
+  const Vector<std::string> &getArguments() const { return Args; }
+
+  // Adds the given argument before "-ignore_remaining_args=1", or at the end
+  // if that flag isn't present.
+  void addArgument(const std::string &Arg) {
+    Args.insert(endMutableArgs(), Arg);
+  }
+
+  // Adds all given arguments before "-ignore_remaining_args=1", or at the end
+  // if that flag isn't present.
+  void addArguments(const Vector<std::string> &ArgsToAdd) {
+    Args.insert(endMutableArgs(), ArgsToAdd.begin(), ArgsToAdd.end());
+  }
+
+  // Removes the given argument from the command argument list.  Ignores any
+  // occurrences after "-ignore_remaining_args=1", if present.
+  void removeArgument(const std::string &Arg) {
+    auto i = endMutableArgs();
+    Args.erase(std::remove(Args.begin(), i, Arg), i);
+  }
+
+  // Like hasArgument, but checks for "-[Flag]=...".
+  bool hasFlag(const std::string &Flag) const {
+    std::string Arg("-" + Flag + "=");
+    auto IsMatch = [&](const std::string &Other) {
+      return Arg.compare(0, std::string::npos, Other, 0, Arg.length()) == 0;
+    };
+    return std::any_of(Args.begin(), endMutableArgs(), IsMatch);
+  }
+
+  // Returns the value of the first instance of a given flag, or an empty string
+  // if the flag isn't present.  Ignores any occurrences after
+  // "-ignore_remaining_args=1", if present.
+  std::string getFlagValue(const std::string &Flag) const {
+    std::string Arg("-" + Flag + "=");
+    auto IsMatch = [&](const std::string &Other) {
+      return Arg.compare(0, std::string::npos, Other, 0, Arg.length()) == 0;
+    };
+    auto i = endMutableArgs();
+    auto j = std::find_if(Args.begin(), i, IsMatch);
+    std::string result;
+    if (j != i) {
+      result = j->substr(Arg.length());
+    }
+    return result;
+  }
+
+  // Like AddArgument, but adds "-[Flag]=[Value]".
+  void addFlag(const std::string &Flag, const std::string &Value) {
+    addArgument("-" + Flag + "=" + Value);
+  }
+
+  // Like RemoveArgument, but removes "-[Flag]=...".
+  void removeFlag(const std::string &Flag) {
+    std::string Arg("-" + Flag + "=");
+    auto IsMatch = [&](const std::string &Other) {
+      return Arg.compare(0, std::string::npos, Other, 0, Arg.length()) == 0;
+    };
+    auto i = endMutableArgs();
+    Args.erase(std::remove_if(Args.begin(), i, IsMatch), i);
+  }
+
+  // Returns whether the command's stdout is being written to an output file.
+  bool hasOutputFile() const { return !OutputFile.empty(); }
+
+  // Returns the currently set output file.
+  const std::string &getOutputFile() const { return OutputFile; }
+
+  // Configures the command to redirect its output to the name file.
+  void setOutputFile(const std::string &FileName) { OutputFile = FileName; }
+
+  // Returns whether the command's stderr is redirected to stdout.
+  bool isOutAndErrCombined() const { return CombinedOutAndErr; }
+
+  // Sets whether to redirect the command's stderr to its stdout.
+  void combineOutAndErr(bool combine = true) { CombinedOutAndErr = combine; }
+
+  // Returns a string representation of the command.  On many systems this will
+  // be the equivalent command line.
+  std::string toString() const {
+    std::stringstream SS;
+    for (auto arg : getArguments())
+      SS << arg << " ";
+    if (hasOutputFile())
+      SS << ">" << getOutputFile() << " ";
+    if (isOutAndErrCombined())
+      SS << "2>&1 ";
+    std::string result = SS.str();
+    if (!result.empty())
+      result = result.substr(0, result.length() - 1);
+    return result;
+  }
+
+private:
+  Command(Command &&Other) = delete;
+  Command &operator=(Command &&Other) = delete;
+
+  Vector<std::string>::iterator endMutableArgs() {
+    return std::find(Args.begin(), Args.end(), ignoreRemainingArgs());
+  }
+
+  Vector<std::string>::const_iterator endMutableArgs() const {
+    return std::find(Args.begin(), Args.end(), ignoreRemainingArgs());
+  }
+
+  // The command arguments.  Args[0] is the command name.
+  Vector<std::string> Args;
+
+  // True indicates stderr is redirected to stdout.
+  bool CombinedOutAndErr;
+
+  // If not empty, stdout is redirected to the named file.
+  std::string OutputFile;
+};
+
+} // namespace fuzzer
+
+#endif // LLVM_FUZZER_COMMAND_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerCorpus.h b/tools/fuzzing/libfuzzer/FuzzerCorpus.h
new file mode 100644
index 0000000000..54d1e09ec6
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerCorpus.h
@@ -0,0 +1,533 @@
+//===- FuzzerCorpus.h - Internal header for the Fuzzer ----------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::InputCorpus
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_CORPUS
+#define LLVM_FUZZER_CORPUS
+
+#include "FuzzerDataFlowTrace.h"
+#include "FuzzerDefs.h"
+#include "FuzzerIO.h"
+#include "FuzzerRandom.h"
+#include "FuzzerSHA1.h"
+#include "FuzzerTracePC.h"
+#include <algorithm>
+#include <numeric>
+#include <random>
+#include <unordered_set>
+
+namespace fuzzer {
+
+struct InputInfo {
+  Unit U;  // The actual input data.
+  uint8_t Sha1[kSHA1NumBytes];  // Checksum.
+  // Number of features that this input has and no smaller input has.
+  size_t NumFeatures = 0;
+  size_t Tmp = 0; // Used by ValidateFeatureSet.
+  // Stats.
+  size_t NumExecutedMutations = 0;
+  size_t NumSuccessfullMutations = 0;
+  bool MayDeleteFile = false;
+  bool Reduced = false;
+  bool HasFocusFunction = false;
+  Vector<uint32_t> UniqFeatureSet;
+  Vector<uint8_t> DataFlowTraceForFocusFunction;
+  // Power schedule.
+  bool NeedsEnergyUpdate = false;
+  double Energy = 0.0;
+  size_t SumIncidence = 0;
+  Vector<std::pair<uint32_t, uint16_t>> FeatureFreqs;
+
+  // Delete feature Idx and its frequency from FeatureFreqs.
+  bool DeleteFeatureFreq(uint32_t Idx) {
+    if (FeatureFreqs.empty())
+      return false;
+
+    // Binary search over local feature frequencies sorted by index.
+    auto Lower = std::lower_bound(FeatureFreqs.begin(), FeatureFreqs.end(),
+                                  std::pair<uint32_t, uint16_t>(Idx, 0));
+
+    if (Lower != FeatureFreqs.end() && Lower->first == Idx) {
+      FeatureFreqs.erase(Lower);
+      return true;
+    }
+    return false;
+  }
+
+  // Assign more energy to a high-entropy seed, i.e., that reveals more
+  // information about the globally rare features in the neighborhood
+  // of the seed. Since we do not know the entropy of a seed that has
+  // never been executed we assign fresh seeds maximum entropy and
+  // let II->Energy approach the true entropy from above.
+  void UpdateEnergy(size_t GlobalNumberOfFeatures) {
+    Energy = 0.0;
+    SumIncidence = 0;
+
+    // Apply add-one smoothing to locally discovered features.
+    for (auto F : FeatureFreqs) {
+      size_t LocalIncidence = F.second + 1;
+      Energy -= LocalIncidence * logl(LocalIncidence);
+      SumIncidence += LocalIncidence;
+    }
+
+    // Apply add-one smoothing to locally undiscovered features.
+    //   PreciseEnergy -= 0; // since logl(1.0) == 0)
+    SumIncidence += (GlobalNumberOfFeatures - FeatureFreqs.size());
+
+    // Add a single locally abundant feature apply add-one smoothing.
+    size_t AbdIncidence = NumExecutedMutations + 1;
+    Energy -= AbdIncidence * logl(AbdIncidence);
+    SumIncidence += AbdIncidence;
+
+    // Normalize.
+    if (SumIncidence != 0)
+      Energy = (Energy / SumIncidence) + logl(SumIncidence);
+  }
+
+  // Increment the frequency of the feature Idx.
+  void UpdateFeatureFrequency(uint32_t Idx) {
+    NeedsEnergyUpdate = true;
+
+    // The local feature frequencies is an ordered vector of pairs.
+    // If there are no local feature frequencies, push_back preserves order.
+    // Set the feature frequency for feature Idx32 to 1.
+    if (FeatureFreqs.empty()) {
+      FeatureFreqs.push_back(std::pair<uint32_t, uint16_t>(Idx, 1));
+      return;
+    }
+
+    // Binary search over local feature frequencies sorted by index.
+    auto Lower = std::lower_bound(FeatureFreqs.begin(), FeatureFreqs.end(),
+                                  std::pair<uint32_t, uint16_t>(Idx, 0));
+
+    // If feature Idx32 already exists, increment its frequency.
+    // Otherwise, insert a new pair right after the next lower index.
+    if (Lower != FeatureFreqs.end() && Lower->first == Idx) {
+      Lower->second++;
+    } else {
+      FeatureFreqs.insert(Lower, std::pair<uint32_t, uint16_t>(Idx, 1));
+    }
+  }
+};
+
+struct EntropicOptions {
+  bool Enabled;
+  size_t NumberOfRarestFeatures;
+  size_t FeatureFrequencyThreshold;
+};
+
+class InputCorpus {
+  static const uint32_t kFeatureSetSize = 1 << 21;
+  static const uint8_t kMaxMutationFactor = 20;
+  static const size_t kSparseEnergyUpdates = 100;
+
+  size_t NumExecutedMutations = 0;
+
+  EntropicOptions Entropic;
+
+public:
+  InputCorpus(const std::string &OutputCorpus, EntropicOptions Entropic)
+      : Entropic(Entropic), OutputCorpus(OutputCorpus) {
+    memset(InputSizesPerFeature, 0, sizeof(InputSizesPerFeature));
+    memset(SmallestElementPerFeature, 0, sizeof(SmallestElementPerFeature));
+  }
+  ~InputCorpus() {
+    for (auto II : Inputs)
+      delete II;
+  }
+  size_t size() const { return Inputs.size(); }
+  size_t SizeInBytes() const {
+    size_t Res = 0;
+    for (auto II : Inputs)
+      Res += II->U.size();
+    return Res;
+  }
+  size_t NumActiveUnits() const {
+    size_t Res = 0;
+    for (auto II : Inputs)
+      Res += !II->U.empty();
+    return Res;
+  }
+  size_t MaxInputSize() const {
+    size_t Res = 0;
+    for (auto II : Inputs)
+        Res = std::max(Res, II->U.size());
+    return Res;
+  }
+  void IncrementNumExecutedMutations() { NumExecutedMutations++; }
+
+  size_t NumInputsThatTouchFocusFunction() {
+    return std::count_if(Inputs.begin(), Inputs.end(), [](const InputInfo *II) {
+      return II->HasFocusFunction;
+    });
+  }
+
+  size_t NumInputsWithDataFlowTrace() {
+    return std::count_if(Inputs.begin(), Inputs.end(), [](const InputInfo *II) {
+      return !II->DataFlowTraceForFocusFunction.empty();
+    });
+  }
+
+  bool empty() const { return Inputs.empty(); }
+  const Unit &operator[] (size_t Idx) const { return Inputs[Idx]->U; }
+  InputInfo *AddToCorpus(const Unit &U, size_t NumFeatures, bool MayDeleteFile,
+                         bool HasFocusFunction,
+                         const Vector<uint32_t> &FeatureSet,
+                         const DataFlowTrace &DFT, const InputInfo *BaseII) {
+    assert(!U.empty());
+    if (FeatureDebug)
+      Printf("ADD_TO_CORPUS %zd NF %zd\n", Inputs.size(), NumFeatures);
+    Inputs.push_back(new InputInfo());
+    InputInfo &II = *Inputs.back();
+    II.U = U;
+    II.NumFeatures = NumFeatures;
+    II.MayDeleteFile = MayDeleteFile;
+    II.UniqFeatureSet = FeatureSet;
+    II.HasFocusFunction = HasFocusFunction;
+    // Assign maximal energy to the new seed.
+    II.Energy = RareFeatures.empty() ? 1.0 : log(RareFeatures.size());
+    II.SumIncidence = RareFeatures.size();
+    II.NeedsEnergyUpdate = false;
+    std::sort(II.UniqFeatureSet.begin(), II.UniqFeatureSet.end());
+    ComputeSHA1(U.data(), U.size(), II.Sha1);
+    auto Sha1Str = Sha1ToString(II.Sha1);
+    Hashes.insert(Sha1Str);
+    if (HasFocusFunction)
+      if (auto V = DFT.Get(Sha1Str))
+        II.DataFlowTraceForFocusFunction = *V;
+    // This is a gross heuristic.
+    // Ideally, when we add an element to a corpus we need to know its DFT.
+    // But if we don't, we'll use the DFT of its base input.
+    if (II.DataFlowTraceForFocusFunction.empty() && BaseII)
+      II.DataFlowTraceForFocusFunction = BaseII->DataFlowTraceForFocusFunction;
+    DistributionNeedsUpdate = true;
+    PrintCorpus();
+    // ValidateFeatureSet();
+    return &II;
+  }
+
+  // Debug-only
+  void PrintUnit(const Unit &U) {
+    if (!FeatureDebug) return;
+    for (uint8_t C : U) {
+      if (C != 'F' && C != 'U' && C != 'Z')
+        C = '.';
+      Printf("%c", C);
+    }
+  }
+
+  // Debug-only
+  void PrintFeatureSet(const Vector<uint32_t> &FeatureSet) {
+    if (!FeatureDebug) return;
+    Printf("{");
+    for (uint32_t Feature: FeatureSet)
+      Printf("%u,", Feature);
+    Printf("}");
+  }
+
+  // Debug-only
+  void PrintCorpus() {
+    if (!FeatureDebug) return;
+    Printf("======= CORPUS:\n");
+    int i = 0;
+    for (auto II : Inputs) {
+      if (std::find(II->U.begin(), II->U.end(), 'F') != II->U.end()) {
+        Printf("[%2d] ", i);
+        Printf("%s sz=%zd ", Sha1ToString(II->Sha1).c_str(), II->U.size());
+        PrintUnit(II->U);
+        Printf(" ");
+        PrintFeatureSet(II->UniqFeatureSet);
+        Printf("\n");
+      }
+      i++;
+    }
+  }
+
+  void Replace(InputInfo *II, const Unit &U) {
+    assert(II->U.size() > U.size());
+    Hashes.erase(Sha1ToString(II->Sha1));
+    DeleteFile(*II);
+    ComputeSHA1(U.data(), U.size(), II->Sha1);
+    Hashes.insert(Sha1ToString(II->Sha1));
+    II->U = U;
+    II->Reduced = true;
+    DistributionNeedsUpdate = true;
+  }
+
+  bool HasUnit(const Unit &U) { return Hashes.count(Hash(U)); }
+  bool HasUnit(const std::string &H) { return Hashes.count(H); }
+  InputInfo &ChooseUnitToMutate(Random &Rand) {
+    InputInfo &II = *Inputs[ChooseUnitIdxToMutate(Rand)];
+    assert(!II.U.empty());
+    return II;
+  }
+
+  // Returns an index of random unit from the corpus to mutate.
+  size_t ChooseUnitIdxToMutate(Random &Rand) {
+    UpdateCorpusDistribution(Rand);
+    size_t Idx = static_cast<size_t>(CorpusDistribution(Rand));
+    assert(Idx < Inputs.size());
+    return Idx;
+  }
+
+  void PrintStats() {
+    for (size_t i = 0; i < Inputs.size(); i++) {
+      const auto &II = *Inputs[i];
+      Printf("  [% 3zd %s] sz: % 5zd runs: % 5zd succ: % 5zd focus: %d\n", i,
+             Sha1ToString(II.Sha1).c_str(), II.U.size(),
+             II.NumExecutedMutations, II.NumSuccessfullMutations, II.HasFocusFunction);
+    }
+  }
+
+  void PrintFeatureSet() {
+    for (size_t i = 0; i < kFeatureSetSize; i++) {
+      if(size_t Sz = GetFeature(i))
+        Printf("[%zd: id %zd sz%zd] ", i, SmallestElementPerFeature[i], Sz);
+    }
+    Printf("\n\t");
+    for (size_t i = 0; i < Inputs.size(); i++)
+      if (size_t N = Inputs[i]->NumFeatures)
+        Printf(" %zd=>%zd ", i, N);
+    Printf("\n");
+  }
+
+  void DeleteFile(const InputInfo &II) {
+    if (!OutputCorpus.empty() && II.MayDeleteFile)
+      RemoveFile(DirPlusFile(OutputCorpus, Sha1ToString(II.Sha1)));
+  }
+
+  void DeleteInput(size_t Idx) {
+    InputInfo &II = *Inputs[Idx];
+    DeleteFile(II);
+    Unit().swap(II.U);
+    II.Energy = 0.0;
+    II.NeedsEnergyUpdate = false;
+    DistributionNeedsUpdate = true;
+    if (FeatureDebug)
+      Printf("EVICTED %zd\n", Idx);
+  }
+
+  void AddRareFeature(uint32_t Idx) {
+    // Maintain *at least* TopXRarestFeatures many rare features
+    // and all features with a frequency below ConsideredRare.
+    // Remove all other features.
+    while (RareFeatures.size() > Entropic.NumberOfRarestFeatures &&
+           FreqOfMostAbundantRareFeature > Entropic.FeatureFrequencyThreshold) {
+
+      // Find most and second most abbundant feature.
+      uint32_t MostAbundantRareFeatureIndices[2] = {RareFeatures[0],
+                                                    RareFeatures[0]};
+      size_t Delete = 0;
+      for (size_t i = 0; i < RareFeatures.size(); i++) {
+        uint32_t Idx2 = RareFeatures[i];
+        if (GlobalFeatureFreqs[Idx2] >=
+            GlobalFeatureFreqs[MostAbundantRareFeatureIndices[0]]) {
+          MostAbundantRareFeatureIndices[1] = MostAbundantRareFeatureIndices[0];
+          MostAbundantRareFeatureIndices[0] = Idx2;
+          Delete = i;
+        }
+      }
+
+      // Remove most abundant rare feature.
+      RareFeatures[Delete] = RareFeatures.back();
+      RareFeatures.pop_back();
+
+      for (auto II : Inputs) {
+        if (II->DeleteFeatureFreq(MostAbundantRareFeatureIndices[0]))
+          II->NeedsEnergyUpdate = true;
+      }
+
+      // Set 2nd most abundant as the new most abundant feature count.
+      FreqOfMostAbundantRareFeature =
+          GlobalFeatureFreqs[MostAbundantRareFeatureIndices[1]];
+    }
+
+    // Add rare feature, handle collisions, and update energy.
+    RareFeatures.push_back(Idx);
+    GlobalFeatureFreqs[Idx] = 0;
+    for (auto II : Inputs) {
+      II->DeleteFeatureFreq(Idx);
+
+      // Apply add-one smoothing to this locally undiscovered feature.
+      // Zero energy seeds will never be fuzzed and remain zero energy.
+      if (II->Energy > 0.0) {
+        II->SumIncidence += 1;
+        II->Energy += logl(II->SumIncidence) / II->SumIncidence;
+      }
+    }
+
+    DistributionNeedsUpdate = true;
+  }
+
+  bool AddFeature(size_t Idx, uint32_t NewSize, bool Shrink) {
+    assert(NewSize);
+    Idx = Idx % kFeatureSetSize;
+    uint32_t OldSize = GetFeature(Idx);
+    if (OldSize == 0 || (Shrink && OldSize > NewSize)) {
+      if (OldSize > 0) {
+        size_t OldIdx = SmallestElementPerFeature[Idx];
+        InputInfo &II = *Inputs[OldIdx];
+        assert(II.NumFeatures > 0);
+        II.NumFeatures--;
+        if (II.NumFeatures == 0)
+          DeleteInput(OldIdx);
+      } else {
+        NumAddedFeatures++;
+        if (Entropic.Enabled)
+          AddRareFeature((uint32_t)Idx);
+      }
+      NumUpdatedFeatures++;
+      if (FeatureDebug)
+        Printf("ADD FEATURE %zd sz %d\n", Idx, NewSize);
+      SmallestElementPerFeature[Idx] = Inputs.size();
+      InputSizesPerFeature[Idx] = NewSize;
+      return true;
+    }
+    return false;
+  }
+
+  // Increment frequency of feature Idx globally and locally.
+  void UpdateFeatureFrequency(InputInfo *II, size_t Idx) {
+    uint32_t Idx32 = Idx % kFeatureSetSize;
+
+    // Saturated increment.
+    if (GlobalFeatureFreqs[Idx32] == 0xFFFF)
+      return;
+    uint16_t Freq = GlobalFeatureFreqs[Idx32]++;
+
+    // Skip if abundant.
+    if (Freq > FreqOfMostAbundantRareFeature ||
+        std::find(RareFeatures.begin(), RareFeatures.end(), Idx32) ==
+            RareFeatures.end())
+      return;
+
+    // Update global frequencies.
+    if (Freq == FreqOfMostAbundantRareFeature)
+      FreqOfMostAbundantRareFeature++;
+
+    // Update local frequencies.
+    if (II)
+      II->UpdateFeatureFrequency(Idx32);
+  }
+
+  size_t NumFeatures() const { return NumAddedFeatures; }
+  size_t NumFeatureUpdates() const { return NumUpdatedFeatures; }
+
+private:
+
+  static const bool FeatureDebug = false;
+
+  size_t GetFeature(size_t Idx) const { return InputSizesPerFeature[Idx]; }
+
+  void ValidateFeatureSet() {
+    if (FeatureDebug)
+      PrintFeatureSet();
+    for (size_t Idx = 0; Idx < kFeatureSetSize; Idx++)
+      if (GetFeature(Idx))
+        Inputs[SmallestElementPerFeature[Idx]]->Tmp++;
+    for (auto II: Inputs) {
+      if (II->Tmp != II->NumFeatures)
+        Printf("ZZZ %zd %zd\n", II->Tmp, II->NumFeatures);
+      assert(II->Tmp == II->NumFeatures);
+      II->Tmp = 0;
+    }
+  }
+
+  // Updates the probability distribution for the units in the corpus.
+  // Must be called whenever the corpus or unit weights are changed.
+  //
+  // Hypothesis: inputs that maximize information about globally rare features
+  // are interesting.
+  void UpdateCorpusDistribution(Random &Rand) {
+    // Skip update if no seeds or rare features were added/deleted.
+    // Sparse updates for local change of feature frequencies,
+    // i.e., randomly do not skip.
+    if (!DistributionNeedsUpdate &&
+        (!Entropic.Enabled || Rand(kSparseEnergyUpdates)))
+      return;
+
+    DistributionNeedsUpdate = false;
+
+    size_t N = Inputs.size();
+    assert(N);
+    Intervals.resize(N + 1);
+    Weights.resize(N);
+    std::iota(Intervals.begin(), Intervals.end(), 0);
+
+    bool VanillaSchedule = true;
+    if (Entropic.Enabled) {
+      for (auto II : Inputs) {
+        if (II->NeedsEnergyUpdate && II->Energy != 0.0) {
+          II->NeedsEnergyUpdate = false;
+          II->UpdateEnergy(RareFeatures.size());
+        }
+      }
+
+      for (size_t i = 0; i < N; i++) {
+
+        if (Inputs[i]->NumFeatures == 0) {
+          // If the seed doesn't represent any features, assign zero energy.
+          Weights[i] = 0.;
+        } else if (Inputs[i]->NumExecutedMutations / kMaxMutationFactor >
+                   NumExecutedMutations / Inputs.size()) {
+          // If the seed was fuzzed a lot more than average, assign zero energy.
+          Weights[i] = 0.;
+        } else {
+          // Otherwise, simply assign the computed energy.
+          Weights[i] = Inputs[i]->Energy;
+        }
+
+        // If energy for all seeds is zero, fall back to vanilla schedule.
+        if (Weights[i] > 0.0)
+          VanillaSchedule = false;
+      }
+    }
+
+    if (VanillaSchedule) {
+      for (size_t i = 0; i < N; i++)
+        Weights[i] = Inputs[i]->NumFeatures
+                         ? (i + 1) * (Inputs[i]->HasFocusFunction ? 1000 : 1)
+                         : 0.;
+    }
+
+    if (FeatureDebug) {
+      for (size_t i = 0; i < N; i++)
+        Printf("%zd ", Inputs[i]->NumFeatures);
+      Printf("SCORE\n");
+      for (size_t i = 0; i < N; i++)
+        Printf("%f ", Weights[i]);
+      Printf("Weights\n");
+    }
+    CorpusDistribution = std::piecewise_constant_distribution<double>(
+        Intervals.begin(), Intervals.end(), Weights.begin());
+  }
+  std::piecewise_constant_distribution<double> CorpusDistribution;
+
+  Vector<double> Intervals;
+  Vector<double> Weights;
+
+  std::unordered_set<std::string> Hashes;
+  Vector<InputInfo*> Inputs;
+
+  size_t NumAddedFeatures = 0;
+  size_t NumUpdatedFeatures = 0;
+  uint32_t InputSizesPerFeature[kFeatureSetSize];
+  uint32_t SmallestElementPerFeature[kFeatureSetSize];
+
+  bool DistributionNeedsUpdate = true;
+  uint16_t FreqOfMostAbundantRareFeature = 0;
+  uint16_t GlobalFeatureFreqs[kFeatureSetSize] = {};
+  Vector<uint32_t> RareFeatures;
+
+  std::string OutputCorpus;
+};
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_CORPUS
diff --git a/tools/fuzzing/libfuzzer/FuzzerCrossOver.cpp b/tools/fuzzing/libfuzzer/FuzzerCrossOver.cpp
new file mode 100644
index 0000000000..83d9f8d47c
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerCrossOver.cpp
@@ -0,0 +1,51 @@
+//===- FuzzerCrossOver.cpp - Cross over two test inputs -------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Cross over test inputs.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#include "FuzzerMutate.h"
+#include "FuzzerRandom.h"
+#include <cstring>
+
+namespace fuzzer {
+
+// Cross Data1 and Data2, store the result (up to MaxOutSize bytes) in Out.
+size_t MutationDispatcher::CrossOver(const uint8_t *Data1, size_t Size1,
+                                     const uint8_t *Data2, size_t Size2,
+                                     uint8_t *Out, size_t MaxOutSize) {
+  assert(Size1 || Size2);
+  MaxOutSize = Rand(MaxOutSize) + 1;
+  size_t OutPos = 0;
+  size_t Pos1 = 0;
+  size_t Pos2 = 0;
+  size_t *InPos = &Pos1;
+  size_t InSize = Size1;
+  const uint8_t *Data = Data1;
+  bool CurrentlyUsingFirstData = true;
+  while (OutPos < MaxOutSize && (Pos1 < Size1 || Pos2 < Size2)) {
+    // Merge a part of Data into Out.
+    size_t OutSizeLeft = MaxOutSize - OutPos;
+    if (*InPos < InSize) {
+      size_t InSizeLeft = InSize - *InPos;
+      size_t MaxExtraSize = std::min(OutSizeLeft, InSizeLeft);
+      size_t ExtraSize = Rand(MaxExtraSize) + 1;
+      memcpy(Out + OutPos, Data + *InPos, ExtraSize);
+      OutPos += ExtraSize;
+      (*InPos) += ExtraSize;
+    }
+    // Use the other input data on the next iteration.
+    InPos  = CurrentlyUsingFirstData ? &Pos2 : &Pos1;
+    InSize = CurrentlyUsingFirstData ? Size2 : Size1;
+    Data   = CurrentlyUsingFirstData ? Data2 : Data1;
+    CurrentlyUsingFirstData = !CurrentlyUsingFirstData;
+  }
+  return OutPos;
+}
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.cpp b/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.cpp
new file mode 100644
index 0000000000..0e9cdf7e66
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.cpp
@@ -0,0 +1,286 @@
+//===- FuzzerDataFlowTrace.cpp - DataFlowTrace                ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::DataFlowTrace
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDataFlowTrace.h"
+
+#include "FuzzerCommand.h"
+#include "FuzzerIO.h"
+#include "FuzzerRandom.h"
+#include "FuzzerSHA1.h"
+#include "FuzzerUtil.h"
+
+#include <cstdlib>
+#include <fstream>
+#include <numeric>
+#include <queue>
+#include <sstream>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+namespace fuzzer {
+static const char *kFunctionsTxt = "functions.txt";
+
+bool BlockCoverage::AppendCoverage(const std::string &S) {
+  std::stringstream SS(S);
+  return AppendCoverage(SS);
+}
+
+// Coverage lines have this form:
+// CN X Y Z T
+// where N is the number of the function, T is the total number of instrumented
+// BBs, and X,Y,Z, if present, are the indecies of covered BB.
+// BB #0, which is the entry block, is not explicitly listed.
+bool BlockCoverage::AppendCoverage(std::istream &IN) {
+  std::string L;
+  while (std::getline(IN, L, '\n')) {
+    if (L.empty())
+      continue;
+    std::stringstream SS(L.c_str() + 1);
+    size_t FunctionId  = 0;
+    SS >> FunctionId;
+    if (L[0] == 'F') {
+      FunctionsWithDFT.insert(FunctionId);
+      continue;
+    }
+    if (L[0] != 'C') continue;
+    Vector<uint32_t> CoveredBlocks;
+    while (true) {
+      uint32_t BB = 0;
+      SS >> BB;
+      if (!SS) break;
+      CoveredBlocks.push_back(BB);
+    }
+    if (CoveredBlocks.empty()) return false;
+    uint32_t NumBlocks = CoveredBlocks.back();
+    CoveredBlocks.pop_back();
+    for (auto BB : CoveredBlocks)
+      if (BB >= NumBlocks) return false;
+    auto It = Functions.find(FunctionId);
+    auto &Counters =
+        It == Functions.end()
+            ? Functions.insert({FunctionId, Vector<uint32_t>(NumBlocks)})
+                  .first->second
+            : It->second;
+
+    if (Counters.size() != NumBlocks) return false;  // wrong number of blocks.
+
+    Counters[0]++;
+    for (auto BB : CoveredBlocks)
+      Counters[BB]++;
+  }
+  return true;
+}
+
+// Assign weights to each function.
+// General principles:
+//   * any uncovered function gets weight 0.
+//   * a function with lots of uncovered blocks gets bigger weight.
+//   * a function with a less frequently executed code gets bigger weight.
+Vector<double> BlockCoverage::FunctionWeights(size_t NumFunctions) const {
+  Vector<double> Res(NumFunctions);
+  for (auto It : Functions) {
+    auto FunctionID = It.first;
+    auto Counters = It.second;
+    assert(FunctionID < NumFunctions);
+    auto &Weight = Res[FunctionID];
+    // Give higher weight if the function has a DFT.
+    Weight = FunctionsWithDFT.count(FunctionID) ? 1000. : 1;
+    // Give higher weight to functions with less frequently seen basic blocks.
+    Weight /= SmallestNonZeroCounter(Counters);
+    // Give higher weight to functions with the most uncovered basic blocks.
+    Weight *= NumberOfUncoveredBlocks(Counters) + 1;
+  }
+  return Res;
+}
+
+void DataFlowTrace::ReadCoverage(const std::string &DirPath) {
+  Vector<SizedFile> Files;
+  GetSizedFilesFromDir(DirPath, &Files);
+  for (auto &SF : Files) {
+    auto Name = Basename(SF.File);
+    if (Name == kFunctionsTxt) continue;
+    if (!CorporaHashes.count(Name)) continue;
+    std::ifstream IF(SF.File);
+    Coverage.AppendCoverage(IF);
+  }
+}
+
+static void DFTStringAppendToVector(Vector<uint8_t> *DFT,
+                                    const std::string &DFTString) {
+  assert(DFT->size() == DFTString.size());
+  for (size_t I = 0, Len = DFT->size(); I < Len; I++)
+    (*DFT)[I] = DFTString[I] == '1';
+}
+
+// converts a string of '0' and '1' into a Vector<uint8_t>
+static Vector<uint8_t> DFTStringToVector(const std::string &DFTString) {
+  Vector<uint8_t> DFT(DFTString.size());
+  DFTStringAppendToVector(&DFT, DFTString);
+  return DFT;
+}
+
+static bool ParseError(const char *Err, const std::string &Line) {
+  Printf("DataFlowTrace: parse error: %s: Line: %s\n", Err, Line.c_str());
+  return false;
+}
+
+// TODO(metzman): replace std::string with std::string_view for
+// better performance. Need to figure our how to use string_view on Windows.
+static bool ParseDFTLine(const std::string &Line, size_t *FunctionNum,
+                         std::string *DFTString) {
+  if (!Line.empty() && Line[0] != 'F')
+    return false; // Ignore coverage.
+  size_t SpacePos = Line.find(' ');
+  if (SpacePos == std::string::npos)
+    return ParseError("no space in the trace line", Line);
+  if (Line.empty() || Line[0] != 'F')
+    return ParseError("the trace line doesn't start with 'F'", Line);
+  *FunctionNum = std::atol(Line.c_str() + 1);
+  const char *Beg = Line.c_str() + SpacePos + 1;
+  const char *End = Line.c_str() + Line.size();
+  assert(Beg < End);
+  size_t Len = End - Beg;
+  for (size_t I = 0; I < Len; I++) {
+    if (Beg[I] != '0' && Beg[I] != '1')
+      return ParseError("the trace should contain only 0 or 1", Line);
+  }
+  *DFTString = Beg;
+  return true;
+}
+
+bool DataFlowTrace::Init(const std::string &DirPath, std::string *FocusFunction,
+                         Vector<SizedFile> &CorporaFiles, Random &Rand) {
+  if (DirPath.empty()) return false;
+  Printf("INFO: DataFlowTrace: reading from '%s'\n", DirPath.c_str());
+  Vector<SizedFile> Files;
+  GetSizedFilesFromDir(DirPath, &Files);
+  std::string L;
+  size_t FocusFuncIdx = SIZE_MAX;
+  Vector<std::string> FunctionNames;
+
+  // Collect the hashes of the corpus files.
+  for (auto &SF : CorporaFiles)
+    CorporaHashes.insert(Hash(FileToVector(SF.File)));
+
+  // Read functions.txt
+  std::ifstream IF(DirPlusFile(DirPath, kFunctionsTxt));
+  size_t NumFunctions = 0;
+  while (std::getline(IF, L, '\n')) {
+    FunctionNames.push_back(L);
+    NumFunctions++;
+    if (*FocusFunction == L)
+      FocusFuncIdx = NumFunctions - 1;
+  }
+  if (!NumFunctions)
+    return false;
+
+  if (*FocusFunction == "auto") {
+    // AUTOFOCUS works like this:
+    // * reads the coverage data from the DFT files.
+    // * assigns weights to functions based on coverage.
+    // * chooses a random function according to the weights.
+    ReadCoverage(DirPath);
+    auto Weights = Coverage.FunctionWeights(NumFunctions);
+    Vector<double> Intervals(NumFunctions + 1);
+    std::iota(Intervals.begin(), Intervals.end(), 0);
+    auto Distribution = std::piecewise_constant_distribution<double>(
+        Intervals.begin(), Intervals.end(), Weights.begin());
+    FocusFuncIdx = static_cast<size_t>(Distribution(Rand));
+    *FocusFunction = FunctionNames[FocusFuncIdx];
+    assert(FocusFuncIdx < NumFunctions);
+    Printf("INFO: AUTOFOCUS: %zd %s\n", FocusFuncIdx,
+           FunctionNames[FocusFuncIdx].c_str());
+    for (size_t i = 0; i < NumFunctions; i++) {
+      if (!Weights[i]) continue;
+      Printf("  [%zd] W %g\tBB-tot %u\tBB-cov %u\tEntryFreq %u:\t%s\n", i,
+             Weights[i], Coverage.GetNumberOfBlocks(i),
+             Coverage.GetNumberOfCoveredBlocks(i), Coverage.GetCounter(i, 0),
+             FunctionNames[i].c_str());
+    }
+  }
+
+  if (!NumFunctions || FocusFuncIdx == SIZE_MAX || Files.size() <= 1)
+    return false;
+
+  // Read traces.
+  size_t NumTraceFiles = 0;
+  size_t NumTracesWithFocusFunction = 0;
+  for (auto &SF : Files) {
+    auto Name = Basename(SF.File);
+    if (Name == kFunctionsTxt) continue;
+    if (!CorporaHashes.count(Name)) continue;  // not in the corpus.
+    NumTraceFiles++;
+    // Printf("=== %s\n", Name.c_str());
+    std::ifstream IF(SF.File);
+    while (std::getline(IF, L, '\n')) {
+      size_t FunctionNum = 0;
+      std::string DFTString;
+      if (ParseDFTLine(L, &FunctionNum, &DFTString) &&
+          FunctionNum == FocusFuncIdx) {
+        NumTracesWithFocusFunction++;
+
+        if (FunctionNum >= NumFunctions)
+          return ParseError("N is greater than the number of functions", L);
+        Traces[Name] = DFTStringToVector(DFTString);
+        // Print just a few small traces.
+        if (NumTracesWithFocusFunction <= 3 && DFTString.size() <= 16)
+          Printf("%s => |%s|\n", Name.c_str(), std::string(DFTString).c_str());
+        break; // No need to parse the following lines.
+      }
+    }
+  }
+  Printf("INFO: DataFlowTrace: %zd trace files, %zd functions, "
+         "%zd traces with focus function\n",
+         NumTraceFiles, NumFunctions, NumTracesWithFocusFunction);
+  return NumTraceFiles > 0;
+}
+
+int CollectDataFlow(const std::string &DFTBinary, const std::string &DirPath,
+                    const Vector<SizedFile> &CorporaFiles) {
+  Printf("INFO: collecting data flow: bin: %s dir: %s files: %zd\n",
+         DFTBinary.c_str(), DirPath.c_str(), CorporaFiles.size());
+  if (CorporaFiles.empty()) {
+    Printf("ERROR: can't collect data flow without corpus provided.");
+    return 1;
+  }
+
+  static char DFSanEnv[] = "DFSAN_OPTIONS=warn_unimplemented=0";
+  putenv(DFSanEnv);
+  MkDir(DirPath);
+  for (auto &F : CorporaFiles) {
+    // For every input F we need to collect the data flow and the coverage.
+    // Data flow collection may fail if we request too many DFSan tags at once.
+    // So, we start from requesting all tags in range [0,Size) and if that fails
+    // we then request tags in [0,Size/2) and [Size/2, Size), and so on.
+    // Function number => DFT.
+    auto OutPath = DirPlusFile(DirPath, Hash(FileToVector(F.File)));
+    std::unordered_map<size_t, Vector<uint8_t>> DFTMap;
+    std::unordered_set<std::string> Cov;
+    Command Cmd;
+    Cmd.addArgument(DFTBinary);
+    Cmd.addArgument(F.File);
+    Cmd.addArgument(OutPath);
+    Printf("CMD: %s\n", Cmd.toString().c_str());
+    ExecuteCommand(Cmd);
+  }
+  // Write functions.txt if it's currently empty or doesn't exist.
+  auto FunctionsTxtPath = DirPlusFile(DirPath, kFunctionsTxt);
+  if (FileToString(FunctionsTxtPath).empty()) {
+    Command Cmd;
+    Cmd.addArgument(DFTBinary);
+    Cmd.setOutputFile(FunctionsTxtPath);
+    ExecuteCommand(Cmd);
+  }
+  return 0;
+}
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.h b/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.h
new file mode 100644
index 0000000000..d6e3de30a4
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.h
@@ -0,0 +1,135 @@
+//===- FuzzerDataFlowTrace.h - Internal header for the Fuzzer ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::DataFlowTrace; reads and handles a data-flow trace.
+//
+// A data flow trace is generated by e.g. dataflow/DataFlow.cpp
+// and is stored on disk in a separate directory.
+//
+// The trace dir contains a file 'functions.txt' which lists function names,
+// oner per line, e.g.
+// ==> functions.txt <==
+// Func2
+// LLVMFuzzerTestOneInput
+// Func1
+//
+// All other files in the dir are the traces, see dataflow/DataFlow.cpp.
+// The name of the file is sha1 of the input used to generate the trace.
+//
+// Current status:
+//   the data is parsed and the summary is printed, but the data is not yet
+//   used in any other way.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_DATA_FLOW_TRACE
+#define LLVM_FUZZER_DATA_FLOW_TRACE
+
+#include "FuzzerDefs.h"
+#include "FuzzerIO.h"
+
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+#include <string>
+
+namespace fuzzer {
+
+int CollectDataFlow(const std::string &DFTBinary, const std::string &DirPath,
+                    const Vector<SizedFile> &CorporaFiles);
+
+class BlockCoverage {
+ public:
+  bool AppendCoverage(std::istream &IN);
+  bool AppendCoverage(const std::string &S);
+
+  size_t NumCoveredFunctions() const { return Functions.size(); }
+
+  uint32_t GetCounter(size_t FunctionId, size_t BasicBlockId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    if (BasicBlockId < Counters.size())
+      return Counters[BasicBlockId];
+    return 0;
+  }
+
+  uint32_t GetNumberOfBlocks(size_t FunctionId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    return Counters.size();
+  }
+
+  uint32_t GetNumberOfCoveredBlocks(size_t FunctionId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    uint32_t Result = 0;
+    for (auto Cnt: Counters)
+      if (Cnt)
+        Result++;
+    return Result;
+  }
+
+  Vector<double> FunctionWeights(size_t NumFunctions) const;
+  void clear() { Functions.clear(); }
+
+ private:
+
+  typedef Vector<uint32_t> CoverageVector;
+
+  uint32_t NumberOfCoveredBlocks(const CoverageVector &Counters) const {
+    uint32_t Res = 0;
+    for (auto Cnt : Counters)
+      if (Cnt)
+        Res++;
+    return Res;
+  }
+
+  uint32_t NumberOfUncoveredBlocks(const CoverageVector &Counters) const {
+    return Counters.size() - NumberOfCoveredBlocks(Counters);
+  }
+
+  uint32_t SmallestNonZeroCounter(const CoverageVector &Counters) const {
+    assert(!Counters.empty());
+    uint32_t Res = Counters[0];
+    for (auto Cnt : Counters)
+      if (Cnt)
+        Res = Min(Res, Cnt);
+    assert(Res);
+    return Res;
+  }
+
+  // Function ID => vector of counters.
+  // Each counter represents how many input files trigger the given basic block.
+  std::unordered_map<size_t, CoverageVector> Functions;
+  // Functions that have DFT entry.
+  std::unordered_set<size_t> FunctionsWithDFT;
+};
+
+class DataFlowTrace {
+ public:
+  void ReadCoverage(const std::string &DirPath);
+  bool Init(const std::string &DirPath, std::string *FocusFunction,
+            Vector<SizedFile> &CorporaFiles, Random &Rand);
+  void Clear() { Traces.clear(); }
+  const Vector<uint8_t> *Get(const std::string &InputSha1) const {
+    auto It = Traces.find(InputSha1);
+    if (It != Traces.end())
+      return &It->second;
+    return nullptr;
+  }
+
+ private:
+  // Input's sha1 => DFT for the FocusFunction.
+  std::unordered_map<std::string, Vector<uint8_t> > Traces;
+  BlockCoverage Coverage;
+  std::unordered_set<std::string> CorporaHashes;
+};
+}  // namespace fuzzer
+
+#endif // LLVM_FUZZER_DATA_FLOW_TRACE
diff --git a/tools/fuzzing/libfuzzer/FuzzerDefs.h b/tools/fuzzing/libfuzzer/FuzzerDefs.h
new file mode 100644
index 0000000000..3952ac51ef
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerDefs.h
@@ -0,0 +1,75 @@
+//===- FuzzerDefs.h - Internal header for the Fuzzer ------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Basic definitions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_DEFS_H
+#define LLVM_FUZZER_DEFS_H
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <memory>
+#include <set>
+#include <string>
+#include <vector>
+
+
+namespace fuzzer {
+
+template <class T> T Min(T a, T b) { return a < b ? a : b; }
+template <class T> T Max(T a, T b) { return a > b ? a : b; }
+
+class Random;
+class Dictionary;
+class DictionaryEntry;
+class MutationDispatcher;
+struct FuzzingOptions;
+class InputCorpus;
+struct InputInfo;
+struct ExternalFunctions;
+
+// Global interface to functions that may or may not be available.
+extern ExternalFunctions *EF;
+
+// We are using a custom allocator to give a different symbol name to STL
+// containers in order to avoid ODR violations.
+template<typename T>
+  class fuzzer_allocator: public std::allocator<T> {
+    public:
+      fuzzer_allocator() = default;
+
+      template<class U>
+      explicit fuzzer_allocator(const fuzzer_allocator<U>&) {}
+
+      template<class Other>
+      struct rebind { typedef fuzzer_allocator<Other> other;  };
+  };
+
+template<typename T>
+using Vector = std::vector<T, fuzzer_allocator<T>>;
+
+template<typename T>
+using Set = std::set<T, std::less<T>, fuzzer_allocator<T>>;
+
+typedef Vector<uint8_t> Unit;
+typedef Vector<Unit> UnitVector;
+typedef int (*UserCallback)(const uint8_t *Data, size_t Size);
+
+int FuzzerDriver(int *argc, char ***argv, UserCallback Callback);
+
+uint8_t *ExtraCountersBegin();
+uint8_t *ExtraCountersEnd();
+void ClearExtraCounters();
+
+extern bool RunningUserCallback;
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_DEFS_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerDictionary.h b/tools/fuzzing/libfuzzer/FuzzerDictionary.h
new file mode 100644
index 0000000000..301c5d9afe
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerDictionary.h
@@ -0,0 +1,118 @@
+//===- FuzzerDictionary.h - Internal header for the Fuzzer ------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::Dictionary
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_DICTIONARY_H
+#define LLVM_FUZZER_DICTIONARY_H
+
+#include "FuzzerDefs.h"
+#include "FuzzerIO.h"
+#include "FuzzerUtil.h"
+#include <algorithm>
+#include <limits>
+
+namespace fuzzer {
+// A simple POD sized array of bytes.
+template <size_t kMaxSizeT> class FixedWord {
+public:
+  static const size_t kMaxSize = kMaxSizeT;
+  FixedWord() {}
+  FixedWord(const uint8_t *B, uint8_t S) { Set(B, S); }
+
+  void Set(const uint8_t *B, uint8_t S) {
+    assert(S <= kMaxSize);
+    memcpy(Data, B, S);
+    Size = S;
+  }
+
+  bool operator==(const FixedWord<kMaxSize> &w) const {
+    return Size == w.Size && 0 == memcmp(Data, w.Data, Size);
+  }
+
+  static size_t GetMaxSize() { return kMaxSize; }
+  const uint8_t *data() const { return Data; }
+  uint8_t size() const { return Size; }
+
+private:
+  uint8_t Size = 0;
+  uint8_t Data[kMaxSize];
+};
+
+typedef FixedWord<64> Word;
+
+class DictionaryEntry {
+ public:
+  DictionaryEntry() {}
+  DictionaryEntry(Word W) : W(W) {}
+  DictionaryEntry(Word W, size_t PositionHint) : W(W), PositionHint(PositionHint) {}
+  const Word &GetW() const { return W; }
+
+  bool HasPositionHint() const { return PositionHint != std::numeric_limits<size_t>::max(); }
+  size_t GetPositionHint() const {
+    assert(HasPositionHint());
+    return PositionHint;
+  }
+  void IncUseCount() { UseCount++; }
+  void IncSuccessCount() { SuccessCount++; }
+  size_t GetUseCount() const { return UseCount; }
+  size_t GetSuccessCount() const {return SuccessCount; }
+
+  void Print(const char *PrintAfter = "\n") {
+    PrintASCII(W.data(), W.size());
+    if (HasPositionHint())
+      Printf("@%zd", GetPositionHint());
+    Printf("%s", PrintAfter);
+  }
+
+private:
+  Word W;
+  size_t PositionHint = std::numeric_limits<size_t>::max();
+  size_t UseCount = 0;
+  size_t SuccessCount = 0;
+};
+
+class Dictionary {
+ public:
+  static const size_t kMaxDictSize = 1 << 14;
+
+  bool ContainsWord(const Word &W) const {
+    return std::any_of(begin(), end(), [&](const DictionaryEntry &DE) {
+      return DE.GetW() == W;
+    });
+  }
+  const DictionaryEntry *begin() const { return &DE[0]; }
+  const DictionaryEntry *end() const { return begin() + Size; }
+  DictionaryEntry & operator[] (size_t Idx) {
+    assert(Idx < Size);
+    return DE[Idx];
+  }
+  void push_back(DictionaryEntry DE) {
+    if (Size < kMaxDictSize)
+      this->DE[Size++] = DE;
+  }
+  void clear() { Size = 0; }
+  bool empty() const { return Size == 0; }
+  size_t size() const { return Size; }
+
+private:
+  DictionaryEntry DE[kMaxDictSize];
+  size_t Size = 0;
+};
+
+// Parses one dictionary entry.
+// If successful, write the enty to Unit and returns true,
+// otherwise returns false.
+bool ParseOneDictionaryEntry(const std::string &Str, Unit *U);
+// Parses the dictionary file, fills Units, returns true iff all lines
+// were parsed successfully.
+bool ParseDictionaryFile(const std::string &Text, Vector<Unit> *Units);
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_DICTIONARY_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerDriver.cpp b/tools/fuzzing/libfuzzer/FuzzerDriver.cpp
new file mode 100644
index 0000000000..cd72020084
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerDriver.cpp
@@ -0,0 +1,871 @@
+//===- FuzzerDriver.cpp - FuzzerDriver function and flags -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// FuzzerDriver and flag parsing.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerCommand.h"
+#include "FuzzerCorpus.h"
+#include "FuzzerFork.h"
+#include "FuzzerIO.h"
+#include "FuzzerInterface.h"
+#include "FuzzerInternal.h"
+#include "FuzzerMerge.h"
+#include "FuzzerMutate.h"
+#include "FuzzerPlatform.h"
+#include "FuzzerRandom.h"
+#include "FuzzerTracePC.h"
+#include <algorithm>
+#include <atomic>
+#include <chrono>
+#include <cstdlib>
+#include <cstring>
+#include <mutex>
+#include <string>
+#include <thread>
+#include <fstream>
+
+// This function should be present in the libFuzzer so that the client
+// binary can test for its existence.
+#if LIBFUZZER_MSVC
+extern "C" void __libfuzzer_is_present() {}
+#if defined(_M_IX86) || defined(__i386__)
+#pragma comment(linker, "/include:___libfuzzer_is_present")
+#else
+#pragma comment(linker, "/include:__libfuzzer_is_present")
+#endif
+#else
+extern "C" __attribute__((used)) void __libfuzzer_is_present() {}
+#endif  // LIBFUZZER_MSVC
+
+namespace fuzzer {
+
+// Program arguments.
+struct FlagDescription {
+  const char *Name;
+  const char *Description;
+  int   Default;
+  int   *IntFlag;
+  const char **StrFlag;
+  unsigned int *UIntFlag;
+};
+
+struct {
+#define FUZZER_DEPRECATED_FLAG(Name)
+#define FUZZER_FLAG_INT(Name, Default, Description) int Name;
+#define FUZZER_FLAG_UNSIGNED(Name, Default, Description) unsigned int Name;
+#define FUZZER_FLAG_STRING(Name, Description) const char *Name;
+#include "FuzzerFlags.def"
+#undef FUZZER_DEPRECATED_FLAG
+#undef FUZZER_FLAG_INT
+#undef FUZZER_FLAG_UNSIGNED
+#undef FUZZER_FLAG_STRING
+} Flags;
+
+static const FlagDescription FlagDescriptions [] {
+#define FUZZER_DEPRECATED_FLAG(Name)                                           \
+  {#Name, "Deprecated; don't use", 0, nullptr, nullptr, nullptr},
+#define FUZZER_FLAG_INT(Name, Default, Description)                            \
+  {#Name, Description, Default, &Flags.Name, nullptr, nullptr},
+#define FUZZER_FLAG_UNSIGNED(Name, Default, Description)                       \
+  {#Name,   Description, static_cast<int>(Default),                            \
+   nullptr, nullptr, &Flags.Name},
+#define FUZZER_FLAG_STRING(Name, Description)                                  \
+  {#Name, Description, 0, nullptr, &Flags.Name, nullptr},
+#include "FuzzerFlags.def"
+#undef FUZZER_DEPRECATED_FLAG
+#undef FUZZER_FLAG_INT
+#undef FUZZER_FLAG_UNSIGNED
+#undef FUZZER_FLAG_STRING
+};
+
+static const size_t kNumFlags =
+    sizeof(FlagDescriptions) / sizeof(FlagDescriptions[0]);
+
+static Vector<std::string> *Inputs;
+static std::string *ProgName;
+
+static void PrintHelp() {
+  Printf("Usage:\n");
+  auto Prog = ProgName->c_str();
+  Printf("\nTo run fuzzing pass 0 or more directories.\n");
+  Printf("%s [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]\n", Prog);
+
+  Printf("\nTo run individual tests without fuzzing pass 1 or more files:\n");
+  Printf("%s [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]\n", Prog);
+
+  Printf("\nFlags: (strictly in form -flag=value)\n");
+  size_t MaxFlagLen = 0;
+  for (size_t F = 0; F < kNumFlags; F++)
+    MaxFlagLen = std::max(strlen(FlagDescriptions[F].Name), MaxFlagLen);
+
+  for (size_t F = 0; F < kNumFlags; F++) {
+    const auto &D = FlagDescriptions[F];
+    if (strstr(D.Description, "internal flag") == D.Description) continue;
+    Printf(" %s", D.Name);
+    for (size_t i = 0, n = MaxFlagLen - strlen(D.Name); i < n; i++)
+      Printf(" ");
+    Printf("\t");
+    Printf("%d\t%s\n", D.Default, D.Description);
+  }
+  Printf("\nFlags starting with '--' will be ignored and "
+            "will be passed verbatim to subprocesses.\n");
+}
+
+static const char *FlagValue(const char *Param, const char *Name) {
+  size_t Len = strlen(Name);
+  if (Param[0] == '-' && strstr(Param + 1, Name) == Param + 1 &&
+      Param[Len + 1] == '=')
+      return &Param[Len + 2];
+  return nullptr;
+}
+
+// Avoid calling stol as it triggers a bug in clang/glibc build.
+static long MyStol(const char *Str) {
+  long Res = 0;
+  long Sign = 1;
+  if (*Str == '-') {
+    Str++;
+    Sign = -1;
+  }
+  for (size_t i = 0; Str[i]; i++) {
+    char Ch = Str[i];
+    if (Ch < '0' || Ch > '9')
+      return Res;
+    Res = Res * 10 + (Ch - '0');
+  }
+  return Res * Sign;
+}
+
+static bool ParseOneFlag(const char *Param) {
+  if (Param[0] != '-') return false;
+  if (Param[1] == '-') {
+    static bool PrintedWarning = false;
+    if (!PrintedWarning) {
+      PrintedWarning = true;
+      Printf("INFO: libFuzzer ignores flags that start with '--'\n");
+    }
+    for (size_t F = 0; F < kNumFlags; F++)
+      if (FlagValue(Param + 1, FlagDescriptions[F].Name))
+        Printf("WARNING: did you mean '%s' (single dash)?\n", Param + 1);
+    return true;
+  }
+  for (size_t F = 0; F < kNumFlags; F++) {
+    const char *Name = FlagDescriptions[F].Name;
+    const char *Str = FlagValue(Param, Name);
+    if (Str)  {
+      if (FlagDescriptions[F].IntFlag) {
+        int Val = MyStol(Str);
+        *FlagDescriptions[F].IntFlag = Val;
+        if (Flags.verbosity >= 2)
+          Printf("Flag: %s %d\n", Name, Val);
+        return true;
+      } else if (FlagDescriptions[F].UIntFlag) {
+        unsigned int Val = std::stoul(Str);
+        *FlagDescriptions[F].UIntFlag = Val;
+        if (Flags.verbosity >= 2)
+          Printf("Flag: %s %u\n", Name, Val);
+        return true;
+      } else if (FlagDescriptions[F].StrFlag) {
+        *FlagDescriptions[F].StrFlag = Str;
+        if (Flags.verbosity >= 2)
+          Printf("Flag: %s %s\n", Name, Str);
+        return true;
+      } else {  // Deprecated flag.
+        Printf("Flag: %s: deprecated, don't use\n", Name);
+        return true;
+      }
+    }
+  }
+  Printf("\n\nWARNING: unrecognized flag '%s'; "
+         "use -help=1 to list all flags\n\n", Param);
+  return true;
+}
+
+// We don't use any library to minimize dependencies.
+static void ParseFlags(const Vector<std::string> &Args,
+                       const ExternalFunctions *EF) {
+  for (size_t F = 0; F < kNumFlags; F++) {
+    if (FlagDescriptions[F].IntFlag)
+      *FlagDescriptions[F].IntFlag = FlagDescriptions[F].Default;
+    if (FlagDescriptions[F].UIntFlag)
+      *FlagDescriptions[F].UIntFlag =
+          static_cast<unsigned int>(FlagDescriptions[F].Default);
+    if (FlagDescriptions[F].StrFlag)
+      *FlagDescriptions[F].StrFlag = nullptr;
+  }
+
+  // Disable len_control by default, if LLVMFuzzerCustomMutator is used.
+  if (EF->LLVMFuzzerCustomMutator) {
+    Flags.len_control = 0;
+    Printf("INFO: found LLVMFuzzerCustomMutator (%p). "
+           "Disabling -len_control by default.\n", EF->LLVMFuzzerCustomMutator);
+  }
+
+  Inputs = new Vector<std::string>;
+  for (size_t A = 1; A < Args.size(); A++) {
+    if (ParseOneFlag(Args[A].c_str())) {
+      if (Flags.ignore_remaining_args)
+        break;
+      continue;
+    }
+    Inputs->push_back(Args[A]);
+  }
+}
+
+static std::mutex Mu;
+
+static void PulseThread() {
+  while (true) {
+    SleepSeconds(600);
+    std::lock_guard<std::mutex> Lock(Mu);
+    Printf("pulse...\n");
+  }
+}
+
+static void WorkerThread(const Command &BaseCmd, std::atomic<unsigned> *Counter,
+                         unsigned NumJobs, std::atomic<bool> *HasErrors) {
+  while (true) {
+    unsigned C = (*Counter)++;
+    if (C >= NumJobs) break;
+    std::string Log = "fuzz-" + std::to_string(C) + ".log";
+    Command Cmd(BaseCmd);
+    Cmd.setOutputFile(Log);
+    Cmd.combineOutAndErr();
+    if (Flags.verbosity) {
+      std::string CommandLine = Cmd.toString();
+      Printf("%s\n", CommandLine.c_str());
+    }
+    int ExitCode = ExecuteCommand(Cmd);
+    if (ExitCode != 0)
+      *HasErrors = true;
+    std::lock_guard<std::mutex> Lock(Mu);
+    Printf("================== Job %u exited with exit code %d ============\n",
+           C, ExitCode);
+    fuzzer::CopyFileToErr(Log);
+  }
+}
+
+std::string CloneArgsWithoutX(const Vector<std::string> &Args,
+                              const char *X1, const char *X2) {
+  std::string Cmd;
+  for (auto &S : Args) {
+    if (FlagValue(S.c_str(), X1) || FlagValue(S.c_str(), X2))
+      continue;
+    Cmd += S + " ";
+  }
+  return Cmd;
+}
+
+static int RunInMultipleProcesses(const Vector<std::string> &Args,
+                                  unsigned NumWorkers, unsigned NumJobs) {
+  std::atomic<unsigned> Counter(0);
+  std::atomic<bool> HasErrors(false);
+  Command Cmd(Args);
+  Cmd.removeFlag("jobs");
+  Cmd.removeFlag("workers");
+  Vector<std::thread> V;
+  std::thread Pulse(PulseThread);
+  Pulse.detach();
+  for (unsigned i = 0; i < NumWorkers; i++)
+    V.push_back(std::thread(WorkerThread, std::ref(Cmd), &Counter, NumJobs, &HasErrors));
+  for (auto &T : V)
+    T.join();
+  return HasErrors ? 1 : 0;
+}
+
+static void RssThread(Fuzzer *F, size_t RssLimitMb) {
+  while (true) {
+    SleepSeconds(1);
+    size_t Peak = GetPeakRSSMb();
+    if (Peak > RssLimitMb)
+      F->RssLimitCallback();
+  }
+}
+
+static void StartRssThread(Fuzzer *F, size_t RssLimitMb) {
+  if (!RssLimitMb)
+    return;
+  std::thread T(RssThread, F, RssLimitMb);
+  T.detach();
+}
+
+int RunOneTest(Fuzzer *F, const char *InputFilePath, size_t MaxLen) {
+  Unit U = FileToVector(InputFilePath);
+  if (MaxLen && MaxLen < U.size())
+    U.resize(MaxLen);
+  F->ExecuteCallback(U.data(), U.size());
+  F->TryDetectingAMemoryLeak(U.data(), U.size(), true);
+  return 0;
+}
+
+static bool AllInputsAreFiles() {
+  if (Inputs->empty()) return false;
+  for (auto &Path : *Inputs)
+    if (!IsFile(Path))
+      return false;
+  return true;
+}
+
+static std::string GetDedupTokenFromCmdOutput(const std::string &S) {
+  auto Beg = S.find("DEDUP_TOKEN:");
+  if (Beg == std::string::npos)
+    return "";
+  auto End = S.find('\n', Beg);
+  if (End == std::string::npos)
+    return "";
+  return S.substr(Beg, End - Beg);
+}
+
+int CleanseCrashInput(const Vector<std::string> &Args,
+                       const FuzzingOptions &Options) {
+  if (Inputs->size() != 1 || !Flags.exact_artifact_path) {
+    Printf("ERROR: -cleanse_crash should be given one input file and"
+          " -exact_artifact_path\n");
+    exit(1);
+  }
+  std::string InputFilePath = Inputs->at(0);
+  std::string OutputFilePath = Flags.exact_artifact_path;
+  Command Cmd(Args);
+  Cmd.removeFlag("cleanse_crash");
+
+  assert(Cmd.hasArgument(InputFilePath));
+  Cmd.removeArgument(InputFilePath);
+
+  auto TmpFilePath = TempPath("CleanseCrashInput", ".repro");
+  Cmd.addArgument(TmpFilePath);
+  Cmd.setOutputFile(getDevNull());
+  Cmd.combineOutAndErr();
+
+  std::string CurrentFilePath = InputFilePath;
+  auto U = FileToVector(CurrentFilePath);
+  size_t Size = U.size();
+
+  const Vector<uint8_t> ReplacementBytes = {' ', 0xff};
+  for (int NumAttempts = 0; NumAttempts < 5; NumAttempts++) {
+    bool Changed = false;
+    for (size_t Idx = 0; Idx < Size; Idx++) {
+      Printf("CLEANSE[%d]: Trying to replace byte %zd of %zd\n", NumAttempts,
+             Idx, Size);
+      uint8_t OriginalByte = U[Idx];
+      if (ReplacementBytes.end() != std::find(ReplacementBytes.begin(),
+                                              ReplacementBytes.end(),
+                                              OriginalByte))
+        continue;
+      for (auto NewByte : ReplacementBytes) {
+        U[Idx] = NewByte;
+        WriteToFile(U, TmpFilePath);
+        auto ExitCode = ExecuteCommand(Cmd);
+        RemoveFile(TmpFilePath);
+        if (!ExitCode) {
+          U[Idx] = OriginalByte;
+        } else {
+          Changed = true;
+          Printf("CLEANSE: Replaced byte %zd with 0x%x\n", Idx, NewByte);
+          WriteToFile(U, OutputFilePath);
+          break;
+        }
+      }
+    }
+    if (!Changed) break;
+  }
+  return 0;
+}
+
+int MinimizeCrashInput(const Vector<std::string> &Args,
+                       const FuzzingOptions &Options) {
+  if (Inputs->size() != 1) {
+    Printf("ERROR: -minimize_crash should be given one input file\n");
+    exit(1);
+  }
+  std::string InputFilePath = Inputs->at(0);
+  Command BaseCmd(Args);
+  BaseCmd.removeFlag("minimize_crash");
+  BaseCmd.removeFlag("exact_artifact_path");
+  assert(BaseCmd.hasArgument(InputFilePath));
+  BaseCmd.removeArgument(InputFilePath);
+  if (Flags.runs <= 0 && Flags.max_total_time == 0) {
+    Printf("INFO: you need to specify -runs=N or "
+           "-max_total_time=N with -minimize_crash=1\n"
+           "INFO: defaulting to -max_total_time=600\n");
+    BaseCmd.addFlag("max_total_time", "600");
+  }
+
+  BaseCmd.combineOutAndErr();
+
+  std::string CurrentFilePath = InputFilePath;
+  while (true) {
+    Unit U = FileToVector(CurrentFilePath);
+    Printf("CRASH_MIN: minimizing crash input: '%s' (%zd bytes)\n",
+           CurrentFilePath.c_str(), U.size());
+
+    Command Cmd(BaseCmd);
+    Cmd.addArgument(CurrentFilePath);
+
+    Printf("CRASH_MIN: executing: %s\n", Cmd.toString().c_str());
+    std::string CmdOutput;
+    bool Success = ExecuteCommand(Cmd, &CmdOutput);
+    if (Success) {
+      Printf("ERROR: the input %s did not crash\n", CurrentFilePath.c_str());
+      exit(1);
+    }
+    Printf("CRASH_MIN: '%s' (%zd bytes) caused a crash. Will try to minimize "
+           "it further\n",
+           CurrentFilePath.c_str(), U.size());
+    auto DedupToken1 = GetDedupTokenFromCmdOutput(CmdOutput);
+    if (!DedupToken1.empty())
+      Printf("CRASH_MIN: DedupToken1: %s\n", DedupToken1.c_str());
+
+    std::string ArtifactPath =
+        Flags.exact_artifact_path
+            ? Flags.exact_artifact_path
+            : Options.ArtifactPrefix + "minimized-from-" + Hash(U);
+    Cmd.addFlag("minimize_crash_internal_step", "1");
+    Cmd.addFlag("exact_artifact_path", ArtifactPath);
+    Printf("CRASH_MIN: executing: %s\n", Cmd.toString().c_str());
+    CmdOutput.clear();
+    Success = ExecuteCommand(Cmd, &CmdOutput);
+    Printf("%s", CmdOutput.c_str());
+    if (Success) {
+      if (Flags.exact_artifact_path) {
+        CurrentFilePath = Flags.exact_artifact_path;
+        WriteToFile(U, CurrentFilePath);
+      }
+      Printf("CRASH_MIN: failed to minimize beyond %s (%d bytes), exiting\n",
+             CurrentFilePath.c_str(), U.size());
+      break;
+    }
+    auto DedupToken2 = GetDedupTokenFromCmdOutput(CmdOutput);
+    if (!DedupToken2.empty())
+      Printf("CRASH_MIN: DedupToken2: %s\n", DedupToken2.c_str());
+
+    if (DedupToken1 != DedupToken2) {
+      if (Flags.exact_artifact_path) {
+        CurrentFilePath = Flags.exact_artifact_path;
+        WriteToFile(U, CurrentFilePath);
+      }
+      Printf("CRASH_MIN: mismatch in dedup tokens"
+             " (looks like a different bug). Won't minimize further\n");
+      break;
+    }
+
+    CurrentFilePath = ArtifactPath;
+    Printf("*********************************\n");
+  }
+  return 0;
+}
+
+int MinimizeCrashInputInternalStep(Fuzzer *F, InputCorpus *Corpus) {
+  assert(Inputs->size() == 1);
+  std::string InputFilePath = Inputs->at(0);
+  Unit U = FileToVector(InputFilePath);
+  Printf("INFO: Starting MinimizeCrashInputInternalStep: %zd\n", U.size());
+  if (U.size() < 2) {
+    Printf("INFO: The input is small enough, exiting\n");
+    exit(0);
+  }
+  F->SetMaxInputLen(U.size());
+  F->SetMaxMutationLen(U.size() - 1);
+  F->MinimizeCrashLoop(U);
+  Printf("INFO: Done MinimizeCrashInputInternalStep, no crashes found\n");
+  exit(0);
+  return 0;
+}
+
+void Merge(Fuzzer *F, FuzzingOptions &Options, const Vector<std::string> &Args,
+           const Vector<std::string> &Corpora, const char *CFPathOrNull) {
+  if (Corpora.size() < 2) {
+    Printf("INFO: Merge requires two or more corpus dirs\n");
+    exit(0);
+  }
+
+  Vector<SizedFile> OldCorpus, NewCorpus;
+  GetSizedFilesFromDir(Corpora[0], &OldCorpus);
+  for (size_t i = 1; i < Corpora.size(); i++)
+    GetSizedFilesFromDir(Corpora[i], &NewCorpus);
+  std::sort(OldCorpus.begin(), OldCorpus.end());
+  std::sort(NewCorpus.begin(), NewCorpus.end());
+
+  std::string CFPath = CFPathOrNull ? CFPathOrNull : TempPath("Merge", ".txt");
+  Vector<std::string> NewFiles;
+  Set<uint32_t> NewFeatures, NewCov;
+  CrashResistantMerge(Args, OldCorpus, NewCorpus, &NewFiles, {}, &NewFeatures,
+                      {}, &NewCov, CFPath, true);
+  for (auto &Path : NewFiles)
+    F->WriteToOutputCorpus(FileToVector(Path, Options.MaxLen));
+  // We are done, delete the control file if it was a temporary one.
+  if (!Flags.merge_control_file)
+    RemoveFile(CFPath);
+
+  exit(0);
+}
+
+int AnalyzeDictionary(Fuzzer *F, const Vector<Unit>& Dict,
+                      UnitVector& Corpus) {
+  Printf("Started dictionary minimization (up to %d tests)\n",
+         Dict.size() * Corpus.size() * 2);
+
+  // Scores and usage count for each dictionary unit.
+  Vector<int> Scores(Dict.size());
+  Vector<int> Usages(Dict.size());
+
+  Vector<size_t> InitialFeatures;
+  Vector<size_t> ModifiedFeatures;
+  for (auto &C : Corpus) {
+    // Get coverage for the testcase without modifications.
+    F->ExecuteCallback(C.data(), C.size());
+    InitialFeatures.clear();
+    TPC.CollectFeatures([&](size_t Feature) {
+      InitialFeatures.push_back(Feature);
+    });
+
+    for (size_t i = 0; i < Dict.size(); ++i) {
+      Vector<uint8_t> Data = C;
+      auto StartPos = std::search(Data.begin(), Data.end(),
+                                  Dict[i].begin(), Dict[i].end());
+      // Skip dictionary unit, if the testcase does not contain it.
+      if (StartPos == Data.end())
+        continue;
+
+      ++Usages[i];
+      while (StartPos != Data.end()) {
+        // Replace all occurrences of dictionary unit in the testcase.
+        auto EndPos = StartPos + Dict[i].size();
+        for (auto It = StartPos; It != EndPos; ++It)
+          *It ^= 0xFF;
+
+        StartPos = std::search(EndPos, Data.end(),
+                               Dict[i].begin(), Dict[i].end());
+      }
+
+      // Get coverage for testcase with masked occurrences of dictionary unit.
+      F->ExecuteCallback(Data.data(), Data.size());
+      ModifiedFeatures.clear();
+      TPC.CollectFeatures([&](size_t Feature) {
+        ModifiedFeatures.push_back(Feature);
+      });
+
+      if (InitialFeatures == ModifiedFeatures)
+        --Scores[i];
+      else
+        Scores[i] += 2;
+    }
+  }
+
+  Printf("###### Useless dictionary elements. ######\n");
+  for (size_t i = 0; i < Dict.size(); ++i) {
+    // Dictionary units with positive score are treated as useful ones.
+    if (Scores[i] > 0)
+       continue;
+
+    Printf("\"");
+    PrintASCII(Dict[i].data(), Dict[i].size(), "\"");
+    Printf(" # Score: %d, Used: %d\n", Scores[i], Usages[i]);
+  }
+  Printf("###### End of useless dictionary elements. ######\n");
+  return 0;
+}
+
+Vector<std::string> ParseSeedInuts(const char *seed_inputs) {
+  // Parse -seed_inputs=file1,file2,... or -seed_inputs=@seed_inputs_file
+  Vector<std::string> Files;
+  if (!seed_inputs) return Files;
+  std::string SeedInputs;
+  if (Flags.seed_inputs[0] == '@')
+    SeedInputs = FileToString(Flags.seed_inputs + 1); // File contains list.
+  else
+    SeedInputs = Flags.seed_inputs; // seed_inputs contains the list.
+  if (SeedInputs.empty()) {
+    Printf("seed_inputs is empty or @file does not exist.\n");
+    exit(1);
+  }
+  // Parse SeedInputs.
+  size_t comma_pos = 0;
+  while ((comma_pos = SeedInputs.find_last_of(',')) != std::string::npos) {
+    Files.push_back(SeedInputs.substr(comma_pos + 1));
+    SeedInputs = SeedInputs.substr(0, comma_pos);
+  }
+  Files.push_back(SeedInputs);
+  return Files;
+}
+
+static Vector<SizedFile> ReadCorpora(const Vector<std::string> &CorpusDirs,
+    const Vector<std::string> &ExtraSeedFiles) {
+  Vector<SizedFile> SizedFiles;
+  size_t LastNumFiles = 0;
+  for (auto &Dir : CorpusDirs) {
+    GetSizedFilesFromDir(Dir, &SizedFiles);
+    Printf("INFO: % 8zd files found in %s\n", SizedFiles.size() - LastNumFiles,
+           Dir.c_str());
+    LastNumFiles = SizedFiles.size();
+  }
+  for (auto &File : ExtraSeedFiles)
+    if (auto Size = FileSize(File))
+      SizedFiles.push_back({File, Size});
+  return SizedFiles;
+}
+
+int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
+  using namespace fuzzer;
+  assert(argc && argv && "Argument pointers cannot be nullptr");
+  std::string Argv0((*argv)[0]);
+  if (!EF)
+    EF = new ExternalFunctions();
+  if (EF->LLVMFuzzerInitialize)
+    EF->LLVMFuzzerInitialize(argc, argv);
+  if (EF->__msan_scoped_disable_interceptor_checks)
+    EF->__msan_scoped_disable_interceptor_checks();
+  const Vector<std::string> Args(*argv, *argv + *argc);
+  assert(!Args.empty());
+  ProgName = new std::string(Args[0]);
+  if (Argv0 != *ProgName) {
+    Printf("ERROR: argv[0] has been modified in LLVMFuzzerInitialize\n");
+    exit(1);
+  }
+  ParseFlags(Args, EF);
+  if (Flags.help) {
+    PrintHelp();
+    return 0;
+  }
+
+  if (Flags.close_fd_mask & 2)
+    DupAndCloseStderr();
+  if (Flags.close_fd_mask & 1)
+    CloseStdout();
+
+  if (Flags.jobs > 0 && Flags.workers == 0) {
+    Flags.workers = std::min(NumberOfCpuCores() / 2, Flags.jobs);
+    if (Flags.workers > 1)
+      Printf("Running %u workers\n", Flags.workers);
+  }
+
+  if (Flags.workers > 0 && Flags.jobs > 0)
+    return RunInMultipleProcesses(Args, Flags.workers, Flags.jobs);
+
+  FuzzingOptions Options;
+  Options.Verbosity = Flags.verbosity;
+  Options.MaxLen = Flags.max_len;
+  Options.LenControl = Flags.len_control;
+  Options.UnitTimeoutSec = Flags.timeout;
+  Options.ErrorExitCode = Flags.error_exitcode;
+  Options.TimeoutExitCode = Flags.timeout_exitcode;
+  Options.IgnoreTimeouts = Flags.ignore_timeouts;
+  Options.IgnoreOOMs = Flags.ignore_ooms;
+  Options.IgnoreCrashes = Flags.ignore_crashes;
+  Options.MaxTotalTimeSec = Flags.max_total_time;
+  Options.DoCrossOver = Flags.cross_over;
+  Options.MutateDepth = Flags.mutate_depth;
+  Options.ReduceDepth = Flags.reduce_depth;
+  Options.UseCounters = Flags.use_counters;
+  Options.UseMemmem = Flags.use_memmem;
+  Options.UseCmp = Flags.use_cmp;
+  Options.UseValueProfile = Flags.use_value_profile;
+  Options.Shrink = Flags.shrink;
+  Options.ReduceInputs = Flags.reduce_inputs;
+  Options.ShuffleAtStartUp = Flags.shuffle;
+  Options.PreferSmall = Flags.prefer_small;
+  Options.ReloadIntervalSec = Flags.reload;
+  Options.OnlyASCII = Flags.only_ascii;
+  Options.DetectLeaks = Flags.detect_leaks;
+  Options.PurgeAllocatorIntervalSec = Flags.purge_allocator_interval;
+  Options.TraceMalloc = Flags.trace_malloc;
+  Options.RssLimitMb = Flags.rss_limit_mb;
+  Options.MallocLimitMb = Flags.malloc_limit_mb;
+  if (!Options.MallocLimitMb)
+    Options.MallocLimitMb = Options.RssLimitMb;
+  if (Flags.runs >= 0)
+    Options.MaxNumberOfRuns = Flags.runs;
+  if (!Inputs->empty() && !Flags.minimize_crash_internal_step)
+    Options.OutputCorpus = (*Inputs)[0];
+  Options.ReportSlowUnits = Flags.report_slow_units;
+  if (Flags.artifact_prefix)
+    Options.ArtifactPrefix = Flags.artifact_prefix;
+  if (Flags.exact_artifact_path)
+    Options.ExactArtifactPath = Flags.exact_artifact_path;
+  Vector<Unit> Dictionary;
+  if (Flags.dict)
+    if (!ParseDictionaryFile(FileToString(Flags.dict), &Dictionary))
+      return 1;
+  if (Flags.verbosity > 0 && !Dictionary.empty())
+    Printf("Dictionary: %zd entries\n", Dictionary.size());
+  bool RunIndividualFiles = AllInputsAreFiles();
+  Options.SaveArtifacts =
+      !RunIndividualFiles || Flags.minimize_crash_internal_step;
+  Options.PrintNewCovPcs = Flags.print_pcs;
+  Options.PrintNewCovFuncs = Flags.print_funcs;
+  Options.PrintFinalStats = Flags.print_final_stats;
+  Options.PrintCorpusStats = Flags.print_corpus_stats;
+  Options.PrintCoverage = Flags.print_coverage;
+  if (Flags.exit_on_src_pos)
+    Options.ExitOnSrcPos = Flags.exit_on_src_pos;
+  if (Flags.exit_on_item)
+    Options.ExitOnItem = Flags.exit_on_item;
+  if (Flags.focus_function)
+    Options.FocusFunction = Flags.focus_function;
+  if (Flags.data_flow_trace)
+    Options.DataFlowTrace = Flags.data_flow_trace;
+  if (Flags.features_dir)
+    Options.FeaturesDir = Flags.features_dir;
+  if (Flags.collect_data_flow)
+    Options.CollectDataFlow = Flags.collect_data_flow;
+  if (Flags.stop_file)
+    Options.StopFile = Flags.stop_file;
+  Options.Entropic = Flags.entropic;
+  Options.EntropicFeatureFrequencyThreshold =
+      (size_t)Flags.entropic_feature_frequency_threshold;
+  Options.EntropicNumberOfRarestFeatures =
+      (size_t)Flags.entropic_number_of_rarest_features;
+  if (Options.Entropic) {
+    if (!Options.FocusFunction.empty()) {
+      Printf("ERROR: The parameters `--entropic` and `--focus_function` cannot "
+             "be used together.\n");
+      exit(1);
+    }
+    Printf("INFO: Running with entropic power schedule (0x%X, %d).\n",
+           Options.EntropicFeatureFrequencyThreshold,
+           Options.EntropicNumberOfRarestFeatures);
+  }
+  struct EntropicOptions Entropic;
+  Entropic.Enabled = Options.Entropic;
+  Entropic.FeatureFrequencyThreshold =
+      Options.EntropicFeatureFrequencyThreshold;
+  Entropic.NumberOfRarestFeatures = Options.EntropicNumberOfRarestFeatures;
+
+  unsigned Seed = Flags.seed;
+  // Initialize Seed.
+  if (Seed == 0)
+    Seed =
+        std::chrono::system_clock::now().time_since_epoch().count() + GetPid();
+  if (Flags.verbosity)
+    Printf("INFO: Seed: %u\n", Seed);
+
+  if (Flags.collect_data_flow && !Flags.fork && !Flags.merge) {
+    if (RunIndividualFiles)
+      return CollectDataFlow(Flags.collect_data_flow, Flags.data_flow_trace,
+                        ReadCorpora({}, *Inputs));
+    else
+      return CollectDataFlow(Flags.collect_data_flow, Flags.data_flow_trace,
+                        ReadCorpora(*Inputs, {}));
+  }
+
+  Random Rand(Seed);
+  auto *MD = new MutationDispatcher(Rand, Options);
+  auto *Corpus = new InputCorpus(Options.OutputCorpus, Entropic);
+  auto *F = new Fuzzer(Callback, *Corpus, *MD, Options);
+
+  for (auto &U: Dictionary)
+    if (U.size() <= Word::GetMaxSize())
+      MD->AddWordToManualDictionary(Word(U.data(), U.size()));
+
+      // Threads are only supported by Chrome. Don't use them with emscripten
+      // for now.
+#if !LIBFUZZER_EMSCRIPTEN
+  StartRssThread(F, Flags.rss_limit_mb);
+#endif // LIBFUZZER_EMSCRIPTEN
+
+  Options.HandleAbrt = Flags.handle_abrt;
+  Options.HandleBus = Flags.handle_bus;
+  Options.HandleFpe = Flags.handle_fpe;
+  Options.HandleIll = Flags.handle_ill;
+  Options.HandleInt = Flags.handle_int;
+  Options.HandleSegv = Flags.handle_segv;
+  Options.HandleTerm = Flags.handle_term;
+  Options.HandleXfsz = Flags.handle_xfsz;
+  Options.HandleUsr1 = Flags.handle_usr1;
+  Options.HandleUsr2 = Flags.handle_usr2;
+  SetSignalHandler(Options);
+
+  std::atexit(Fuzzer::StaticExitCallback);
+
+  if (Flags.minimize_crash)
+    return MinimizeCrashInput(Args, Options);
+
+  if (Flags.minimize_crash_internal_step)
+    return MinimizeCrashInputInternalStep(F, Corpus);
+
+  if (Flags.cleanse_crash)
+    return CleanseCrashInput(Args, Options);
+
+  if (RunIndividualFiles) {
+    Options.SaveArtifacts = false;
+    int Runs = std::max(1, Flags.runs);
+    Printf("%s: Running %zd inputs %d time(s) each.\n", ProgName->c_str(),
+           Inputs->size(), Runs);
+    for (auto &Path : *Inputs) {
+      auto StartTime = system_clock::now();
+      Printf("Running: %s\n", Path.c_str());
+      for (int Iter = 0; Iter < Runs; Iter++)
+        RunOneTest(F, Path.c_str(), Options.MaxLen);
+      auto StopTime = system_clock::now();
+      auto MS = duration_cast<milliseconds>(StopTime - StartTime).count();
+      Printf("Executed %s in %zd ms\n", Path.c_str(), (long)MS);
+    }
+    Printf("***\n"
+           "*** NOTE: fuzzing was not performed, you have only\n"
+           "***       executed the target code on a fixed set of inputs.\n"
+           "***\n");
+    F->PrintFinalStats();
+    exit(0);
+  }
+
+  if (Flags.fork)
+    FuzzWithFork(F->GetMD().GetRand(), Options, Args, *Inputs, Flags.fork);
+
+  if (Flags.merge)
+    Merge(F, Options, Args, *Inputs, Flags.merge_control_file);
+
+  if (Flags.merge_inner) {
+    const size_t kDefaultMaxMergeLen = 1 << 20;
+    if (Options.MaxLen == 0)
+      F->SetMaxInputLen(kDefaultMaxMergeLen);
+    assert(Flags.merge_control_file);
+    F->CrashResistantMergeInternalStep(Flags.merge_control_file);
+    exit(0);
+  }
+
+  if (Flags.analyze_dict) {
+    size_t MaxLen = INT_MAX;  // Large max length.
+    UnitVector InitialCorpus;
+    for (auto &Inp : *Inputs) {
+      Printf("Loading corpus dir: %s\n", Inp.c_str());
+      ReadDirToVectorOfUnits(Inp.c_str(), &InitialCorpus, nullptr,
+                             MaxLen, /*ExitOnError=*/false);
+    }
+
+    if (Dictionary.empty() || Inputs->empty()) {
+      Printf("ERROR: can't analyze dict without dict and corpus provided\n");
+      return 1;
+    }
+    if (AnalyzeDictionary(F, Dictionary, InitialCorpus)) {
+      Printf("Dictionary analysis failed\n");
+      exit(1);
+    }
+    Printf("Dictionary analysis succeeded\n");
+    exit(0);
+  }
+
+  auto CorporaFiles = ReadCorpora(*Inputs, ParseSeedInuts(Flags.seed_inputs));
+  F->Loop(CorporaFiles);
+
+  if (Flags.verbosity)
+    Printf("Done %zd runs in %zd second(s)\n", F->getTotalNumberOfRuns(),
+           F->secondsSinceProcessStartUp());
+  F->PrintFinalStats();
+
+  exit(0);  // Don't let F destroy itself.
+}
+
+extern "C" ATTRIBUTE_INTERFACE int
+LLVMFuzzerRunDriver(int *argc, char ***argv,
+                    int (*UserCb)(const uint8_t *Data, size_t Size)) {
+  return FuzzerDriver(argc, argv, UserCb);
+}
+
+// Storage for global ExternalFunctions object.
+ExternalFunctions *EF = nullptr;
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtFunctions.def b/tools/fuzzing/libfuzzer/FuzzerExtFunctions.def
new file mode 100644
index 0000000000..51edf8444e
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtFunctions.def
@@ -0,0 +1,50 @@
+//===- FuzzerExtFunctions.def - External functions --------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// This defines the external function pointers that
+// ``fuzzer::ExternalFunctions`` should contain and try to initialize.  The
+// EXT_FUNC macro must be defined at the point of inclusion. The signature of
+// the macro is:
+//
+// EXT_FUNC(<name>, <return_type>, <function_signature>, <warn_if_missing>)
+//===----------------------------------------------------------------------===//
+
+// Optional user functions
+EXT_FUNC(LLVMFuzzerInitialize, int, (int *argc, char ***argv), false);
+EXT_FUNC(LLVMFuzzerCustomMutator, size_t,
+         (uint8_t *Data, size_t Size, size_t MaxSize, unsigned int Seed),
+         false);
+EXT_FUNC(LLVMFuzzerCustomCrossOver, size_t,
+         (const uint8_t *Data1, size_t Size1,
+          const uint8_t *Data2, size_t Size2,
+          uint8_t *Out, size_t MaxOutSize, unsigned int Seed),
+         false);
+
+// Sanitizer functions
+EXT_FUNC(__lsan_enable, void, (), false);
+EXT_FUNC(__lsan_disable, void, (), false);
+EXT_FUNC(__lsan_do_recoverable_leak_check, int, (), false);
+EXT_FUNC(__sanitizer_acquire_crash_state, int, (), true);
+EXT_FUNC(__sanitizer_install_malloc_and_free_hooks, int,
+         (void (*malloc_hook)(const volatile void *, size_t),
+          void (*free_hook)(const volatile void *)),
+         false);
+EXT_FUNC(__sanitizer_log_write, void, (const char *buf, size_t len), false);
+EXT_FUNC(__sanitizer_purge_allocator, void, (), false);
+EXT_FUNC(__sanitizer_print_memory_profile, void, (size_t, size_t), false);
+EXT_FUNC(__sanitizer_print_stack_trace, void, (), true);
+EXT_FUNC(__sanitizer_symbolize_pc, void,
+         (void *, const char *fmt, char *out_buf, size_t out_buf_size), false);
+EXT_FUNC(__sanitizer_get_module_and_offset_for_pc, int,
+         (void *pc, char *module_path,
+         size_t module_path_len,void **pc_offset), false);
+EXT_FUNC(__sanitizer_set_death_callback, void, (void (*)(void)), true);
+EXT_FUNC(__sanitizer_set_report_fd, void, (void*), false);
+EXT_FUNC(__msan_scoped_disable_interceptor_checks, void, (), false);
+EXT_FUNC(__msan_scoped_enable_interceptor_checks, void, (), false);
+EXT_FUNC(__msan_unpoison, void, (const volatile void *, size_t size), false);
+EXT_FUNC(__msan_unpoison_param, void, (size_t n), false);
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtFunctions.h b/tools/fuzzing/libfuzzer/FuzzerExtFunctions.h
new file mode 100644
index 0000000000..c88aac4e67
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtFunctions.h
@@ -0,0 +1,34 @@
+//===- FuzzerExtFunctions.h - Interface to external functions ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Defines an interface to (possibly optional) functions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_EXT_FUNCTIONS_H
+#define LLVM_FUZZER_EXT_FUNCTIONS_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+namespace fuzzer {
+
+struct ExternalFunctions {
+  // Initialize function pointers. Functions that are not available will be set
+  // to nullptr.  Do not call this constructor  before ``main()`` has been
+  // entered.
+  ExternalFunctions();
+
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  RETURN_TYPE(*NAME) FUNC_SIG = nullptr
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+};
+} // namespace fuzzer
+
+#endif
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtFunctionsDlsym.cpp b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsDlsym.cpp
new file mode 100644
index 0000000000..95233d2a10
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsDlsym.cpp
@@ -0,0 +1,51 @@
+//===- FuzzerExtFunctionsDlsym.cpp - Interface to external functions ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Implementation for operating systems that support dlsym(). We only use it on
+// Apple platforms for now. We don't use this approach on Linux because it
+// requires that clients of LibFuzzer pass ``--export-dynamic`` to the linker.
+// That is a complication we don't wish to expose to clients right now.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_APPLE
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include <dlfcn.h>
+
+using namespace fuzzer;
+
+template <typename T>
+static T GetFnPtr(const char *FnName, bool WarnIfMissing) {
+  dlerror(); // Clear any previous errors.
+  void *Fn = dlsym(RTLD_DEFAULT, FnName);
+  if (Fn == nullptr) {
+    if (WarnIfMissing) {
+      const char *ErrorMsg = dlerror();
+      Printf("WARNING: Failed to find function \"%s\".", FnName);
+      if (ErrorMsg)
+        Printf(" Reason %s.", ErrorMsg);
+      Printf("\n");
+    }
+  }
+  return reinterpret_cast<T>(Fn);
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = GetFnPtr<decltype(ExternalFunctions::NAME)>(#NAME, WARN)
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_APPLE
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWeak.cpp b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWeak.cpp
new file mode 100644
index 0000000000..24ddc57d47
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWeak.cpp
@@ -0,0 +1,54 @@
+//===- FuzzerExtFunctionsWeak.cpp - Interface to external functions -------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Implementation for Linux. This relies on the linker's support for weak
+// symbols. We don't use this approach on Apple platforms because it requires
+// clients of LibFuzzer to pass ``-U _<symbol_name>`` to the linker to allow
+// weak symbols to be undefined. That is a complication we don't want to expose
+// to clients right now.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FUCHSIA ||                \
+    LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+extern "C" {
+// Declare these symbols as weak to allow them to be optionally defined.
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  __attribute__((weak, visibility("default"))) RETURN_TYPE NAME FUNC_SIG
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+using namespace fuzzer;
+
+static void CheckFnPtr(void *FnPtr, const char *FnName, bool WarnIfMissing) {
+  if (FnPtr == nullptr && WarnIfMissing) {
+    Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+  }
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = ::NAME;                                                         \
+  CheckFnPtr(reinterpret_cast<void *>(reinterpret_cast<uintptr_t>(::NAME)),    \
+             #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+} // namespace fuzzer
+
+#endif
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWindows.cpp b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWindows.cpp
new file mode 100644
index 0000000000..688bad1d51
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtFunctionsWindows.cpp
@@ -0,0 +1,82 @@
+//=== FuzzerExtWindows.cpp - Interface to external functions --------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Implementation of FuzzerExtFunctions for Windows. Uses alternatename when
+// compiled with MSVC. Uses weak aliases when compiled with clang. Unfortunately
+// the method each compiler supports is not supported by the other.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_WINDOWS
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+using namespace fuzzer;
+
+// Intermediate macro to ensure the parameter is expanded before stringified.
+#define STRINGIFY_(A) #A
+#define STRINGIFY(A) STRINGIFY_(A)
+
+#if LIBFUZZER_MSVC
+// Copied from compiler-rt/lib/sanitizer_common/sanitizer_win_defs.h
+#if defined(_M_IX86) || defined(__i386__)
+#define WIN_SYM_PREFIX "_"
+#else
+#define WIN_SYM_PREFIX
+#endif
+
+// Declare external functions as having alternativenames, so that we can
+// determine if they are not defined.
+#define EXTERNAL_FUNC(Name, Default)                                   \
+  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY( \
+                               Name) "=" WIN_SYM_PREFIX STRINGIFY(Default)))
+#else
+// Declare external functions as weak to allow them to default to a specified
+// function if not defined explicitly. We must use weak symbols because clang's
+// support for alternatename is not 100%, see
+// https://bugs.llvm.org/show_bug.cgi?id=40218 for more details.
+#define EXTERNAL_FUNC(Name, Default) \
+  __attribute__((weak, alias(STRINGIFY(Default))))
+#endif  // LIBFUZZER_MSVC
+
+extern "C" {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)         \
+  RETURN_TYPE NAME##Def FUNC_SIG {                          \
+    Printf("ERROR: Function \"%s\" not defined.\n", #NAME); \
+    exit(1);                                                \
+  }                                                         \
+  EXTERNAL_FUNC(NAME, NAME##Def) RETURN_TYPE NAME FUNC_SIG
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+template <typename T>
+static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
+  if (Fun == FunDef) {
+    if (WarnIfMissing)
+      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+    return nullptr;
+  }
+  return Fun;
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN) \
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+}  // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS
diff --git a/tools/fuzzing/libfuzzer/FuzzerExtraCounters.cpp b/tools/fuzzing/libfuzzer/FuzzerExtraCounters.cpp
new file mode 100644
index 0000000000..d36beba1b1
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerExtraCounters.cpp
@@ -0,0 +1,42 @@
+//===- FuzzerExtraCounters.cpp - Extra coverage counters ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Extra coverage counters defined by user code.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerPlatform.h"
+#include <cstdint>
+
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FREEBSD ||                \
+    LIBFUZZER_OPENBSD || LIBFUZZER_FUCHSIA || LIBFUZZER_EMSCRIPTEN
+__attribute__((weak)) extern uint8_t __start___libfuzzer_extra_counters;
+__attribute__((weak)) extern uint8_t __stop___libfuzzer_extra_counters;
+
+namespace fuzzer {
+uint8_t *ExtraCountersBegin() { return &__start___libfuzzer_extra_counters; }
+uint8_t *ExtraCountersEnd() { return &__stop___libfuzzer_extra_counters; }
+ATTRIBUTE_NO_SANITIZE_ALL
+void ClearExtraCounters() {  // hand-written memset, don't asan-ify.
+  uintptr_t *Beg = reinterpret_cast<uintptr_t*>(ExtraCountersBegin());
+  uintptr_t *End = reinterpret_cast<uintptr_t*>(ExtraCountersEnd());
+  for (; Beg < End; Beg++) {
+    *Beg = 0;
+    __asm__ __volatile__("" : : : "memory");
+  }
+}
+
+}  // namespace fuzzer
+
+#else
+// TODO: implement for other platforms.
+namespace fuzzer {
+uint8_t *ExtraCountersBegin() { return nullptr; }
+uint8_t *ExtraCountersEnd() { return nullptr; }
+void ClearExtraCounters() {}
+}  // namespace fuzzer
+
+#endif
diff --git a/tools/fuzzing/libfuzzer/FuzzerFlags.def b/tools/fuzzing/libfuzzer/FuzzerFlags.def
new file mode 100644
index 0000000000..832224a705
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerFlags.def
@@ -0,0 +1,169 @@
+//===- FuzzerFlags.def - Run-time flags -------------------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Flags. FUZZER_FLAG_INT/FUZZER_FLAG_STRING macros should be defined at the
+// point of inclusion. We are not using any flag parsing library for better
+// portability and independence.
+//===----------------------------------------------------------------------===//
+FUZZER_FLAG_INT(verbosity, 1, "Verbosity level.")
+FUZZER_FLAG_UNSIGNED(seed, 0, "Random seed. If 0, seed is generated.")
+FUZZER_FLAG_INT(runs, -1,
+            "Number of individual test runs (-1 for infinite runs).")
+FUZZER_FLAG_INT(max_len, 0, "Maximum length of the test input. "
+    "If 0, libFuzzer tries to guess a good value based on the corpus "
+    "and reports it. ")
+FUZZER_FLAG_INT(len_control, 100, "Try generating small inputs first, "
+  "then try larger inputs over time.  Specifies the rate at which the length "
+  "limit is increased (smaller == faster).  If 0, immediately try inputs with "
+  "size up to max_len. Default value is 0, if LLVMFuzzerCustomMutator is used.")
+FUZZER_FLAG_STRING(seed_inputs, "A comma-separated list of input files "
+  "to use as an additional seed corpus. Alternatively, an \"@\" followed by "
+  "the name of a file containing the comma-separated list.")
+FUZZER_FLAG_INT(cross_over, 1, "If 1, cross over inputs.")
+FUZZER_FLAG_INT(mutate_depth, 5,
+            "Apply this number of consecutive mutations to each input.")
+FUZZER_FLAG_INT(reduce_depth, 0, "Experimental/internal. "
+                "Reduce depth if mutations lose unique features")
+FUZZER_FLAG_INT(shuffle, 1, "Shuffle inputs at startup")
+FUZZER_FLAG_INT(prefer_small, 1,
+    "If 1, always prefer smaller inputs during the corpus shuffle.")
+FUZZER_FLAG_INT(
+    timeout, 1200,
+    "Timeout in seconds (if positive). "
+    "If one unit runs more than this number of seconds the process will abort.")
+FUZZER_FLAG_INT(error_exitcode, 77, "When libFuzzer itself reports a bug "
+  "this exit code will be used.")
+FUZZER_FLAG_INT(timeout_exitcode, 70, "When libFuzzer reports a timeout "
+  "this exit code will be used.")
+FUZZER_FLAG_INT(max_total_time, 0, "If positive, indicates the maximal total "
+                                   "time in seconds to run the fuzzer.")
+FUZZER_FLAG_INT(help, 0, "Print help.")
+FUZZER_FLAG_INT(fork, 0, "Experimental mode where fuzzing happens "
+                "in a subprocess")
+FUZZER_FLAG_INT(ignore_timeouts, 1, "Ignore timeouts in fork mode")
+FUZZER_FLAG_INT(ignore_ooms, 1, "Ignore OOMs in fork mode")
+FUZZER_FLAG_INT(ignore_crashes, 0, "Ignore crashes in fork mode")
+FUZZER_FLAG_INT(merge, 0, "If 1, the 2-nd, 3-rd, etc corpora will be "
+  "merged into the 1-st corpus. Only interesting units will be taken. "
+  "This flag can be used to minimize a corpus.")
+FUZZER_FLAG_STRING(stop_file, "Stop fuzzing ASAP if this file exists")
+FUZZER_FLAG_STRING(merge_inner, "internal flag")
+FUZZER_FLAG_STRING(merge_control_file,
+                   "Specify a control file used for the merge process. "
+                   "If a merge process gets killed it tries to leave this file "
+                   "in a state suitable for resuming the merge. "
+                   "By default a temporary file will be used."
+                   "The same file can be used for multistep merge process.")
+FUZZER_FLAG_INT(minimize_crash, 0, "If 1, minimizes the provided"
+  " crash input. Use with -runs=N or -max_total_time=N to limit "
+  "the number attempts."
+  " Use with -exact_artifact_path to specify the output."
+  " Combine with ASAN_OPTIONS=dedup_token_length=3 (or similar) to ensure that"
+  " the minimized input triggers the same crash."
+  )
+FUZZER_FLAG_INT(cleanse_crash, 0, "If 1, tries to cleanse the provided"
+  " crash input to make it contain fewer original bytes."
+  " Use with -exact_artifact_path to specify the output."
+  )
+FUZZER_FLAG_INT(minimize_crash_internal_step, 0, "internal flag")
+FUZZER_FLAG_STRING(features_dir, "internal flag. Used to dump feature sets on disk."
+  "Every time a new input is added to the corpus, a corresponding file in the features_dir"
+  " is created containing the unique features of that input."
+  " Features are stored in binary format.")
+FUZZER_FLAG_INT(use_counters, 1, "Use coverage counters")
+FUZZER_FLAG_INT(use_memmem, 1,
+                "Use hints from intercepting memmem, strstr, etc")
+FUZZER_FLAG_INT(use_value_profile, 0,
+                "Experimental. Use value profile to guide fuzzing.")
+FUZZER_FLAG_INT(use_cmp, 1, "Use CMP traces to guide mutations")
+FUZZER_FLAG_INT(shrink, 0, "Experimental. Try to shrink corpus inputs.")
+FUZZER_FLAG_INT(reduce_inputs, 1,
+  "Try to reduce the size of inputs while preserving their full feature sets")
+FUZZER_FLAG_UNSIGNED(jobs, 0, "Number of jobs to run. If jobs >= 1 we spawn"
+                          " this number of jobs in separate worker processes"
+                          " with stdout/stderr redirected to fuzz-JOB.log.")
+FUZZER_FLAG_UNSIGNED(workers, 0,
+            "Number of simultaneous worker processes to run the jobs."
+            " If zero, \"min(jobs,NumberOfCpuCores()/2)\" is used.")
+FUZZER_FLAG_INT(reload, 1,
+                "Reload the main corpus every <N> seconds to get new units"
+                " discovered by other processes. If 0, disabled")
+FUZZER_FLAG_INT(report_slow_units, 10,
+    "Report slowest units if they run for more than this number of seconds.")
+FUZZER_FLAG_INT(only_ascii, 0,
+                "If 1, generate only ASCII (isprint+isspace) inputs.")
+FUZZER_FLAG_STRING(dict, "Experimental. Use the dictionary file.")
+FUZZER_FLAG_STRING(artifact_prefix, "Write fuzzing artifacts (crash, "
+                                    "timeout, or slow inputs) as "
+                                    "$(artifact_prefix)file")
+FUZZER_FLAG_STRING(exact_artifact_path,
+                   "Write the single artifact on failure (crash, timeout) "
+                   "as $(exact_artifact_path). This overrides -artifact_prefix "
+                   "and will not use checksum in the file name. Do not "
+                   "use the same path for several parallel processes.")
+FUZZER_FLAG_INT(print_pcs, 0, "If 1, print out newly covered PCs.")
+FUZZER_FLAG_INT(print_funcs, 2, "If >=1, print out at most this number of "
+                                "newly covered functions.")
+FUZZER_FLAG_INT(print_final_stats, 0, "If 1, print statistics at exit.")
+FUZZER_FLAG_INT(print_corpus_stats, 0,
+  "If 1, print statistics on corpus elements at exit.")
+FUZZER_FLAG_INT(print_coverage, 0, "If 1, print coverage information as text"
+                                   " at exit.")
+FUZZER_FLAG_INT(dump_coverage, 0, "Deprecated.")
+FUZZER_FLAG_INT(handle_segv, 1, "If 1, try to intercept SIGSEGV.")
+FUZZER_FLAG_INT(handle_bus, 1, "If 1, try to intercept SIGBUS.")
+FUZZER_FLAG_INT(handle_abrt, 1, "If 1, try to intercept SIGABRT.")
+FUZZER_FLAG_INT(handle_ill, 1, "If 1, try to intercept SIGILL.")
+FUZZER_FLAG_INT(handle_fpe, 1, "If 1, try to intercept SIGFPE.")
+FUZZER_FLAG_INT(handle_int, 1, "If 1, try to intercept SIGINT.")
+FUZZER_FLAG_INT(handle_term, 1, "If 1, try to intercept SIGTERM.")
+FUZZER_FLAG_INT(handle_xfsz, 1, "If 1, try to intercept SIGXFSZ.")
+FUZZER_FLAG_INT(handle_usr1, 1, "If 1, try to intercept SIGUSR1.")
+FUZZER_FLAG_INT(handle_usr2, 1, "If 1, try to intercept SIGUSR2.")
+FUZZER_FLAG_INT(close_fd_mask, 0, "If 1, close stdout at startup; "
+    "if 2, close stderr; if 3, close both. "
+    "Be careful, this will also close e.g. stderr of asan.")
+FUZZER_FLAG_INT(detect_leaks, 1, "If 1, and if LeakSanitizer is enabled "
+    "try to detect memory leaks during fuzzing (i.e. not only at shut down).")
+FUZZER_FLAG_INT(purge_allocator_interval, 1, "Purge allocator caches and "
+    "quarantines every <N> seconds. When rss_limit_mb is specified (>0), "
+    "purging starts when RSS exceeds 50% of rss_limit_mb. Pass "
+    "purge_allocator_interval=-1 to disable this functionality.")
+FUZZER_FLAG_INT(trace_malloc, 0, "If >= 1 will print all mallocs/frees. "
+    "If >= 2 will also print stack traces.")
+FUZZER_FLAG_INT(rss_limit_mb, 2048, "If non-zero, the fuzzer will exit upon"
+    "reaching this limit of RSS memory usage.")
+FUZZER_FLAG_INT(malloc_limit_mb, 0, "If non-zero, the fuzzer will exit "
+    "if the target tries to allocate this number of Mb with one malloc call. "
+    "If zero (default) same limit as rss_limit_mb is applied.")
+FUZZER_FLAG_STRING(exit_on_src_pos, "Exit if a newly found PC originates"
+    " from the given source location. Example: -exit_on_src_pos=foo.cc:123. "
+    "Used primarily for testing libFuzzer itself.")
+FUZZER_FLAG_STRING(exit_on_item, "Exit if an item with a given sha1 sum"
+    " was added to the corpus. "
+    "Used primarily for testing libFuzzer itself.")
+FUZZER_FLAG_INT(ignore_remaining_args, 0, "If 1, ignore all arguments passed "
+                "after this one. Useful for fuzzers that need to do their own "
+                "argument parsing.")
+FUZZER_FLAG_STRING(focus_function, "Experimental. "
+     "Fuzzing will focus on inputs that trigger calls to this function. "
+     "If -focus_function=auto and -data_flow_trace is used, libFuzzer "
+     "will choose the focus functions automatically.")
+FUZZER_FLAG_INT(entropic, 0, "Experimental. Enables entropic power schedule.")
+FUZZER_FLAG_INT(entropic_feature_frequency_threshold, 0xFF, "Experimental. If "
+     "entropic is enabled, all features which are observed less often than "
+     "the specified value are considered as rare.")
+FUZZER_FLAG_INT(entropic_number_of_rarest_features, 100, "Experimental. If "
+     "entropic is enabled, we keep track of the frequencies only for the "
+     "Top-X least abundant features (union features that are considered as "
+     "rare).")
+
+FUZZER_FLAG_INT(analyze_dict, 0, "Experimental")
+FUZZER_DEPRECATED_FLAG(use_clang_coverage)
+FUZZER_FLAG_STRING(data_flow_trace, "Experimental: use the data flow trace")
+FUZZER_FLAG_STRING(collect_data_flow,
+                   "Experimental: collect the data flow trace")
diff --git a/tools/fuzzing/libfuzzer/FuzzerFork.cpp b/tools/fuzzing/libfuzzer/FuzzerFork.cpp
new file mode 100644
index 0000000000..d9e6b79443
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerFork.cpp
@@ -0,0 +1,409 @@
+//===- FuzzerFork.cpp - run fuzzing in separate subprocesses --------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Spawn and orchestrate separate fuzzing processes.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerCommand.h"
+#include "FuzzerFork.h"
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include "FuzzerMerge.h"
+#include "FuzzerSHA1.h"
+#include "FuzzerTracePC.h"
+#include "FuzzerUtil.h"
+
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <fstream>
+#include <memory>
+#include <mutex>
+#include <queue>
+#include <sstream>
+#include <thread>
+
+namespace fuzzer {
+
+struct Stats {
+  size_t number_of_executed_units = 0;
+  size_t peak_rss_mb = 0;
+  size_t average_exec_per_sec = 0;
+};
+
+static Stats ParseFinalStatsFromLog(const std::string &LogPath) {
+  std::ifstream In(LogPath);
+  std::string Line;
+  Stats Res;
+  struct {
+    const char *Name;
+    size_t *Var;
+  } NameVarPairs[] = {
+      {"stat::number_of_executed_units:", &Res.number_of_executed_units},
+      {"stat::peak_rss_mb:", &Res.peak_rss_mb},
+      {"stat::average_exec_per_sec:", &Res.average_exec_per_sec},
+      {nullptr, nullptr},
+  };
+  while (std::getline(In, Line, '\n')) {
+    if (Line.find("stat::") != 0) continue;
+    std::istringstream ISS(Line);
+    std::string Name;
+    size_t Val;
+    ISS >> Name >> Val;
+    for (size_t i = 0; NameVarPairs[i].Name; i++)
+      if (Name == NameVarPairs[i].Name)
+        *NameVarPairs[i].Var = Val;
+  }
+  return Res;
+}
+
+struct FuzzJob {
+  // Inputs.
+  Command Cmd;
+  std::string CorpusDir;
+  std::string FeaturesDir;
+  std::string LogPath;
+  std::string SeedListPath;
+  std::string CFPath;
+  size_t      JobId;
+
+  int         DftTimeInSeconds = 0;
+
+  // Fuzzing Outputs.
+  int ExitCode;
+
+  ~FuzzJob() {
+    RemoveFile(CFPath);
+    RemoveFile(LogPath);
+    RemoveFile(SeedListPath);
+    RmDirRecursive(CorpusDir);
+    RmDirRecursive(FeaturesDir);
+  }
+};
+
+struct GlobalEnv {
+  Vector<std::string> Args;
+  Vector<std::string> CorpusDirs;
+  std::string MainCorpusDir;
+  std::string TempDir;
+  std::string DFTDir;
+  std::string DataFlowBinary;
+  Set<uint32_t> Features, Cov;
+  Set<std::string> FilesWithDFT;
+  Vector<std::string> Files;
+  Random *Rand;
+  std::chrono::system_clock::time_point ProcessStartTime;
+  int Verbosity = 0;
+
+  size_t NumTimeouts = 0;
+  size_t NumOOMs = 0;
+  size_t NumCrashes = 0;
+
+
+  size_t NumRuns = 0;
+
+  std::string StopFile() { return DirPlusFile(TempDir, "STOP"); }
+
+  size_t secondsSinceProcessStartUp() const {
+    return std::chrono::duration_cast<std::chrono::seconds>(
+               std::chrono::system_clock::now() - ProcessStartTime)
+        .count();
+  }
+
+  FuzzJob *CreateNewJob(size_t JobId) {
+    Command Cmd(Args);
+    Cmd.removeFlag("fork");
+    Cmd.removeFlag("runs");
+    Cmd.removeFlag("collect_data_flow");
+    for (auto &C : CorpusDirs) // Remove all corpora from the args.
+      Cmd.removeArgument(C);
+    Cmd.addFlag("reload", "0");  // working in an isolated dir, no reload.
+    Cmd.addFlag("print_final_stats", "1");
+    Cmd.addFlag("print_funcs", "0");  // no need to spend time symbolizing.
+    Cmd.addFlag("max_total_time", std::to_string(std::min((size_t)300, JobId)));
+    Cmd.addFlag("stop_file", StopFile());
+    if (!DataFlowBinary.empty()) {
+      Cmd.addFlag("data_flow_trace", DFTDir);
+      if (!Cmd.hasFlag("focus_function"))
+        Cmd.addFlag("focus_function", "auto");
+    }
+    auto Job = new FuzzJob;
+    std::string Seeds;
+    if (size_t CorpusSubsetSize =
+            std::min(Files.size(), (size_t)sqrt(Files.size() + 2))) {
+      auto Time1 = std::chrono::system_clock::now();
+      for (size_t i = 0; i < CorpusSubsetSize; i++) {
+        auto &SF = Files[Rand->SkewTowardsLast(Files.size())];
+        Seeds += (Seeds.empty() ? "" : ",") + SF;
+        CollectDFT(SF);
+      }
+      auto Time2 = std::chrono::system_clock::now();
+      Job->DftTimeInSeconds = duration_cast<seconds>(Time2 - Time1).count();
+    }
+    if (!Seeds.empty()) {
+      Job->SeedListPath =
+          DirPlusFile(TempDir, std::to_string(JobId) + ".seeds");
+      WriteToFile(Seeds, Job->SeedListPath);
+      Cmd.addFlag("seed_inputs", "@" + Job->SeedListPath);
+    }
+    Job->LogPath = DirPlusFile(TempDir, std::to_string(JobId) + ".log");
+    Job->CorpusDir = DirPlusFile(TempDir, "C" + std::to_string(JobId));
+    Job->FeaturesDir = DirPlusFile(TempDir, "F" + std::to_string(JobId));
+    Job->CFPath = DirPlusFile(TempDir, std::to_string(JobId) + ".merge");
+    Job->JobId = JobId;
+
+
+    Cmd.addArgument(Job->CorpusDir);
+    Cmd.addFlag("features_dir", Job->FeaturesDir);
+
+    for (auto &D : {Job->CorpusDir, Job->FeaturesDir}) {
+      RmDirRecursive(D);
+      MkDir(D);
+    }
+
+    Cmd.setOutputFile(Job->LogPath);
+    Cmd.combineOutAndErr();
+
+    Job->Cmd = Cmd;
+
+    if (Verbosity >= 2)
+      Printf("Job %zd/%p Created: %s\n", JobId, Job,
+             Job->Cmd.toString().c_str());
+    // Start from very short runs and gradually increase them.
+    return Job;
+  }
+
+  void RunOneMergeJob(FuzzJob *Job) {
+    auto Stats = ParseFinalStatsFromLog(Job->LogPath);
+    NumRuns += Stats.number_of_executed_units;
+
+    Vector<SizedFile> TempFiles, MergeCandidates;
+    // Read all newly created inputs and their feature sets.
+    // Choose only those inputs that have new features.
+    GetSizedFilesFromDir(Job->CorpusDir, &TempFiles);
+    std::sort(TempFiles.begin(), TempFiles.end());
+    for (auto &F : TempFiles) {
+      auto FeatureFile = F.File;
+      FeatureFile.replace(0, Job->CorpusDir.size(), Job->FeaturesDir);
+      auto FeatureBytes = FileToVector(FeatureFile, 0, false);
+      assert((FeatureBytes.size() % sizeof(uint32_t)) == 0);
+      Vector<uint32_t> NewFeatures(FeatureBytes.size() / sizeof(uint32_t));
+      memcpy(NewFeatures.data(), FeatureBytes.data(), FeatureBytes.size());
+      for (auto Ft : NewFeatures) {
+        if (!Features.count(Ft)) {
+          MergeCandidates.push_back(F);
+          break;
+        }
+      }
+    }
+    // if (!FilesToAdd.empty() || Job->ExitCode != 0)
+    Printf("#%zd: cov: %zd ft: %zd corp: %zd exec/s %zd "
+           "oom/timeout/crash: %zd/%zd/%zd time: %zds job: %zd dft_time: %d\n",
+           NumRuns, Cov.size(), Features.size(), Files.size(),
+           Stats.average_exec_per_sec, NumOOMs, NumTimeouts, NumCrashes,
+           secondsSinceProcessStartUp(), Job->JobId, Job->DftTimeInSeconds);
+
+    if (MergeCandidates.empty()) return;
+
+    Vector<std::string> FilesToAdd;
+    Set<uint32_t> NewFeatures, NewCov;
+    CrashResistantMerge(Args, {}, MergeCandidates, &FilesToAdd, Features,
+                        &NewFeatures, Cov, &NewCov, Job->CFPath, false);
+    for (auto &Path : FilesToAdd) {
+      auto U = FileToVector(Path);
+      auto NewPath = DirPlusFile(MainCorpusDir, Hash(U));
+      WriteToFile(U, NewPath);
+      Files.push_back(NewPath);
+    }
+    Features.insert(NewFeatures.begin(), NewFeatures.end());
+    Cov.insert(NewCov.begin(), NewCov.end());
+    for (auto Idx : NewCov)
+      if (auto *TE = TPC.PCTableEntryByIdx(Idx))
+        if (TPC.PcIsFuncEntry(TE))
+          PrintPC("  NEW_FUNC: %p %F %L\n", "",
+                  TPC.GetNextInstructionPc(TE->PC));
+
+  }
+
+
+  void CollectDFT(const std::string &InputPath) {
+    if (DataFlowBinary.empty()) return;
+    if (!FilesWithDFT.insert(InputPath).second) return;
+    Command Cmd(Args);
+    Cmd.removeFlag("fork");
+    Cmd.removeFlag("runs");
+    Cmd.addFlag("data_flow_trace", DFTDir);
+    Cmd.addArgument(InputPath);
+    for (auto &C : CorpusDirs) // Remove all corpora from the args.
+      Cmd.removeArgument(C);
+    Cmd.setOutputFile(DirPlusFile(TempDir, "dft.log"));
+    Cmd.combineOutAndErr();
+    // Printf("CollectDFT: %s\n", Cmd.toString().c_str());
+    ExecuteCommand(Cmd);
+  }
+
+};
+
+struct JobQueue {
+  std::queue<FuzzJob *> Qu;
+  std::mutex Mu;
+  std::condition_variable Cv;
+
+  void Push(FuzzJob *Job) {
+    {
+      std::lock_guard<std::mutex> Lock(Mu);
+      Qu.push(Job);
+    }
+    Cv.notify_one();
+  }
+  FuzzJob *Pop() {
+    std::unique_lock<std::mutex> Lk(Mu);
+    // std::lock_guard<std::mutex> Lock(Mu);
+    Cv.wait(Lk, [&]{return !Qu.empty();});
+    assert(!Qu.empty());
+    auto Job = Qu.front();
+    Qu.pop();
+    return Job;
+  }
+};
+
+void WorkerThread(JobQueue *FuzzQ, JobQueue *MergeQ) {
+  while (auto Job = FuzzQ->Pop()) {
+    // Printf("WorkerThread: job %p\n", Job);
+    Job->ExitCode = ExecuteCommand(Job->Cmd);
+    MergeQ->Push(Job);
+  }
+}
+
+// This is just a skeleton of an experimental -fork=1 feature.
+void FuzzWithFork(Random &Rand, const FuzzingOptions &Options,
+                  const Vector<std::string> &Args,
+                  const Vector<std::string> &CorpusDirs, int NumJobs) {
+  Printf("INFO: -fork=%d: fuzzing in separate process(s)\n", NumJobs);
+
+  GlobalEnv Env;
+  Env.Args = Args;
+  Env.CorpusDirs = CorpusDirs;
+  Env.Rand = &Rand;
+  Env.Verbosity = Options.Verbosity;
+  Env.ProcessStartTime = std::chrono::system_clock::now();
+  Env.DataFlowBinary = Options.CollectDataFlow;
+
+  Vector<SizedFile> SeedFiles;
+  for (auto &Dir : CorpusDirs)
+    GetSizedFilesFromDir(Dir, &SeedFiles);
+  std::sort(SeedFiles.begin(), SeedFiles.end());
+  Env.TempDir = TempPath("FuzzWithFork", ".dir");
+  Env.DFTDir = DirPlusFile(Env.TempDir, "DFT");
+  RmDirRecursive(Env.TempDir);  // in case there is a leftover from old runs.
+  MkDir(Env.TempDir);
+  MkDir(Env.DFTDir);
+
+
+  if (CorpusDirs.empty())
+    MkDir(Env.MainCorpusDir = DirPlusFile(Env.TempDir, "C"));
+  else
+    Env.MainCorpusDir = CorpusDirs[0];
+
+  auto CFPath = DirPlusFile(Env.TempDir, "merge.txt");
+  CrashResistantMerge(Env.Args, {}, SeedFiles, &Env.Files, {}, &Env.Features,
+                      {}, &Env.Cov,
+                      CFPath, false);
+  RemoveFile(CFPath);
+  Printf("INFO: -fork=%d: %zd seed inputs, starting to fuzz in %s\n", NumJobs,
+         Env.Files.size(), Env.TempDir.c_str());
+
+  int ExitCode = 0;
+
+  JobQueue FuzzQ, MergeQ;
+
+  auto StopJobs = [&]() {
+    for (int i = 0; i < NumJobs; i++)
+      FuzzQ.Push(nullptr);
+    MergeQ.Push(nullptr);
+    WriteToFile(Unit({1}), Env.StopFile());
+  };
+
+  size_t JobId = 1;
+  Vector<std::thread> Threads;
+  for (int t = 0; t < NumJobs; t++) {
+    Threads.push_back(std::thread(WorkerThread, &FuzzQ, &MergeQ));
+    FuzzQ.Push(Env.CreateNewJob(JobId++));
+  }
+
+  while (true) {
+    std::unique_ptr<FuzzJob> Job(MergeQ.Pop());
+    if (!Job)
+      break;
+    ExitCode = Job->ExitCode;
+    if (ExitCode == Options.InterruptExitCode) {
+      Printf("==%lu== libFuzzer: a child was interrupted; exiting\n", GetPid());
+      StopJobs();
+      break;
+    }
+    Fuzzer::MaybeExitGracefully();
+
+    Env.RunOneMergeJob(Job.get());
+
+    // Continue if our crash is one of the ignorred ones.
+    if (Options.IgnoreTimeouts && ExitCode == Options.TimeoutExitCode)
+      Env.NumTimeouts++;
+    else if (Options.IgnoreOOMs && ExitCode == Options.OOMExitCode)
+      Env.NumOOMs++;
+    else if (ExitCode != 0) {
+      Env.NumCrashes++;
+      if (Options.IgnoreCrashes) {
+        std::ifstream In(Job->LogPath);
+        std::string Line;
+        while (std::getline(In, Line, '\n'))
+          if (Line.find("ERROR:") != Line.npos ||
+              Line.find("runtime error:") != Line.npos)
+            Printf("%s\n", Line.c_str());
+      } else {
+        // And exit if we don't ignore this crash.
+        Printf("INFO: log from the inner process:\n%s",
+               FileToString(Job->LogPath).c_str());
+        StopJobs();
+        break;
+      }
+    }
+
+    // Stop if we are over the time budget.
+    // This is not precise, since other threads are still running
+    // and we will wait while joining them.
+    // We also don't stop instantly: other jobs need to finish.
+    if (Options.MaxTotalTimeSec > 0 &&
+        Env.secondsSinceProcessStartUp() >= (size_t)Options.MaxTotalTimeSec) {
+      Printf("INFO: fuzzed for %zd seconds, wrapping up soon\n",
+             Env.secondsSinceProcessStartUp());
+      StopJobs();
+      break;
+    }
+    if (Env.NumRuns >= Options.MaxNumberOfRuns) {
+      Printf("INFO: fuzzed for %zd iterations, wrapping up soon\n",
+             Env.NumRuns);
+      StopJobs();
+      break;
+    }
+
+    FuzzQ.Push(Env.CreateNewJob(JobId++));
+  }
+
+  for (auto &T : Threads)
+    T.join();
+
+  // The workers have terminated. Don't try to remove the directory before they
+  // terminate to avoid a race condition preventing cleanup on Windows.
+  RmDirRecursive(Env.TempDir);
+
+  // Use the exit code from the last child process.
+  Printf("INFO: exiting: %d time: %zds\n", ExitCode,
+         Env.secondsSinceProcessStartUp());
+  exit(ExitCode);
+}
+
+} // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerFork.h b/tools/fuzzing/libfuzzer/FuzzerFork.h
new file mode 100644
index 0000000000..b29a43e13f
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerFork.h
@@ -0,0 +1,24 @@
+//===- FuzzerFork.h - run fuzzing in sub-processes --------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_FORK_H
+#define LLVM_FUZZER_FORK_H
+
+#include "FuzzerDefs.h"
+#include "FuzzerOptions.h"
+#include "FuzzerRandom.h"
+
+#include <string>
+
+namespace fuzzer {
+void FuzzWithFork(Random &Rand, const FuzzingOptions &Options,
+                  const Vector<std::string> &Args,
+                  const Vector<std::string> &CorpusDirs, int NumJobs);
+} // namespace fuzzer
+
+#endif // LLVM_FUZZER_FORK_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerIO.cpp b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
new file mode 100644
index 0000000000..0053ef39f2
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
@@ -0,0 +1,160 @@
+//===- FuzzerIO.cpp - IO utils. -------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// IO functions.
+//===----------------------------------------------------------------------===//
+
+#include "mozilla/Unused.h"
+#include "FuzzerDefs.h"
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include "FuzzerUtil.h"
+#include <algorithm>
+#include <cstdarg>
+#include <fstream>
+#include <iterator>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+namespace fuzzer {
+
+static FILE *OutputFile = stderr;
+
+long GetEpoch(const std::string &Path) {
+  struct stat St;
+  if (stat(Path.c_str(), &St))
+    return 0;  // Can't stat, be conservative.
+  return St.st_mtime;
+}
+
+Unit FileToVector(const std::string &Path, size_t MaxSize, bool ExitOnError) {
+  std::ifstream T(Path, std::ios::binary);
+  if (ExitOnError && !T) {
+    Printf("No such directory: %s; exiting\n", Path.c_str());
+    exit(1);
+  }
+
+  T.seekg(0, T.end);
+  auto EndPos = T.tellg();
+  if (EndPos < 0) return {};
+  size_t FileLen = EndPos;
+  if (MaxSize)
+    FileLen = std::min(FileLen, MaxSize);
+
+  T.seekg(0, T.beg);
+  Unit Res(FileLen);
+  T.read(reinterpret_cast<char *>(Res.data()), FileLen);
+  return Res;
+}
+
+std::string FileToString(const std::string &Path) {
+  std::ifstream T(Path, std::ios::binary);
+  return std::string((std::istreambuf_iterator<char>(T)),
+                     std::istreambuf_iterator<char>());
+}
+
+void CopyFileToErr(const std::string &Path) {
+  Printf("%s", FileToString(Path).c_str());
+}
+
+void WriteToFile(const Unit &U, const std::string &Path) {
+  WriteToFile(U.data(), U.size(), Path);
+}
+
+void WriteToFile(const std::string &Data, const std::string &Path) {
+  WriteToFile(reinterpret_cast<const uint8_t *>(Data.c_str()), Data.size(),
+              Path);
+}
+
+void WriteToFile(const uint8_t *Data, size_t Size, const std::string &Path) {
+  // Use raw C interface because this function may be called from a sig handler.
+  FILE *Out = fopen(Path.c_str(), "wb");
+  if (!Out) return;
+  mozilla::Unused << fwrite(Data, sizeof(Data[0]), Size, Out);
+  fclose(Out);
+}
+
+void ReadDirToVectorOfUnits(const char *Path, Vector<Unit> *V,
+                            long *Epoch, size_t MaxSize, bool ExitOnError) {
+  long E = Epoch ? *Epoch : 0;
+  Vector<std::string> Files;
+  ListFilesInDirRecursive(Path, Epoch, &Files, /*TopDir*/true);
+  size_t NumLoaded = 0;
+  for (size_t i = 0; i < Files.size(); i++) {
+    auto &X = Files[i];
+    if (Epoch && GetEpoch(X) < E) continue;
+    NumLoaded++;
+    if ((NumLoaded & (NumLoaded - 1)) == 0 && NumLoaded >= 1024)
+      Printf("Loaded %zd/%zd files from %s\n", NumLoaded, Files.size(), Path);
+    auto S = FileToVector(X, MaxSize, ExitOnError);
+    if (!S.empty())
+      V->push_back(S);
+  }
+}
+
+
+void GetSizedFilesFromDir(const std::string &Dir, Vector<SizedFile> *V) {
+  Vector<std::string> Files;
+  ListFilesInDirRecursive(Dir, 0, &Files, /*TopDir*/true);
+  for (auto &File : Files)
+    if (size_t Size = FileSize(File))
+      V->push_back({File, Size});
+}
+
+std::string DirPlusFile(const std::string &DirPath,
+                        const std::string &FileName) {
+  return DirPath + GetSeparator() + FileName;
+}
+
+void DupAndCloseStderr() {
+  int OutputFd = DuplicateFile(2);
+  if (OutputFd >= 0) {
+    FILE *NewOutputFile = OpenFile(OutputFd, "w");
+    if (NewOutputFile) {
+      OutputFile = NewOutputFile;
+      if (EF->__sanitizer_set_report_fd)
+        EF->__sanitizer_set_report_fd(
+            reinterpret_cast<void *>(GetHandleFromFd(OutputFd)));
+      DiscardOutput(2);
+    }
+  }
+}
+
+void CloseStdout() {
+  DiscardOutput(1);
+}
+
+void Printf(const char *Fmt, ...) {
+  va_list ap;
+  va_start(ap, Fmt);
+  vfprintf(OutputFile, Fmt, ap);
+  va_end(ap);
+  fflush(OutputFile);
+}
+
+void VPrintf(bool Verbose, const char *Fmt, ...) {
+  if (!Verbose) return;
+  va_list ap;
+  va_start(ap, Fmt);
+  vfprintf(OutputFile, Fmt, ap);
+  va_end(ap);
+  fflush(OutputFile);
+}
+
+void RmDirRecursive(const std::string &Dir) {
+  IterateDirRecursive(
+      Dir, [](const std::string &Path) {},
+      [](const std::string &Path) { RmDir(Path); },
+      [](const std::string &Path) { RemoveFile(Path); });
+}
+
+std::string TempPath(const char *Prefix, const char *Extension) {
+  return DirPlusFile(TmpDir(), std::string("libFuzzerTemp.") + Prefix +
+                                   std::to_string(GetPid()) + Extension);
+}
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerIO.h b/tools/fuzzing/libfuzzer/FuzzerIO.h
new file mode 100644
index 0000000000..6e4368b971
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerIO.h
@@ -0,0 +1,106 @@
+//===- FuzzerIO.h - Internal header for IO utils ----------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// IO interface.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_IO_H
+#define LLVM_FUZZER_IO_H
+
+#include "FuzzerDefs.h"
+
+namespace fuzzer {
+
+long GetEpoch(const std::string &Path);
+
+Unit FileToVector(const std::string &Path, size_t MaxSize = 0,
+                  bool ExitOnError = true);
+
+std::string FileToString(const std::string &Path);
+
+void CopyFileToErr(const std::string &Path);
+
+void WriteToFile(const uint8_t *Data, size_t Size, const std::string &Path);
+// Write Data.c_str() to the file without terminating null character.
+void WriteToFile(const std::string &Data, const std::string &Path);
+void WriteToFile(const Unit &U, const std::string &Path);
+
+void ReadDirToVectorOfUnits(const char *Path, Vector<Unit> *V,
+                            long *Epoch, size_t MaxSize, bool ExitOnError);
+
+// Returns "Dir/FileName" or equivalent for the current OS.
+std::string DirPlusFile(const std::string &DirPath,
+                        const std::string &FileName);
+
+// Returns the name of the dir, similar to the 'dirname' utility.
+std::string DirName(const std::string &FileName);
+
+// Returns path to a TmpDir.
+std::string TmpDir();
+
+std::string TempPath(const char *Prefix, const char *Extension);
+
+bool IsInterestingCoverageFile(const std::string &FileName);
+
+void DupAndCloseStderr();
+
+void CloseStdout();
+
+void Printf(const char *Fmt, ...);
+void VPrintf(bool Verbose, const char *Fmt, ...);
+
+// Print using raw syscalls, useful when printing at early init stages.
+void RawPrint(const char *Str);
+
+// Platform specific functions:
+bool IsFile(const std::string &Path);
+size_t FileSize(const std::string &Path);
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             Vector<std::string> *V, bool TopDir);
+
+void RmDirRecursive(const std::string &Dir);
+
+// Iterate files and dirs inside Dir, recursively.
+// Call DirPreCallback/DirPostCallback on dirs before/after
+// calling FileCallback on files.
+void IterateDirRecursive(const std::string &Dir,
+                         void (*DirPreCallback)(const std::string &Dir),
+                         void (*DirPostCallback)(const std::string &Dir),
+                         void (*FileCallback)(const std::string &Dir));
+
+struct SizedFile {
+  std::string File;
+  size_t Size;
+  bool operator<(const SizedFile &B) const { return Size < B.Size; }
+};
+
+void GetSizedFilesFromDir(const std::string &Dir, Vector<SizedFile> *V);
+
+char GetSeparator();
+// Similar to the basename utility: returns the file name w/o the dir prefix.
+std::string Basename(const std::string &Path);
+
+FILE* OpenFile(int Fd, const char *Mode);
+
+int CloseFile(int Fd);
+
+int DuplicateFile(int Fd);
+
+void RemoveFile(const std::string &Path);
+void RenameFile(const std::string &OldPath, const std::string &NewPath);
+
+intptr_t GetHandleFromFd(int fd);
+
+void MkDir(const std::string &Path);
+void RmDir(const std::string &Path);
+
+const std::string &getDevNull();
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_IO_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
new file mode 100644
index 0000000000..4b453d286c
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
@@ -0,0 +1,177 @@
+//===- FuzzerIOPosix.cpp - IO utils for Posix. ----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// IO functions implementation using Posix API.
+//===----------------------------------------------------------------------===//
+#include "mozilla/Unused.h"
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_POSIX || LIBFUZZER_FUCHSIA
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include <cstdarg>
+#include <cstdio>
+#include <dirent.h>
+#include <fstream>
+#include <iterator>
+#include <libgen.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+namespace fuzzer {
+
+bool IsFile(const std::string &Path) {
+  struct stat St;
+  if (stat(Path.c_str(), &St))
+    return false;
+  return S_ISREG(St.st_mode);
+}
+
+static bool IsDirectory(const std::string &Path) {
+  struct stat St;
+  if (stat(Path.c_str(), &St))
+    return false;
+  return S_ISDIR(St.st_mode);
+}
+
+size_t FileSize(const std::string &Path) {
+  struct stat St;
+  if (stat(Path.c_str(), &St))
+    return 0;
+  return St.st_size;
+}
+
+std::string Basename(const std::string &Path) {
+  size_t Pos = Path.rfind(GetSeparator());
+  if (Pos == std::string::npos) return Path;
+  assert(Pos < Path.size());
+  return Path.substr(Pos + 1);
+}
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             Vector<std::string> *V, bool TopDir) {
+  auto E = GetEpoch(Dir);
+  if (Epoch)
+    if (E && *Epoch >= E) return;
+
+  DIR *D = opendir(Dir.c_str());
+  if (!D) {
+    Printf("%s: %s; exiting\n", strerror(errno), Dir.c_str());
+    exit(1);
+  }
+  while (auto E = readdir(D)) {
+    std::string Path = DirPlusFile(Dir, E->d_name);
+    if (E->d_type == DT_REG || E->d_type == DT_LNK ||
+        (E->d_type == DT_UNKNOWN && IsFile(Path)))
+      V->push_back(Path);
+    else if ((E->d_type == DT_DIR ||
+             (E->d_type == DT_UNKNOWN && IsDirectory(Path))) &&
+             *E->d_name != '.')
+      ListFilesInDirRecursive(Path, Epoch, V, false);
+  }
+  closedir(D);
+  if (Epoch && TopDir)
+    *Epoch = E;
+}
+
+
+void IterateDirRecursive(const std::string &Dir,
+                         void (*DirPreCallback)(const std::string &Dir),
+                         void (*DirPostCallback)(const std::string &Dir),
+                         void (*FileCallback)(const std::string &Dir)) {
+  DirPreCallback(Dir);
+  DIR *D = opendir(Dir.c_str());
+  if (!D) return;
+  while (auto E = readdir(D)) {
+    std::string Path = DirPlusFile(Dir, E->d_name);
+    if (E->d_type == DT_REG || E->d_type == DT_LNK ||
+        (E->d_type == DT_UNKNOWN && IsFile(Path)))
+      FileCallback(Path);
+    else if ((E->d_type == DT_DIR ||
+             (E->d_type == DT_UNKNOWN && IsDirectory(Path))) &&
+             *E->d_name != '.')
+      IterateDirRecursive(Path, DirPreCallback, DirPostCallback, FileCallback);
+  }
+  closedir(D);
+  DirPostCallback(Dir);
+}
+
+char GetSeparator() {
+  return '/';
+}
+
+FILE* OpenFile(int Fd, const char* Mode) {
+  return fdopen(Fd, Mode);
+}
+
+int CloseFile(int fd) {
+  return close(fd);
+}
+
+int DuplicateFile(int Fd) {
+  return dup(Fd);
+}
+
+void RemoveFile(const std::string &Path) {
+  unlink(Path.c_str());
+}
+
+void RenameFile(const std::string &OldPath, const std::string &NewPath) {
+  rename(OldPath.c_str(), NewPath.c_str());
+}
+
+intptr_t GetHandleFromFd(int fd) {
+  return static_cast<intptr_t>(fd);
+}
+
+std::string DirName(const std::string &FileName) {
+  char *Tmp = new char[FileName.size() + 1];
+  memcpy(Tmp, FileName.c_str(), FileName.size() + 1);
+  std::string Res = dirname(Tmp);
+  delete [] Tmp;
+  return Res;
+}
+
+std::string TmpDir() {
+  if (auto Env = getenv("TMPDIR"))
+    return Env;
+  return "/tmp";
+}
+
+bool IsInterestingCoverageFile(const std::string &FileName) {
+  if (FileName.find("compiler-rt/lib/") != std::string::npos)
+    return false; // sanitizer internal.
+  if (FileName.find("/usr/lib/") != std::string::npos)
+    return false;
+  if (FileName.find("/usr/include/") != std::string::npos)
+    return false;
+  if (FileName == "<null>")
+    return false;
+  return true;
+}
+
+void RawPrint(const char *Str) {
+  mozilla::Unused << write(2, Str, strlen(Str));
+}
+
+void MkDir(const std::string &Path) {
+  mkdir(Path.c_str(), 0700);
+}
+
+void RmDir(const std::string &Path) {
+  rmdir(Path.c_str());
+}
+
+const std::string &getDevNull() {
+  static const std::string devNull = "/dev/null";
+  return devNull;
+}
+
+}  // namespace fuzzer
+
+#endif // LIBFUZZER_POSIX
diff --git a/tools/fuzzing/libfuzzer/FuzzerIOWindows.cpp b/tools/fuzzing/libfuzzer/FuzzerIOWindows.cpp
new file mode 100644
index 0000000000..651283a551
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerIOWindows.cpp
@@ -0,0 +1,413 @@
+//===- FuzzerIOWindows.cpp - IO utils for Windows. ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// IO functions implementation for Windows.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_WINDOWS
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include <cstdarg>
+#include <cstdio>
+#include <fstream>
+#include <io.h>
+#include <iterator>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <windows.h>
+
+namespace fuzzer {
+
+static bool IsFile(const std::string &Path, const DWORD &FileAttributes) {
+
+  if (FileAttributes & FILE_ATTRIBUTE_NORMAL)
+    return true;
+
+  if (FileAttributes & FILE_ATTRIBUTE_DIRECTORY)
+    return false;
+
+  HANDLE FileHandle(
+      CreateFileA(Path.c_str(), 0, FILE_SHARE_READ, NULL, OPEN_EXISTING,
+                  FILE_FLAG_BACKUP_SEMANTICS, 0));
+
+  if (FileHandle == INVALID_HANDLE_VALUE) {
+    Printf("CreateFileA() failed for \"%s\" (Error code: %lu).\n", Path.c_str(),
+        GetLastError());
+    return false;
+  }
+
+  DWORD FileType = GetFileType(FileHandle);
+
+  if (FileType == FILE_TYPE_UNKNOWN) {
+    Printf("GetFileType() failed for \"%s\" (Error code: %lu).\n", Path.c_str(),
+        GetLastError());
+    CloseHandle(FileHandle);
+    return false;
+  }
+
+  if (FileType != FILE_TYPE_DISK) {
+    CloseHandle(FileHandle);
+    return false;
+  }
+
+  CloseHandle(FileHandle);
+  return true;
+}
+
+bool IsFile(const std::string &Path) {
+  DWORD Att = GetFileAttributesA(Path.c_str());
+
+  if (Att == INVALID_FILE_ATTRIBUTES) {
+    Printf("GetFileAttributesA() failed for \"%s\" (Error code: %lu).\n",
+        Path.c_str(), GetLastError());
+    return false;
+  }
+
+  return IsFile(Path, Att);
+}
+
+static bool IsDir(DWORD FileAttrs) {
+  if (FileAttrs == INVALID_FILE_ATTRIBUTES) return false;
+  return FileAttrs & FILE_ATTRIBUTE_DIRECTORY;
+}
+
+std::string Basename(const std::string &Path) {
+  size_t Pos = Path.find_last_of("/\\");
+  if (Pos == std::string::npos) return Path;
+  assert(Pos < Path.size());
+  return Path.substr(Pos + 1);
+}
+
+size_t FileSize(const std::string &Path) {
+  WIN32_FILE_ATTRIBUTE_DATA attr;
+  if (!GetFileAttributesExA(Path.c_str(), GetFileExInfoStandard, &attr)) {
+    DWORD LastError = GetLastError();
+    if (LastError != ERROR_FILE_NOT_FOUND)
+      Printf("GetFileAttributesExA() failed for \"%s\" (Error code: %lu).\n",
+             Path.c_str(), LastError);
+    return 0;
+  }
+  ULARGE_INTEGER size;
+  size.HighPart = attr.nFileSizeHigh;
+  size.LowPart = attr.nFileSizeLow;
+  return size.QuadPart;
+}
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             Vector<std::string> *V, bool TopDir) {
+  auto E = GetEpoch(Dir);
+  if (Epoch)
+    if (E && *Epoch >= E) return;
+
+  std::string Path(Dir);
+  assert(!Path.empty());
+  if (Path.back() != '\\')
+      Path.push_back('\\');
+  Path.push_back('*');
+
+  // Get the first directory entry.
+  WIN32_FIND_DATAA FindInfo;
+  HANDLE FindHandle(FindFirstFileA(Path.c_str(), &FindInfo));
+  if (FindHandle == INVALID_HANDLE_VALUE)
+  {
+    if (GetLastError() == ERROR_FILE_NOT_FOUND)
+      return;
+    Printf("No such file or directory: %s; exiting\n", Dir.c_str());
+    exit(1);
+  }
+
+  do {
+    std::string FileName = DirPlusFile(Dir, FindInfo.cFileName);
+
+    if (FindInfo.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) {
+      size_t FilenameLen = strlen(FindInfo.cFileName);
+      if ((FilenameLen == 1 && FindInfo.cFileName[0] == '.') ||
+          (FilenameLen == 2 && FindInfo.cFileName[0] == '.' &&
+                               FindInfo.cFileName[1] == '.'))
+        continue;
+
+      ListFilesInDirRecursive(FileName, Epoch, V, false);
+    }
+    else if (IsFile(FileName, FindInfo.dwFileAttributes))
+      V->push_back(FileName);
+  } while (FindNextFileA(FindHandle, &FindInfo));
+
+  DWORD LastError = GetLastError();
+  if (LastError != ERROR_NO_MORE_FILES)
+    Printf("FindNextFileA failed (Error code: %lu).\n", LastError);
+
+  FindClose(FindHandle);
+
+  if (Epoch && TopDir)
+    *Epoch = E;
+}
+
+
+void IterateDirRecursive(const std::string &Dir,
+                         void (*DirPreCallback)(const std::string &Dir),
+                         void (*DirPostCallback)(const std::string &Dir),
+                         void (*FileCallback)(const std::string &Dir)) {
+  // TODO(metzman): Implement ListFilesInDirRecursive via this function.
+  DirPreCallback(Dir);
+
+  DWORD DirAttrs = GetFileAttributesA(Dir.c_str());
+  if (!IsDir(DirAttrs)) return;
+
+  std::string TargetDir(Dir);
+  assert(!TargetDir.empty());
+  if (TargetDir.back() != '\\') TargetDir.push_back('\\');
+  TargetDir.push_back('*');
+
+  WIN32_FIND_DATAA FindInfo;
+  // Find the directory's first file.
+  HANDLE FindHandle = FindFirstFileA(TargetDir.c_str(), &FindInfo);
+  if (FindHandle == INVALID_HANDLE_VALUE) {
+    DWORD LastError = GetLastError();
+    if (LastError != ERROR_FILE_NOT_FOUND) {
+      // If the directory isn't empty, then something abnormal is going on.
+      Printf("FindFirstFileA failed for %s (Error code: %lu).\n", Dir.c_str(),
+             LastError);
+    }
+    return;
+  }
+
+  do {
+    std::string Path = DirPlusFile(Dir, FindInfo.cFileName);
+    DWORD PathAttrs = FindInfo.dwFileAttributes;
+    if (IsDir(PathAttrs)) {
+      // Is Path the current directory (".") or the parent ("..")?
+      if (strcmp(FindInfo.cFileName, ".") == 0 ||
+          strcmp(FindInfo.cFileName, "..") == 0)
+        continue;
+      IterateDirRecursive(Path, DirPreCallback, DirPostCallback, FileCallback);
+    } else if (PathAttrs != INVALID_FILE_ATTRIBUTES) {
+      FileCallback(Path);
+    }
+  } while (FindNextFileA(FindHandle, &FindInfo));
+
+  DWORD LastError = GetLastError();
+  if (LastError != ERROR_NO_MORE_FILES)
+    Printf("FindNextFileA failed for %s (Error code: %lu).\n", Dir.c_str(),
+           LastError);
+
+  FindClose(FindHandle);
+  DirPostCallback(Dir);
+}
+
+char GetSeparator() {
+  return '\\';
+}
+
+FILE* OpenFile(int Fd, const char* Mode) {
+  return _fdopen(Fd, Mode);
+}
+
+int CloseFile(int Fd) {
+  return _close(Fd);
+}
+
+int DuplicateFile(int Fd) {
+  return _dup(Fd);
+}
+
+void RemoveFile(const std::string &Path) {
+  _unlink(Path.c_str());
+}
+
+void RenameFile(const std::string &OldPath, const std::string &NewPath) {
+  rename(OldPath.c_str(), NewPath.c_str());
+}
+
+intptr_t GetHandleFromFd(int fd) {
+  return _get_osfhandle(fd);
+}
+
+static bool IsSeparator(char C) {
+  return C == '\\' || C == '/';
+}
+
+// Parse disk designators, like "C:\". If Relative == true, also accepts: "C:".
+// Returns number of characters considered if successful.
+static size_t ParseDrive(const std::string &FileName, const size_t Offset,
+                         bool Relative = true) {
+  if (Offset + 1 >= FileName.size() || FileName[Offset + 1] != ':')
+    return 0;
+  if (Offset + 2 >= FileName.size() || !IsSeparator(FileName[Offset + 2])) {
+    if (!Relative) // Accept relative path?
+      return 0;
+    else
+      return 2;
+  }
+  return 3;
+}
+
+// Parse a file name, like: SomeFile.txt
+// Returns number of characters considered if successful.
+static size_t ParseFileName(const std::string &FileName, const size_t Offset) {
+  size_t Pos = Offset;
+  const size_t End = FileName.size();
+  for(; Pos < End && !IsSeparator(FileName[Pos]); ++Pos)
+    ;
+  return Pos - Offset;
+}
+
+// Parse a directory ending in separator, like: `SomeDir\`
+// Returns number of characters considered if successful.
+static size_t ParseDir(const std::string &FileName, const size_t Offset) {
+  size_t Pos = Offset;
+  const size_t End = FileName.size();
+  if (Pos >= End || IsSeparator(FileName[Pos]))
+    return 0;
+  for(; Pos < End && !IsSeparator(FileName[Pos]); ++Pos)
+    ;
+  if (Pos >= End)
+    return 0;
+  ++Pos; // Include separator.
+  return Pos - Offset;
+}
+
+// Parse a servername and share, like: `SomeServer\SomeShare\`
+// Returns number of characters considered if successful.
+static size_t ParseServerAndShare(const std::string &FileName,
+                                  const size_t Offset) {
+  size_t Pos = Offset, Res;
+  if (!(Res = ParseDir(FileName, Pos)))
+    return 0;
+  Pos += Res;
+  if (!(Res = ParseDir(FileName, Pos)))
+    return 0;
+  Pos += Res;
+  return Pos - Offset;
+}
+
+// Parse the given Ref string from the position Offset, to exactly match the given
+// string Patt.
+// Returns number of characters considered if successful.
+static size_t ParseCustomString(const std::string &Ref, size_t Offset,
+                                const char *Patt) {
+  size_t Len = strlen(Patt);
+  if (Offset + Len > Ref.size())
+    return 0;
+  return Ref.compare(Offset, Len, Patt) == 0 ? Len : 0;
+}
+
+// Parse a location, like:
+// \\?\UNC\Server\Share\  \\?\C:\  \\Server\Share\  \  C:\  C:
+// Returns number of characters considered if successful.
+static size_t ParseLocation(const std::string &FileName) {
+  size_t Pos = 0, Res;
+
+  if ((Res = ParseCustomString(FileName, Pos, R"(\\?\)"))) {
+    Pos += Res;
+    if ((Res = ParseCustomString(FileName, Pos, R"(UNC\)"))) {
+      Pos += Res;
+      if ((Res = ParseServerAndShare(FileName, Pos)))
+        return Pos + Res;
+      return 0;
+    }
+    if ((Res = ParseDrive(FileName, Pos, false)))
+      return Pos + Res;
+    return 0;
+  }
+
+  if (Pos < FileName.size() && IsSeparator(FileName[Pos])) {
+    ++Pos;
+    if (Pos < FileName.size() && IsSeparator(FileName[Pos])) {
+      ++Pos;
+      if ((Res = ParseServerAndShare(FileName, Pos)))
+        return Pos + Res;
+      return 0;
+    }
+    return Pos;
+  }
+
+  if ((Res = ParseDrive(FileName, Pos)))
+    return Pos + Res;
+
+  return Pos;
+}
+
+std::string DirName(const std::string &FileName) {
+  size_t LocationLen = ParseLocation(FileName);
+  size_t DirLen = 0, Res;
+  while ((Res = ParseDir(FileName, LocationLen + DirLen)))
+    DirLen += Res;
+  size_t FileLen = ParseFileName(FileName, LocationLen + DirLen);
+
+  if (LocationLen + DirLen + FileLen != FileName.size()) {
+    Printf("DirName() failed for \"%s\", invalid path.\n", FileName.c_str());
+    exit(1);
+  }
+
+  if (DirLen) {
+    --DirLen; // Remove trailing separator.
+    if (!FileLen) { // Path ended in separator.
+      assert(DirLen);
+      // Remove file name from Dir.
+      while (DirLen && !IsSeparator(FileName[LocationLen + DirLen - 1]))
+        --DirLen;
+      if (DirLen) // Remove trailing separator.
+        --DirLen;
+    }
+  }
+
+  if (!LocationLen) { // Relative path.
+    if (!DirLen)
+      return ".";
+    return std::string(".\\").append(FileName, 0, DirLen);
+  }
+
+  return FileName.substr(0, LocationLen + DirLen);
+}
+
+std::string TmpDir() {
+  std::string Tmp;
+  Tmp.resize(MAX_PATH + 1);
+  DWORD Size = GetTempPathA(Tmp.size(), &Tmp[0]);
+  if (Size == 0) {
+    Printf("Couldn't get Tmp path.\n");
+    exit(1);
+  }
+  Tmp.resize(Size);
+  return Tmp;
+}
+
+bool IsInterestingCoverageFile(const std::string &FileName) {
+  if (FileName.find("Program Files") != std::string::npos)
+    return false;
+  if (FileName.find("compiler-rt\\lib\\") != std::string::npos)
+    return false; // sanitizer internal.
+  if (FileName == "<null>")
+    return false;
+  return true;
+}
+
+void RawPrint(const char *Str) {
+  _write(2, Str, strlen(Str));
+}
+
+void MkDir(const std::string &Path) {
+  if (CreateDirectoryA(Path.c_str(), nullptr)) return;
+  Printf("CreateDirectoryA failed for %s (Error code: %lu).\n", Path.c_str(),
+         GetLastError());
+}
+
+void RmDir(const std::string &Path) {
+  if (RemoveDirectoryA(Path.c_str())) return;
+  Printf("RemoveDirectoryA failed for %s (Error code: %lu).\n", Path.c_str(),
+         GetLastError());
+}
+
+const std::string &getDevNull() {
+  static const std::string devNull = "NUL";
+  return devNull;
+}
+
+}  // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS
diff --git a/tools/fuzzing/libfuzzer/FuzzerInterceptors.cpp b/tools/fuzzing/libfuzzer/FuzzerInterceptors.cpp
new file mode 100644
index 0000000000..a1a64780de
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerInterceptors.cpp
@@ -0,0 +1,235 @@
+//===-- FuzzerInterceptors.cpp --------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Intercept certain libc functions to aid fuzzing.
+// Linked only when other RTs that define their own interceptors are not linked.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerPlatform.h"
+
+#if LIBFUZZER_LINUX
+
+#define GET_CALLER_PC() __builtin_return_address(0)
+
+#define PTR_TO_REAL(x) real_##x
+#define REAL(x) __interception::PTR_TO_REAL(x)
+#define FUNC_TYPE(x) x##_type
+#define DEFINE_REAL(ret_type, func, ...)                                       \
+  typedef ret_type (*FUNC_TYPE(func))(__VA_ARGS__);                            \
+  namespace __interception {                                                   \
+  FUNC_TYPE(func) PTR_TO_REAL(func);                                           \
+  }
+
+#include <cassert>
+#include <cstdint>
+#include <dlfcn.h> // for dlsym()
+#include <sanitizer/common_interface_defs.h>
+
+static void *getFuncAddr(const char *name, uintptr_t wrapper_addr) {
+  void *addr = dlsym(RTLD_NEXT, name);
+  if (!addr) {
+    // If the lookup using RTLD_NEXT failed, the sanitizer runtime library is
+    // later in the library search order than the DSO that we are trying to
+    // intercept, which means that we cannot intercept this function. We still
+    // want the address of the real definition, though, so look it up using
+    // RTLD_DEFAULT.
+    addr = dlsym(RTLD_DEFAULT, name);
+
+    // In case `name' is not loaded, dlsym ends up finding the actual wrapper.
+    // We don't want to intercept the wrapper and have it point to itself.
+    if (reinterpret_cast<uintptr_t>(addr) == wrapper_addr)
+      addr = nullptr;
+  }
+  return addr;
+}
+
+static int FuzzerInited = 0;
+static bool FuzzerInitIsRunning;
+
+static void fuzzerInit();
+
+static void ensureFuzzerInited() {
+  assert(!FuzzerInitIsRunning);
+  if (!FuzzerInited) {
+    fuzzerInit();
+  }
+}
+
+static int internal_strcmp_strncmp(const char *s1, const char *s2, bool strncmp,
+                                   size_t n) {
+  size_t i = 0;
+  while (true) {
+    if (strncmp) {
+      if (i == n)
+        break;
+      i++;
+    }
+    unsigned c1 = *s1;
+    unsigned c2 = *s2;
+    if (c1 != c2)
+      return (c1 < c2) ? -1 : 1;
+    if (c1 == 0)
+      break;
+    s1++;
+    s2++;
+  }
+  return 0;
+}
+
+static int internal_strncmp(const char *s1, const char *s2, size_t n) {
+  return internal_strcmp_strncmp(s1, s2, true, n);
+}
+
+static int internal_strcmp(const char *s1, const char *s2) {
+  return internal_strcmp_strncmp(s1, s2, false, 0);
+}
+
+static int internal_memcmp(const void *s1, const void *s2, size_t n) {
+  const uint8_t *t1 = static_cast<const uint8_t *>(s1);
+  const uint8_t *t2 = static_cast<const uint8_t *>(s2);
+  for (size_t i = 0; i < n; ++i, ++t1, ++t2)
+    if (*t1 != *t2)
+      return *t1 < *t2 ? -1 : 1;
+  return 0;
+}
+
+static size_t internal_strlen(const char *s) {
+  size_t i = 0;
+  while (s[i])
+    i++;
+  return i;
+}
+
+static char *internal_strstr(const char *haystack, const char *needle) {
+  // This is O(N^2), but we are not using it in hot places.
+  size_t len1 = internal_strlen(haystack);
+  size_t len2 = internal_strlen(needle);
+  if (len1 < len2)
+    return nullptr;
+  for (size_t pos = 0; pos <= len1 - len2; pos++) {
+    if (internal_memcmp(haystack + pos, needle, len2) == 0)
+      return const_cast<char *>(haystack) + pos;
+  }
+  return nullptr;
+}
+
+extern "C" {
+
+DEFINE_REAL(int, bcmp, const void *, const void *, size_t)
+DEFINE_REAL(int, memcmp, const void *, const void *, size_t)
+DEFINE_REAL(int, strncmp, const char *, const char *, size_t)
+DEFINE_REAL(int, strcmp, const char *, const char *)
+DEFINE_REAL(int, strncasecmp, const char *, const char *, size_t)
+DEFINE_REAL(int, strcasecmp, const char *, const char *)
+DEFINE_REAL(char *, strstr, const char *, const char *)
+DEFINE_REAL(char *, strcasestr, const char *, const char *)
+DEFINE_REAL(void *, memmem, const void *, size_t, const void *, size_t)
+
+ATTRIBUTE_INTERFACE int bcmp(const char *s1, const char *s2, size_t n) {
+  if (!FuzzerInited)
+    return internal_memcmp(s1, s2, n);
+  int result = REAL(bcmp)(s1, s2, n);
+  __sanitizer_weak_hook_memcmp(GET_CALLER_PC(), s1, s2, n, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE int memcmp(const void *s1, const void *s2, size_t n) {
+  if (!FuzzerInited)
+    return internal_memcmp(s1, s2, n);
+  int result = REAL(memcmp)(s1, s2, n);
+  __sanitizer_weak_hook_memcmp(GET_CALLER_PC(), s1, s2, n, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE int strncmp(const char *s1, const char *s2, size_t n) {
+  if (!FuzzerInited)
+    return internal_strncmp(s1, s2, n);
+  int result = REAL(strncmp)(s1, s2, n);
+  __sanitizer_weak_hook_strncmp(GET_CALLER_PC(), s1, s2, n, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE int strcmp(const char *s1, const char *s2) {
+  if (!FuzzerInited)
+    return internal_strcmp(s1, s2);
+  int result = REAL(strcmp)(s1, s2);
+  __sanitizer_weak_hook_strcmp(GET_CALLER_PC(), s1, s2, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE int strncasecmp(const char *s1, const char *s2, size_t n) {
+  ensureFuzzerInited();
+  int result = REAL(strncasecmp)(s1, s2, n);
+  __sanitizer_weak_hook_strncasecmp(GET_CALLER_PC(), s1, s2, n, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE int strcasecmp(const char *s1, const char *s2) {
+  ensureFuzzerInited();
+  int result = REAL(strcasecmp)(s1, s2);
+  __sanitizer_weak_hook_strcasecmp(GET_CALLER_PC(), s1, s2, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE char *strstr(const char *s1, const char *s2) {
+  if (!FuzzerInited)
+    return internal_strstr(s1, s2);
+  char *result = REAL(strstr)(s1, s2);
+  __sanitizer_weak_hook_strstr(GET_CALLER_PC(), s1, s2, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE char *strcasestr(const char *s1, const char *s2) {
+  ensureFuzzerInited();
+  char *result = REAL(strcasestr)(s1, s2);
+  __sanitizer_weak_hook_strcasestr(GET_CALLER_PC(), s1, s2, result);
+  return result;
+}
+
+ATTRIBUTE_INTERFACE
+void *memmem(const void *s1, size_t len1, const void *s2, size_t len2) {
+  ensureFuzzerInited();
+  void *result = REAL(memmem)(s1, len1, s2, len2);
+  __sanitizer_weak_hook_memmem(GET_CALLER_PC(), s1, len1, s2, len2, result);
+  return result;
+}
+
+__attribute__((section(".preinit_array"),
+               used)) static void (*__local_fuzzer_preinit)(void) = fuzzerInit;
+
+} // extern "C"
+
+static void fuzzerInit() {
+  assert(!FuzzerInitIsRunning);
+  if (FuzzerInited)
+    return;
+  FuzzerInitIsRunning = true;
+
+  REAL(bcmp) = reinterpret_cast<memcmp_type>(
+      getFuncAddr("bcmp", reinterpret_cast<uintptr_t>(&bcmp)));
+  REAL(memcmp) = reinterpret_cast<memcmp_type>(
+      getFuncAddr("memcmp", reinterpret_cast<uintptr_t>(&memcmp)));
+  REAL(strncmp) = reinterpret_cast<strncmp_type>(
+      getFuncAddr("strncmp", reinterpret_cast<uintptr_t>(&strncmp)));
+  REAL(strcmp) = reinterpret_cast<strcmp_type>(
+      getFuncAddr("strcmp", reinterpret_cast<uintptr_t>(&strcmp)));
+  REAL(strncasecmp) = reinterpret_cast<strncasecmp_type>(
+      getFuncAddr("strncasecmp", reinterpret_cast<uintptr_t>(&strncasecmp)));
+  REAL(strcasecmp) = reinterpret_cast<strcasecmp_type>(
+      getFuncAddr("strcasecmp", reinterpret_cast<uintptr_t>(&strcasecmp)));
+  REAL(strstr) = reinterpret_cast<strstr_type>(
+      getFuncAddr("strstr", reinterpret_cast<uintptr_t>(&strstr)));
+  REAL(strcasestr) = reinterpret_cast<strcasestr_type>(
+      getFuncAddr("strcasestr", reinterpret_cast<uintptr_t>(&strcasestr)));
+  REAL(memmem) = reinterpret_cast<memmem_type>(
+      getFuncAddr("memmem", reinterpret_cast<uintptr_t>(&memmem)));
+
+  FuzzerInitIsRunning = false;
+  FuzzerInited = 1;
+}
+
+#endif
diff --git a/tools/fuzzing/libfuzzer/FuzzerInterface.h b/tools/fuzzing/libfuzzer/FuzzerInterface.h
new file mode 100644
index 0000000000..4f62822eac
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerInterface.h
@@ -0,0 +1,79 @@
+//===- FuzzerInterface.h - Interface header for the Fuzzer ------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Define the interface between libFuzzer and the library being tested.
+//===----------------------------------------------------------------------===//
+
+// NOTE: the libFuzzer interface is thin and in the majority of cases
+// you should not include this file into your target. In 95% of cases
+// all you need is to define the following function in your file:
+// extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+
+// WARNING: keep the interface in C.
+
+#ifndef LLVM_FUZZER_INTERFACE_H
+#define LLVM_FUZZER_INTERFACE_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif  // __cplusplus
+
+// Define FUZZER_INTERFACE_VISIBILITY to set default visibility in a way that
+// doesn't break MSVC.
+#if defined(_WIN32)
+#define FUZZER_INTERFACE_VISIBILITY __declspec(dllexport)
+#else
+#define FUZZER_INTERFACE_VISIBILITY __attribute__((visibility("default")))
+#endif
+
+// Mandatory user-provided target function.
+// Executes the code under test with [Data, Data+Size) as the input.
+// libFuzzer will invoke this function *many* times with different inputs.
+// Must return 0.
+FUZZER_INTERFACE_VISIBILITY int
+LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+
+// Optional user-provided initialization function.
+// If provided, this function will be called by libFuzzer once at startup.
+// It may read and modify argc/argv.
+// Must return 0.
+FUZZER_INTERFACE_VISIBILITY int LLVMFuzzerInitialize(int *argc, char ***argv);
+
+// Optional user-provided custom mutator.
+// Mutates raw data in [Data, Data+Size) inplace.
+// Returns the new size, which is not greater than MaxSize.
+// Given the same Seed produces the same mutation.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size, size_t MaxSize,
+                        unsigned int Seed);
+
+// Optional user-provided custom cross-over function.
+// Combines pieces of Data1 & Data2 together into Out.
+// Returns the new size, which is not greater than MaxOutSize.
+// Should produce the same mutation given the same Seed.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerCustomCrossOver(const uint8_t *Data1, size_t Size1,
+                          const uint8_t *Data2, size_t Size2, uint8_t *Out,
+                          size_t MaxOutSize, unsigned int Seed);
+
+// Experimental, may go away in future.
+// libFuzzer-provided function to be used inside LLVMFuzzerCustomMutator.
+// Mutates raw data in [Data, Data+Size) inplace.
+// Returns the new size, which is not greater than MaxSize.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize);
+
+#undef FUZZER_INTERFACE_VISIBILITY
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus
+
+#endif  // LLVM_FUZZER_INTERFACE_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerInternal.h b/tools/fuzzing/libfuzzer/FuzzerInternal.h
new file mode 100644
index 0000000000..1f7d671ed8
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerInternal.h
@@ -0,0 +1,172 @@
+//===- FuzzerInternal.h - Internal header for the Fuzzer --------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Define the main class fuzzer::Fuzzer and most functions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_INTERNAL_H
+#define LLVM_FUZZER_INTERNAL_H
+
+#include "FuzzerDataFlowTrace.h"
+#include "FuzzerDefs.h"
+#include "FuzzerExtFunctions.h"
+#include "FuzzerInterface.h"
+#include "FuzzerOptions.h"
+#include "FuzzerSHA1.h"
+#include "FuzzerValueBitMap.h"
+#include <algorithm>
+#include <atomic>
+#include <chrono>
+#include <climits>
+#include <cstdlib>
+#include <string.h>
+
+namespace fuzzer {
+
+using namespace std::chrono;
+
+class Fuzzer {
+public:
+
+  Fuzzer(UserCallback CB, InputCorpus &Corpus, MutationDispatcher &MD,
+         FuzzingOptions Options);
+  ~Fuzzer();
+  void Loop(Vector<SizedFile> &CorporaFiles);
+  void ReadAndExecuteSeedCorpora(Vector<SizedFile> &CorporaFiles);
+  void MinimizeCrashLoop(const Unit &U);
+  void RereadOutputCorpus(size_t MaxSize);
+
+  size_t secondsSinceProcessStartUp() {
+    return duration_cast<seconds>(system_clock::now() - ProcessStartTime)
+        .count();
+  }
+
+  bool TimedOut() {
+    return Options.MaxTotalTimeSec > 0 &&
+           secondsSinceProcessStartUp() >
+               static_cast<size_t>(Options.MaxTotalTimeSec);
+  }
+
+  size_t execPerSec() {
+    size_t Seconds = secondsSinceProcessStartUp();
+    return Seconds ? TotalNumberOfRuns / Seconds : 0;
+  }
+
+  size_t getTotalNumberOfRuns() { return TotalNumberOfRuns; }
+
+  static void StaticAlarmCallback();
+  static void StaticCrashSignalCallback();
+  static void StaticExitCallback();
+  static void StaticInterruptCallback();
+  static void StaticFileSizeExceedCallback();
+  static void StaticGracefulExitCallback();
+
+  int ExecuteCallback(const uint8_t *Data, size_t Size);
+  bool RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile = false,
+              InputInfo *II = nullptr, bool *FoundUniqFeatures = nullptr);
+
+  // Merge Corpora[1:] into Corpora[0].
+  void Merge(const Vector<std::string> &Corpora);
+  void CrashResistantMergeInternalStep(const std::string &ControlFilePath);
+  MutationDispatcher &GetMD() { return MD; }
+  void PrintFinalStats();
+  void SetMaxInputLen(size_t MaxInputLen);
+  void SetMaxMutationLen(size_t MaxMutationLen);
+  void RssLimitCallback();
+
+  bool InFuzzingThread() const { return IsMyThread; }
+  size_t GetCurrentUnitInFuzzingThead(const uint8_t **Data) const;
+  void TryDetectingAMemoryLeak(const uint8_t *Data, size_t Size,
+                               bool DuringInitialCorpusExecution);
+
+  void HandleMalloc(size_t Size);
+  static void MaybeExitGracefully();
+  std::string WriteToOutputCorpus(const Unit &U);
+
+private:
+  void AlarmCallback();
+  void CrashCallback();
+  void ExitCallback();
+  void CrashOnOverwrittenData();
+  void InterruptCallback();
+  void MutateAndTestOne();
+  void PurgeAllocator();
+  void ReportNewCoverage(InputInfo *II, const Unit &U);
+  void PrintPulseAndReportSlowInput(const uint8_t *Data, size_t Size);
+  void WriteUnitToFileWithPrefix(const Unit &U, const char *Prefix);
+  void PrintStats(const char *Where, const char *End = "\n", size_t Units = 0,
+                  size_t Features = 0);
+  void PrintStatusForNewUnit(const Unit &U, const char *Text);
+  void CheckExitOnSrcPosOrItem();
+
+  static void StaticDeathCallback();
+  void DumpCurrentUnit(const char *Prefix);
+  void DeathCallback();
+
+  void AllocateCurrentUnitData();
+  uint8_t *CurrentUnitData = nullptr;
+  std::atomic<size_t> CurrentUnitSize;
+  uint8_t BaseSha1[kSHA1NumBytes];  // Checksum of the base unit.
+
+  bool GracefulExitRequested = false;
+
+  size_t TotalNumberOfRuns = 0;
+  size_t NumberOfNewUnitsAdded = 0;
+
+  size_t LastCorpusUpdateRun = 0;
+
+  bool HasMoreMallocsThanFrees = false;
+  size_t NumberOfLeakDetectionAttempts = 0;
+
+  system_clock::time_point LastAllocatorPurgeAttemptTime = system_clock::now();
+
+  UserCallback CB;
+  InputCorpus &Corpus;
+  MutationDispatcher &MD;
+  FuzzingOptions Options;
+  DataFlowTrace DFT;
+
+  system_clock::time_point ProcessStartTime = system_clock::now();
+  system_clock::time_point UnitStartTime, UnitStopTime;
+  long TimeOfLongestUnitInSeconds = 0;
+  long EpochOfLastReadOfOutputCorpus = 0;
+
+  size_t MaxInputLen = 0;
+  size_t MaxMutationLen = 0;
+  size_t TmpMaxMutationLen = 0;
+
+  Vector<uint32_t> UniqFeatureSetTmp;
+
+  // Need to know our own thread.
+  static thread_local bool IsMyThread;
+};
+
+struct ScopedEnableMsanInterceptorChecks {
+  ScopedEnableMsanInterceptorChecks() {
+    if (EF->__msan_scoped_enable_interceptor_checks)
+      EF->__msan_scoped_enable_interceptor_checks();
+  }
+  ~ScopedEnableMsanInterceptorChecks() {
+    if (EF->__msan_scoped_disable_interceptor_checks)
+      EF->__msan_scoped_disable_interceptor_checks();
+  }
+};
+
+struct ScopedDisableMsanInterceptorChecks {
+  ScopedDisableMsanInterceptorChecks() {
+    if (EF->__msan_scoped_disable_interceptor_checks)
+      EF->__msan_scoped_disable_interceptor_checks();
+  }
+  ~ScopedDisableMsanInterceptorChecks() {
+    if (EF->__msan_scoped_enable_interceptor_checks)
+      EF->__msan_scoped_enable_interceptor_checks();
+  }
+};
+
+} // namespace fuzzer
+
+#endif // LLVM_FUZZER_INTERNAL_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerLoop.cpp b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
new file mode 100644
index 0000000000..4c4e8c271b
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
@@ -0,0 +1,883 @@
+//===- FuzzerLoop.cpp - Fuzzer's main loop --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Fuzzer's main loop.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerCorpus.h"
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include "FuzzerMutate.h"
+#include "FuzzerPlatform.h"
+#include "FuzzerRandom.h"
+#include "FuzzerTracePC.h"
+#include <algorithm>
+#include <cstring>
+#include <memory>
+#include <mutex>
+#include <set>
+
+#if defined(__has_include)
+#if __has_include(<sanitizer / lsan_interface.h>)
+#include <sanitizer/lsan_interface.h>
+#endif
+#endif
+
+#define NO_SANITIZE_MEMORY
+#if defined(__has_feature)
+#if __has_feature(memory_sanitizer)
+#undef NO_SANITIZE_MEMORY
+#define NO_SANITIZE_MEMORY __attribute__((no_sanitize_memory))
+#endif
+#endif
+
+namespace fuzzer {
+static const size_t kMaxUnitSizeToPrint = 256;
+
+thread_local bool Fuzzer::IsMyThread;
+
+bool RunningUserCallback = false;
+
+// Only one Fuzzer per process.
+static Fuzzer *F;
+
+// Leak detection is expensive, so we first check if there were more mallocs
+// than frees (using the sanitizer malloc hooks) and only then try to call lsan.
+struct MallocFreeTracer {
+  void Start(int TraceLevel) {
+    this->TraceLevel = TraceLevel;
+    if (TraceLevel)
+      Printf("MallocFreeTracer: START\n");
+    Mallocs = 0;
+    Frees = 0;
+  }
+  // Returns true if there were more mallocs than frees.
+  bool Stop() {
+    if (TraceLevel)
+      Printf("MallocFreeTracer: STOP %zd %zd (%s)\n", Mallocs.load(),
+             Frees.load(), Mallocs == Frees ? "same" : "DIFFERENT");
+    bool Result = Mallocs > Frees;
+    Mallocs = 0;
+    Frees = 0;
+    TraceLevel = 0;
+    return Result;
+  }
+  std::atomic<size_t> Mallocs;
+  std::atomic<size_t> Frees;
+  int TraceLevel = 0;
+
+  std::recursive_mutex TraceMutex;
+  bool TraceDisabled = false;
+};
+
+static MallocFreeTracer AllocTracer;
+
+// Locks printing and avoids nested hooks triggered from mallocs/frees in
+// sanitizer.
+class TraceLock {
+public:
+  TraceLock() : Lock(AllocTracer.TraceMutex) {
+    AllocTracer.TraceDisabled = !AllocTracer.TraceDisabled;
+  }
+  ~TraceLock() { AllocTracer.TraceDisabled = !AllocTracer.TraceDisabled; }
+
+  bool IsDisabled() const {
+    // This is already inverted value.
+    return !AllocTracer.TraceDisabled;
+  }
+
+private:
+  std::lock_guard<std::recursive_mutex> Lock;
+};
+
+ATTRIBUTE_NO_SANITIZE_MEMORY
+void MallocHook(const volatile void *ptr, size_t size) {
+  size_t N = AllocTracer.Mallocs++;
+  F->HandleMalloc(size);
+  if (int TraceLevel = AllocTracer.TraceLevel) {
+    TraceLock Lock;
+    if (Lock.IsDisabled())
+      return;
+    Printf("MALLOC[%zd] %p %zd\n", N, ptr, size);
+    if (TraceLevel >= 2 && EF)
+      PrintStackTrace();
+  }
+}
+
+ATTRIBUTE_NO_SANITIZE_MEMORY
+void FreeHook(const volatile void *ptr) {
+  size_t N = AllocTracer.Frees++;
+  if (int TraceLevel = AllocTracer.TraceLevel) {
+    TraceLock Lock;
+    if (Lock.IsDisabled())
+      return;
+    Printf("FREE[%zd]   %p\n", N, ptr);
+    if (TraceLevel >= 2 && EF)
+      PrintStackTrace();
+  }
+}
+
+// Crash on a single malloc that exceeds the rss limit.
+void Fuzzer::HandleMalloc(size_t Size) {
+  if (!Options.MallocLimitMb || (Size >> 20) < (size_t)Options.MallocLimitMb)
+    return;
+  Printf("==%d== ERROR: libFuzzer: out-of-memory (malloc(%zd))\n", GetPid(),
+         Size);
+  Printf("   To change the out-of-memory limit use -rss_limit_mb=<N>\n\n");
+  PrintStackTrace();
+  DumpCurrentUnit("oom-");
+  Printf("SUMMARY: libFuzzer: out-of-memory\n");
+  PrintFinalStats();
+  _Exit(Options.OOMExitCode); // Stop right now.
+}
+
+Fuzzer::Fuzzer(UserCallback CB, InputCorpus &Corpus, MutationDispatcher &MD,
+               FuzzingOptions Options)
+    : CB(CB), Corpus(Corpus), MD(MD), Options(Options) {
+  if (EF->__sanitizer_set_death_callback)
+    EF->__sanitizer_set_death_callback(StaticDeathCallback);
+  assert(!F);
+  F = this;
+  TPC.ResetMaps();
+  IsMyThread = true;
+  if (Options.DetectLeaks && EF->__sanitizer_install_malloc_and_free_hooks)
+    EF->__sanitizer_install_malloc_and_free_hooks(MallocHook, FreeHook);
+  TPC.SetUseCounters(Options.UseCounters);
+  TPC.SetUseValueProfileMask(Options.UseValueProfile);
+
+  if (Options.Verbosity)
+    TPC.PrintModuleInfo();
+  if (!Options.OutputCorpus.empty() && Options.ReloadIntervalSec)
+    EpochOfLastReadOfOutputCorpus = GetEpoch(Options.OutputCorpus);
+  MaxInputLen = MaxMutationLen = Options.MaxLen;
+  TmpMaxMutationLen = 0;  // Will be set once we load the corpus.
+  AllocateCurrentUnitData();
+  CurrentUnitSize = 0;
+  memset(BaseSha1, 0, sizeof(BaseSha1));
+}
+
+Fuzzer::~Fuzzer() {}
+
+void Fuzzer::AllocateCurrentUnitData() {
+  if (CurrentUnitData || MaxInputLen == 0)
+    return;
+  CurrentUnitData = new uint8_t[MaxInputLen];
+}
+
+void Fuzzer::StaticDeathCallback() {
+  assert(F);
+  F->DeathCallback();
+}
+
+void Fuzzer::DumpCurrentUnit(const char *Prefix) {
+  if (!CurrentUnitData)
+    return; // Happens when running individual inputs.
+  ScopedDisableMsanInterceptorChecks S;
+  MD.PrintMutationSequence();
+  Printf("; base unit: %s\n", Sha1ToString(BaseSha1).c_str());
+  size_t UnitSize = CurrentUnitSize;
+  if (UnitSize <= kMaxUnitSizeToPrint) {
+    PrintHexArray(CurrentUnitData, UnitSize, "\n");
+    PrintASCII(CurrentUnitData, UnitSize, "\n");
+  }
+  WriteUnitToFileWithPrefix({CurrentUnitData, CurrentUnitData + UnitSize},
+                            Prefix);
+}
+
+NO_SANITIZE_MEMORY
+void Fuzzer::DeathCallback() {
+  DumpCurrentUnit("crash-");
+  PrintFinalStats();
+}
+
+void Fuzzer::StaticAlarmCallback() {
+  assert(F);
+  F->AlarmCallback();
+}
+
+void Fuzzer::StaticCrashSignalCallback() {
+  assert(F);
+  F->CrashCallback();
+}
+
+void Fuzzer::StaticExitCallback() {
+  assert(F);
+  F->ExitCallback();
+}
+
+void Fuzzer::StaticInterruptCallback() {
+  assert(F);
+  F->InterruptCallback();
+}
+
+void Fuzzer::StaticGracefulExitCallback() {
+  assert(F);
+  F->GracefulExitRequested = true;
+  Printf("INFO: signal received, trying to exit gracefully\n");
+}
+
+void Fuzzer::StaticFileSizeExceedCallback() {
+  Printf("==%lu== ERROR: libFuzzer: file size exceeded\n", GetPid());
+  exit(1);
+}
+
+void Fuzzer::CrashCallback() {
+  if (EF->__sanitizer_acquire_crash_state &&
+      !EF->__sanitizer_acquire_crash_state())
+    return;
+  Printf("==%lu== ERROR: libFuzzer: deadly signal\n", GetPid());
+  PrintStackTrace();
+  Printf("NOTE: libFuzzer has rudimentary signal handlers.\n"
+         "      Combine libFuzzer with AddressSanitizer or similar for better "
+         "crash reports.\n");
+  Printf("SUMMARY: libFuzzer: deadly signal\n");
+  DumpCurrentUnit("crash-");
+  PrintFinalStats();
+  _Exit(Options.ErrorExitCode); // Stop right now.
+}
+
+void Fuzzer::ExitCallback() {
+  if (!RunningUserCallback)
+    return; // This exit did not come from the user callback
+  if (EF->__sanitizer_acquire_crash_state &&
+      !EF->__sanitizer_acquire_crash_state())
+    return;
+  Printf("==%lu== ERROR: libFuzzer: fuzz target exited\n", GetPid());
+  PrintStackTrace();
+  Printf("SUMMARY: libFuzzer: fuzz target exited\n");
+  DumpCurrentUnit("crash-");
+  PrintFinalStats();
+  _Exit(Options.ErrorExitCode);
+}
+
+void Fuzzer::MaybeExitGracefully() {
+  if (!F->GracefulExitRequested) return;
+  Printf("==%lu== INFO: libFuzzer: exiting as requested\n", GetPid());
+  RmDirRecursive(TempPath("FuzzWithFork", ".dir"));
+  F->PrintFinalStats();
+  _Exit(0);
+}
+
+void Fuzzer::InterruptCallback() {
+  Printf("==%lu== libFuzzer: run interrupted; exiting\n", GetPid());
+  PrintFinalStats();
+  ScopedDisableMsanInterceptorChecks S; // RmDirRecursive may call opendir().
+  RmDirRecursive(TempPath("FuzzWithFork", ".dir"));
+  // Stop right now, don't perform any at-exit actions.
+  _Exit(Options.InterruptExitCode);
+}
+
+NO_SANITIZE_MEMORY
+void Fuzzer::AlarmCallback() {
+  assert(Options.UnitTimeoutSec > 0);
+  // In Windows and Fuchsia, Alarm callback is executed by a different thread.
+  // NetBSD's current behavior needs this change too.
+#if !LIBFUZZER_WINDOWS && !LIBFUZZER_NETBSD && !LIBFUZZER_FUCHSIA
+  if (!InFuzzingThread())
+    return;
+#endif
+  if (!RunningUserCallback)
+    return; // We have not started running units yet.
+  size_t Seconds =
+      duration_cast<seconds>(system_clock::now() - UnitStartTime).count();
+  if (Seconds == 0)
+    return;
+  if (Options.Verbosity >= 2)
+    Printf("AlarmCallback %zd\n", Seconds);
+  if (Seconds >= (size_t)Options.UnitTimeoutSec) {
+    if (EF->__sanitizer_acquire_crash_state &&
+        !EF->__sanitizer_acquire_crash_state())
+      return;
+    Printf("ALARM: working on the last Unit for %zd seconds\n", Seconds);
+    Printf("       and the timeout value is %d (use -timeout=N to change)\n",
+           Options.UnitTimeoutSec);
+    DumpCurrentUnit("timeout-");
+    Printf("==%lu== ERROR: libFuzzer: timeout after %d seconds\n", GetPid(),
+           Seconds);
+    PrintStackTrace();
+    Printf("SUMMARY: libFuzzer: timeout\n");
+    PrintFinalStats();
+    _Exit(Options.TimeoutExitCode); // Stop right now.
+  }
+}
+
+void Fuzzer::RssLimitCallback() {
+  if (EF->__sanitizer_acquire_crash_state &&
+      !EF->__sanitizer_acquire_crash_state())
+    return;
+  Printf(
+      "==%lu== ERROR: libFuzzer: out-of-memory (used: %zdMb; limit: %zdMb)\n",
+      GetPid(), GetPeakRSSMb(), Options.RssLimitMb);
+  Printf("   To change the out-of-memory limit use -rss_limit_mb=<N>\n\n");
+  PrintMemoryProfile();
+  DumpCurrentUnit("oom-");
+  Printf("SUMMARY: libFuzzer: out-of-memory\n");
+  PrintFinalStats();
+  _Exit(Options.OOMExitCode); // Stop right now.
+}
+
+void Fuzzer::PrintStats(const char *Where, const char *End, size_t Units,
+                        size_t Features) {
+  size_t ExecPerSec = execPerSec();
+  if (!Options.Verbosity)
+    return;
+  Printf("#%zd\t%s", TotalNumberOfRuns, Where);
+  if (size_t N = TPC.GetTotalPCCoverage())
+    Printf(" cov: %zd", N);
+  if (size_t N = Features ? Features : Corpus.NumFeatures())
+    Printf(" ft: %zd", N);
+  if (!Corpus.empty()) {
+    Printf(" corp: %zd", Corpus.NumActiveUnits());
+    if (size_t N = Corpus.SizeInBytes()) {
+      if (N < (1 << 14))
+        Printf("/%zdb", N);
+      else if (N < (1 << 24))
+        Printf("/%zdKb", N >> 10);
+      else
+        Printf("/%zdMb", N >> 20);
+    }
+    if (size_t FF = Corpus.NumInputsThatTouchFocusFunction())
+      Printf(" focus: %zd", FF);
+  }
+  if (TmpMaxMutationLen)
+    Printf(" lim: %zd", TmpMaxMutationLen);
+  if (Units)
+    Printf(" units: %zd", Units);
+
+  Printf(" exec/s: %zd", ExecPerSec);
+  Printf(" rss: %zdMb", GetPeakRSSMb());
+  Printf("%s", End);
+}
+
+void Fuzzer::PrintFinalStats() {
+  if (Options.PrintCoverage)
+    TPC.PrintCoverage();
+  if (Options.PrintCorpusStats)
+    Corpus.PrintStats();
+  if (!Options.PrintFinalStats)
+    return;
+  size_t ExecPerSec = execPerSec();
+  Printf("stat::number_of_executed_units: %zd\n", TotalNumberOfRuns);
+  Printf("stat::average_exec_per_sec:     %zd\n", ExecPerSec);
+  Printf("stat::new_units_added:          %zd\n", NumberOfNewUnitsAdded);
+  Printf("stat::slowest_unit_time_sec:    %zd\n", TimeOfLongestUnitInSeconds);
+  Printf("stat::peak_rss_mb:              %zd\n", GetPeakRSSMb());
+}
+
+void Fuzzer::SetMaxInputLen(size_t MaxInputLen) {
+  assert(this->MaxInputLen == 0); // Can only reset MaxInputLen from 0 to non-0.
+  assert(MaxInputLen);
+  this->MaxInputLen = MaxInputLen;
+  this->MaxMutationLen = MaxInputLen;
+  AllocateCurrentUnitData();
+  Printf("INFO: -max_len is not provided; "
+         "libFuzzer will not generate inputs larger than %zd bytes\n",
+         MaxInputLen);
+}
+
+void Fuzzer::SetMaxMutationLen(size_t MaxMutationLen) {
+  assert(MaxMutationLen && MaxMutationLen <= MaxInputLen);
+  this->MaxMutationLen = MaxMutationLen;
+}
+
+void Fuzzer::CheckExitOnSrcPosOrItem() {
+  if (!Options.ExitOnSrcPos.empty()) {
+    static auto *PCsSet = new Set<uintptr_t>;
+    auto HandlePC = [&](const TracePC::PCTableEntry *TE) {
+      if (!PCsSet->insert(TE->PC).second)
+        return;
+      std::string Descr = DescribePC("%F %L", TE->PC + 1);
+      if (Descr.find(Options.ExitOnSrcPos) != std::string::npos) {
+        Printf("INFO: found line matching '%s', exiting.\n",
+               Options.ExitOnSrcPos.c_str());
+        _Exit(0);
+      }
+    };
+    TPC.ForEachObservedPC(HandlePC);
+  }
+  if (!Options.ExitOnItem.empty()) {
+    if (Corpus.HasUnit(Options.ExitOnItem)) {
+      Printf("INFO: found item with checksum '%s', exiting.\n",
+             Options.ExitOnItem.c_str());
+      _Exit(0);
+    }
+  }
+}
+
+void Fuzzer::RereadOutputCorpus(size_t MaxSize) {
+  if (Options.OutputCorpus.empty() || !Options.ReloadIntervalSec)
+    return;
+  Vector<Unit> AdditionalCorpus;
+  ReadDirToVectorOfUnits(Options.OutputCorpus.c_str(), &AdditionalCorpus,
+                         &EpochOfLastReadOfOutputCorpus, MaxSize,
+                         /*ExitOnError*/ false);
+  if (Options.Verbosity >= 2)
+    Printf("Reload: read %zd new units.\n", AdditionalCorpus.size());
+  bool Reloaded = false;
+  for (auto &U : AdditionalCorpus) {
+    if (U.size() > MaxSize)
+      U.resize(MaxSize);
+    if (!Corpus.HasUnit(U)) {
+      if (RunOne(U.data(), U.size())) {
+        CheckExitOnSrcPosOrItem();
+        Reloaded = true;
+      }
+    }
+  }
+  if (Reloaded)
+    PrintStats("RELOAD");
+}
+
+void Fuzzer::PrintPulseAndReportSlowInput(const uint8_t *Data, size_t Size) {
+  auto TimeOfUnit =
+      duration_cast<seconds>(UnitStopTime - UnitStartTime).count();
+  if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)) &&
+      secondsSinceProcessStartUp() >= 2)
+    PrintStats("pulse ");
+  if (TimeOfUnit > TimeOfLongestUnitInSeconds * 1.1 &&
+      TimeOfUnit >= Options.ReportSlowUnits) {
+    TimeOfLongestUnitInSeconds = TimeOfUnit;
+    Printf("Slowest unit: %zd s:\n", TimeOfLongestUnitInSeconds);
+    WriteUnitToFileWithPrefix({Data, Data + Size}, "slow-unit-");
+  }
+}
+
+static void WriteFeatureSetToFile(const std::string &FeaturesDir,
+                                  const std::string &FileName,
+                                  const Vector<uint32_t> &FeatureSet) {
+  if (FeaturesDir.empty() || FeatureSet.empty()) return;
+  WriteToFile(reinterpret_cast<const uint8_t *>(FeatureSet.data()),
+              FeatureSet.size() * sizeof(FeatureSet[0]),
+              DirPlusFile(FeaturesDir, FileName));
+}
+
+static void RenameFeatureSetFile(const std::string &FeaturesDir,
+                                 const std::string &OldFile,
+                                 const std::string &NewFile) {
+  if (FeaturesDir.empty()) return;
+  RenameFile(DirPlusFile(FeaturesDir, OldFile),
+             DirPlusFile(FeaturesDir, NewFile));
+}
+
+bool Fuzzer::RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile,
+                    InputInfo *II, bool *FoundUniqFeatures) {
+  if (!Size)
+    return false;
+
+  if (ExecuteCallback(Data, Size) > 0) {
+    return false;
+  }
+
+  UniqFeatureSetTmp.clear();
+  size_t FoundUniqFeaturesOfII = 0;
+  size_t NumUpdatesBefore = Corpus.NumFeatureUpdates();
+  TPC.CollectFeatures([&](size_t Feature) {
+    if (Corpus.AddFeature(Feature, Size, Options.Shrink))
+      UniqFeatureSetTmp.push_back(Feature);
+    if (Options.Entropic)
+      Corpus.UpdateFeatureFrequency(II, Feature);
+    if (Options.ReduceInputs && II)
+      if (std::binary_search(II->UniqFeatureSet.begin(),
+                             II->UniqFeatureSet.end(), Feature))
+        FoundUniqFeaturesOfII++;
+  });
+  if (FoundUniqFeatures)
+    *FoundUniqFeatures = FoundUniqFeaturesOfII;
+  PrintPulseAndReportSlowInput(Data, Size);
+  size_t NumNewFeatures = Corpus.NumFeatureUpdates() - NumUpdatesBefore;
+  if (NumNewFeatures) {
+    TPC.UpdateObservedPCs();
+    auto NewII = Corpus.AddToCorpus({Data, Data + Size}, NumNewFeatures,
+                                    MayDeleteFile, TPC.ObservedFocusFunction(),
+                                    UniqFeatureSetTmp, DFT, II);
+    WriteFeatureSetToFile(Options.FeaturesDir, Sha1ToString(NewII->Sha1),
+                          NewII->UniqFeatureSet);
+    return true;
+  }
+  if (II && FoundUniqFeaturesOfII &&
+      II->DataFlowTraceForFocusFunction.empty() &&
+      FoundUniqFeaturesOfII == II->UniqFeatureSet.size() &&
+      II->U.size() > Size) {
+    auto OldFeaturesFile = Sha1ToString(II->Sha1);
+    Corpus.Replace(II, {Data, Data + Size});
+    RenameFeatureSetFile(Options.FeaturesDir, OldFeaturesFile,
+                         Sha1ToString(II->Sha1));
+    return true;
+  }
+  return false;
+}
+
+size_t Fuzzer::GetCurrentUnitInFuzzingThead(const uint8_t **Data) const {
+  assert(InFuzzingThread());
+  *Data = CurrentUnitData;
+  return CurrentUnitSize;
+}
+
+void Fuzzer::CrashOnOverwrittenData() {
+  Printf("==%d== ERROR: libFuzzer: fuzz target overwrites its const input\n",
+         GetPid());
+  PrintStackTrace();
+  Printf("SUMMARY: libFuzzer: overwrites-const-input\n");
+  DumpCurrentUnit("crash-");
+  PrintFinalStats();
+  _Exit(Options.ErrorExitCode); // Stop right now.
+}
+
+// Compare two arrays, but not all bytes if the arrays are large.
+static bool LooseMemeq(const uint8_t *A, const uint8_t *B, size_t Size) {
+  const size_t Limit = 64;
+  if (Size <= 64)
+    return !memcmp(A, B, Size);
+  // Compare first and last Limit/2 bytes.
+  return !memcmp(A, B, Limit / 2) &&
+         !memcmp(A + Size - Limit / 2, B + Size - Limit / 2, Limit / 2);
+}
+
+int Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {
+  TPC.RecordInitialStack();
+  TotalNumberOfRuns++;
+  assert(InFuzzingThread());
+  // We copy the contents of Unit into a separate heap buffer
+  // so that we reliably find buffer overflows in it.
+  uint8_t *DataCopy = new uint8_t[Size];
+  memcpy(DataCopy, Data, Size);
+  if (EF->__msan_unpoison)
+    EF->__msan_unpoison(DataCopy, Size);
+  if (EF->__msan_unpoison_param)
+    EF->__msan_unpoison_param(2);
+  if (CurrentUnitData && CurrentUnitData != Data)
+    memcpy(CurrentUnitData, Data, Size);
+  CurrentUnitSize = Size;
+  int Res = 0;
+  {
+    ScopedEnableMsanInterceptorChecks S;
+    AllocTracer.Start(Options.TraceMalloc);
+    UnitStartTime = system_clock::now();
+    TPC.ResetMaps();
+    RunningUserCallback = true;
+    Res = CB(DataCopy, Size);
+    RunningUserCallback = false;
+    UnitStopTime = system_clock::now();
+    assert(Res >= 0);
+    HasMoreMallocsThanFrees = AllocTracer.Stop();
+  }
+  if (!LooseMemeq(DataCopy, Data, Size))
+    CrashOnOverwrittenData();
+  CurrentUnitSize = 0;
+  delete[] DataCopy;
+  return Res;
+}
+
+std::string Fuzzer::WriteToOutputCorpus(const Unit &U) {
+  if (Options.OnlyASCII)
+    assert(IsASCII(U));
+  if (Options.OutputCorpus.empty())
+    return "";
+  std::string Path = DirPlusFile(Options.OutputCorpus, Hash(U));
+  WriteToFile(U, Path);
+  if (Options.Verbosity >= 2)
+    Printf("Written %zd bytes to %s\n", U.size(), Path.c_str());
+  return Path;
+}
+
+void Fuzzer::WriteUnitToFileWithPrefix(const Unit &U, const char *Prefix) {
+  if (!Options.SaveArtifacts)
+    return;
+  std::string Path = Options.ArtifactPrefix + Prefix + Hash(U);
+  if (!Options.ExactArtifactPath.empty())
+    Path = Options.ExactArtifactPath; // Overrides ArtifactPrefix.
+  WriteToFile(U, Path);
+  Printf("artifact_prefix='%s'; Test unit written to %s\n",
+         Options.ArtifactPrefix.c_str(), Path.c_str());
+  if (U.size() <= kMaxUnitSizeToPrint)
+    Printf("Base64: %s\n", Base64(U).c_str());
+}
+
+void Fuzzer::PrintStatusForNewUnit(const Unit &U, const char *Text) {
+  if (!Options.PrintNEW)
+    return;
+  PrintStats(Text, "");
+  if (Options.Verbosity) {
+    Printf(" L: %zd/%zd ", U.size(), Corpus.MaxInputSize());
+    MD.PrintMutationSequence();
+    Printf("\n");
+  }
+}
+
+void Fuzzer::ReportNewCoverage(InputInfo *II, const Unit &U) {
+  II->NumSuccessfullMutations++;
+  MD.RecordSuccessfulMutationSequence();
+  PrintStatusForNewUnit(U, II->Reduced ? "REDUCE" : "NEW   ");
+  WriteToOutputCorpus(U);
+  NumberOfNewUnitsAdded++;
+  CheckExitOnSrcPosOrItem(); // Check only after the unit is saved to corpus.
+  LastCorpusUpdateRun = TotalNumberOfRuns;
+}
+
+// Tries detecting a memory leak on the particular input that we have just
+// executed before calling this function.
+void Fuzzer::TryDetectingAMemoryLeak(const uint8_t *Data, size_t Size,
+                                     bool DuringInitialCorpusExecution) {
+  if (!HasMoreMallocsThanFrees)
+    return; // mallocs==frees, a leak is unlikely.
+  if (!Options.DetectLeaks)
+    return;
+  if (!DuringInitialCorpusExecution &&
+      TotalNumberOfRuns >= Options.MaxNumberOfRuns)
+    return;
+  if (!&(EF->__lsan_enable) || !&(EF->__lsan_disable) ||
+      !(EF->__lsan_do_recoverable_leak_check))
+    return; // No lsan.
+  // Run the target once again, but with lsan disabled so that if there is
+  // a real leak we do not report it twice.
+  EF->__lsan_disable();
+  ExecuteCallback(Data, Size);
+  EF->__lsan_enable();
+  if (!HasMoreMallocsThanFrees)
+    return; // a leak is unlikely.
+  if (NumberOfLeakDetectionAttempts++ > 1000) {
+    Options.DetectLeaks = false;
+    Printf("INFO: libFuzzer disabled leak detection after every mutation.\n"
+           "      Most likely the target function accumulates allocated\n"
+           "      memory in a global state w/o actually leaking it.\n"
+           "      You may try running this binary with -trace_malloc=[12]"
+           "      to get a trace of mallocs and frees.\n"
+           "      If LeakSanitizer is enabled in this process it will still\n"
+           "      run on the process shutdown.\n");
+    return;
+  }
+  // Now perform the actual lsan pass. This is expensive and we must ensure
+  // we don't call it too often.
+  if (EF->__lsan_do_recoverable_leak_check()) { // Leak is found, report it.
+    if (DuringInitialCorpusExecution)
+      Printf("\nINFO: a leak has been found in the initial corpus.\n\n");
+    Printf("INFO: to ignore leaks on libFuzzer side use -detect_leaks=0.\n\n");
+    CurrentUnitSize = Size;
+    DumpCurrentUnit("leak-");
+    PrintFinalStats();
+    _Exit(Options.ErrorExitCode); // not exit() to disable lsan further on.
+  }
+}
+
+void Fuzzer::MutateAndTestOne() {
+  MD.StartMutationSequence();
+
+  auto &II = Corpus.ChooseUnitToMutate(MD.GetRand());
+  if (Options.DoCrossOver)
+    MD.SetCrossOverWith(&Corpus.ChooseUnitToMutate(MD.GetRand()).U);
+  const auto &U = II.U;
+  memcpy(BaseSha1, II.Sha1, sizeof(BaseSha1));
+  assert(CurrentUnitData);
+  size_t Size = U.size();
+  assert(Size <= MaxInputLen && "Oversized Unit");
+  memcpy(CurrentUnitData, U.data(), Size);
+
+  assert(MaxMutationLen > 0);
+
+  size_t CurrentMaxMutationLen =
+      Min(MaxMutationLen, Max(U.size(), TmpMaxMutationLen));
+  assert(CurrentMaxMutationLen > 0);
+
+  for (int i = 0; i < Options.MutateDepth; i++) {
+    if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
+      break;
+    MaybeExitGracefully();
+    size_t NewSize = 0;
+    if (II.HasFocusFunction && !II.DataFlowTraceForFocusFunction.empty() &&
+        Size <= CurrentMaxMutationLen)
+      NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
+                                  II.DataFlowTraceForFocusFunction);
+
+    // If MutateWithMask either failed or wasn't called, call default Mutate.
+    if (!NewSize)
+      NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
+
+    if (!NewSize)
+      continue;
+
+    assert(NewSize > 0 && "Mutator returned empty unit");
+    assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");
+    Size = NewSize;
+    II.NumExecutedMutations++;
+    Corpus.IncrementNumExecutedMutations();
+
+    bool FoundUniqFeatures = false;
+    bool NewCov = RunOne(CurrentUnitData, Size, /*MayDeleteFile=*/true, &II,
+                         &FoundUniqFeatures);
+    TryDetectingAMemoryLeak(CurrentUnitData, Size,
+                            /*DuringInitialCorpusExecution*/ false);
+    if (NewCov) {
+      ReportNewCoverage(&II, {CurrentUnitData, CurrentUnitData + Size});
+      break;  // We will mutate this input more in the next rounds.
+    }
+    if (Options.ReduceDepth && !FoundUniqFeatures)
+      break;
+  }
+
+  II.NeedsEnergyUpdate = true;
+}
+
+void Fuzzer::PurgeAllocator() {
+  if (Options.PurgeAllocatorIntervalSec < 0 || !EF->__sanitizer_purge_allocator)
+    return;
+  if (duration_cast<seconds>(system_clock::now() -
+                             LastAllocatorPurgeAttemptTime)
+          .count() < Options.PurgeAllocatorIntervalSec)
+    return;
+
+  if (Options.RssLimitMb <= 0 ||
+      GetPeakRSSMb() > static_cast<size_t>(Options.RssLimitMb) / 2)
+    EF->__sanitizer_purge_allocator();
+
+  LastAllocatorPurgeAttemptTime = system_clock::now();
+}
+
+void Fuzzer::ReadAndExecuteSeedCorpora(Vector<SizedFile> &CorporaFiles) {
+  const size_t kMaxSaneLen = 1 << 20;
+  const size_t kMinDefaultLen = 4096;
+  size_t MaxSize = 0;
+  size_t MinSize = -1;
+  size_t TotalSize = 0;
+  for (auto &File : CorporaFiles) {
+    MaxSize = Max(File.Size, MaxSize);
+    MinSize = Min(File.Size, MinSize);
+    TotalSize += File.Size;
+  }
+  if (Options.MaxLen == 0)
+    SetMaxInputLen(std::min(std::max(kMinDefaultLen, MaxSize), kMaxSaneLen));
+  assert(MaxInputLen > 0);
+
+  // Test the callback with empty input and never try it again.
+  uint8_t dummy = 0;
+  ExecuteCallback(&dummy, 0);
+
+  if (CorporaFiles.empty()) {
+    Printf("INFO: A corpus is not provided, starting from an empty corpus\n");
+    Unit U({'\n'}); // Valid ASCII input.
+    RunOne(U.data(), U.size());
+  } else {
+    Printf("INFO: seed corpus: files: %zd min: %zdb max: %zdb total: %zdb"
+           " rss: %zdMb\n",
+           CorporaFiles.size(), MinSize, MaxSize, TotalSize, GetPeakRSSMb());
+    if (Options.ShuffleAtStartUp)
+      std::shuffle(CorporaFiles.begin(), CorporaFiles.end(), MD.GetRand());
+
+    if (Options.PreferSmall) {
+      std::stable_sort(CorporaFiles.begin(), CorporaFiles.end());
+      assert(CorporaFiles.front().Size <= CorporaFiles.back().Size);
+    }
+
+    // Load and execute inputs one by one.
+    for (auto &SF : CorporaFiles) {
+      auto U = FileToVector(SF.File, MaxInputLen, /*ExitOnError=*/false);
+      assert(U.size() <= MaxInputLen);
+      RunOne(U.data(), U.size());
+      CheckExitOnSrcPosOrItem();
+      TryDetectingAMemoryLeak(U.data(), U.size(),
+                              /*DuringInitialCorpusExecution*/ true);
+    }
+  }
+
+  PrintStats("INITED");
+  if (!Options.FocusFunction.empty()) {
+    Printf("INFO: %zd/%zd inputs touch the focus function\n",
+           Corpus.NumInputsThatTouchFocusFunction(), Corpus.size());
+    if (!Options.DataFlowTrace.empty())
+      Printf("INFO: %zd/%zd inputs have the Data Flow Trace\n",
+             Corpus.NumInputsWithDataFlowTrace(),
+             Corpus.NumInputsThatTouchFocusFunction());
+  }
+
+  if (Corpus.empty() && Options.MaxNumberOfRuns) {
+    Printf("ERROR: no interesting inputs were found. "
+           "Is the code instrumented for coverage? Exiting.\n");
+    exit(1);
+  }
+}
+
+void Fuzzer::Loop(Vector<SizedFile> &CorporaFiles) {
+  auto FocusFunctionOrAuto = Options.FocusFunction;
+  DFT.Init(Options.DataFlowTrace, &FocusFunctionOrAuto, CorporaFiles,
+           MD.GetRand());
+  TPC.SetFocusFunction(FocusFunctionOrAuto);
+  ReadAndExecuteSeedCorpora(CorporaFiles);
+  DFT.Clear();  // No need for DFT any more.
+  TPC.SetPrintNewPCs(Options.PrintNewCovPcs);
+  TPC.SetPrintNewFuncs(Options.PrintNewCovFuncs);
+  system_clock::time_point LastCorpusReload = system_clock::now();
+
+  TmpMaxMutationLen =
+      Min(MaxMutationLen, Max(size_t(4), Corpus.MaxInputSize()));
+
+  while (true) {
+    auto Now = system_clock::now();
+    if (!Options.StopFile.empty() &&
+        !FileToVector(Options.StopFile, 1, false).empty())
+      break;
+    if (duration_cast<seconds>(Now - LastCorpusReload).count() >=
+        Options.ReloadIntervalSec) {
+      RereadOutputCorpus(MaxInputLen);
+      LastCorpusReload = system_clock::now();
+    }
+    if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
+      break;
+    if (TimedOut())
+      break;
+
+    // Update TmpMaxMutationLen
+    if (Options.LenControl) {
+      if (TmpMaxMutationLen < MaxMutationLen &&
+          TotalNumberOfRuns - LastCorpusUpdateRun >
+              Options.LenControl * Log(TmpMaxMutationLen)) {
+        TmpMaxMutationLen =
+            Min(MaxMutationLen, TmpMaxMutationLen + Log(TmpMaxMutationLen));
+        LastCorpusUpdateRun = TotalNumberOfRuns;
+      }
+    } else {
+      TmpMaxMutationLen = MaxMutationLen;
+    }
+
+    // Perform several mutations and runs.
+    MutateAndTestOne();
+
+    PurgeAllocator();
+  }
+
+  PrintStats("DONE  ", "\n");
+  MD.PrintRecommendedDictionary();
+}
+
+void Fuzzer::MinimizeCrashLoop(const Unit &U) {
+  if (U.size() <= 1)
+    return;
+  while (!TimedOut() && TotalNumberOfRuns < Options.MaxNumberOfRuns) {
+    MD.StartMutationSequence();
+    memcpy(CurrentUnitData, U.data(), U.size());
+    for (int i = 0; i < Options.MutateDepth; i++) {
+      size_t NewSize = MD.Mutate(CurrentUnitData, U.size(), MaxMutationLen);
+      assert(NewSize <= MaxMutationLen);
+      if (!NewSize)
+        continue;
+      ExecuteCallback(CurrentUnitData, NewSize);
+      PrintPulseAndReportSlowInput(CurrentUnitData, NewSize);
+      TryDetectingAMemoryLeak(CurrentUnitData, NewSize,
+                              /*DuringInitialCorpusExecution*/ false);
+    }
+  }
+}
+
+} // namespace fuzzer
+
+extern "C" {
+
+ATTRIBUTE_INTERFACE size_t
+LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize) {
+  assert(fuzzer::F);
+  return fuzzer::F->GetMD().DefaultMutate(Data, Size, MaxSize);
+}
+
+} // extern "C"
diff --git a/tools/fuzzing/libfuzzer/FuzzerMain.cpp b/tools/fuzzing/libfuzzer/FuzzerMain.cpp
new file mode 100644
index 0000000000..75f2f8e75c
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerMain.cpp
@@ -0,0 +1,21 @@
+//===- FuzzerMain.cpp - main() function and flags -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// main() and flags.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#include "FuzzerPlatform.h"
+
+extern "C" {
+// This function should be defined by the user.
+int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+}  // extern "C"
+
+ATTRIBUTE_INTERFACE int main(int argc, char **argv) {
+  return fuzzer::FuzzerDriver(&argc, &argv, LLVMFuzzerTestOneInput);
+}
diff --git a/tools/fuzzing/libfuzzer/FuzzerMerge.cpp b/tools/fuzzing/libfuzzer/FuzzerMerge.cpp
new file mode 100644
index 0000000000..919eea8485
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerMerge.cpp
@@ -0,0 +1,404 @@
+//===- FuzzerMerge.cpp - merging corpora ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Merging corpora.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerCommand.h"
+#include "FuzzerMerge.h"
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include "FuzzerTracePC.h"
+#include "FuzzerUtil.h"
+
+#include <fstream>
+#include <iterator>
+#include <set>
+#include <sstream>
+#include <unordered_set>
+
+namespace fuzzer {
+
+bool Merger::Parse(const std::string &Str, bool ParseCoverage) {
+  std::istringstream SS(Str);
+  return Parse(SS, ParseCoverage);
+}
+
+void Merger::ParseOrExit(std::istream &IS, bool ParseCoverage) {
+  if (!Parse(IS, ParseCoverage)) {
+    Printf("MERGE: failed to parse the control file (unexpected error)\n");
+    exit(1);
+  }
+}
+
+// The control file example:
+//
+// 3 # The number of inputs
+// 1 # The number of inputs in the first corpus, <= the previous number
+// file0
+// file1
+// file2  # One file name per line.
+// STARTED 0 123  # FileID, file size
+// FT 0 1 4 6 8  # FileID COV1 COV2 ...
+// COV 0 7 8 9 # FileID COV1 COV1
+// STARTED 1 456  # If FT is missing, the input crashed while processing.
+// STARTED 2 567
+// FT 2 8 9
+// COV 2 11 12
+bool Merger::Parse(std::istream &IS, bool ParseCoverage) {
+  LastFailure.clear();
+  std::string Line;
+
+  // Parse NumFiles.
+  if (!std::getline(IS, Line, '\n')) return false;
+  std::istringstream L1(Line);
+  size_t NumFiles = 0;
+  L1 >> NumFiles;
+  if (NumFiles == 0 || NumFiles > 10000000) return false;
+
+  // Parse NumFilesInFirstCorpus.
+  if (!std::getline(IS, Line, '\n')) return false;
+  std::istringstream L2(Line);
+  NumFilesInFirstCorpus = NumFiles + 1;
+  L2 >> NumFilesInFirstCorpus;
+  if (NumFilesInFirstCorpus > NumFiles) return false;
+
+  // Parse file names.
+  Files.resize(NumFiles);
+  for (size_t i = 0; i < NumFiles; i++)
+    if (!std::getline(IS, Files[i].Name, '\n'))
+      return false;
+
+  // Parse STARTED, FT, and COV lines.
+  size_t ExpectedStartMarker = 0;
+  const size_t kInvalidStartMarker = -1;
+  size_t LastSeenStartMarker = kInvalidStartMarker;
+  Vector<uint32_t> TmpFeatures;
+  Set<uint32_t> PCs;
+  while (std::getline(IS, Line, '\n')) {
+    std::istringstream ISS1(Line);
+    std::string Marker;
+    size_t N;
+    ISS1 >> Marker;
+    ISS1 >> N;
+    if (Marker == "STARTED") {
+      // STARTED FILE_ID FILE_SIZE
+      if (ExpectedStartMarker != N)
+        return false;
+      ISS1 >> Files[ExpectedStartMarker].Size;
+      LastSeenStartMarker = ExpectedStartMarker;
+      assert(ExpectedStartMarker < Files.size());
+      ExpectedStartMarker++;
+    } else if (Marker == "FT") {
+      // FT FILE_ID COV1 COV2 COV3 ...
+      size_t CurrentFileIdx = N;
+      if (CurrentFileIdx != LastSeenStartMarker)
+        return false;
+      LastSeenStartMarker = kInvalidStartMarker;
+      if (ParseCoverage) {
+        TmpFeatures.clear();  // use a vector from outer scope to avoid resizes.
+        while (ISS1 >> N)
+          TmpFeatures.push_back(N);
+        std::sort(TmpFeatures.begin(), TmpFeatures.end());
+        Files[CurrentFileIdx].Features = TmpFeatures;
+      }
+    } else if (Marker == "COV") {
+      size_t CurrentFileIdx = N;
+      if (ParseCoverage)
+        while (ISS1 >> N)
+          if (PCs.insert(N).second)
+            Files[CurrentFileIdx].Cov.push_back(N);
+    } else {
+      return false;
+    }
+  }
+  if (LastSeenStartMarker != kInvalidStartMarker)
+    LastFailure = Files[LastSeenStartMarker].Name;
+
+  FirstNotProcessedFile = ExpectedStartMarker;
+  return true;
+}
+
+size_t Merger::ApproximateMemoryConsumption() const  {
+  size_t Res = 0;
+  for (const auto &F: Files)
+    Res += sizeof(F) + F.Features.size() * sizeof(F.Features[0]);
+  return Res;
+}
+
+// Decides which files need to be merged (add those to NewFiles).
+// Returns the number of new features added.
+size_t Merger::Merge(const Set<uint32_t> &InitialFeatures,
+                     Set<uint32_t> *NewFeatures,
+                     const Set<uint32_t> &InitialCov, Set<uint32_t> *NewCov,
+                     Vector<std::string> *NewFiles) {
+  NewFiles->clear();
+  assert(NumFilesInFirstCorpus <= Files.size());
+  Set<uint32_t> AllFeatures = InitialFeatures;
+
+  // What features are in the initial corpus?
+  for (size_t i = 0; i < NumFilesInFirstCorpus; i++) {
+    auto &Cur = Files[i].Features;
+    AllFeatures.insert(Cur.begin(), Cur.end());
+  }
+  // Remove all features that we already know from all other inputs.
+  for (size_t i = NumFilesInFirstCorpus; i < Files.size(); i++) {
+    auto &Cur = Files[i].Features;
+    Vector<uint32_t> Tmp;
+    std::set_difference(Cur.begin(), Cur.end(), AllFeatures.begin(),
+                        AllFeatures.end(), std::inserter(Tmp, Tmp.begin()));
+    Cur.swap(Tmp);
+  }
+
+  // Sort. Give preference to
+  //   * smaller files
+  //   * files with more features.
+  std::sort(Files.begin() + NumFilesInFirstCorpus, Files.end(),
+            [&](const MergeFileInfo &a, const MergeFileInfo &b) -> bool {
+              if (a.Size != b.Size)
+                return a.Size < b.Size;
+              return a.Features.size() > b.Features.size();
+            });
+
+  // One greedy pass: add the file's features to AllFeatures.
+  // If new features were added, add this file to NewFiles.
+  for (size_t i = NumFilesInFirstCorpus; i < Files.size(); i++) {
+    auto &Cur = Files[i].Features;
+    // Printf("%s -> sz %zd ft %zd\n", Files[i].Name.c_str(),
+    //       Files[i].Size, Cur.size());
+    bool FoundNewFeatures = false;
+    for (auto Fe: Cur) {
+      if (AllFeatures.insert(Fe).second) {
+        FoundNewFeatures = true;
+        NewFeatures->insert(Fe);
+      }
+    }
+    if (FoundNewFeatures)
+      NewFiles->push_back(Files[i].Name);
+    for (auto Cov : Files[i].Cov)
+      if (InitialCov.find(Cov) == InitialCov.end())
+        NewCov->insert(Cov);
+  }
+  return NewFeatures->size();
+}
+
+Set<uint32_t> Merger::AllFeatures() const {
+  Set<uint32_t> S;
+  for (auto &File : Files)
+    S.insert(File.Features.begin(), File.Features.end());
+  return S;
+}
+
+// Inner process. May crash if the target crashes.
+void Fuzzer::CrashResistantMergeInternalStep(const std::string &CFPath) {
+  Printf("MERGE-INNER: using the control file '%s'\n", CFPath.c_str());
+  Merger M;
+  std::ifstream IF(CFPath);
+  M.ParseOrExit(IF, false);
+  IF.close();
+  if (!M.LastFailure.empty())
+    Printf("MERGE-INNER: '%s' caused a failure at the previous merge step\n",
+           M.LastFailure.c_str());
+
+  Printf("MERGE-INNER: %zd total files;"
+         " %zd processed earlier; will process %zd files now\n",
+         M.Files.size(), M.FirstNotProcessedFile,
+         M.Files.size() - M.FirstNotProcessedFile);
+
+  std::ofstream OF(CFPath, std::ofstream::out | std::ofstream::app);
+  Set<size_t> AllFeatures;
+  auto PrintStatsWrapper = [this, &AllFeatures](const char* Where) {
+    this->PrintStats(Where, "\n", 0, AllFeatures.size());
+  };
+  Set<const TracePC::PCTableEntry *> AllPCs;
+  for (size_t i = M.FirstNotProcessedFile; i < M.Files.size(); i++) {
+    Fuzzer::MaybeExitGracefully();
+    auto U = FileToVector(M.Files[i].Name);
+    if (U.size() > MaxInputLen) {
+      U.resize(MaxInputLen);
+      U.shrink_to_fit();
+    }
+
+    // Write the pre-run marker.
+    OF << "STARTED " << i << " " << U.size() << "\n";
+    OF.flush();  // Flush is important since Command::Execute may crash.
+    // Run.
+    TPC.ResetMaps();
+    if (ExecuteCallback(U.data(), U.size()) > 0) {
+      continue;
+    }
+    // Collect coverage. We are iterating over the files in this order:
+    // * First, files in the initial corpus ordered by size, smallest first.
+    // * Then, all other files, smallest first.
+    // So it makes no sense to record all features for all files, instead we
+    // only record features that were not seen before.
+    Set<size_t> UniqFeatures;
+    TPC.CollectFeatures([&](size_t Feature) {
+      if (AllFeatures.insert(Feature).second)
+        UniqFeatures.insert(Feature);
+    });
+    TPC.UpdateObservedPCs();
+    // Show stats.
+    if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)))
+      PrintStatsWrapper("pulse ");
+    if (TotalNumberOfRuns == M.NumFilesInFirstCorpus)
+      PrintStatsWrapper("LOADED");
+    // Write the post-run marker and the coverage.
+    OF << "FT " << i;
+    for (size_t F : UniqFeatures)
+      OF << " " << F;
+    OF << "\n";
+    OF << "COV " << i;
+    TPC.ForEachObservedPC([&](const TracePC::PCTableEntry *TE) {
+      if (AllPCs.insert(TE).second)
+        OF << " " << TPC.PCTableEntryIdx(TE);
+    });
+    OF << "\n";
+    OF.flush();
+  }
+  PrintStatsWrapper("DONE  ");
+}
+
+static size_t WriteNewControlFile(const std::string &CFPath,
+                                  const Vector<SizedFile> &OldCorpus,
+                                  const Vector<SizedFile> &NewCorpus,
+                                  const Vector<MergeFileInfo> &KnownFiles) {
+  std::unordered_set<std::string> FilesToSkip;
+  for (auto &SF: KnownFiles)
+    FilesToSkip.insert(SF.Name);
+
+  Vector<std::string> FilesToUse;
+  auto MaybeUseFile = [=, &FilesToUse](std::string Name) {
+    if (FilesToSkip.find(Name) == FilesToSkip.end())
+      FilesToUse.push_back(Name);
+  };
+  for (auto &SF: OldCorpus)
+    MaybeUseFile(SF.File);
+  auto FilesToUseFromOldCorpus = FilesToUse.size();
+  for (auto &SF: NewCorpus)
+    MaybeUseFile(SF.File);
+
+  RemoveFile(CFPath);
+  std::ofstream ControlFile(CFPath);
+  ControlFile << FilesToUse.size() << "\n";
+  ControlFile << FilesToUseFromOldCorpus << "\n";
+  for (auto &FN: FilesToUse)
+    ControlFile << FN << "\n";
+
+  if (!ControlFile) {
+    Printf("MERGE-OUTER: failed to write to the control file: %s\n",
+           CFPath.c_str());
+    exit(1);
+  }
+
+  return FilesToUse.size();
+}
+
+// Outer process. Does not call the target code and thus should not fail.
+void CrashResistantMerge(const Vector<std::string> &Args,
+                         const Vector<SizedFile> &OldCorpus,
+                         const Vector<SizedFile> &NewCorpus,
+                         Vector<std::string> *NewFiles,
+                         const Set<uint32_t> &InitialFeatures,
+                         Set<uint32_t> *NewFeatures,
+                         const Set<uint32_t> &InitialCov,
+                         Set<uint32_t> *NewCov,
+                         const std::string &CFPath,
+                         bool V /*Verbose*/) {
+  if (NewCorpus.empty() && OldCorpus.empty()) return;  // Nothing to merge.
+  size_t NumAttempts = 0;
+  Vector<MergeFileInfo> KnownFiles;
+  if (FileSize(CFPath)) {
+    VPrintf(V, "MERGE-OUTER: non-empty control file provided: '%s'\n",
+           CFPath.c_str());
+    Merger M;
+    std::ifstream IF(CFPath);
+    if (M.Parse(IF, /*ParseCoverage=*/true)) {
+      VPrintf(V, "MERGE-OUTER: control file ok, %zd files total,"
+             " first not processed file %zd\n",
+             M.Files.size(), M.FirstNotProcessedFile);
+      if (!M.LastFailure.empty())
+        VPrintf(V, "MERGE-OUTER: '%s' will be skipped as unlucky "
+               "(merge has stumbled on it the last time)\n",
+               M.LastFailure.c_str());
+      if (M.FirstNotProcessedFile >= M.Files.size()) {
+        // Merge has already been completed with the given merge control file.
+        if (M.Files.size() == OldCorpus.size() + NewCorpus.size()) {
+          VPrintf(
+              V,
+              "MERGE-OUTER: nothing to do, merge has been completed before\n");
+          exit(0);
+        }
+
+        // Number of input files likely changed, start merge from scratch, but
+        // reuse coverage information from the given merge control file.
+        VPrintf(
+            V,
+            "MERGE-OUTER: starting merge from scratch, but reusing coverage "
+            "information from the given control file\n");
+        KnownFiles = M.Files;
+      } else {
+        // There is a merge in progress, continue.
+        NumAttempts = M.Files.size() - M.FirstNotProcessedFile;
+      }
+    } else {
+      VPrintf(V, "MERGE-OUTER: bad control file, will overwrite it\n");
+    }
+  }
+
+  if (!NumAttempts) {
+    // The supplied control file is empty or bad, create a fresh one.
+    VPrintf(V, "MERGE-OUTER: "
+            "%zd files, %zd in the initial corpus, %zd processed earlier\n",
+            OldCorpus.size() + NewCorpus.size(), OldCorpus.size(),
+            KnownFiles.size());
+    NumAttempts = WriteNewControlFile(CFPath, OldCorpus, NewCorpus, KnownFiles);
+  }
+
+  // Execute the inner process until it passes.
+  // Every inner process should execute at least one input.
+  Command BaseCmd(Args);
+  BaseCmd.removeFlag("merge");
+  BaseCmd.removeFlag("fork");
+  BaseCmd.removeFlag("collect_data_flow");
+  for (size_t Attempt = 1; Attempt <= NumAttempts; Attempt++) {
+    Fuzzer::MaybeExitGracefully();
+    VPrintf(V, "MERGE-OUTER: attempt %zd\n", Attempt);
+    Command Cmd(BaseCmd);
+    Cmd.addFlag("merge_control_file", CFPath);
+    Cmd.addFlag("merge_inner", "1");
+    if (!V) {
+      Cmd.setOutputFile(getDevNull());
+      Cmd.combineOutAndErr();
+    }
+    auto ExitCode = ExecuteCommand(Cmd);
+    if (!ExitCode) {
+      VPrintf(V, "MERGE-OUTER: succesfull in %zd attempt(s)\n", Attempt);
+      break;
+    }
+  }
+  // Read the control file and do the merge.
+  Merger M;
+  std::ifstream IF(CFPath);
+  IF.seekg(0, IF.end);
+  VPrintf(V, "MERGE-OUTER: the control file has %zd bytes\n",
+          (size_t)IF.tellg());
+  IF.seekg(0, IF.beg);
+  M.ParseOrExit(IF, true);
+  IF.close();
+  VPrintf(V,
+          "MERGE-OUTER: consumed %zdMb (%zdMb rss) to parse the control file\n",
+          M.ApproximateMemoryConsumption() >> 20, GetPeakRSSMb());
+
+  M.Files.insert(M.Files.end(), KnownFiles.begin(), KnownFiles.end());
+  M.Merge(InitialFeatures, NewFeatures, InitialCov, NewCov, NewFiles);
+  VPrintf(V, "MERGE-OUTER: %zd new files with %zd new features added; "
+          "%zd new coverage edges\n",
+         NewFiles->size(), NewFeatures->size(), NewCov->size());
+}
+
+} // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerMerge.h b/tools/fuzzing/libfuzzer/FuzzerMerge.h
new file mode 100644
index 0000000000..e0c6bc539b
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerMerge.h
@@ -0,0 +1,87 @@
+//===- FuzzerMerge.h - merging corpa ----------------------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Merging Corpora.
+//
+// The task:
+//   Take the existing corpus (possibly empty) and merge new inputs into
+//   it so that only inputs with new coverage ('features') are added.
+//   The process should tolerate the crashes, OOMs, leaks, etc.
+//
+// Algorithm:
+//   The outer process collects the set of files and writes their names
+//   into a temporary "control" file, then repeatedly launches the inner
+//   process until all inputs are processed.
+//   The outer process does not actually execute the target code.
+//
+//   The inner process reads the control file and sees a) list of all the inputs
+//   and b) the last processed input. Then it starts processing the inputs one
+//   by one. Before processing every input it writes one line to control file:
+//   STARTED INPUT_ID INPUT_SIZE
+//   After processing an input it writes the following lines:
+//   FT INPUT_ID Feature1 Feature2 Feature3 ...
+//   COV INPUT_ID Coverage1 Coverage2 Coverage3 ...
+//   If a crash happens while processing an input the last line in the control
+//   file will be "STARTED INPUT_ID" and so the next process will know
+//   where to resume.
+//
+//   Once all inputs are processed by the inner process(es) the outer process
+//   reads the control files and does the merge based entirely on the contents
+//   of control file.
+//   It uses a single pass greedy algorithm choosing first the smallest inputs
+//   within the same size the inputs that have more new features.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_MERGE_H
+#define LLVM_FUZZER_MERGE_H
+
+#include "FuzzerDefs.h"
+
+#include <istream>
+#include <ostream>
+#include <set>
+#include <vector>
+
+namespace fuzzer {
+
+struct MergeFileInfo {
+  std::string Name;
+  size_t Size = 0;
+  Vector<uint32_t> Features, Cov;
+};
+
+struct Merger {
+  Vector<MergeFileInfo> Files;
+  size_t NumFilesInFirstCorpus = 0;
+  size_t FirstNotProcessedFile = 0;
+  std::string LastFailure;
+
+  bool Parse(std::istream &IS, bool ParseCoverage);
+  bool Parse(const std::string &Str, bool ParseCoverage);
+  void ParseOrExit(std::istream &IS, bool ParseCoverage);
+  size_t Merge(const Set<uint32_t> &InitialFeatures, Set<uint32_t> *NewFeatures,
+               const Set<uint32_t> &InitialCov, Set<uint32_t> *NewCov,
+               Vector<std::string> *NewFiles);
+  size_t ApproximateMemoryConsumption() const;
+  Set<uint32_t> AllFeatures() const;
+};
+
+void CrashResistantMerge(const Vector<std::string> &Args,
+                         const Vector<SizedFile> &OldCorpus,
+                         const Vector<SizedFile> &NewCorpus,
+                         Vector<std::string> *NewFiles,
+                         const Set<uint32_t> &InitialFeatures,
+                         Set<uint32_t> *NewFeatures,
+                         const Set<uint32_t> &InitialCov,
+                         Set<uint32_t> *NewCov,
+                         const std::string &CFPath,
+                         bool Verbose);
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_MERGE_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerMutate.cpp b/tools/fuzzing/libfuzzer/FuzzerMutate.cpp
new file mode 100644
index 0000000000..29541eac5d
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerMutate.cpp
@@ -0,0 +1,562 @@
+//===- FuzzerMutate.cpp - Mutate a test input -----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Mutate a test input.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include "FuzzerMutate.h"
+#include "FuzzerOptions.h"
+#include "FuzzerTracePC.h"
+
+namespace fuzzer {
+
+const size_t Dictionary::kMaxDictSize;
+
+static void PrintASCII(const Word &W, const char *PrintAfter) {
+  PrintASCII(W.data(), W.size(), PrintAfter);
+}
+
+MutationDispatcher::MutationDispatcher(Random &Rand,
+                                       const FuzzingOptions &Options)
+    : Rand(Rand), Options(Options) {
+  DefaultMutators.insert(
+      DefaultMutators.begin(),
+      {
+          {&MutationDispatcher::Mutate_EraseBytes, "EraseBytes"},
+          {&MutationDispatcher::Mutate_InsertByte, "InsertByte"},
+          {&MutationDispatcher::Mutate_InsertRepeatedBytes,
+           "InsertRepeatedBytes"},
+          {&MutationDispatcher::Mutate_ChangeByte, "ChangeByte"},
+          {&MutationDispatcher::Mutate_ChangeBit, "ChangeBit"},
+          {&MutationDispatcher::Mutate_ShuffleBytes, "ShuffleBytes"},
+          {&MutationDispatcher::Mutate_ChangeASCIIInteger, "ChangeASCIIInt"},
+          {&MutationDispatcher::Mutate_ChangeBinaryInteger, "ChangeBinInt"},
+          {&MutationDispatcher::Mutate_CopyPart, "CopyPart"},
+          {&MutationDispatcher::Mutate_CrossOver, "CrossOver"},
+          {&MutationDispatcher::Mutate_AddWordFromManualDictionary,
+           "ManualDict"},
+          {&MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary,
+           "PersAutoDict"},
+      });
+  if(Options.UseCmp)
+    DefaultMutators.push_back(
+        {&MutationDispatcher::Mutate_AddWordFromTORC, "CMP"});
+
+  if (EF->LLVMFuzzerCustomMutator)
+    Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom"});
+  else
+    Mutators = DefaultMutators;
+
+  if (EF->LLVMFuzzerCustomCrossOver)
+    Mutators.push_back(
+        {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
+}
+
+static char RandCh(Random &Rand) {
+  if (Rand.RandBool()) return Rand(256);
+  const char Special[] = "!*'();:@&=+$,/?%#[]012Az-`~.\xff\x00";
+  return Special[Rand(sizeof(Special) - 1)];
+}
+
+size_t MutationDispatcher::Mutate_Custom(uint8_t *Data, size_t Size,
+                                         size_t MaxSize) {
+  return EF->LLVMFuzzerCustomMutator(Data, Size, MaxSize, Rand.Rand());
+}
+
+size_t MutationDispatcher::Mutate_CustomCrossOver(uint8_t *Data, size_t Size,
+                                                  size_t MaxSize) {
+  if (Size == 0)
+    return 0;
+  if (!CrossOverWith) return 0;
+  const Unit &Other = *CrossOverWith;
+  if (Other.empty())
+    return 0;
+  CustomCrossOverInPlaceHere.resize(MaxSize);
+  auto &U = CustomCrossOverInPlaceHere;
+  size_t NewSize = EF->LLVMFuzzerCustomCrossOver(
+      Data, Size, Other.data(), Other.size(), U.data(), U.size(), Rand.Rand());
+  if (!NewSize)
+    return 0;
+  assert(NewSize <= MaxSize && "CustomCrossOver returned overisized unit");
+  memcpy(Data, U.data(), NewSize);
+  return NewSize;
+}
+
+size_t MutationDispatcher::Mutate_ShuffleBytes(uint8_t *Data, size_t Size,
+                                               size_t MaxSize) {
+  if (Size > MaxSize || Size == 0) return 0;
+  size_t ShuffleAmount =
+      Rand(std::min(Size, (size_t)8)) + 1; // [1,8] and <= Size.
+  size_t ShuffleStart = Rand(Size - ShuffleAmount);
+  assert(ShuffleStart + ShuffleAmount <= Size);
+  std::shuffle(Data + ShuffleStart, Data + ShuffleStart + ShuffleAmount, Rand);
+  return Size;
+}
+
+size_t MutationDispatcher::Mutate_EraseBytes(uint8_t *Data, size_t Size,
+                                             size_t MaxSize) {
+  if (Size <= 1) return 0;
+  size_t N = Rand(Size / 2) + 1;
+  assert(N < Size);
+  size_t Idx = Rand(Size - N + 1);
+  // Erase Data[Idx:Idx+N].
+  memmove(Data + Idx, Data + Idx + N, Size - Idx - N);
+  // Printf("Erase: %zd %zd => %zd; Idx %zd\n", N, Size, Size - N, Idx);
+  return Size - N;
+}
+
+size_t MutationDispatcher::Mutate_InsertByte(uint8_t *Data, size_t Size,
+                                             size_t MaxSize) {
+  if (Size >= MaxSize) return 0;
+  size_t Idx = Rand(Size + 1);
+  // Insert new value at Data[Idx].
+  memmove(Data + Idx + 1, Data + Idx, Size - Idx);
+  Data[Idx] = RandCh(Rand);
+  return Size + 1;
+}
+
+size_t MutationDispatcher::Mutate_InsertRepeatedBytes(uint8_t *Data,
+                                                      size_t Size,
+                                                      size_t MaxSize) {
+  const size_t kMinBytesToInsert = 3;
+  if (Size + kMinBytesToInsert >= MaxSize) return 0;
+  size_t MaxBytesToInsert = std::min(MaxSize - Size, (size_t)128);
+  size_t N = Rand(MaxBytesToInsert - kMinBytesToInsert + 1) + kMinBytesToInsert;
+  assert(Size + N <= MaxSize && N);
+  size_t Idx = Rand(Size + 1);
+  // Insert new values at Data[Idx].
+  memmove(Data + Idx + N, Data + Idx, Size - Idx);
+  // Give preference to 0x00 and 0xff.
+  uint8_t Byte = Rand.RandBool() ? Rand(256) : (Rand.RandBool() ? 0 : 255);
+  for (size_t i = 0; i < N; i++)
+    Data[Idx + i] = Byte;
+  return Size + N;
+}
+
+size_t MutationDispatcher::Mutate_ChangeByte(uint8_t *Data, size_t Size,
+                                             size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  size_t Idx = Rand(Size);
+  Data[Idx] = RandCh(Rand);
+  return Size;
+}
+
+size_t MutationDispatcher::Mutate_ChangeBit(uint8_t *Data, size_t Size,
+                                            size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  size_t Idx = Rand(Size);
+  Data[Idx] ^= 1 << Rand(8);
+  return Size;
+}
+
+size_t MutationDispatcher::Mutate_AddWordFromManualDictionary(uint8_t *Data,
+                                                              size_t Size,
+                                                              size_t MaxSize) {
+  return AddWordFromDictionary(ManualDictionary, Data, Size, MaxSize);
+}
+
+size_t MutationDispatcher::ApplyDictionaryEntry(uint8_t *Data, size_t Size,
+                                                size_t MaxSize,
+                                                DictionaryEntry &DE) {
+  const Word &W = DE.GetW();
+  bool UsePositionHint = DE.HasPositionHint() &&
+                         DE.GetPositionHint() + W.size() < Size &&
+                         Rand.RandBool();
+  if (Rand.RandBool()) {  // Insert W.
+    if (Size + W.size() > MaxSize) return 0;
+    size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size + 1);
+    memmove(Data + Idx + W.size(), Data + Idx, Size - Idx);
+    memcpy(Data + Idx, W.data(), W.size());
+    Size += W.size();
+  } else {  // Overwrite some bytes with W.
+    if (W.size() > Size) return 0;
+    size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size - W.size());
+    memcpy(Data + Idx, W.data(), W.size());
+  }
+  return Size;
+}
+
+// Somewhere in the past we have observed a comparison instructions
+// with arguments Arg1 Arg2. This function tries to guess a dictionary
+// entry that will satisfy that comparison.
+// It first tries to find one of the arguments (possibly swapped) in the
+// input and if it succeeds it creates a DE with a position hint.
+// Otherwise it creates a DE with one of the arguments w/o a position hint.
+DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
+    const void *Arg1, const void *Arg2,
+    const void *Arg1Mutation, const void *Arg2Mutation,
+    size_t ArgSize, const uint8_t *Data,
+    size_t Size) {
+  bool HandleFirst = Rand.RandBool();
+  const void *ExistingBytes, *DesiredBytes;
+  Word W;
+  const uint8_t *End = Data + Size;
+  for (int Arg = 0; Arg < 2; Arg++) {
+    ExistingBytes = HandleFirst ? Arg1 : Arg2;
+    DesiredBytes = HandleFirst ? Arg2Mutation : Arg1Mutation;
+    HandleFirst = !HandleFirst;
+    W.Set(reinterpret_cast<const uint8_t*>(DesiredBytes), ArgSize);
+    const size_t kMaxNumPositions = 8;
+    size_t Positions[kMaxNumPositions];
+    size_t NumPositions = 0;
+    for (const uint8_t *Cur = Data;
+         Cur < End && NumPositions < kMaxNumPositions; Cur++) {
+      Cur =
+          (const uint8_t *)SearchMemory(Cur, End - Cur, ExistingBytes, ArgSize);
+      if (!Cur) break;
+      Positions[NumPositions++] = Cur - Data;
+    }
+    if (!NumPositions) continue;
+    return DictionaryEntry(W, Positions[Rand(NumPositions)]);
+  }
+  DictionaryEntry DE(W);
+  return DE;
+}
+
+
+template <class T>
+DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
+    T Arg1, T Arg2, const uint8_t *Data, size_t Size) {
+  if (Rand.RandBool()) Arg1 = Bswap(Arg1);
+  if (Rand.RandBool()) Arg2 = Bswap(Arg2);
+  T Arg1Mutation = Arg1 + Rand(-1, 1);
+  T Arg2Mutation = Arg2 + Rand(-1, 1);
+  return MakeDictionaryEntryFromCMP(&Arg1, &Arg2, &Arg1Mutation, &Arg2Mutation,
+                                    sizeof(Arg1), Data, Size);
+}
+
+DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
+    const Word &Arg1, const Word &Arg2, const uint8_t *Data, size_t Size) {
+  return MakeDictionaryEntryFromCMP(Arg1.data(), Arg2.data(), Arg1.data(),
+                                    Arg2.data(), Arg1.size(), Data, Size);
+}
+
+size_t MutationDispatcher::Mutate_AddWordFromTORC(
+    uint8_t *Data, size_t Size, size_t MaxSize) {
+  Word W;
+  DictionaryEntry DE;
+  switch (Rand(4)) {
+  case 0: {
+    auto X = TPC.TORC8.Get(Rand.Rand());
+    DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
+  } break;
+  case 1: {
+    auto X = TPC.TORC4.Get(Rand.Rand());
+    if ((X.A >> 16) == 0 && (X.B >> 16) == 0 && Rand.RandBool())
+      DE = MakeDictionaryEntryFromCMP((uint16_t)X.A, (uint16_t)X.B, Data, Size);
+    else
+      DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
+  } break;
+  case 2: {
+    auto X = TPC.TORCW.Get(Rand.Rand());
+    DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
+  } break;
+  case 3: if (Options.UseMemmem) {
+    auto X = TPC.MMT.Get(Rand.Rand());
+    DE = DictionaryEntry(X);
+  } break;
+  default:
+    assert(0);
+  }
+  if (!DE.GetW().size()) return 0;
+  Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
+  if (!Size) return 0;
+  DictionaryEntry &DERef =
+      CmpDictionaryEntriesDeque[CmpDictionaryEntriesDequeIdx++ %
+                                kCmpDictionaryEntriesDequeSize];
+  DERef = DE;
+  CurrentDictionaryEntrySequence.push_back(&DERef);
+  return Size;
+}
+
+size_t MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary(
+    uint8_t *Data, size_t Size, size_t MaxSize) {
+  return AddWordFromDictionary(PersistentAutoDictionary, Data, Size, MaxSize);
+}
+
+size_t MutationDispatcher::AddWordFromDictionary(Dictionary &D, uint8_t *Data,
+                                                 size_t Size, size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  if (D.empty()) return 0;
+  DictionaryEntry &DE = D[Rand(D.size())];
+  Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
+  if (!Size) return 0;
+  DE.IncUseCount();
+  CurrentDictionaryEntrySequence.push_back(&DE);
+  return Size;
+}
+
+// Overwrites part of To[0,ToSize) with a part of From[0,FromSize).
+// Returns ToSize.
+size_t MutationDispatcher::CopyPartOf(const uint8_t *From, size_t FromSize,
+                                      uint8_t *To, size_t ToSize) {
+  // Copy From[FromBeg, FromBeg + CopySize) into To[ToBeg, ToBeg + CopySize).
+  size_t ToBeg = Rand(ToSize);
+  size_t CopySize = Rand(ToSize - ToBeg) + 1;
+  assert(ToBeg + CopySize <= ToSize);
+  CopySize = std::min(CopySize, FromSize);
+  size_t FromBeg = Rand(FromSize - CopySize + 1);
+  assert(FromBeg + CopySize <= FromSize);
+  memmove(To + ToBeg, From + FromBeg, CopySize);
+  return ToSize;
+}
+
+// Inserts part of From[0,ToSize) into To.
+// Returns new size of To on success or 0 on failure.
+size_t MutationDispatcher::InsertPartOf(const uint8_t *From, size_t FromSize,
+                                        uint8_t *To, size_t ToSize,
+                                        size_t MaxToSize) {
+  if (ToSize >= MaxToSize) return 0;
+  size_t AvailableSpace = MaxToSize - ToSize;
+  size_t MaxCopySize = std::min(AvailableSpace, FromSize);
+  size_t CopySize = Rand(MaxCopySize) + 1;
+  size_t FromBeg = Rand(FromSize - CopySize + 1);
+  assert(FromBeg + CopySize <= FromSize);
+  size_t ToInsertPos = Rand(ToSize + 1);
+  assert(ToInsertPos + CopySize <= MaxToSize);
+  size_t TailSize = ToSize - ToInsertPos;
+  if (To == From) {
+    MutateInPlaceHere.resize(MaxToSize);
+    memcpy(MutateInPlaceHere.data(), From + FromBeg, CopySize);
+    memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
+    memmove(To + ToInsertPos, MutateInPlaceHere.data(), CopySize);
+  } else {
+    memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
+    memmove(To + ToInsertPos, From + FromBeg, CopySize);
+  }
+  return ToSize + CopySize;
+}
+
+size_t MutationDispatcher::Mutate_CopyPart(uint8_t *Data, size_t Size,
+                                           size_t MaxSize) {
+  if (Size > MaxSize || Size == 0) return 0;
+  // If Size == MaxSize, `InsertPartOf(...)` will
+  // fail so there's no point using it in this case.
+  if (Size == MaxSize || Rand.RandBool())
+    return CopyPartOf(Data, Size, Data, Size);
+  else
+    return InsertPartOf(Data, Size, Data, Size, MaxSize);
+}
+
+size_t MutationDispatcher::Mutate_ChangeASCIIInteger(uint8_t *Data, size_t Size,
+                                                     size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  size_t B = Rand(Size);
+  while (B < Size && !isdigit(Data[B])) B++;
+  if (B == Size) return 0;
+  size_t E = B;
+  while (E < Size && isdigit(Data[E])) E++;
+  assert(B < E);
+  // now we have digits in [B, E).
+  // strtol and friends don't accept non-zero-teminated data, parse it manually.
+  uint64_t Val = Data[B] - '0';
+  for (size_t i = B + 1; i < E; i++)
+    Val = Val * 10 + Data[i] - '0';
+
+  // Mutate the integer value.
+  switch(Rand(5)) {
+    case 0: Val++; break;
+    case 1: Val--; break;
+    case 2: Val /= 2; break;
+    case 3: Val *= 2; break;
+    case 4: Val = Rand(Val * Val); break;
+    default: assert(0);
+  }
+  // Just replace the bytes with the new ones, don't bother moving bytes.
+  for (size_t i = B; i < E; i++) {
+    size_t Idx = E + B - i - 1;
+    assert(Idx >= B && Idx < E);
+    Data[Idx] = (Val % 10) + '0';
+    Val /= 10;
+  }
+  return Size;
+}
+
+template<class T>
+size_t ChangeBinaryInteger(uint8_t *Data, size_t Size, Random &Rand) {
+  if (Size < sizeof(T)) return 0;
+  size_t Off = Rand(Size - sizeof(T) + 1);
+  assert(Off + sizeof(T) <= Size);
+  T Val;
+  if (Off < 64 && !Rand(4)) {
+    Val = Size;
+    if (Rand.RandBool())
+      Val = Bswap(Val);
+  } else {
+    memcpy(&Val, Data + Off, sizeof(Val));
+    T Add = Rand(21);
+    Add -= 10;
+    if (Rand.RandBool())
+      Val = Bswap(T(Bswap(Val) + Add)); // Add assuming different endiannes.
+    else
+      Val = Val + Add;               // Add assuming current endiannes.
+    if (Add == 0 || Rand.RandBool()) // Maybe negate.
+      Val = -Val;
+  }
+  memcpy(Data + Off, &Val, sizeof(Val));
+  return Size;
+}
+
+size_t MutationDispatcher::Mutate_ChangeBinaryInteger(uint8_t *Data,
+                                                      size_t Size,
+                                                      size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  switch (Rand(4)) {
+    case 3: return ChangeBinaryInteger<uint64_t>(Data, Size, Rand);
+    case 2: return ChangeBinaryInteger<uint32_t>(Data, Size, Rand);
+    case 1: return ChangeBinaryInteger<uint16_t>(Data, Size, Rand);
+    case 0: return ChangeBinaryInteger<uint8_t>(Data, Size, Rand);
+    default: assert(0);
+  }
+  return 0;
+}
+
+size_t MutationDispatcher::Mutate_CrossOver(uint8_t *Data, size_t Size,
+                                            size_t MaxSize) {
+  if (Size > MaxSize) return 0;
+  if (Size == 0) return 0;
+  if (!CrossOverWith) return 0;
+  const Unit &O = *CrossOverWith;
+  if (O.empty()) return 0;
+  MutateInPlaceHere.resize(MaxSize);
+  auto &U = MutateInPlaceHere;
+  size_t NewSize = 0;
+  switch(Rand(3)) {
+    case 0:
+      NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
+      break;
+    case 1:
+      NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
+      if (!NewSize)
+        NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+      break;
+    case 2:
+      NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
+      break;
+    default: assert(0);
+  }
+  assert(NewSize > 0 && "CrossOver returned empty unit");
+  assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
+  memcpy(Data, U.data(), NewSize);
+  return NewSize;
+}
+
+void MutationDispatcher::StartMutationSequence() {
+  CurrentMutatorSequence.clear();
+  CurrentDictionaryEntrySequence.clear();
+}
+
+// Copy successful dictionary entries to PersistentAutoDictionary.
+void MutationDispatcher::RecordSuccessfulMutationSequence() {
+  for (auto DE : CurrentDictionaryEntrySequence) {
+    // PersistentAutoDictionary.AddWithSuccessCountOne(DE);
+    DE->IncSuccessCount();
+    assert(DE->GetW().size());
+    // Linear search is fine here as this happens seldom.
+    if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
+      PersistentAutoDictionary.push_back({DE->GetW(), 1});
+  }
+}
+
+void MutationDispatcher::PrintRecommendedDictionary() {
+  Vector<DictionaryEntry> V;
+  for (auto &DE : PersistentAutoDictionary)
+    if (!ManualDictionary.ContainsWord(DE.GetW()))
+      V.push_back(DE);
+  if (V.empty()) return;
+  Printf("###### Recommended dictionary. ######\n");
+  for (auto &DE: V) {
+    assert(DE.GetW().size());
+    Printf("\"");
+    PrintASCII(DE.GetW(), "\"");
+    Printf(" # Uses: %zd\n", DE.GetUseCount());
+  }
+  Printf("###### End of recommended dictionary. ######\n");
+}
+
+void MutationDispatcher::PrintMutationSequence() {
+  Printf("MS: %zd ", CurrentMutatorSequence.size());
+  for (auto M : CurrentMutatorSequence)
+    Printf("%s-", M.Name);
+  if (!CurrentDictionaryEntrySequence.empty()) {
+    Printf(" DE: ");
+    for (auto DE : CurrentDictionaryEntrySequence) {
+      Printf("\"");
+      PrintASCII(DE->GetW(), "\"-");
+    }
+  }
+}
+
+size_t MutationDispatcher::Mutate(uint8_t *Data, size_t Size, size_t MaxSize) {
+  return MutateImpl(Data, Size, MaxSize, Mutators);
+}
+
+size_t MutationDispatcher::DefaultMutate(uint8_t *Data, size_t Size,
+                                         size_t MaxSize) {
+  return MutateImpl(Data, Size, MaxSize, DefaultMutators);
+}
+
+// Mutates Data in place, returns new size.
+size_t MutationDispatcher::MutateImpl(uint8_t *Data, size_t Size,
+                                      size_t MaxSize,
+                                      Vector<Mutator> &Mutators) {
+  assert(MaxSize > 0);
+  // Some mutations may fail (e.g. can't insert more bytes if Size == MaxSize),
+  // in which case they will return 0.
+  // Try several times before returning un-mutated data.
+  for (int Iter = 0; Iter < 100; Iter++) {
+    auto M = Mutators[Rand(Mutators.size())];
+    size_t NewSize = (this->*(M.Fn))(Data, Size, MaxSize);
+    if (NewSize && NewSize <= MaxSize) {
+      if (Options.OnlyASCII)
+        ToASCII(Data, NewSize);
+      CurrentMutatorSequence.push_back(M);
+      return NewSize;
+    }
+  }
+  *Data = ' ';
+  return 1;   // Fallback, should not happen frequently.
+}
+
+// Mask represents the set of Data bytes that are worth mutating.
+size_t MutationDispatcher::MutateWithMask(uint8_t *Data, size_t Size,
+                                          size_t MaxSize,
+                                          const Vector<uint8_t> &Mask) {
+  size_t MaskedSize = std::min(Size, Mask.size());
+  // * Copy the worthy bytes into a temporary array T
+  // * Mutate T
+  // * Copy T back.
+  // This is totally unoptimized.
+  auto &T = MutateWithMaskTemp;
+  if (T.size() < Size)
+    T.resize(Size);
+  size_t OneBits = 0;
+  for (size_t I = 0; I < MaskedSize; I++)
+    if (Mask[I])
+      T[OneBits++] = Data[I];
+
+  if (!OneBits) return 0;
+  assert(!T.empty());
+  size_t NewSize = Mutate(T.data(), OneBits, OneBits);
+  assert(NewSize <= OneBits);
+  (void)NewSize;
+  // Even if NewSize < OneBits we still use all OneBits bytes.
+  for (size_t I = 0, J = 0; I < MaskedSize; I++)
+    if (Mask[I])
+      Data[I] = T[J++];
+  return Size;
+}
+
+void MutationDispatcher::AddWordToManualDictionary(const Word &W) {
+  ManualDictionary.push_back(
+      {W, std::numeric_limits<size_t>::max()});
+}
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerMutate.h b/tools/fuzzing/libfuzzer/FuzzerMutate.h
new file mode 100644
index 0000000000..6cbce80276
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerMutate.h
@@ -0,0 +1,156 @@
+//===- FuzzerMutate.h - Internal header for the Fuzzer ----------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::MutationDispatcher
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_MUTATE_H
+#define LLVM_FUZZER_MUTATE_H
+
+#include "FuzzerDefs.h"
+#include "FuzzerDictionary.h"
+#include "FuzzerOptions.h"
+#include "FuzzerRandom.h"
+
+namespace fuzzer {
+
+class MutationDispatcher {
+public:
+  MutationDispatcher(Random &Rand, const FuzzingOptions &Options);
+  ~MutationDispatcher() {}
+  /// Indicate that we are about to start a new sequence of mutations.
+  void StartMutationSequence();
+  /// Print the current sequence of mutations.
+  void PrintMutationSequence();
+  /// Indicate that the current sequence of mutations was successful.
+  void RecordSuccessfulMutationSequence();
+  /// Mutates data by invoking user-provided mutator.
+  size_t Mutate_Custom(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by invoking user-provided crossover.
+  size_t Mutate_CustomCrossOver(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by shuffling bytes.
+  size_t Mutate_ShuffleBytes(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by erasing bytes.
+  size_t Mutate_EraseBytes(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by inserting a byte.
+  size_t Mutate_InsertByte(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by inserting several repeated bytes.
+  size_t Mutate_InsertRepeatedBytes(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by chanding one byte.
+  size_t Mutate_ChangeByte(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by chanding one bit.
+  size_t Mutate_ChangeBit(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Mutates data by copying/inserting a part of data into a different place.
+  size_t Mutate_CopyPart(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// Mutates data by adding a word from the manual dictionary.
+  size_t Mutate_AddWordFromManualDictionary(uint8_t *Data, size_t Size,
+                                            size_t MaxSize);
+
+  /// Mutates data by adding a word from the TORC.
+  size_t Mutate_AddWordFromTORC(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// Mutates data by adding a word from the persistent automatic dictionary.
+  size_t Mutate_AddWordFromPersistentAutoDictionary(uint8_t *Data, size_t Size,
+                                                    size_t MaxSize);
+
+  /// Tries to find an ASCII integer in Data, changes it to another ASCII int.
+  size_t Mutate_ChangeASCIIInteger(uint8_t *Data, size_t Size, size_t MaxSize);
+  /// Change a 1-, 2-, 4-, or 8-byte integer in interesting ways.
+  size_t Mutate_ChangeBinaryInteger(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// CrossOver Data with CrossOverWith.
+  size_t Mutate_CrossOver(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// Applies one of the configured mutations.
+  /// Returns the new size of data which could be up to MaxSize.
+  size_t Mutate(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// Applies one of the configured mutations to the bytes of Data
+  /// that have '1' in Mask.
+  /// Mask.size() should be >= Size.
+  size_t MutateWithMask(uint8_t *Data, size_t Size, size_t MaxSize,
+                        const Vector<uint8_t> &Mask);
+
+  /// Applies one of the default mutations. Provided as a service
+  /// to mutation authors.
+  size_t DefaultMutate(uint8_t *Data, size_t Size, size_t MaxSize);
+
+  /// Creates a cross-over of two pieces of Data, returns its size.
+  size_t CrossOver(const uint8_t *Data1, size_t Size1, const uint8_t *Data2,
+                   size_t Size2, uint8_t *Out, size_t MaxOutSize);
+
+  void AddWordToManualDictionary(const Word &W);
+
+  void PrintRecommendedDictionary();
+
+  void SetCrossOverWith(const Unit *U) { CrossOverWith = U; }
+
+  Random &GetRand() { return Rand; }
+
+ private:
+  struct Mutator {
+    size_t (MutationDispatcher::*Fn)(uint8_t *Data, size_t Size, size_t Max);
+    const char *Name;
+  };
+
+  size_t AddWordFromDictionary(Dictionary &D, uint8_t *Data, size_t Size,
+                               size_t MaxSize);
+  size_t MutateImpl(uint8_t *Data, size_t Size, size_t MaxSize,
+                    Vector<Mutator> &Mutators);
+
+  size_t InsertPartOf(const uint8_t *From, size_t FromSize, uint8_t *To,
+                      size_t ToSize, size_t MaxToSize);
+  size_t CopyPartOf(const uint8_t *From, size_t FromSize, uint8_t *To,
+                    size_t ToSize);
+  size_t ApplyDictionaryEntry(uint8_t *Data, size_t Size, size_t MaxSize,
+                              DictionaryEntry &DE);
+
+  template <class T>
+  DictionaryEntry MakeDictionaryEntryFromCMP(T Arg1, T Arg2,
+                                             const uint8_t *Data, size_t Size);
+  DictionaryEntry MakeDictionaryEntryFromCMP(const Word &Arg1, const Word &Arg2,
+                                             const uint8_t *Data, size_t Size);
+  DictionaryEntry MakeDictionaryEntryFromCMP(const void *Arg1, const void *Arg2,
+                                             const void *Arg1Mutation,
+                                             const void *Arg2Mutation,
+                                             size_t ArgSize,
+                                             const uint8_t *Data, size_t Size);
+
+  Random &Rand;
+  const FuzzingOptions Options;
+
+  // Dictionary provided by the user via -dict=DICT_FILE.
+  Dictionary ManualDictionary;
+  // Temporary dictionary modified by the fuzzer itself,
+  // recreated periodically.
+  Dictionary TempAutoDictionary;
+  // Persistent dictionary modified by the fuzzer, consists of
+  // entries that led to successful discoveries in the past mutations.
+  Dictionary PersistentAutoDictionary;
+
+  Vector<DictionaryEntry *> CurrentDictionaryEntrySequence;
+
+  static const size_t kCmpDictionaryEntriesDequeSize = 16;
+  DictionaryEntry CmpDictionaryEntriesDeque[kCmpDictionaryEntriesDequeSize];
+  size_t CmpDictionaryEntriesDequeIdx = 0;
+
+  const Unit *CrossOverWith = nullptr;
+  Vector<uint8_t> MutateInPlaceHere;
+  Vector<uint8_t> MutateWithMaskTemp;
+  // CustomCrossOver needs its own buffer as a custom implementation may call
+  // LLVMFuzzerMutate, which in turn may resize MutateInPlaceHere.
+  Vector<uint8_t> CustomCrossOverInPlaceHere;
+
+  Vector<Mutator> Mutators;
+  Vector<Mutator> DefaultMutators;
+  Vector<Mutator> CurrentMutatorSequence;
+};
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_MUTATE_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerOptions.h b/tools/fuzzing/libfuzzer/FuzzerOptions.h
new file mode 100644
index 0000000000..9d975bd61f
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerOptions.h
@@ -0,0 +1,85 @@
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::FuzzingOptions
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_OPTIONS_H
+#define LLVM_FUZZER_OPTIONS_H
+
+#include "FuzzerDefs.h"
+
+namespace fuzzer {
+
+struct FuzzingOptions {
+  int Verbosity = 1;
+  size_t MaxLen = 0;
+  size_t LenControl = 1000;
+  int UnitTimeoutSec = 300;
+  int TimeoutExitCode = 70;
+  int OOMExitCode = 71;
+  int InterruptExitCode = 72;
+  int ErrorExitCode = 77;
+  bool IgnoreTimeouts = true;
+  bool IgnoreOOMs = true;
+  bool IgnoreCrashes = false;
+  int MaxTotalTimeSec = 0;
+  int RssLimitMb = 0;
+  int MallocLimitMb = 0;
+  bool DoCrossOver = true;
+  int MutateDepth = 5;
+  bool ReduceDepth = false;
+  bool UseCounters = false;
+  bool UseMemmem = true;
+  bool UseCmp = false;
+  int UseValueProfile = false;
+  bool Shrink = false;
+  bool ReduceInputs = false;
+  int ReloadIntervalSec = 1;
+  bool ShuffleAtStartUp = true;
+  bool PreferSmall = true;
+  size_t MaxNumberOfRuns = -1L;
+  int ReportSlowUnits = 10;
+  bool OnlyASCII = false;
+  bool Entropic = false;
+  size_t EntropicFeatureFrequencyThreshold = 0xFF;
+  size_t EntropicNumberOfRarestFeatures = 100;
+  std::string OutputCorpus;
+  std::string ArtifactPrefix = "./";
+  std::string ExactArtifactPath;
+  std::string ExitOnSrcPos;
+  std::string ExitOnItem;
+  std::string FocusFunction;
+  std::string DataFlowTrace;
+  std::string CollectDataFlow;
+  std::string FeaturesDir;
+  std::string StopFile;
+  bool SaveArtifacts = true;
+  bool PrintNEW = true; // Print a status line when new units are found;
+  bool PrintNewCovPcs = false;
+  int PrintNewCovFuncs = 0;
+  bool PrintFinalStats = false;
+  bool PrintCorpusStats = false;
+  bool PrintCoverage = false;
+  bool DumpCoverage = false;
+  bool DetectLeaks = true;
+  int PurgeAllocatorIntervalSec = 1;
+  int  TraceMalloc = 0;
+  bool HandleAbrt = false;
+  bool HandleBus = false;
+  bool HandleFpe = false;
+  bool HandleIll = false;
+  bool HandleInt = false;
+  bool HandleSegv = false;
+  bool HandleTerm = false;
+  bool HandleXfsz = false;
+  bool HandleUsr1 = false;
+  bool HandleUsr2 = false;
+};
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_OPTIONS_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerPlatform.h b/tools/fuzzing/libfuzzer/FuzzerPlatform.h
new file mode 100644
index 0000000000..8befdb882c
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerPlatform.h
@@ -0,0 +1,163 @@
+//===-- FuzzerPlatform.h --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Common platform macros.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_PLATFORM_H
+#define LLVM_FUZZER_PLATFORM_H
+
+// Platform detection.
+#ifdef __linux__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 1
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __APPLE__
+#define LIBFUZZER_APPLE 1
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __NetBSD__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 1
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __FreeBSD__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 1
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __OpenBSD__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 1
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif _WIN32
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 1
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __Fuchsia__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 1
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __EMSCRIPTEN__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 1
+#else
+#error "Support for your platform has not been implemented"
+#endif
+
+#if defined(_MSC_VER) && !defined(__clang__)
+// MSVC compiler is being used.
+#define LIBFUZZER_MSVC 1
+#else
+#define LIBFUZZER_MSVC 0
+#endif
+
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+#define LIBFUZZER_POSIX                                                        \
+  (LIBFUZZER_APPLE || LIBFUZZER_LINUX || LIBFUZZER_NETBSD ||                   \
+   LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN)
+
+#ifdef __x86_64
+#if __has_attribute(target)
+#define ATTRIBUTE_TARGET_POPCNT __attribute__((target("popcnt")))
+#else
+#define ATTRIBUTE_TARGET_POPCNT
+#endif
+#else
+#define ATTRIBUTE_TARGET_POPCNT
+#endif
+
+#ifdef __clang__ // avoid gcc warning.
+#if __has_attribute(no_sanitize)
+#define ATTRIBUTE_NO_SANITIZE_MEMORY __attribute__((no_sanitize("memory")))
+#else
+#define ATTRIBUTE_NO_SANITIZE_MEMORY
+#endif
+#define ALWAYS_INLINE __attribute__((always_inline))
+#else
+#define ATTRIBUTE_NO_SANITIZE_MEMORY
+#define ALWAYS_INLINE
+#endif // __clang__
+
+#if LIBFUZZER_WINDOWS
+#define ATTRIBUTE_NO_SANITIZE_ADDRESS
+#else
+#define ATTRIBUTE_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
+#endif
+
+#if LIBFUZZER_WINDOWS
+#define ATTRIBUTE_ALIGNED(X) __declspec(align(X))
+#define ATTRIBUTE_INTERFACE __declspec(dllexport)
+// This is used for __sancov_lowest_stack which is needed for
+// -fsanitize-coverage=stack-depth. That feature is not yet available on
+// Windows, so make the symbol static to avoid linking errors.
+#define ATTRIBUTES_INTERFACE_TLS_INITIAL_EXEC static
+#define ATTRIBUTE_NOINLINE __declspec(noinline)
+#else
+#define ATTRIBUTE_ALIGNED(X) __attribute__((aligned(X)))
+#define ATTRIBUTE_INTERFACE __attribute__((visibility("default")))
+#define ATTRIBUTES_INTERFACE_TLS_INITIAL_EXEC                                  \
+  ATTRIBUTE_INTERFACE __attribute__((tls_model("initial-exec"))) thread_local
+
+#define ATTRIBUTE_NOINLINE __attribute__((noinline))
+#endif
+
+#if defined(__has_feature)
+#if __has_feature(address_sanitizer)
+#define ATTRIBUTE_NO_SANITIZE_ALL ATTRIBUTE_NO_SANITIZE_ADDRESS
+#elif __has_feature(memory_sanitizer)
+#define ATTRIBUTE_NO_SANITIZE_ALL ATTRIBUTE_NO_SANITIZE_MEMORY
+#else
+#define ATTRIBUTE_NO_SANITIZE_ALL
+#endif
+#else
+#define ATTRIBUTE_NO_SANITIZE_ALL
+#endif
+
+#endif // LLVM_FUZZER_PLATFORM_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerRandom.h b/tools/fuzzing/libfuzzer/FuzzerRandom.h
new file mode 100644
index 0000000000..659283eee2
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerRandom.h
@@ -0,0 +1,38 @@
+//===- FuzzerRandom.h - Internal header for the Fuzzer ----------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::Random
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_RANDOM_H
+#define LLVM_FUZZER_RANDOM_H
+
+#include <random>
+
+namespace fuzzer {
+class Random : public std::minstd_rand {
+ public:
+  Random(unsigned int seed) : std::minstd_rand(seed) {}
+  result_type operator()() { return this->std::minstd_rand::operator()(); }
+  size_t Rand() { return this->operator()(); }
+  size_t RandBool() { return Rand() % 2; }
+  size_t SkewTowardsLast(size_t n) {
+    size_t T = this->operator()(n * n);
+    size_t Res = sqrt(T);
+    return Res;
+  }
+  size_t operator()(size_t n) { return n ? Rand() % n : 0; }
+  intptr_t operator()(intptr_t From, intptr_t To) {
+    assert(From < To);
+    intptr_t RangeSize = To - From + 1;
+    return operator()(RangeSize) + From;
+  }
+};
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_RANDOM_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerSHA1.cpp b/tools/fuzzing/libfuzzer/FuzzerSHA1.cpp
new file mode 100644
index 0000000000..2005dc7003
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerSHA1.cpp
@@ -0,0 +1,223 @@
+//===- FuzzerSHA1.h - Private copy of the SHA1 implementation ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// This code is taken from public domain
+// (http://oauth.googlecode.com/svn/code/c/liboauth/src/sha1.c)
+// and modified by adding anonymous namespace, adding an interface
+// function fuzzer::ComputeSHA1() and removing unnecessary code.
+//
+// lib/Fuzzer can not use SHA1 implementation from openssl because
+// openssl may not be available and because we may be fuzzing openssl itself.
+// For the same reason we do not want to depend on SHA1 from LLVM tree.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerSHA1.h"
+#include "FuzzerDefs.h"
+#include "FuzzerPlatform.h"
+
+/* This code is public-domain - it is based on libcrypt
+ * placed in the public domain by Wei Dai and other contributors.
+ */
+
+#include <iomanip>
+#include <sstream>
+#include <stdint.h>
+#include <string.h>
+
+namespace {  // Added for LibFuzzer
+
+#ifdef __BIG_ENDIAN__
+# define SHA_BIG_ENDIAN
+// Windows is always little endian and MSVC doesn't have <endian.h>
+#elif defined __LITTLE_ENDIAN__ || LIBFUZZER_WINDOWS
+/* override */
+#elif defined __BYTE_ORDER
+# if __BYTE_ORDER__ ==  __ORDER_BIG_ENDIAN__
+# define SHA_BIG_ENDIAN
+# endif
+#else // ! defined __LITTLE_ENDIAN__
+# include <endian.h> // machine/endian.h
+# if __BYTE_ORDER__ ==  __ORDER_BIG_ENDIAN__
+#  define SHA_BIG_ENDIAN
+# endif
+#endif
+
+
+/* header */
+
+#define HASH_LENGTH 20
+#define BLOCK_LENGTH 64
+
+typedef struct sha1nfo {
+	uint32_t buffer[BLOCK_LENGTH/4];
+	uint32_t state[HASH_LENGTH/4];
+	uint32_t byteCount;
+	uint8_t bufferOffset;
+	uint8_t keyBuffer[BLOCK_LENGTH];
+	uint8_t innerHash[HASH_LENGTH];
+} sha1nfo;
+
+/* public API - prototypes - TODO: doxygen*/
+
+/**
+ */
+void sha1_init(sha1nfo *s);
+/**
+ */
+void sha1_writebyte(sha1nfo *s, uint8_t data);
+/**
+ */
+void sha1_write(sha1nfo *s, const char *data, size_t len);
+/**
+ */
+uint8_t* sha1_result(sha1nfo *s);
+
+
+/* code */
+#define SHA1_K0  0x5a827999
+#define SHA1_K20 0x6ed9eba1
+#define SHA1_K40 0x8f1bbcdc
+#define SHA1_K60 0xca62c1d6
+
+void sha1_init(sha1nfo *s) {
+	s->state[0] = 0x67452301;
+	s->state[1] = 0xefcdab89;
+	s->state[2] = 0x98badcfe;
+	s->state[3] = 0x10325476;
+	s->state[4] = 0xc3d2e1f0;
+	s->byteCount = 0;
+	s->bufferOffset = 0;
+}
+
+uint32_t sha1_rol32(uint32_t number, uint8_t bits) {
+	return ((number << bits) | (number >> (32-bits)));
+}
+
+void sha1_hashBlock(sha1nfo *s) {
+	uint8_t i;
+	uint32_t a,b,c,d,e,t;
+
+	a=s->state[0];
+	b=s->state[1];
+	c=s->state[2];
+	d=s->state[3];
+	e=s->state[4];
+	for (i=0; i<80; i++) {
+		if (i>=16) {
+			t = s->buffer[(i+13)&15] ^ s->buffer[(i+8)&15] ^ s->buffer[(i+2)&15] ^ s->buffer[i&15];
+			s->buffer[i&15] = sha1_rol32(t,1);
+		}
+		if (i<20) {
+			t = (d ^ (b & (c ^ d))) + SHA1_K0;
+		} else if (i<40) {
+			t = (b ^ c ^ d) + SHA1_K20;
+		} else if (i<60) {
+			t = ((b & c) | (d & (b | c))) + SHA1_K40;
+		} else {
+			t = (b ^ c ^ d) + SHA1_K60;
+		}
+		t+=sha1_rol32(a,5) + e + s->buffer[i&15];
+		e=d;
+		d=c;
+		c=sha1_rol32(b,30);
+		b=a;
+		a=t;
+	}
+	s->state[0] += a;
+	s->state[1] += b;
+	s->state[2] += c;
+	s->state[3] += d;
+	s->state[4] += e;
+}
+
+void sha1_addUncounted(sha1nfo *s, uint8_t data) {
+	uint8_t * const b = (uint8_t*) s->buffer;
+#ifdef SHA_BIG_ENDIAN
+	b[s->bufferOffset] = data;
+#else
+	b[s->bufferOffset ^ 3] = data;
+#endif
+	s->bufferOffset++;
+	if (s->bufferOffset == BLOCK_LENGTH) {
+		sha1_hashBlock(s);
+		s->bufferOffset = 0;
+	}
+}
+
+void sha1_writebyte(sha1nfo *s, uint8_t data) {
+	++s->byteCount;
+	sha1_addUncounted(s, data);
+}
+
+void sha1_write(sha1nfo *s, const char *data, size_t len) {
+	for (;len--;) sha1_writebyte(s, (uint8_t) *data++);
+}
+
+void sha1_pad(sha1nfo *s) {
+	// Implement SHA-1 padding (fips180-2 §5.1.1)
+
+	// Pad with 0x80 followed by 0x00 until the end of the block
+	sha1_addUncounted(s, 0x80);
+	while (s->bufferOffset != 56) sha1_addUncounted(s, 0x00);
+
+	// Append length in the last 8 bytes
+	sha1_addUncounted(s, 0); // We're only using 32 bit lengths
+	sha1_addUncounted(s, 0); // But SHA-1 supports 64 bit lengths
+	sha1_addUncounted(s, 0); // So zero pad the top bits
+	sha1_addUncounted(s, s->byteCount >> 29); // Shifting to multiply by 8
+	sha1_addUncounted(s, s->byteCount >> 21); // as SHA-1 supports bitstreams as well as
+	sha1_addUncounted(s, s->byteCount >> 13); // byte.
+	sha1_addUncounted(s, s->byteCount >> 5);
+	sha1_addUncounted(s, s->byteCount << 3);
+}
+
+uint8_t* sha1_result(sha1nfo *s) {
+	// Pad to complete the last block
+	sha1_pad(s);
+
+#ifndef SHA_BIG_ENDIAN
+	// Swap byte order back
+	int i;
+	for (i=0; i<5; i++) {
+		s->state[i]=
+			  (((s->state[i])<<24)& 0xff000000)
+			| (((s->state[i])<<8) & 0x00ff0000)
+			| (((s->state[i])>>8) & 0x0000ff00)
+			| (((s->state[i])>>24)& 0x000000ff);
+	}
+#endif
+
+	// Return pointer to hash (20 characters)
+	return (uint8_t*) s->state;
+}
+
+}  // namespace; Added for LibFuzzer
+
+namespace fuzzer {
+
+// The rest is added for LibFuzzer
+void ComputeSHA1(const uint8_t *Data, size_t Len, uint8_t *Out) {
+  sha1nfo s;
+  sha1_init(&s);
+  sha1_write(&s, (const char*)Data, Len);
+  memcpy(Out, sha1_result(&s), HASH_LENGTH);
+}
+
+std::string Sha1ToString(const uint8_t Sha1[kSHA1NumBytes]) {
+  std::stringstream SS;
+  for (int i = 0; i < kSHA1NumBytes; i++)
+    SS << std::hex << std::setfill('0') << std::setw(2) << (unsigned)Sha1[i];
+  return SS.str();
+}
+
+std::string Hash(const Unit &U) {
+  uint8_t Hash[kSHA1NumBytes];
+  ComputeSHA1(U.data(), U.size(), Hash);
+  return Sha1ToString(Hash);
+}
+
+}
diff --git a/tools/fuzzing/libfuzzer/FuzzerSHA1.h b/tools/fuzzing/libfuzzer/FuzzerSHA1.h
new file mode 100644
index 0000000000..05cbacda87
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerSHA1.h
@@ -0,0 +1,32 @@
+//===- FuzzerSHA1.h - Internal header for the SHA1 utils --------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// SHA1 utils.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_SHA1_H
+#define LLVM_FUZZER_SHA1_H
+
+#include "FuzzerDefs.h"
+#include <cstddef>
+#include <stdint.h>
+
+namespace fuzzer {
+
+// Private copy of SHA1 implementation.
+static const int kSHA1NumBytes = 20;
+
+// Computes SHA1 hash of 'Len' bytes in 'Data', writes kSHA1NumBytes to 'Out'.
+void ComputeSHA1(const uint8_t *Data, size_t Len, uint8_t *Out);
+
+std::string Sha1ToString(const uint8_t Sha1[kSHA1NumBytes]);
+
+std::string Hash(const Unit &U);
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_SHA1_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerTracePC.cpp b/tools/fuzzing/libfuzzer/FuzzerTracePC.cpp
new file mode 100644
index 0000000000..b2ca7693e5
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerTracePC.cpp
@@ -0,0 +1,657 @@
+//===- FuzzerTracePC.cpp - PC tracing--------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Trace PCs.
+// This module implements __sanitizer_cov_trace_pc_guard[_init],
+// the callback required for -fsanitize-coverage=trace-pc-guard instrumentation.
+//
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerTracePC.h"
+#include "FuzzerBuiltins.h"
+#include "FuzzerBuiltinsMsvc.h"
+#include "FuzzerCorpus.h"
+#include "FuzzerDefs.h"
+#include "FuzzerDictionary.h"
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include "FuzzerPlatform.h"
+#include "FuzzerUtil.h"
+#include "FuzzerValueBitMap.h"
+#include <set>
+
+// Used by -fsanitize-coverage=stack-depth to track stack depth
+ATTRIBUTES_INTERFACE_TLS_INITIAL_EXEC uintptr_t __sancov_lowest_stack;
+
+namespace fuzzer {
+
+TracePC TPC;
+
+size_t TracePC::GetTotalPCCoverage() {
+  return ObservedPCs.size();
+}
+
+
+void TracePC::HandleInline8bitCountersInit(uint8_t *Start, uint8_t *Stop) {
+  if (Start == Stop) return;
+  if (NumModules &&
+      Modules[NumModules - 1].Start() == Start)
+    return;
+  assert(NumModules <
+         sizeof(Modules) / sizeof(Modules[0]));
+  auto &M = Modules[NumModules++];
+  uint8_t *AlignedStart = RoundUpByPage(Start);
+  uint8_t *AlignedStop  = RoundDownByPage(Stop);
+  size_t NumFullPages = AlignedStop > AlignedStart ?
+                        (AlignedStop - AlignedStart) / PageSize() : 0;
+  bool NeedFirst = Start < AlignedStart || !NumFullPages;
+  bool NeedLast  = Stop > AlignedStop && AlignedStop >= AlignedStart;
+  M.NumRegions = NumFullPages + NeedFirst + NeedLast;;
+  assert(M.NumRegions > 0);
+  M.Regions = new Module::Region[M.NumRegions];
+  assert(M.Regions);
+  size_t R = 0;
+  if (NeedFirst)
+    M.Regions[R++] = {Start, std::min(Stop, AlignedStart), true, false};
+  for (uint8_t *P = AlignedStart; P < AlignedStop; P += PageSize())
+    M.Regions[R++] = {P, P + PageSize(), true, true};
+  if (NeedLast)
+    M.Regions[R++] = {AlignedStop, Stop, true, false};
+  assert(R == M.NumRegions);
+  assert(M.Size() == (size_t)(Stop - Start));
+  assert(M.Stop() == Stop);
+  assert(M.Start() == Start);
+  NumInline8bitCounters += M.Size();
+}
+
+void TracePC::HandlePCsInit(const uintptr_t *Start, const uintptr_t *Stop) {
+  const PCTableEntry *B = reinterpret_cast<const PCTableEntry *>(Start);
+  const PCTableEntry *E = reinterpret_cast<const PCTableEntry *>(Stop);
+  if (NumPCTables && ModulePCTable[NumPCTables - 1].Start == B) return;
+  assert(NumPCTables < sizeof(ModulePCTable) / sizeof(ModulePCTable[0]));
+  ModulePCTable[NumPCTables++] = {B, E};
+  NumPCsInPCTables += E - B;
+}
+
+void TracePC::PrintModuleInfo() {
+  if (NumModules) {
+    Printf("INFO: Loaded %zd modules   (%zd inline 8-bit counters): ",
+           NumModules, NumInline8bitCounters);
+    for (size_t i = 0; i < NumModules; i++)
+      Printf("%zd [%p, %p), ", Modules[i].Size(), Modules[i].Start(),
+             Modules[i].Stop());
+    Printf("\n");
+  }
+  if (NumPCTables) {
+    Printf("INFO: Loaded %zd PC tables (%zd PCs): ", NumPCTables,
+           NumPCsInPCTables);
+    for (size_t i = 0; i < NumPCTables; i++) {
+      Printf("%zd [%p,%p), ", ModulePCTable[i].Stop - ModulePCTable[i].Start,
+             ModulePCTable[i].Start, ModulePCTable[i].Stop);
+    }
+    Printf("\n");
+
+    if (NumInline8bitCounters && NumInline8bitCounters != NumPCsInPCTables) {
+      Printf("ERROR: The size of coverage PC tables does not match the\n"
+             "number of instrumented PCs. This might be a compiler bug,\n"
+             "please contact the libFuzzer developers.\n"
+             "Also check https://bugs.llvm.org/show_bug.cgi?id=34636\n"
+             "for possible workarounds (tl;dr: don't use the old GNU ld)\n");
+      _Exit(1);
+    }
+  }
+  if (size_t NumExtraCounters = ExtraCountersEnd() - ExtraCountersBegin())
+    Printf("INFO: %zd Extra Counters\n", NumExtraCounters);
+}
+
+ATTRIBUTE_NO_SANITIZE_ALL
+void TracePC::HandleCallerCallee(uintptr_t Caller, uintptr_t Callee) {
+  const uintptr_t kBits = 12;
+  const uintptr_t kMask = (1 << kBits) - 1;
+  uintptr_t Idx = (Caller & kMask) | ((Callee & kMask) << kBits);
+  ValueProfileMap.AddValueModPrime(Idx);
+}
+
+/// \return the address of the previous instruction.
+/// Note: the logic is copied from `sanitizer_common/sanitizer_stacktrace.h`
+inline ALWAYS_INLINE uintptr_t GetPreviousInstructionPc(uintptr_t PC) {
+#if defined(__arm__)
+  // T32 (Thumb) branch instructions might be 16 or 32 bit long,
+  // so we return (pc-2) in that case in order to be safe.
+  // For A32 mode we return (pc-4) because all instructions are 32 bit long.
+  return (PC - 3) & (~1);
+#elif defined(__powerpc__) || defined(__powerpc64__) || defined(__aarch64__)
+  // PCs are always 4 byte aligned.
+  return PC - 4;
+#elif defined(__sparc__) || defined(__mips__)
+  return PC - 8;
+#else
+  return PC - 1;
+#endif
+}
+
+/// \return the address of the next instruction.
+/// Note: the logic is copied from `sanitizer_common/sanitizer_stacktrace.cpp`
+ALWAYS_INLINE uintptr_t TracePC::GetNextInstructionPc(uintptr_t PC) {
+#if defined(__mips__)
+  return PC + 8;
+#elif defined(__powerpc__) || defined(__sparc__) || defined(__arm__) || \
+    defined(__aarch64__)
+  return PC + 4;
+#else
+  return PC + 1;
+#endif
+}
+
+void TracePC::UpdateObservedPCs() {
+  Vector<uintptr_t> CoveredFuncs;
+  auto ObservePC = [&](const PCTableEntry *TE) {
+    if (ObservedPCs.insert(TE).second && DoPrintNewPCs) {
+      PrintPC("\tNEW_PC: %p %F %L", "\tNEW_PC: %p",
+              GetNextInstructionPc(TE->PC));
+      Printf("\n");
+    }
+  };
+
+  auto Observe = [&](const PCTableEntry *TE) {
+    if (PcIsFuncEntry(TE))
+      if (++ObservedFuncs[TE->PC] == 1 && NumPrintNewFuncs)
+        CoveredFuncs.push_back(TE->PC);
+    ObservePC(TE);
+  };
+
+  if (NumPCsInPCTables) {
+    if (NumInline8bitCounters == NumPCsInPCTables) {
+      for (size_t i = 0; i < NumModules; i++) {
+        auto &M = Modules[i];
+        assert(M.Size() ==
+               (size_t)(ModulePCTable[i].Stop - ModulePCTable[i].Start));
+        for (size_t r = 0; r < M.NumRegions; r++) {
+          auto &R = M.Regions[r];
+          if (!R.Enabled) continue;
+          for (uint8_t *P = R.Start; P < R.Stop; P++)
+            if (*P)
+              Observe(&ModulePCTable[i].Start[M.Idx(P)]);
+        }
+      }
+    }
+  }
+
+  for (size_t i = 0, N = Min(CoveredFuncs.size(), NumPrintNewFuncs); i < N;
+       i++) {
+    Printf("\tNEW_FUNC[%zd/%zd]: ", i + 1, CoveredFuncs.size());
+    PrintPC("%p %F %L", "%p", GetNextInstructionPc(CoveredFuncs[i]));
+    Printf("\n");
+  }
+}
+
+uintptr_t TracePC::PCTableEntryIdx(const PCTableEntry *TE) {
+  size_t TotalTEs = 0;
+  for (size_t i = 0; i < NumPCTables; i++) {
+    auto &M = ModulePCTable[i];
+    if (TE >= M.Start && TE < M.Stop)
+      return TotalTEs + TE - M.Start;
+    TotalTEs += M.Stop - M.Start;
+  }
+  assert(0);
+  return 0;
+}
+
+const TracePC::PCTableEntry *TracePC::PCTableEntryByIdx(uintptr_t Idx) {
+  for (size_t i = 0; i < NumPCTables; i++) {
+    auto &M = ModulePCTable[i];
+    size_t Size = M.Stop - M.Start;
+    if (Idx < Size) return &M.Start[Idx];
+    Idx -= Size;
+  }
+  return nullptr;
+}
+
+static std::string GetModuleName(uintptr_t PC) {
+  char ModulePathRaw[4096] = "";  // What's PATH_MAX in portable C++?
+  void *OffsetRaw = nullptr;
+  if (!EF->__sanitizer_get_module_and_offset_for_pc(
+      reinterpret_cast<void *>(PC), ModulePathRaw,
+      sizeof(ModulePathRaw), &OffsetRaw))
+    return "";
+  return ModulePathRaw;
+}
+
+template<class CallBack>
+void TracePC::IterateCoveredFunctions(CallBack CB) {
+  for (size_t i = 0; i < NumPCTables; i++) {
+    auto &M = ModulePCTable[i];
+    assert(M.Start < M.Stop);
+    auto ModuleName = GetModuleName(M.Start->PC);
+    for (auto NextFE = M.Start; NextFE < M.Stop; ) {
+      auto FE = NextFE;
+      assert(PcIsFuncEntry(FE) && "Not a function entry point");
+      do {
+        NextFE++;
+      } while (NextFE < M.Stop && !(PcIsFuncEntry(NextFE)));
+      CB(FE, NextFE, ObservedFuncs[FE->PC]);
+    }
+  }
+}
+
+void TracePC::SetFocusFunction(const std::string &FuncName) {
+  // This function should be called once.
+  assert(!FocusFunctionCounterPtr);
+  // "auto" is not a valid function name. If this function is called with "auto"
+  // that means the auto focus functionality failed.
+  if (FuncName.empty() || FuncName == "auto")
+    return;
+  for (size_t M = 0; M < NumModules; M++) {
+    auto &PCTE = ModulePCTable[M];
+    size_t N = PCTE.Stop - PCTE.Start;
+    for (size_t I = 0; I < N; I++) {
+      if (!(PcIsFuncEntry(&PCTE.Start[I]))) continue;  // not a function entry.
+      auto Name = DescribePC("%F", GetNextInstructionPc(PCTE.Start[I].PC));
+      if (Name[0] == 'i' && Name[1] == 'n' && Name[2] == ' ')
+        Name = Name.substr(3, std::string::npos);
+      if (FuncName != Name) continue;
+      Printf("INFO: Focus function is set to '%s'\n", Name.c_str());
+      FocusFunctionCounterPtr = Modules[M].Start() + I;
+      return;
+    }
+  }
+
+  Printf("ERROR: Failed to set focus function. Make sure the function name is "
+         "valid (%s) and symbolization is enabled.\n", FuncName.c_str());
+  exit(1);
+}
+
+bool TracePC::ObservedFocusFunction() {
+  return FocusFunctionCounterPtr && *FocusFunctionCounterPtr;
+}
+
+void TracePC::PrintCoverage() {
+  if (!EF->__sanitizer_symbolize_pc ||
+      !EF->__sanitizer_get_module_and_offset_for_pc) {
+    Printf("INFO: __sanitizer_symbolize_pc or "
+           "__sanitizer_get_module_and_offset_for_pc is not available,"
+           " not printing coverage\n");
+    return;
+  }
+  Printf("COVERAGE:\n");
+  auto CoveredFunctionCallback = [&](const PCTableEntry *First,
+                                     const PCTableEntry *Last,
+                                     uintptr_t Counter) {
+    assert(First < Last);
+    auto VisualizePC = GetNextInstructionPc(First->PC);
+    std::string FileStr = DescribePC("%s", VisualizePC);
+    if (!IsInterestingCoverageFile(FileStr))
+      return;
+    std::string FunctionStr = DescribePC("%F", VisualizePC);
+    if (FunctionStr.find("in ") == 0)
+      FunctionStr = FunctionStr.substr(3);
+    std::string LineStr = DescribePC("%l", VisualizePC);
+    size_t NumEdges = Last - First;
+    Vector<uintptr_t> UncoveredPCs;
+    for (auto TE = First; TE < Last; TE++)
+      if (!ObservedPCs.count(TE))
+        UncoveredPCs.push_back(TE->PC);
+    Printf("%sCOVERED_FUNC: hits: %zd", Counter ? "" : "UN", Counter);
+    Printf(" edges: %zd/%zd", NumEdges - UncoveredPCs.size(), NumEdges);
+    Printf(" %s %s:%s\n", FunctionStr.c_str(), FileStr.c_str(),
+           LineStr.c_str());
+    if (Counter)
+      for (auto PC : UncoveredPCs)
+        Printf("  UNCOVERED_PC: %s\n",
+               DescribePC("%s:%l", GetNextInstructionPc(PC)).c_str());
+  };
+
+  IterateCoveredFunctions(CoveredFunctionCallback);
+}
+
+// Value profile.
+// We keep track of various values that affect control flow.
+// These values are inserted into a bit-set-based hash map.
+// Every new bit in the map is treated as a new coverage.
+//
+// For memcmp/strcmp/etc the interesting value is the length of the common
+// prefix of the parameters.
+// For cmp instructions the interesting value is a XOR of the parameters.
+// The interesting value is mixed up with the PC and is then added to the map.
+
+ATTRIBUTE_NO_SANITIZE_ALL
+void TracePC::AddValueForMemcmp(void *caller_pc, const void *s1, const void *s2,
+                                size_t n, bool StopAtZero) {
+  if (!n) return;
+  size_t Len = std::min(n, Word::GetMaxSize());
+  const uint8_t *A1 = reinterpret_cast<const uint8_t *>(s1);
+  const uint8_t *A2 = reinterpret_cast<const uint8_t *>(s2);
+  uint8_t B1[Word::kMaxSize];
+  uint8_t B2[Word::kMaxSize];
+  // Copy the data into locals in this non-msan-instrumented function
+  // to avoid msan complaining further.
+  size_t Hash = 0;  // Compute some simple hash of both strings.
+  for (size_t i = 0; i < Len; i++) {
+    B1[i] = A1[i];
+    B2[i] = A2[i];
+    size_t T = B1[i];
+    Hash ^= (T << 8) | B2[i];
+  }
+  size_t I = 0;
+  uint8_t HammingDistance = 0;
+  for (; I < Len; I++) {
+    if (B1[I] != B2[I] || (StopAtZero && B1[I] == 0)) {
+      HammingDistance = Popcountll(B1[I] ^ B2[I]);
+      break;
+    }
+  }
+  size_t PC = reinterpret_cast<size_t>(caller_pc);
+  size_t Idx = (PC & 4095) | (I << 12);
+  Idx += HammingDistance;
+  ValueProfileMap.AddValue(Idx);
+  TORCW.Insert(Idx ^ Hash, Word(B1, Len), Word(B2, Len));
+}
+
+template <class T>
+ATTRIBUTE_TARGET_POPCNT ALWAYS_INLINE
+ATTRIBUTE_NO_SANITIZE_ALL
+void TracePC::HandleCmp(uintptr_t PC, T Arg1, T Arg2) {
+  uint64_t ArgXor = Arg1 ^ Arg2;
+  if (sizeof(T) == 4)
+      TORC4.Insert(ArgXor, Arg1, Arg2);
+  else if (sizeof(T) == 8)
+      TORC8.Insert(ArgXor, Arg1, Arg2);
+  uint64_t HammingDistance = Popcountll(ArgXor);  // [0,64]
+  uint64_t AbsoluteDistance = (Arg1 == Arg2 ? 0 : Clzll(Arg1 - Arg2) + 1);
+  ValueProfileMap.AddValue(PC * 128 + HammingDistance);
+  ValueProfileMap.AddValue(PC * 128 + 64 + AbsoluteDistance);
+}
+
+static size_t InternalStrnlen(const char *S, size_t MaxLen) {
+  size_t Len = 0;
+  for (; Len < MaxLen && S[Len]; Len++) {}
+  return Len;
+}
+
+// Finds min of (strlen(S1), strlen(S2)).
+// Needed bacause one of these strings may actually be non-zero terminated.
+static size_t InternalStrnlen2(const char *S1, const char *S2) {
+  size_t Len = 0;
+  for (; S1[Len] && S2[Len]; Len++)  {}
+  return Len;
+}
+
+void TracePC::ClearInlineCounters() {
+  IterateCounterRegions([](const Module::Region &R){
+    if (R.Enabled)
+      memset(R.Start, 0, R.Stop - R.Start);
+  });
+}
+
+ATTRIBUTE_NO_SANITIZE_ALL
+void TracePC::RecordInitialStack() {
+  int stack;
+  __sancov_lowest_stack = InitialStack = reinterpret_cast<uintptr_t>(&stack);
+}
+
+uintptr_t TracePC::GetMaxStackOffset() const {
+  return InitialStack - __sancov_lowest_stack;  // Stack grows down
+}
+
+void WarnAboutDeprecatedInstrumentation(const char *flag) {
+  // Use RawPrint because Printf cannot be used on Windows before OutputFile is
+  // initialized.
+  RawPrint(flag);
+  RawPrint(
+      " is no longer supported by libFuzzer.\n"
+      "Please either migrate to a compiler that supports -fsanitize=fuzzer\n"
+      "or use an older version of libFuzzer\n");
+  exit(1);
+}
+
+} // namespace fuzzer
+
+extern "C" {
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+void __sanitizer_cov_trace_pc_guard(uint32_t *Guard) {
+  fuzzer::WarnAboutDeprecatedInstrumentation(
+      "-fsanitize-coverage=trace-pc-guard");
+}
+
+// Best-effort support for -fsanitize-coverage=trace-pc, which is available
+// in both Clang and GCC.
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+void __sanitizer_cov_trace_pc() {
+  fuzzer::WarnAboutDeprecatedInstrumentation("-fsanitize-coverage=trace-pc");
+}
+
+ATTRIBUTE_INTERFACE
+void __sanitizer_cov_trace_pc_guard_init(uint32_t *Start, uint32_t *Stop) {
+  fuzzer::WarnAboutDeprecatedInstrumentation(
+      "-fsanitize-coverage=trace-pc-guard");
+}
+
+ATTRIBUTE_INTERFACE
+void __sanitizer_cov_8bit_counters_init(uint8_t *Start, uint8_t *Stop) {
+  fuzzer::TPC.HandleInline8bitCountersInit(Start, Stop);
+}
+
+ATTRIBUTE_INTERFACE
+void __sanitizer_cov_pcs_init(const uintptr_t *pcs_beg,
+                              const uintptr_t *pcs_end) {
+  fuzzer::TPC.HandlePCsInit(pcs_beg, pcs_end);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+void __sanitizer_cov_trace_pc_indir(uintptr_t Callee) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCallerCallee(PC, Callee);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_cmp8(uint64_t Arg1, uint64_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+// Now the __sanitizer_cov_trace_const_cmp[1248] callbacks just mimic
+// the behaviour of __sanitizer_cov_trace_cmp[1248] ones. This, however,
+// should be changed later to make full use of instrumentation.
+void __sanitizer_cov_trace_const_cmp8(uint64_t Arg1, uint64_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_cmp4(uint32_t Arg1, uint32_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_const_cmp4(uint32_t Arg1, uint32_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_cmp2(uint16_t Arg1, uint16_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_const_cmp2(uint16_t Arg1, uint16_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_cmp1(uint8_t Arg1, uint8_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_const_cmp1(uint8_t Arg1, uint8_t Arg2) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Arg1, Arg2);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases) {
+  uint64_t N = Cases[0];
+  uint64_t ValSizeInBits = Cases[1];
+  uint64_t *Vals = Cases + 2;
+  // Skip the most common and the most boring case: all switch values are small.
+  // We may want to skip this at compile-time, but it will make the
+  // instrumentation less general.
+  if (Vals[N - 1]  < 256)
+    return;
+  // Also skip small inputs values, they won't give good signal.
+  if (Val < 256)
+    return;
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  size_t i;
+  uint64_t Smaller = 0;
+  uint64_t Larger = ~(uint64_t)0;
+  // Find two switch values such that Smaller < Val < Larger.
+  // Use 0 and 0xfff..f as the defaults.
+  for (i = 0; i < N; i++) {
+    if (Val < Vals[i]) {
+      Larger = Vals[i];
+      break;
+    }
+    if (Val > Vals[i]) Smaller = Vals[i];
+  }
+
+  // Apply HandleCmp to {Val,Smaller} and {Val, Larger},
+  // use i as the PC modifier for HandleCmp.
+  if (ValSizeInBits == 16) {
+    fuzzer::TPC.HandleCmp(PC + 2 * i, static_cast<uint16_t>(Val),
+                          (uint16_t)(Smaller));
+    fuzzer::TPC.HandleCmp(PC + 2 * i + 1, static_cast<uint16_t>(Val),
+                          (uint16_t)(Larger));
+  } else if (ValSizeInBits == 32) {
+    fuzzer::TPC.HandleCmp(PC + 2 * i, static_cast<uint32_t>(Val),
+                          (uint32_t)(Smaller));
+    fuzzer::TPC.HandleCmp(PC + 2 * i + 1, static_cast<uint32_t>(Val),
+                          (uint32_t)(Larger));
+  } else {
+    fuzzer::TPC.HandleCmp(PC + 2*i, Val, Smaller);
+    fuzzer::TPC.HandleCmp(PC + 2*i + 1, Val, Larger);
+  }
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_div4(uint32_t Val) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Val, (uint32_t)0);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_div8(uint64_t Val) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Val, (uint64_t)0);
+}
+
+ATTRIBUTE_INTERFACE
+ATTRIBUTE_NO_SANITIZE_ALL
+ATTRIBUTE_TARGET_POPCNT
+void __sanitizer_cov_trace_gep(uintptr_t Idx) {
+  uintptr_t PC = reinterpret_cast<uintptr_t>(GET_CALLER_PC());
+  fuzzer::TPC.HandleCmp(PC, Idx, (uintptr_t)0);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_memcmp(void *caller_pc, const void *s1,
+                                  const void *s2, size_t n, int result) {
+  if (!fuzzer::RunningUserCallback) return;
+  if (result == 0) return;  // No reason to mutate.
+  if (n <= 1) return;  // Not interesting.
+  fuzzer::TPC.AddValueForMemcmp(caller_pc, s1, s2, n, /*StopAtZero*/false);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strncmp(void *caller_pc, const char *s1,
+                                   const char *s2, size_t n, int result) {
+  if (!fuzzer::RunningUserCallback) return;
+  if (result == 0) return;  // No reason to mutate.
+  size_t Len1 = fuzzer::InternalStrnlen(s1, n);
+  size_t Len2 = fuzzer::InternalStrnlen(s2, n);
+  n = std::min(n, Len1);
+  n = std::min(n, Len2);
+  if (n <= 1) return;  // Not interesting.
+  fuzzer::TPC.AddValueForMemcmp(caller_pc, s1, s2, n, /*StopAtZero*/true);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strcmp(void *caller_pc, const char *s1,
+                                   const char *s2, int result) {
+  if (!fuzzer::RunningUserCallback) return;
+  if (result == 0) return;  // No reason to mutate.
+  size_t N = fuzzer::InternalStrnlen2(s1, s2);
+  if (N <= 1) return;  // Not interesting.
+  fuzzer::TPC.AddValueForMemcmp(caller_pc, s1, s2, N, /*StopAtZero*/true);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strncasecmp(void *called_pc, const char *s1,
+                                       const char *s2, size_t n, int result) {
+  if (!fuzzer::RunningUserCallback) return;
+  return __sanitizer_weak_hook_strncmp(called_pc, s1, s2, n, result);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strcasecmp(void *called_pc, const char *s1,
+                                      const char *s2, int result) {
+  if (!fuzzer::RunningUserCallback) return;
+  return __sanitizer_weak_hook_strcmp(called_pc, s1, s2, result);
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strstr(void *called_pc, const char *s1,
+                                  const char *s2, char *result) {
+  if (!fuzzer::RunningUserCallback) return;
+  fuzzer::TPC.MMT.Add(reinterpret_cast<const uint8_t *>(s2), strlen(s2));
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_strcasestr(void *called_pc, const char *s1,
+                                      const char *s2, char *result) {
+  if (!fuzzer::RunningUserCallback) return;
+  fuzzer::TPC.MMT.Add(reinterpret_cast<const uint8_t *>(s2), strlen(s2));
+}
+
+ATTRIBUTE_INTERFACE ATTRIBUTE_NO_SANITIZE_MEMORY
+void __sanitizer_weak_hook_memmem(void *called_pc, const void *s1, size_t len1,
+                                  const void *s2, size_t len2, void *result) {
+  if (!fuzzer::RunningUserCallback) return;
+  fuzzer::TPC.MMT.Add(reinterpret_cast<const uint8_t *>(s2), len2);
+}
+}  // extern "C"
diff --git a/tools/fuzzing/libfuzzer/FuzzerTracePC.h b/tools/fuzzing/libfuzzer/FuzzerTracePC.h
new file mode 100644
index 0000000000..501f3b5449
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerTracePC.h
@@ -0,0 +1,289 @@
+//===- FuzzerTracePC.h - Internal header for the Fuzzer ---------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::TracePC
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_TRACE_PC
+#define LLVM_FUZZER_TRACE_PC
+
+#include "FuzzerDefs.h"
+#include "FuzzerDictionary.h"
+#include "FuzzerValueBitMap.h"
+
+#include <set>
+#include <unordered_map>
+
+namespace fuzzer {
+
+// TableOfRecentCompares (TORC) remembers the most recently performed
+// comparisons of type T.
+// We record the arguments of CMP instructions in this table unconditionally
+// because it seems cheaper this way than to compute some expensive
+// conditions inside __sanitizer_cov_trace_cmp*.
+// After the unit has been executed we may decide to use the contents of
+// this table to populate a Dictionary.
+template<class T, size_t kSizeT>
+struct TableOfRecentCompares {
+  static const size_t kSize = kSizeT;
+  struct Pair {
+    T A, B;
+  };
+  ATTRIBUTE_NO_SANITIZE_ALL
+  void Insert(size_t Idx, const T &Arg1, const T &Arg2) {
+    Idx = Idx % kSize;
+    Table[Idx].A = Arg1;
+    Table[Idx].B = Arg2;
+  }
+
+  Pair Get(size_t I) { return Table[I % kSize]; }
+
+  Pair Table[kSize];
+};
+
+template <size_t kSizeT>
+struct MemMemTable {
+  static const size_t kSize = kSizeT;
+  Word MemMemWords[kSize];
+  Word EmptyWord;
+
+  void Add(const uint8_t *Data, size_t Size) {
+    if (Size <= 2) return;
+    Size = std::min(Size, Word::GetMaxSize());
+    size_t Idx = SimpleFastHash(Data, Size) % kSize;
+    MemMemWords[Idx].Set(Data, Size);
+  }
+  const Word &Get(size_t Idx) {
+    for (size_t i = 0; i < kSize; i++) {
+      const Word &W = MemMemWords[(Idx + i) % kSize];
+      if (W.size()) return W;
+    }
+    EmptyWord.Set(nullptr, 0);
+    return EmptyWord;
+  }
+};
+
+class TracePC {
+ public:
+  void HandleInline8bitCountersInit(uint8_t *Start, uint8_t *Stop);
+  void HandlePCsInit(const uintptr_t *Start, const uintptr_t *Stop);
+  void HandleCallerCallee(uintptr_t Caller, uintptr_t Callee);
+  template <class T> void HandleCmp(uintptr_t PC, T Arg1, T Arg2);
+  size_t GetTotalPCCoverage();
+  void SetUseCounters(bool UC) { UseCounters = UC; }
+  void SetUseValueProfileMask(uint32_t VPMask) { UseValueProfileMask = VPMask; }
+  void SetPrintNewPCs(bool P) { DoPrintNewPCs = P; }
+  void SetPrintNewFuncs(size_t P) { NumPrintNewFuncs = P; }
+  void UpdateObservedPCs();
+  template <class Callback> void CollectFeatures(Callback CB) const;
+
+  void ResetMaps() {
+    ValueProfileMap.Reset();
+    ClearExtraCounters();
+    ClearInlineCounters();
+  }
+
+  void ClearInlineCounters();
+
+  void UpdateFeatureSet(size_t CurrentElementIdx, size_t CurrentElementSize);
+  void PrintFeatureSet();
+
+  void PrintModuleInfo();
+
+  void PrintCoverage();
+
+  template<class CallBack>
+  void IterateCoveredFunctions(CallBack CB);
+
+  void AddValueForMemcmp(void *caller_pc, const void *s1, const void *s2,
+                         size_t n, bool StopAtZero);
+
+  TableOfRecentCompares<uint32_t, 32> TORC4;
+  TableOfRecentCompares<uint64_t, 32> TORC8;
+  TableOfRecentCompares<Word, 32> TORCW;
+  MemMemTable<1024> MMT;
+
+  void RecordInitialStack();
+  uintptr_t GetMaxStackOffset() const;
+
+  template<class CallBack>
+  void ForEachObservedPC(CallBack CB) {
+    for (auto PC : ObservedPCs)
+      CB(PC);
+  }
+
+  void SetFocusFunction(const std::string &FuncName);
+  bool ObservedFocusFunction();
+
+  struct PCTableEntry {
+    uintptr_t PC, PCFlags;
+  };
+
+  uintptr_t PCTableEntryIdx(const PCTableEntry *TE);
+  const PCTableEntry *PCTableEntryByIdx(uintptr_t Idx);
+  static uintptr_t GetNextInstructionPc(uintptr_t PC);
+  bool PcIsFuncEntry(const PCTableEntry *TE) { return TE->PCFlags & 1; }
+
+private:
+  bool UseCounters = false;
+  uint32_t UseValueProfileMask = false;
+  bool DoPrintNewPCs = false;
+  size_t NumPrintNewFuncs = 0;
+
+  // Module represents the array of 8-bit counters split into regions
+  // such that every region, except maybe the first and the last one, is one
+  // full page.
+  struct Module {
+    struct Region {
+      uint8_t *Start, *Stop;
+      bool Enabled;
+      bool OneFullPage;
+    };
+    Region *Regions;
+    size_t NumRegions;
+    uint8_t *Start() { return Regions[0].Start; }
+    uint8_t *Stop()  { return Regions[NumRegions - 1].Stop; }
+    size_t Size()   { return Stop() - Start(); }
+    size_t  Idx(uint8_t *P) {
+      assert(P >= Start() && P < Stop());
+      return P - Start();
+    }
+  };
+
+  Module Modules[4096];
+  size_t NumModules;  // linker-initialized.
+  size_t NumInline8bitCounters;
+
+  template <class Callback>
+  void IterateCounterRegions(Callback CB) {
+    for (size_t m = 0; m < NumModules; m++)
+      for (size_t r = 0; r < Modules[m].NumRegions; r++)
+        CB(Modules[m].Regions[r]);
+  }
+
+  struct { const PCTableEntry *Start, *Stop; } ModulePCTable[4096];
+  size_t NumPCTables;
+  size_t NumPCsInPCTables;
+
+  Set<const PCTableEntry*> ObservedPCs;
+  std::unordered_map<uintptr_t, uintptr_t> ObservedFuncs;  // PC => Counter.
+
+  uint8_t *FocusFunctionCounterPtr = nullptr;
+
+  ValueBitMap ValueProfileMap;
+  uintptr_t InitialStack;
+};
+
+template <class Callback>
+// void Callback(size_t FirstFeature, size_t Idx, uint8_t Value);
+ATTRIBUTE_NO_SANITIZE_ALL
+size_t ForEachNonZeroByte(const uint8_t *Begin, const uint8_t *End,
+                        size_t FirstFeature, Callback Handle8bitCounter) {
+  typedef uintptr_t LargeType;
+  const size_t Step = sizeof(LargeType) / sizeof(uint8_t);
+  const size_t StepMask = Step - 1;
+  auto P = Begin;
+  // Iterate by 1 byte until either the alignment boundary or the end.
+  for (; reinterpret_cast<uintptr_t>(P) & StepMask && P < End; P++)
+    if (uint8_t V = *P)
+      Handle8bitCounter(FirstFeature, P - Begin, V);
+
+  // Iterate by Step bytes at a time.
+  for (; P < End; P += Step)
+    if (LargeType Bundle = *reinterpret_cast<const LargeType *>(P))
+      for (size_t I = 0; I < Step; I++, Bundle >>= 8)
+        if (uint8_t V = Bundle & 0xff)
+          Handle8bitCounter(FirstFeature, P - Begin + I, V);
+
+  // Iterate by 1 byte until the end.
+  for (; P < End; P++)
+    if (uint8_t V = *P)
+      Handle8bitCounter(FirstFeature, P - Begin, V);
+  return End - Begin;
+}
+
+// Given a non-zero Counter returns a number in the range [0,7].
+template<class T>
+unsigned CounterToFeature(T Counter) {
+    // Returns a feature number by placing Counters into buckets as illustrated
+    // below.
+    //
+    // Counter bucket: [1] [2] [3] [4-7] [8-15] [16-31] [32-127] [128+]
+    // Feature number:  0   1   2    3     4       5       6       7
+    //
+    // This is a heuristic taken from AFL (see
+    // http://lcamtuf.coredump.cx/afl/technical_details.txt).
+    //
+    // This implementation may change in the future so clients should
+    // not rely on it.
+    assert(Counter);
+    unsigned Bit = 0;
+    /**/ if (Counter >= 128) Bit = 7;
+    else if (Counter >= 32) Bit = 6;
+    else if (Counter >= 16) Bit = 5;
+    else if (Counter >= 8) Bit = 4;
+    else if (Counter >= 4) Bit = 3;
+    else if (Counter >= 3) Bit = 2;
+    else if (Counter >= 2) Bit = 1;
+    return Bit;
+}
+
+template <class Callback>  // void Callback(size_t Feature)
+ATTRIBUTE_NO_SANITIZE_ADDRESS
+ATTRIBUTE_NOINLINE
+void TracePC::CollectFeatures(Callback HandleFeature) const {
+  auto Handle8bitCounter = [&](size_t FirstFeature,
+                               size_t Idx, uint8_t Counter) {
+    if (UseCounters)
+      HandleFeature(FirstFeature + Idx * 8 + CounterToFeature(Counter));
+    else
+      HandleFeature(FirstFeature + Idx);
+  };
+
+  size_t FirstFeature = 0;
+
+  for (size_t i = 0; i < NumModules; i++) {
+    for (size_t r = 0; r < Modules[i].NumRegions; r++) {
+      if (!Modules[i].Regions[r].Enabled) continue;
+      FirstFeature += 8 * ForEachNonZeroByte(Modules[i].Regions[r].Start,
+                                             Modules[i].Regions[r].Stop,
+                                             FirstFeature, Handle8bitCounter);
+    }
+  }
+
+  FirstFeature +=
+      8 * ForEachNonZeroByte(ExtraCountersBegin(), ExtraCountersEnd(),
+                             FirstFeature, Handle8bitCounter);
+
+  if (UseValueProfileMask) {
+    ValueProfileMap.ForEach([&](size_t Idx) {
+      HandleFeature(FirstFeature + Idx);
+    });
+    FirstFeature += ValueProfileMap.SizeInBits();
+  }
+
+  // Step function, grows similar to 8 * Log_2(A).
+  auto StackDepthStepFunction = [](uint32_t A) -> uint32_t {
+    if (!A) return A;
+    uint32_t Log2 = Log(A);
+    if (Log2 < 3) return A;
+    Log2 -= 3;
+    return (Log2 + 1) * 8 + ((A >> Log2) & 7);
+  };
+  assert(StackDepthStepFunction(1024) == 64);
+  assert(StackDepthStepFunction(1024 * 4) == 80);
+  assert(StackDepthStepFunction(1024 * 1024) == 144);
+
+  if (auto MaxStackOffset = GetMaxStackOffset())
+    HandleFeature(FirstFeature + StackDepthStepFunction(MaxStackOffset / 8));
+}
+
+extern TracePC TPC;
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_TRACE_PC
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtil.cpp b/tools/fuzzing/libfuzzer/FuzzerUtil.cpp
new file mode 100644
index 0000000000..7eecb68d07
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtil.cpp
@@ -0,0 +1,236 @@
+//===- FuzzerUtil.cpp - Misc utils ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerUtil.h"
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include <cassert>
+#include <chrono>
+#include <cstring>
+#include <errno.h>
+#include <mutex>
+#include <signal.h>
+#include <sstream>
+#include <stdio.h>
+#include <sys/types.h>
+#include <thread>
+
+namespace fuzzer {
+
+void PrintHexArray(const uint8_t *Data, size_t Size,
+                   const char *PrintAfter) {
+  for (size_t i = 0; i < Size; i++)
+    Printf("0x%x,", (unsigned)Data[i]);
+  Printf("%s", PrintAfter);
+}
+
+void Print(const Unit &v, const char *PrintAfter) {
+  PrintHexArray(v.data(), v.size(), PrintAfter);
+}
+
+void PrintASCIIByte(uint8_t Byte) {
+  if (Byte == '\\')
+    Printf("\\\\");
+  else if (Byte == '"')
+    Printf("\\\"");
+  else if (Byte >= 32 && Byte < 127)
+    Printf("%c", Byte);
+  else
+    Printf("\\x%02x", Byte);
+}
+
+void PrintASCII(const uint8_t *Data, size_t Size, const char *PrintAfter) {
+  for (size_t i = 0; i < Size; i++)
+    PrintASCIIByte(Data[i]);
+  Printf("%s", PrintAfter);
+}
+
+void PrintASCII(const Unit &U, const char *PrintAfter) {
+  PrintASCII(U.data(), U.size(), PrintAfter);
+}
+
+bool ToASCII(uint8_t *Data, size_t Size) {
+  bool Changed = false;
+  for (size_t i = 0; i < Size; i++) {
+    uint8_t &X = Data[i];
+    auto NewX = X;
+    NewX &= 127;
+    if (!isspace(NewX) && !isprint(NewX))
+      NewX = ' ';
+    Changed |= NewX != X;
+    X = NewX;
+  }
+  return Changed;
+}
+
+bool IsASCII(const Unit &U) { return IsASCII(U.data(), U.size()); }
+
+bool IsASCII(const uint8_t *Data, size_t Size) {
+  for (size_t i = 0; i < Size; i++)
+    if (!(isprint(Data[i]) || isspace(Data[i]))) return false;
+  return true;
+}
+
+bool ParseOneDictionaryEntry(const std::string &Str, Unit *U) {
+  U->clear();
+  if (Str.empty()) return false;
+  size_t L = 0, R = Str.size() - 1;  // We are parsing the range [L,R].
+  // Skip spaces from both sides.
+  while (L < R && isspace(Str[L])) L++;
+  while (R > L && isspace(Str[R])) R--;
+  if (R - L < 2) return false;
+  // Check the closing "
+  if (Str[R] != '"') return false;
+  R--;
+  // Find the opening "
+  while (L < R && Str[L] != '"') L++;
+  if (L >= R) return false;
+  assert(Str[L] == '\"');
+  L++;
+  assert(L <= R);
+  for (size_t Pos = L; Pos <= R; Pos++) {
+    uint8_t V = (uint8_t)Str[Pos];
+    if (!isprint(V) && !isspace(V)) return false;
+    if (V =='\\') {
+      // Handle '\\'
+      if (Pos + 1 <= R && (Str[Pos + 1] == '\\' || Str[Pos + 1] == '"')) {
+        U->push_back(Str[Pos + 1]);
+        Pos++;
+        continue;
+      }
+      // Handle '\xAB'
+      if (Pos + 3 <= R && Str[Pos + 1] == 'x'
+           && isxdigit(Str[Pos + 2]) && isxdigit(Str[Pos + 3])) {
+        char Hex[] = "0xAA";
+        Hex[2] = Str[Pos + 2];
+        Hex[3] = Str[Pos + 3];
+        U->push_back(strtol(Hex, nullptr, 16));
+        Pos += 3;
+        continue;
+      }
+      return false;  // Invalid escape.
+    } else {
+      // Any other character.
+      U->push_back(V);
+    }
+  }
+  return true;
+}
+
+bool ParseDictionaryFile(const std::string &Text, Vector<Unit> *Units) {
+  if (Text.empty()) {
+    Printf("ParseDictionaryFile: file does not exist or is empty\n");
+    return false;
+  }
+  std::istringstream ISS(Text);
+  Units->clear();
+  Unit U;
+  int LineNo = 0;
+  std::string S;
+  while (std::getline(ISS, S, '\n')) {
+    LineNo++;
+    size_t Pos = 0;
+    while (Pos < S.size() && isspace(S[Pos])) Pos++;  // Skip spaces.
+    if (Pos == S.size()) continue;  // Empty line.
+    if (S[Pos] == '#') continue;  // Comment line.
+    if (ParseOneDictionaryEntry(S, &U)) {
+      Units->push_back(U);
+    } else {
+      Printf("ParseDictionaryFile: error in line %d\n\t\t%s\n", LineNo,
+             S.c_str());
+      return false;
+    }
+  }
+  return true;
+}
+
+// Code duplicated (and tested) in llvm/include/llvm/Support/Base64.h
+std::string Base64(const Unit &U) {
+  static const char Table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                              "abcdefghijklmnopqrstuvwxyz"
+                              "0123456789+/";
+  std::string Buffer;
+  Buffer.resize(((U.size() + 2) / 3) * 4);
+
+  size_t i = 0, j = 0;
+  for (size_t n = U.size() / 3 * 3; i < n; i += 3, j += 4) {
+    uint32_t x = ((unsigned char)U[i] << 16) | ((unsigned char)U[i + 1] << 8) |
+                 (unsigned char)U[i + 2];
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = Table[(x >> 6) & 63];
+    Buffer[j + 3] = Table[x & 63];
+  }
+  if (i + 1 == U.size()) {
+    uint32_t x = ((unsigned char)U[i] << 16);
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = '=';
+    Buffer[j + 3] = '=';
+  } else if (i + 2 == U.size()) {
+    uint32_t x = ((unsigned char)U[i] << 16) | ((unsigned char)U[i + 1] << 8);
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = Table[(x >> 6) & 63];
+    Buffer[j + 3] = '=';
+  }
+  return Buffer;
+}
+
+static std::mutex SymbolizeMutex;
+
+std::string DescribePC(const char *SymbolizedFMT, uintptr_t PC) {
+  std::unique_lock<std::mutex> l(SymbolizeMutex, std::try_to_lock);
+  if (!EF->__sanitizer_symbolize_pc || !l.owns_lock())
+    return "<can not symbolize>";
+  char PcDescr[1024] = {};
+  EF->__sanitizer_symbolize_pc(reinterpret_cast<void*>(PC),
+                               SymbolizedFMT, PcDescr, sizeof(PcDescr));
+  PcDescr[sizeof(PcDescr) - 1] = 0;  // Just in case.
+  return PcDescr;
+}
+
+void PrintPC(const char *SymbolizedFMT, const char *FallbackFMT, uintptr_t PC) {
+  if (EF->__sanitizer_symbolize_pc)
+    Printf("%s", DescribePC(SymbolizedFMT, PC).c_str());
+  else
+    Printf(FallbackFMT, PC);
+}
+
+void PrintStackTrace() {
+  std::unique_lock<std::mutex> l(SymbolizeMutex, std::try_to_lock);
+  if (EF->__sanitizer_print_stack_trace && l.owns_lock())
+    EF->__sanitizer_print_stack_trace();
+}
+
+void PrintMemoryProfile() {
+  std::unique_lock<std::mutex> l(SymbolizeMutex, std::try_to_lock);
+  if (EF->__sanitizer_print_memory_profile && l.owns_lock())
+    EF->__sanitizer_print_memory_profile(95, 8);
+}
+
+unsigned NumberOfCpuCores() {
+  unsigned N = std::thread::hardware_concurrency();
+  if (!N) {
+    Printf("WARNING: std::thread::hardware_concurrency not well defined for "
+           "your platform. Assuming CPU count of 1.\n");
+    N = 1;
+  }
+  return N;
+}
+
+size_t SimpleFastHash(const uint8_t *Data, size_t Size) {
+  size_t Res = 0;
+  for (size_t i = 0; i < Size; i++)
+    Res = Res * 11 + Data[i];
+  return Res;
+}
+
+}  // namespace fuzzer
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtil.h b/tools/fuzzing/libfuzzer/FuzzerUtil.h
new file mode 100644
index 0000000000..4ae3583830
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtil.h
@@ -0,0 +1,111 @@
+//===- FuzzerUtil.h - Internal header for the Fuzzer Utils ------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Util functions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_UTIL_H
+#define LLVM_FUZZER_UTIL_H
+
+#include "FuzzerBuiltins.h"
+#include "FuzzerBuiltinsMsvc.h"
+#include "FuzzerCommand.h"
+#include "FuzzerDefs.h"
+
+namespace fuzzer {
+
+void PrintHexArray(const Unit &U, const char *PrintAfter = "");
+
+void PrintHexArray(const uint8_t *Data, size_t Size,
+                   const char *PrintAfter = "");
+
+void PrintASCII(const uint8_t *Data, size_t Size, const char *PrintAfter = "");
+
+void PrintASCII(const Unit &U, const char *PrintAfter = "");
+
+// Changes U to contain only ASCII (isprint+isspace) characters.
+// Returns true iff U has been changed.
+bool ToASCII(uint8_t *Data, size_t Size);
+
+bool IsASCII(const Unit &U);
+
+bool IsASCII(const uint8_t *Data, size_t Size);
+
+std::string Base64(const Unit &U);
+
+void PrintPC(const char *SymbolizedFMT, const char *FallbackFMT, uintptr_t PC);
+
+std::string DescribePC(const char *SymbolizedFMT, uintptr_t PC);
+
+void PrintStackTrace();
+
+void PrintMemoryProfile();
+
+unsigned NumberOfCpuCores();
+
+// Platform specific functions.
+void SetSignalHandler(const FuzzingOptions& Options);
+
+void SleepSeconds(int Seconds);
+
+unsigned long GetPid();
+
+size_t GetPeakRSSMb();
+
+int ExecuteCommand(const Command &Cmd);
+bool ExecuteCommand(const Command &Cmd, std::string *CmdOutput);
+
+// Fuchsia does not have popen/pclose.
+FILE *OpenProcessPipe(const char *Command, const char *Mode);
+int CloseProcessPipe(FILE *F);
+
+const void *SearchMemory(const void *haystack, size_t haystacklen,
+                         const void *needle, size_t needlelen);
+
+std::string CloneArgsWithoutX(const Vector<std::string> &Args,
+                              const char *X1, const char *X2);
+
+inline std::string CloneArgsWithoutX(const Vector<std::string> &Args,
+                                     const char *X) {
+  return CloneArgsWithoutX(Args, X, X);
+}
+
+inline std::pair<std::string, std::string> SplitBefore(std::string X,
+                                                       std::string S) {
+  auto Pos = S.find(X);
+  if (Pos == std::string::npos)
+    return std::make_pair(S, "");
+  return std::make_pair(S.substr(0, Pos), S.substr(Pos));
+}
+
+void DiscardOutput(int Fd);
+
+std::string DisassembleCmd(const std::string &FileName);
+
+std::string SearchRegexCmd(const std::string &Regex);
+
+size_t SimpleFastHash(const uint8_t *Data, size_t Size);
+
+inline uint32_t Log(uint32_t X) { return 32 - Clz(X) - 1; }
+
+inline size_t PageSize() { return 4096; }
+inline uint8_t *RoundUpByPage(uint8_t *P) {
+  uintptr_t X = reinterpret_cast<uintptr_t>(P);
+  size_t Mask = PageSize() - 1;
+  X = (X + Mask) & ~Mask;
+  return reinterpret_cast<uint8_t *>(X);
+}
+inline uint8_t *RoundDownByPage(uint8_t *P) {
+  uintptr_t X = reinterpret_cast<uintptr_t>(P);
+  size_t Mask = PageSize() - 1;
+  X = X & ~Mask;
+  return reinterpret_cast<uint8_t *>(X);
+}
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_UTIL_H
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtilDarwin.cpp b/tools/fuzzing/libfuzzer/FuzzerUtilDarwin.cpp
new file mode 100644
index 0000000000..a5bed658a4
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtilDarwin.cpp
@@ -0,0 +1,170 @@
+//===- FuzzerUtilDarwin.cpp - Misc utils ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils for Darwin.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_APPLE
+#include "FuzzerCommand.h"
+#include "FuzzerIO.h"
+#include <mutex>
+#include <signal.h>
+#include <spawn.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+// There is no header for this on macOS so declare here
+extern "C" char **environ;
+
+namespace fuzzer {
+
+static std::mutex SignalMutex;
+// Global variables used to keep track of how signal handling should be
+// restored. They should **not** be accessed without holding `SignalMutex`.
+static int ActiveThreadCount = 0;
+static struct sigaction OldSigIntAction;
+static struct sigaction OldSigQuitAction;
+static sigset_t OldBlockedSignalsSet;
+
+// This is a reimplementation of Libc's `system()`. On Darwin the Libc
+// implementation contains a mutex which prevents it from being used
+// concurrently. This implementation **can** be used concurrently. It sets the
+// signal handlers when the first thread enters and restores them when the last
+// thread finishes execution of the function and ensures this is not racey by
+// using a mutex.
+int ExecuteCommand(const Command &Cmd) {
+  std::string CmdLine = Cmd.toString();
+  posix_spawnattr_t SpawnAttributes;
+  if (posix_spawnattr_init(&SpawnAttributes))
+    return -1;
+  // Block and ignore signals of the current process when the first thread
+  // enters.
+  {
+    std::lock_guard<std::mutex> Lock(SignalMutex);
+    if (ActiveThreadCount == 0) {
+      static struct sigaction IgnoreSignalAction;
+      sigset_t BlockedSignalsSet;
+      memset(&IgnoreSignalAction, 0, sizeof(IgnoreSignalAction));
+      IgnoreSignalAction.sa_handler = SIG_IGN;
+
+      if (sigaction(SIGINT, &IgnoreSignalAction, &OldSigIntAction) == -1) {
+        Printf("Failed to ignore SIGINT\n");
+        (void)posix_spawnattr_destroy(&SpawnAttributes);
+        return -1;
+      }
+      if (sigaction(SIGQUIT, &IgnoreSignalAction, &OldSigQuitAction) == -1) {
+        Printf("Failed to ignore SIGQUIT\n");
+        // Try our best to restore the signal handlers.
+        (void)sigaction(SIGINT, &OldSigIntAction, NULL);
+        (void)posix_spawnattr_destroy(&SpawnAttributes);
+        return -1;
+      }
+
+      (void)sigemptyset(&BlockedSignalsSet);
+      (void)sigaddset(&BlockedSignalsSet, SIGCHLD);
+      if (sigprocmask(SIG_BLOCK, &BlockedSignalsSet, &OldBlockedSignalsSet) ==
+          -1) {
+        Printf("Failed to block SIGCHLD\n");
+        // Try our best to restore the signal handlers.
+        (void)sigaction(SIGQUIT, &OldSigQuitAction, NULL);
+        (void)sigaction(SIGINT, &OldSigIntAction, NULL);
+        (void)posix_spawnattr_destroy(&SpawnAttributes);
+        return -1;
+      }
+    }
+    ++ActiveThreadCount;
+  }
+
+  // NOTE: Do not introduce any new `return` statements past this
+  // point. It is important that `ActiveThreadCount` always be decremented
+  // when leaving this function.
+
+  // Make sure the child process uses the default handlers for the
+  // following signals rather than inheriting what the parent has.
+  sigset_t DefaultSigSet;
+  (void)sigemptyset(&DefaultSigSet);
+  (void)sigaddset(&DefaultSigSet, SIGQUIT);
+  (void)sigaddset(&DefaultSigSet, SIGINT);
+  (void)posix_spawnattr_setsigdefault(&SpawnAttributes, &DefaultSigSet);
+  // Make sure the child process doesn't block SIGCHLD
+  (void)posix_spawnattr_setsigmask(&SpawnAttributes, &OldBlockedSignalsSet);
+  short SpawnFlags = POSIX_SPAWN_SETSIGDEF | POSIX_SPAWN_SETSIGMASK;
+  (void)posix_spawnattr_setflags(&SpawnAttributes, SpawnFlags);
+
+  pid_t Pid;
+  char **Environ = environ; // Read from global
+  const char *CommandCStr = CmdLine.c_str();
+  char *const Argv[] = {
+    strdup("sh"),
+    strdup("-c"),
+    strdup(CommandCStr),
+    NULL
+  };
+  int ErrorCode = 0, ProcessStatus = 0;
+  // FIXME: We probably shouldn't hardcode the shell path.
+  ErrorCode = posix_spawn(&Pid, "/bin/sh", NULL, &SpawnAttributes,
+                          Argv, Environ);
+  (void)posix_spawnattr_destroy(&SpawnAttributes);
+  if (!ErrorCode) {
+    pid_t SavedPid = Pid;
+    do {
+      // Repeat until call completes uninterrupted.
+      Pid = waitpid(SavedPid, &ProcessStatus, /*options=*/0);
+    } while (Pid == -1 && errno == EINTR);
+    if (Pid == -1) {
+      // Fail for some other reason.
+      ProcessStatus = -1;
+    }
+  } else if (ErrorCode == ENOMEM || ErrorCode == EAGAIN) {
+    // Fork failure.
+    ProcessStatus = -1;
+  } else {
+    // Shell execution failure.
+    ProcessStatus = W_EXITCODE(127, 0);
+  }
+  for (unsigned i = 0, n = sizeof(Argv) / sizeof(Argv[0]); i < n; ++i)
+    free(Argv[i]);
+
+  // Restore the signal handlers of the current process when the last thread
+  // using this function finishes.
+  {
+    std::lock_guard<std::mutex> Lock(SignalMutex);
+    --ActiveThreadCount;
+    if (ActiveThreadCount == 0) {
+      bool FailedRestore = false;
+      if (sigaction(SIGINT, &OldSigIntAction, NULL) == -1) {
+        Printf("Failed to restore SIGINT handling\n");
+        FailedRestore = true;
+      }
+      if (sigaction(SIGQUIT, &OldSigQuitAction, NULL) == -1) {
+        Printf("Failed to restore SIGQUIT handling\n");
+        FailedRestore = true;
+      }
+      if (sigprocmask(SIG_BLOCK, &OldBlockedSignalsSet, NULL) == -1) {
+        Printf("Failed to unblock SIGCHLD\n");
+        FailedRestore = true;
+      }
+      if (FailedRestore)
+        ProcessStatus = -1;
+    }
+  }
+  return ProcessStatus;
+}
+
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("/dev/null", "w");
+  if (!Temp)
+    return;
+  dup2(fileno(Temp), Fd);
+  fclose(Temp);
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_APPLE
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtilFuchsia.cpp b/tools/fuzzing/libfuzzer/FuzzerUtilFuchsia.cpp
new file mode 100644
index 0000000000..190fb78666
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtilFuchsia.cpp
@@ -0,0 +1,565 @@
+//===- FuzzerUtilFuchsia.cpp - Misc utils for Fuchsia. --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils implementation using Fuchsia/Zircon APIs.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+
+#if LIBFUZZER_FUCHSIA
+
+#include "FuzzerInternal.h"
+#include "FuzzerUtil.h"
+#include <cassert>
+#include <cerrno>
+#include <cinttypes>
+#include <cstdint>
+#include <fcntl.h>
+#include <lib/fdio/fdio.h>
+#include <lib/fdio/spawn.h>
+#include <string>
+#include <sys/select.h>
+#include <thread>
+#include <unistd.h>
+#include <zircon/errors.h>
+#include <zircon/process.h>
+#include <zircon/sanitizer.h>
+#include <zircon/status.h>
+#include <zircon/syscalls.h>
+#include <zircon/syscalls/debug.h>
+#include <zircon/syscalls/exception.h>
+#include <zircon/syscalls/object.h>
+#include <zircon/types.h>
+
+#include <vector>
+
+namespace fuzzer {
+
+// Given that Fuchsia doesn't have the POSIX signals that libFuzzer was written
+// around, the general approach is to spin up dedicated threads to watch for
+// each requested condition (alarm, interrupt, crash).  Of these, the crash
+// handler is the most involved, as it requires resuming the crashed thread in
+// order to invoke the sanitizers to get the needed state.
+
+// Forward declaration of assembly trampoline needed to resume crashed threads.
+// This appears to have external linkage to  C++, which is why it's not in the
+// anonymous namespace.  The assembly definition inside MakeTrampoline()
+// actually defines the symbol with internal linkage only.
+void CrashTrampolineAsm() __asm__("CrashTrampolineAsm");
+
+namespace {
+
+// Helper function to handle Zircon syscall failures.
+void ExitOnErr(zx_status_t Status, const char *Syscall) {
+  if (Status != ZX_OK) {
+    Printf("libFuzzer: %s failed: %s\n", Syscall,
+           _zx_status_get_string(Status));
+    exit(1);
+  }
+}
+
+void AlarmHandler(int Seconds) {
+  while (true) {
+    SleepSeconds(Seconds);
+    Fuzzer::StaticAlarmCallback();
+  }
+}
+
+void InterruptHandler() {
+  fd_set readfds;
+  // Ctrl-C sends ETX in Zircon.
+  do {
+    FD_ZERO(&readfds);
+    FD_SET(STDIN_FILENO, &readfds);
+    select(STDIN_FILENO + 1, &readfds, nullptr, nullptr, nullptr);
+  } while(!FD_ISSET(STDIN_FILENO, &readfds) || getchar() != 0x03);
+  Fuzzer::StaticInterruptCallback();
+}
+
+// CFAOffset is used to reference the stack pointer before entering the
+// trampoline (Stack Pointer + CFAOffset = prev Stack Pointer). Before jumping
+// to the trampoline we copy all the registers onto the stack. We need to make
+// sure that the new stack has enough space to store all the registers.
+//
+// The trampoline holds CFI information regarding the registers stored in the
+// stack, which is then used by the unwinder to restore them.
+#if defined(__x86_64__)
+// In x86_64 the crashing function might also be using the red zone (128 bytes
+// on top of their rsp).
+constexpr size_t CFAOffset = 128 + sizeof(zx_thread_state_general_regs_t);
+#elif defined(__aarch64__)
+// In aarch64 we need to always have the stack pointer aligned to 16 bytes, so we
+// make sure that we are keeping that same alignment.
+constexpr size_t CFAOffset = (sizeof(zx_thread_state_general_regs_t) + 15) & -(uintptr_t)16;
+#endif
+
+// For the crash handler, we need to call Fuzzer::StaticCrashSignalCallback
+// without POSIX signal handlers.  To achieve this, we use an assembly function
+// to add the necessary CFI unwinding information and a C function to bridge
+// from that back into C++.
+
+// FIXME: This works as a short-term solution, but this code really shouldn't be
+// architecture dependent. A better long term solution is to implement remote
+// unwinding and expose the necessary APIs through sanitizer_common and/or ASAN
+// to allow the exception handling thread to gather the crash state directly.
+//
+// Alternatively, Fuchsia may in future actually implement basic signal
+// handling for the machine trap signals.
+#if defined(__x86_64__)
+#define FOREACH_REGISTER(OP_REG, OP_NUM) \
+  OP_REG(rax)                            \
+  OP_REG(rbx)                            \
+  OP_REG(rcx)                            \
+  OP_REG(rdx)                            \
+  OP_REG(rsi)                            \
+  OP_REG(rdi)                            \
+  OP_REG(rbp)                            \
+  OP_REG(rsp)                            \
+  OP_REG(r8)                             \
+  OP_REG(r9)                             \
+  OP_REG(r10)                            \
+  OP_REG(r11)                            \
+  OP_REG(r12)                            \
+  OP_REG(r13)                            \
+  OP_REG(r14)                            \
+  OP_REG(r15)                            \
+  OP_REG(rip)
+
+#elif defined(__aarch64__)
+#define FOREACH_REGISTER(OP_REG, OP_NUM) \
+  OP_NUM(0)                              \
+  OP_NUM(1)                              \
+  OP_NUM(2)                              \
+  OP_NUM(3)                              \
+  OP_NUM(4)                              \
+  OP_NUM(5)                              \
+  OP_NUM(6)                              \
+  OP_NUM(7)                              \
+  OP_NUM(8)                              \
+  OP_NUM(9)                              \
+  OP_NUM(10)                             \
+  OP_NUM(11)                             \
+  OP_NUM(12)                             \
+  OP_NUM(13)                             \
+  OP_NUM(14)                             \
+  OP_NUM(15)                             \
+  OP_NUM(16)                             \
+  OP_NUM(17)                             \
+  OP_NUM(18)                             \
+  OP_NUM(19)                             \
+  OP_NUM(20)                             \
+  OP_NUM(21)                             \
+  OP_NUM(22)                             \
+  OP_NUM(23)                             \
+  OP_NUM(24)                             \
+  OP_NUM(25)                             \
+  OP_NUM(26)                             \
+  OP_NUM(27)                             \
+  OP_NUM(28)                             \
+  OP_NUM(29)                             \
+  OP_REG(sp)
+
+#else
+#error "Unsupported architecture for fuzzing on Fuchsia"
+#endif
+
+// Produces a CFI directive for the named or numbered register.
+// The value used refers to an assembler immediate operand with the same name
+// as the register (see ASM_OPERAND_REG).
+#define CFI_OFFSET_REG(reg) ".cfi_offset " #reg ", %c[" #reg "]\n"
+#define CFI_OFFSET_NUM(num) CFI_OFFSET_REG(x##num)
+
+// Produces an assembler immediate operand for the named or numbered register.
+// This operand contains the offset of the register relative to the CFA.
+#define ASM_OPERAND_REG(reg) \
+  [reg] "i"(offsetof(zx_thread_state_general_regs_t, reg) - CFAOffset),
+#define ASM_OPERAND_NUM(num)                                 \
+  [x##num] "i"(offsetof(zx_thread_state_general_regs_t, r[num]) - CFAOffset),
+
+// Trampoline to bridge from the assembly below to the static C++ crash
+// callback.
+__attribute__((noreturn))
+static void StaticCrashHandler() {
+  Fuzzer::StaticCrashSignalCallback();
+  for (;;) {
+    _Exit(1);
+  }
+}
+
+// Creates the trampoline with the necessary CFI information to unwind through
+// to the crashing call stack:
+//  * Defining the CFA so that it points to the stack pointer at the point
+//    of crash.
+//  * Storing all registers at the point of crash in the stack and refer to them
+//    via CFI information (relative to the CFA).
+//  * Setting the return column so the unwinder knows how to continue unwinding.
+//  * (x86_64) making sure rsp is aligned before calling StaticCrashHandler.
+//  * Calling StaticCrashHandler that will trigger the unwinder.
+//
+// The __attribute__((used)) is necessary because the function
+// is never called; it's just a container around the assembly to allow it to
+// use operands for compile-time computed constants.
+__attribute__((used))
+void MakeTrampoline() {
+  __asm__(".cfi_endproc\n"
+    ".pushsection .text.CrashTrampolineAsm\n"
+    ".type CrashTrampolineAsm,STT_FUNC\n"
+"CrashTrampolineAsm:\n"
+    ".cfi_startproc simple\n"
+    ".cfi_signal_frame\n"
+#if defined(__x86_64__)
+    ".cfi_return_column rip\n"
+    ".cfi_def_cfa rsp, %c[CFAOffset]\n"
+    FOREACH_REGISTER(CFI_OFFSET_REG, CFI_OFFSET_NUM)
+    "mov %%rsp, %%rbp\n"
+    ".cfi_def_cfa_register rbp\n"
+    "andq $-16, %%rsp\n"
+    "call %c[StaticCrashHandler]\n"
+    "ud2\n"
+#elif defined(__aarch64__)
+    ".cfi_return_column 33\n"
+    ".cfi_def_cfa sp, %c[CFAOffset]\n"
+    FOREACH_REGISTER(CFI_OFFSET_REG, CFI_OFFSET_NUM)
+    ".cfi_offset 33, %c[pc]\n"
+    ".cfi_offset 30, %c[lr]\n"
+    "bl %c[StaticCrashHandler]\n"
+    "brk 1\n"
+#else
+#error "Unsupported architecture for fuzzing on Fuchsia"
+#endif
+    ".cfi_endproc\n"
+    ".size CrashTrampolineAsm, . - CrashTrampolineAsm\n"
+    ".popsection\n"
+    ".cfi_startproc\n"
+    : // No outputs
+    : FOREACH_REGISTER(ASM_OPERAND_REG, ASM_OPERAND_NUM)
+#if defined(__aarch64__)
+      ASM_OPERAND_REG(pc)
+      ASM_OPERAND_REG(lr)
+#endif
+      [StaticCrashHandler] "i" (StaticCrashHandler),
+      [CFAOffset] "i" (CFAOffset));
+}
+
+void CrashHandler(zx_handle_t *Event) {
+  // This structure is used to ensure we close handles to objects we create in
+  // this handler.
+  struct ScopedHandle {
+    ~ScopedHandle() { _zx_handle_close(Handle); }
+    zx_handle_t Handle = ZX_HANDLE_INVALID;
+  };
+
+  // Create the exception channel.  We need to claim to be a "debugger" so the
+  // kernel will allow us to modify and resume dying threads (see below). Once
+  // the channel is set, we can signal the main thread to continue and wait
+  // for the exception to arrive.
+  ScopedHandle Channel;
+  zx_handle_t Self = _zx_process_self();
+  ExitOnErr(_zx_task_create_exception_channel(
+                Self, ZX_EXCEPTION_CHANNEL_DEBUGGER, &Channel.Handle),
+            "_zx_task_create_exception_channel");
+
+  ExitOnErr(_zx_object_signal(*Event, 0, ZX_USER_SIGNAL_0),
+            "_zx_object_signal");
+
+  // This thread lives as long as the process in order to keep handling
+  // crashes.  In practice, the first crashed thread to reach the end of the
+  // StaticCrashHandler will end the process.
+  while (true) {
+    ExitOnErr(_zx_object_wait_one(Channel.Handle, ZX_CHANNEL_READABLE,
+                                  ZX_TIME_INFINITE, nullptr),
+              "_zx_object_wait_one");
+
+    zx_exception_info_t ExceptionInfo;
+    ScopedHandle Exception;
+    ExitOnErr(_zx_channel_read(Channel.Handle, 0, &ExceptionInfo,
+                               &Exception.Handle, sizeof(ExceptionInfo), 1,
+                               nullptr, nullptr),
+              "_zx_channel_read");
+
+    // Ignore informational synthetic exceptions.
+    if (ZX_EXCP_THREAD_STARTING == ExceptionInfo.type ||
+        ZX_EXCP_THREAD_EXITING == ExceptionInfo.type ||
+        ZX_EXCP_PROCESS_STARTING == ExceptionInfo.type) {
+      continue;
+    }
+
+    // At this point, we want to get the state of the crashing thread, but
+    // libFuzzer and the sanitizers assume this will happen from that same
+    // thread via a POSIX signal handler. "Resurrecting" the thread in the
+    // middle of the appropriate callback is as simple as forcibly setting the
+    // instruction pointer/program counter, provided we NEVER EVER return from
+    // that function (since otherwise our stack will not be valid).
+    ScopedHandle Thread;
+    ExitOnErr(_zx_exception_get_thread(Exception.Handle, &Thread.Handle),
+              "_zx_exception_get_thread");
+
+    zx_thread_state_general_regs_t GeneralRegisters;
+    ExitOnErr(_zx_thread_read_state(Thread.Handle, ZX_THREAD_STATE_GENERAL_REGS,
+                                    &GeneralRegisters,
+                                    sizeof(GeneralRegisters)),
+              "_zx_thread_read_state");
+
+    // To unwind properly, we need to push the crashing thread's register state
+    // onto the stack and jump into a trampoline with CFI instructions on how
+    // to restore it.
+#if defined(__x86_64__)
+    uintptr_t StackPtr = GeneralRegisters.rsp - CFAOffset;
+    __unsanitized_memcpy(reinterpret_cast<void *>(StackPtr), &GeneralRegisters,
+                         sizeof(GeneralRegisters));
+    GeneralRegisters.rsp = StackPtr;
+    GeneralRegisters.rip = reinterpret_cast<zx_vaddr_t>(CrashTrampolineAsm);
+
+#elif defined(__aarch64__)
+    uintptr_t StackPtr = GeneralRegisters.sp - CFAOffset;
+    __unsanitized_memcpy(reinterpret_cast<void *>(StackPtr), &GeneralRegisters,
+                         sizeof(GeneralRegisters));
+    GeneralRegisters.sp = StackPtr;
+    GeneralRegisters.pc = reinterpret_cast<zx_vaddr_t>(CrashTrampolineAsm);
+
+#else
+#error "Unsupported architecture for fuzzing on Fuchsia"
+#endif
+
+    // Now force the crashing thread's state.
+    ExitOnErr(
+        _zx_thread_write_state(Thread.Handle, ZX_THREAD_STATE_GENERAL_REGS,
+                               &GeneralRegisters, sizeof(GeneralRegisters)),
+        "_zx_thread_write_state");
+
+    // Set the exception to HANDLED so it resumes the thread on close.
+    uint32_t ExceptionState = ZX_EXCEPTION_STATE_HANDLED;
+    ExitOnErr(_zx_object_set_property(Exception.Handle, ZX_PROP_EXCEPTION_STATE,
+                                      &ExceptionState, sizeof(ExceptionState)),
+              "zx_object_set_property");
+  }
+}
+
+} // namespace
+
+// Platform specific functions.
+void SetSignalHandler(const FuzzingOptions &Options) {
+  // Make sure information from libFuzzer and the sanitizers are easy to
+  // reassemble. `__sanitizer_log_write` has the added benefit of ensuring the
+  // DSO map is always available for the symbolizer.
+  // A uint64_t fits in 20 chars, so 64 is plenty.
+  char Buf[64];
+  memset(Buf, 0, sizeof(Buf));
+  snprintf(Buf, sizeof(Buf), "==%lu== INFO: libFuzzer starting.\n", GetPid());
+  if (EF->__sanitizer_log_write)
+    __sanitizer_log_write(Buf, sizeof(Buf));
+  Printf("%s", Buf);
+
+  // Set up alarm handler if needed.
+  if (Options.UnitTimeoutSec > 0) {
+    std::thread T(AlarmHandler, Options.UnitTimeoutSec / 2 + 1);
+    T.detach();
+  }
+
+  // Set up interrupt handler if needed.
+  if (Options.HandleInt || Options.HandleTerm) {
+    std::thread T(InterruptHandler);
+    T.detach();
+  }
+
+  // Early exit if no crash handler needed.
+  if (!Options.HandleSegv && !Options.HandleBus && !Options.HandleIll &&
+      !Options.HandleFpe && !Options.HandleAbrt)
+    return;
+
+  // Set up the crash handler and wait until it is ready before proceeding.
+  zx_handle_t Event;
+  ExitOnErr(_zx_event_create(0, &Event), "_zx_event_create");
+
+  std::thread T(CrashHandler, &Event);
+  zx_status_t Status =
+      _zx_object_wait_one(Event, ZX_USER_SIGNAL_0, ZX_TIME_INFINITE, nullptr);
+  _zx_handle_close(Event);
+  ExitOnErr(Status, "_zx_object_wait_one");
+
+  T.detach();
+}
+
+void SleepSeconds(int Seconds) {
+  _zx_nanosleep(_zx_deadline_after(ZX_SEC(Seconds)));
+}
+
+unsigned long GetPid() {
+  zx_status_t rc;
+  zx_info_handle_basic_t Info;
+  if ((rc = _zx_object_get_info(_zx_process_self(), ZX_INFO_HANDLE_BASIC, &Info,
+                                sizeof(Info), NULL, NULL)) != ZX_OK) {
+    Printf("libFuzzer: unable to get info about self: %s\n",
+           _zx_status_get_string(rc));
+    exit(1);
+  }
+  return Info.koid;
+}
+
+size_t GetPeakRSSMb() {
+  zx_status_t rc;
+  zx_info_task_stats_t Info;
+  if ((rc = _zx_object_get_info(_zx_process_self(), ZX_INFO_TASK_STATS, &Info,
+                                sizeof(Info), NULL, NULL)) != ZX_OK) {
+    Printf("libFuzzer: unable to get info about self: %s\n",
+           _zx_status_get_string(rc));
+    exit(1);
+  }
+  return (Info.mem_private_bytes + Info.mem_shared_bytes) >> 20;
+}
+
+template <typename Fn>
+class RunOnDestruction {
+ public:
+  explicit RunOnDestruction(Fn fn) : fn_(fn) {}
+  ~RunOnDestruction() { fn_(); }
+
+ private:
+  Fn fn_;
+};
+
+template <typename Fn>
+RunOnDestruction<Fn> at_scope_exit(Fn fn) {
+  return RunOnDestruction<Fn>(fn);
+}
+
+static fdio_spawn_action_t clone_fd_action(int localFd, int targetFd) {
+  return {
+      .action = FDIO_SPAWN_ACTION_CLONE_FD,
+      .fd =
+          {
+              .local_fd = localFd,
+              .target_fd = targetFd,
+          },
+  };
+}
+
+int ExecuteCommand(const Command &Cmd) {
+  zx_status_t rc;
+
+  // Convert arguments to C array
+  auto Args = Cmd.getArguments();
+  size_t Argc = Args.size();
+  assert(Argc != 0);
+  std::unique_ptr<const char *[]> Argv(new const char *[Argc + 1]);
+  for (size_t i = 0; i < Argc; ++i)
+    Argv[i] = Args[i].c_str();
+  Argv[Argc] = nullptr;
+
+  // Determine output.  On Fuchsia, the fuzzer is typically run as a component
+  // that lacks a mutable working directory. Fortunately, when this is the case
+  // a mutable output directory must be specified using "-artifact_prefix=...",
+  // so write the log file(s) there.
+  // However, we don't want to apply this logic for absolute paths.
+  int FdOut = STDOUT_FILENO;
+  bool discardStdout = false;
+  bool discardStderr = false;
+
+  if (Cmd.hasOutputFile()) {
+    std::string Path = Cmd.getOutputFile();
+    if (Path == getDevNull()) {
+      // On Fuchsia, there's no "/dev/null" like-file, so we
+      // just don't copy the FDs into the spawned process.
+      discardStdout = true;
+    } else {
+      bool IsAbsolutePath = Path.length() > 1 && Path[0] == '/';
+      if (!IsAbsolutePath && Cmd.hasFlag("artifact_prefix"))
+        Path = Cmd.getFlagValue("artifact_prefix") + "/" + Path;
+
+      FdOut = open(Path.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
+      if (FdOut == -1) {
+        Printf("libFuzzer: failed to open %s: %s\n", Path.c_str(),
+               strerror(errno));
+        return ZX_ERR_IO;
+      }
+    }
+  }
+  auto CloseFdOut = at_scope_exit([FdOut]() {
+    if (FdOut != STDOUT_FILENO)
+      close(FdOut);
+  });
+
+  // Determine stderr
+  int FdErr = STDERR_FILENO;
+  if (Cmd.isOutAndErrCombined()) {
+    FdErr = FdOut;
+    if (discardStdout)
+      discardStderr = true;
+  }
+
+  // Clone the file descriptors into the new process
+  std::vector<fdio_spawn_action_t> SpawnActions;
+  SpawnActions.push_back(clone_fd_action(STDIN_FILENO, STDIN_FILENO));
+
+  if (!discardStdout)
+    SpawnActions.push_back(clone_fd_action(FdOut, STDOUT_FILENO));
+  if (!discardStderr)
+    SpawnActions.push_back(clone_fd_action(FdErr, STDERR_FILENO));
+
+  // Start the process.
+  char ErrorMsg[FDIO_SPAWN_ERR_MSG_MAX_LENGTH];
+  zx_handle_t ProcessHandle = ZX_HANDLE_INVALID;
+  rc = fdio_spawn_etc(ZX_HANDLE_INVALID,
+                      FDIO_SPAWN_CLONE_ALL & (~FDIO_SPAWN_CLONE_STDIO), Argv[0],
+                      Argv.get(), nullptr, SpawnActions.size(),
+                      SpawnActions.data(), &ProcessHandle, ErrorMsg);
+
+  if (rc != ZX_OK) {
+    Printf("libFuzzer: failed to launch '%s': %s, %s\n", Argv[0], ErrorMsg,
+           _zx_status_get_string(rc));
+    return rc;
+  }
+  auto CloseHandle = at_scope_exit([&]() { _zx_handle_close(ProcessHandle); });
+
+  // Now join the process and return the exit status.
+  if ((rc = _zx_object_wait_one(ProcessHandle, ZX_PROCESS_TERMINATED,
+                                ZX_TIME_INFINITE, nullptr)) != ZX_OK) {
+    Printf("libFuzzer: failed to join '%s': %s\n", Argv[0],
+           _zx_status_get_string(rc));
+    return rc;
+  }
+
+  zx_info_process_t Info;
+  if ((rc = _zx_object_get_info(ProcessHandle, ZX_INFO_PROCESS, &Info,
+                                sizeof(Info), nullptr, nullptr)) != ZX_OK) {
+    Printf("libFuzzer: unable to get return code from '%s': %s\n", Argv[0],
+           _zx_status_get_string(rc));
+    return rc;
+  }
+
+  return Info.return_code;
+}
+
+bool ExecuteCommand(const Command &BaseCmd, std::string *CmdOutput) {
+  auto LogFilePath = TempPath("SimPopenOut", ".txt");
+  Command Cmd(BaseCmd);
+  Cmd.setOutputFile(LogFilePath);
+  int Ret = ExecuteCommand(Cmd);
+  *CmdOutput = FileToString(LogFilePath);
+  RemoveFile(LogFilePath);
+  return Ret == 0;
+}
+
+const void *SearchMemory(const void *Data, size_t DataLen, const void *Patt,
+                         size_t PattLen) {
+  return memmem(Data, DataLen, Patt, PattLen);
+}
+
+// In fuchsia, accessing /dev/null is not supported. There's nothing
+// similar to a file that discards everything that is written to it.
+// The way of doing something similar in fuchsia is by using
+// fdio_null_create and binding that to a file descriptor.
+void DiscardOutput(int Fd) {
+  fdio_t *fdio_null = fdio_null_create();
+  if (fdio_null == nullptr) return;
+  int nullfd = fdio_bind_to_fd(fdio_null, -1, 0);
+  if (nullfd < 0) return;
+  dup2(nullfd, Fd);
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_FUCHSIA
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtilLinux.cpp b/tools/fuzzing/libfuzzer/FuzzerUtilLinux.cpp
new file mode 100644
index 0000000000..95490b992e
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtilLinux.cpp
@@ -0,0 +1,41 @@
+//===- FuzzerUtilLinux.cpp - Misc utils for Linux. ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils for Linux.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FREEBSD ||                \
+    LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN
+#include "FuzzerCommand.h"
+
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+
+namespace fuzzer {
+
+int ExecuteCommand(const Command &Cmd) {
+  std::string CmdLine = Cmd.toString();
+  int exit_code = system(CmdLine.c_str());
+  if (WIFEXITED(exit_code))
+    return WEXITSTATUS(exit_code);
+  return exit_code;
+}
+
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("/dev/null", "w");
+  if (!Temp)
+    return;
+  dup2(fileno(Temp), Fd);
+  fclose(Temp);
+}
+
+} // namespace fuzzer
+
+#endif
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtilPosix.cpp b/tools/fuzzing/libfuzzer/FuzzerUtilPosix.cpp
new file mode 100644
index 0000000000..fc57b724db
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtilPosix.cpp
@@ -0,0 +1,185 @@
+//===- FuzzerUtilPosix.cpp - Misc utils for Posix. ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils implementation using Posix API.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_POSIX
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include "FuzzerTracePC.h"
+#include <cassert>
+#include <chrono>
+#include <cstring>
+#include <errno.h>
+#include <iomanip>
+#include <signal.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <thread>
+#include <unistd.h>
+
+namespace fuzzer {
+
+static void AlarmHandler(int, siginfo_t *, void *) {
+  Fuzzer::StaticAlarmCallback();
+}
+
+static void (*upstream_segv_handler)(int, siginfo_t *, void *);
+
+static void SegvHandler(int sig, siginfo_t *si, void *ucontext) {
+  assert(si->si_signo == SIGSEGV);
+  if (upstream_segv_handler)
+    return upstream_segv_handler(sig, si, ucontext);
+  Fuzzer::StaticCrashSignalCallback();
+}
+
+static void CrashHandler(int, siginfo_t *, void *) {
+  Fuzzer::StaticCrashSignalCallback();
+}
+
+static void InterruptHandler(int, siginfo_t *, void *) {
+  Fuzzer::StaticInterruptCallback();
+}
+
+static void GracefulExitHandler(int, siginfo_t *, void *) {
+  Fuzzer::StaticGracefulExitCallback();
+}
+
+static void FileSizeExceedHandler(int, siginfo_t *, void *) {
+  Fuzzer::StaticFileSizeExceedCallback();
+}
+
+static void SetSigaction(int signum,
+                         void (*callback)(int, siginfo_t *, void *)) {
+  struct sigaction sigact = {};
+  if (sigaction(signum, nullptr, &sigact)) {
+    Printf("libFuzzer: sigaction failed with %d\n", errno);
+    exit(1);
+  }
+  if (sigact.sa_flags & SA_SIGINFO) {
+    if (sigact.sa_sigaction) {
+      if (signum != SIGSEGV)
+        return;
+      upstream_segv_handler = sigact.sa_sigaction;
+    }
+  } else {
+    if (sigact.sa_handler != SIG_DFL && sigact.sa_handler != SIG_IGN &&
+        sigact.sa_handler != SIG_ERR)
+      return;
+  }
+
+  sigact = {};
+  sigact.sa_flags = SA_SIGINFO;
+  sigact.sa_sigaction = callback;
+  if (sigaction(signum, &sigact, 0)) {
+    Printf("libFuzzer: sigaction failed with %d\n", errno);
+    exit(1);
+  }
+}
+
+// Return true on success, false otherwise.
+bool ExecuteCommand(const Command &Cmd, std::string *CmdOutput) {
+  FILE *Pipe = popen(Cmd.toString().c_str(), "r");
+  if (!Pipe)
+    return false;
+
+  if (CmdOutput) {
+    char TmpBuffer[128];
+    while (fgets(TmpBuffer, sizeof(TmpBuffer), Pipe))
+      CmdOutput->append(TmpBuffer);
+  }
+  return pclose(Pipe) == 0;
+}
+
+void SetTimer(int Seconds) {
+  struct itimerval T {
+    {Seconds, 0}, { Seconds, 0 }
+  };
+  if (setitimer(ITIMER_REAL, &T, nullptr)) {
+    Printf("libFuzzer: setitimer failed with %d\n", errno);
+    exit(1);
+  }
+  SetSigaction(SIGALRM, AlarmHandler);
+}
+
+void SetSignalHandler(const FuzzingOptions& Options) {
+  // setitimer is not implemented in emscripten.
+  if (Options.UnitTimeoutSec > 0 && !LIBFUZZER_EMSCRIPTEN)
+    SetTimer(Options.UnitTimeoutSec / 2 + 1);
+  if (Options.HandleInt)
+    SetSigaction(SIGINT, InterruptHandler);
+  if (Options.HandleTerm)
+    SetSigaction(SIGTERM, InterruptHandler);
+  if (Options.HandleSegv)
+    SetSigaction(SIGSEGV, SegvHandler);
+  if (Options.HandleBus)
+    SetSigaction(SIGBUS, CrashHandler);
+  if (Options.HandleAbrt)
+    SetSigaction(SIGABRT, CrashHandler);
+  if (Options.HandleIll)
+    SetSigaction(SIGILL, CrashHandler);
+  if (Options.HandleFpe)
+    SetSigaction(SIGFPE, CrashHandler);
+  if (Options.HandleXfsz)
+    SetSigaction(SIGXFSZ, FileSizeExceedHandler);
+  if (Options.HandleUsr1)
+    SetSigaction(SIGUSR1, GracefulExitHandler);
+  if (Options.HandleUsr2)
+    SetSigaction(SIGUSR2, GracefulExitHandler);
+}
+
+void SleepSeconds(int Seconds) {
+  sleep(Seconds); // Use C API to avoid coverage from instrumented libc++.
+}
+
+unsigned long GetPid() { return (unsigned long)getpid(); }
+
+size_t GetPeakRSSMb() {
+  struct rusage usage;
+  if (getrusage(RUSAGE_SELF, &usage))
+    return 0;
+  if (LIBFUZZER_LINUX || LIBFUZZER_FREEBSD || LIBFUZZER_NETBSD ||
+      LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN) {
+    // ru_maxrss is in KiB
+    return usage.ru_maxrss >> 10;
+  } else if (LIBFUZZER_APPLE) {
+    // ru_maxrss is in bytes
+    return usage.ru_maxrss >> 20;
+  }
+  assert(0 && "GetPeakRSSMb() is not implemented for your platform");
+  return 0;
+}
+
+FILE *OpenProcessPipe(const char *Command, const char *Mode) {
+  return popen(Command, Mode);
+}
+
+int CloseProcessPipe(FILE *F) {
+  return pclose(F);
+}
+
+const void *SearchMemory(const void *Data, size_t DataLen, const void *Patt,
+                         size_t PattLen) {
+  return memmem(Data, DataLen, Patt, PattLen);
+}
+
+std::string DisassembleCmd(const std::string &FileName) {
+  return "objdump -d " + FileName;
+}
+
+std::string SearchRegexCmd(const std::string &Regex) {
+  return "grep '" + Regex + "'";
+}
+
+}  // namespace fuzzer
+
+#endif // LIBFUZZER_POSIX
diff --git a/tools/fuzzing/libfuzzer/FuzzerUtilWindows.cpp b/tools/fuzzing/libfuzzer/FuzzerUtilWindows.cpp
new file mode 100644
index 0000000000..6c693e3d7e
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerUtilWindows.cpp
@@ -0,0 +1,221 @@
+//===- FuzzerUtilWindows.cpp - Misc utils for Windows. --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Misc utils implementation for Windows.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_WINDOWS
+#include "FuzzerCommand.h"
+#include "FuzzerIO.h"
+#include "FuzzerInternal.h"
+#include <cassert>
+#include <chrono>
+#include <cstring>
+#include <errno.h>
+#include <io.h>
+#include <iomanip>
+#include <signal.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <windows.h>
+
+// This must be included after windows.h.
+#include <psapi.h>
+
+namespace fuzzer {
+
+static const FuzzingOptions* HandlerOpt = nullptr;
+
+static LONG CALLBACK ExceptionHandler(PEXCEPTION_POINTERS ExceptionInfo) {
+  switch (ExceptionInfo->ExceptionRecord->ExceptionCode) {
+    case EXCEPTION_ACCESS_VIOLATION:
+    case EXCEPTION_ARRAY_BOUNDS_EXCEEDED:
+    case EXCEPTION_STACK_OVERFLOW:
+      if (HandlerOpt->HandleSegv)
+        Fuzzer::StaticCrashSignalCallback();
+      break;
+    case EXCEPTION_DATATYPE_MISALIGNMENT:
+    case EXCEPTION_IN_PAGE_ERROR:
+      if (HandlerOpt->HandleBus)
+        Fuzzer::StaticCrashSignalCallback();
+      break;
+    case EXCEPTION_ILLEGAL_INSTRUCTION:
+    case EXCEPTION_PRIV_INSTRUCTION:
+      if (HandlerOpt->HandleIll)
+        Fuzzer::StaticCrashSignalCallback();
+      break;
+    case EXCEPTION_FLT_DENORMAL_OPERAND:
+    case EXCEPTION_FLT_DIVIDE_BY_ZERO:
+    case EXCEPTION_FLT_INEXACT_RESULT:
+    case EXCEPTION_FLT_INVALID_OPERATION:
+    case EXCEPTION_FLT_OVERFLOW:
+    case EXCEPTION_FLT_STACK_CHECK:
+    case EXCEPTION_FLT_UNDERFLOW:
+    case EXCEPTION_INT_DIVIDE_BY_ZERO:
+    case EXCEPTION_INT_OVERFLOW:
+      if (HandlerOpt->HandleFpe)
+        Fuzzer::StaticCrashSignalCallback();
+      break;
+    // TODO: handle (Options.HandleXfsz)
+  }
+  return EXCEPTION_CONTINUE_SEARCH;
+}
+
+BOOL WINAPI CtrlHandler(DWORD dwCtrlType) {
+  switch (dwCtrlType) {
+    case CTRL_C_EVENT:
+      if (HandlerOpt->HandleInt)
+        Fuzzer::StaticInterruptCallback();
+      return TRUE;
+    case CTRL_BREAK_EVENT:
+      if (HandlerOpt->HandleTerm)
+        Fuzzer::StaticInterruptCallback();
+      return TRUE;
+  }
+  return FALSE;
+}
+
+void CALLBACK AlarmHandler(PVOID, BOOLEAN) {
+  Fuzzer::StaticAlarmCallback();
+}
+
+class TimerQ {
+  HANDLE TimerQueue;
+ public:
+  TimerQ() : TimerQueue(NULL) {}
+  ~TimerQ() {
+    if (TimerQueue)
+      DeleteTimerQueueEx(TimerQueue, NULL);
+  }
+  void SetTimer(int Seconds) {
+    if (!TimerQueue) {
+      TimerQueue = CreateTimerQueue();
+      if (!TimerQueue) {
+        Printf("libFuzzer: CreateTimerQueue failed.\n");
+        exit(1);
+      }
+    }
+    HANDLE Timer;
+    if (!CreateTimerQueueTimer(&Timer, TimerQueue, AlarmHandler, NULL,
+        Seconds*1000, Seconds*1000, 0)) {
+      Printf("libFuzzer: CreateTimerQueueTimer failed.\n");
+      exit(1);
+    }
+  }
+};
+
+static TimerQ Timer;
+
+static void CrashHandler(int) { Fuzzer::StaticCrashSignalCallback(); }
+
+void SetSignalHandler(const FuzzingOptions& Options) {
+  HandlerOpt = &Options;
+
+  if (Options.UnitTimeoutSec > 0)
+    Timer.SetTimer(Options.UnitTimeoutSec / 2 + 1);
+
+  if (Options.HandleInt || Options.HandleTerm)
+    if (!SetConsoleCtrlHandler(CtrlHandler, TRUE)) {
+      DWORD LastError = GetLastError();
+      Printf("libFuzzer: SetConsoleCtrlHandler failed (Error code: %lu).\n",
+        LastError);
+      exit(1);
+    }
+
+  if (Options.HandleSegv || Options.HandleBus || Options.HandleIll ||
+      Options.HandleFpe)
+    SetUnhandledExceptionFilter(ExceptionHandler);
+
+  if (Options.HandleAbrt)
+    if (SIG_ERR == signal(SIGABRT, CrashHandler)) {
+      Printf("libFuzzer: signal failed with %d\n", errno);
+      exit(1);
+    }
+}
+
+void SleepSeconds(int Seconds) { Sleep(Seconds * 1000); }
+
+unsigned long GetPid() { return GetCurrentProcessId(); }
+
+size_t GetPeakRSSMb() {
+  PROCESS_MEMORY_COUNTERS info;
+  if (!GetProcessMemoryInfo(GetCurrentProcess(), &info, sizeof(info)))
+    return 0;
+  return info.PeakWorkingSetSize >> 20;
+}
+
+FILE *OpenProcessPipe(const char *Command, const char *Mode) {
+  return _popen(Command, Mode);
+}
+
+int CloseProcessPipe(FILE *F) {
+  return _pclose(F);
+}
+
+int ExecuteCommand(const Command &Cmd) {
+  std::string CmdLine = Cmd.toString();
+  return system(CmdLine.c_str());
+}
+
+bool ExecuteCommand(const Command &Cmd, std::string *CmdOutput) {
+  FILE *Pipe = _popen(Cmd.toString().c_str(), "r");
+  if (!Pipe)
+    return false;
+
+  if (CmdOutput) {
+    char TmpBuffer[128];
+    while (fgets(TmpBuffer, sizeof(TmpBuffer), Pipe))
+      CmdOutput->append(TmpBuffer);
+  }
+  return _pclose(Pipe) == 0;
+}
+
+const void *SearchMemory(const void *Data, size_t DataLen, const void *Patt,
+                         size_t PattLen) {
+  // TODO: make this implementation more efficient.
+  const char *Cdata = (const char *)Data;
+  const char *Cpatt = (const char *)Patt;
+
+  if (!Data || !Patt || DataLen == 0 || PattLen == 0 || DataLen < PattLen)
+    return NULL;
+
+  if (PattLen == 1)
+    return memchr(Data, *Cpatt, DataLen);
+
+  const char *End = Cdata + DataLen - PattLen + 1;
+
+  for (const char *It = Cdata; It < End; ++It)
+    if (It[0] == Cpatt[0] && memcmp(It, Cpatt, PattLen) == 0)
+      return It;
+
+  return NULL;
+}
+
+std::string DisassembleCmd(const std::string &FileName) {
+  Vector<std::string> command_vector;
+  command_vector.push_back("dumpbin /summary > nul");
+  if (ExecuteCommand(Command(command_vector)) == 0)
+    return "dumpbin /disasm " + FileName;
+  Printf("libFuzzer: couldn't find tool to disassemble (dumpbin)\n");
+  exit(1);
+}
+
+std::string SearchRegexCmd(const std::string &Regex) {
+  return "findstr /r \"" + Regex + "\"";
+}
+
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("nul", "w");
+  if (!Temp)
+    return;
+  _dup2(_fileno(Temp), Fd);
+  fclose(Temp);
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS
diff --git a/tools/fuzzing/libfuzzer/FuzzerValueBitMap.h b/tools/fuzzing/libfuzzer/FuzzerValueBitMap.h
new file mode 100644
index 0000000000..ddbfe200af
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/FuzzerValueBitMap.h
@@ -0,0 +1,73 @@
+//===- FuzzerValueBitMap.h - INTERNAL - Bit map -----------------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// ValueBitMap.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_VALUE_BIT_MAP_H
+#define LLVM_FUZZER_VALUE_BIT_MAP_H
+
+#include "FuzzerPlatform.h"
+#include <cstdint>
+
+namespace fuzzer {
+
+// A bit map containing kMapSizeInWords bits.
+struct ValueBitMap {
+  static const size_t kMapSizeInBits = 1 << 16;
+  static const size_t kMapPrimeMod = 65371;  // Largest Prime < kMapSizeInBits;
+  static const size_t kBitsInWord = (sizeof(uintptr_t) * 8);
+  static const size_t kMapSizeInWords = kMapSizeInBits / kBitsInWord;
+ public:
+
+  // Clears all bits.
+  void Reset() { memset(Map, 0, sizeof(Map)); }
+
+  // Computes a hash function of Value and sets the corresponding bit.
+  // Returns true if the bit was changed from 0 to 1.
+  ATTRIBUTE_NO_SANITIZE_ALL
+  inline bool AddValue(uintptr_t Value) {
+    uintptr_t Idx = Value % kMapSizeInBits;
+    uintptr_t WordIdx = Idx / kBitsInWord;
+    uintptr_t BitIdx = Idx % kBitsInWord;
+    uintptr_t Old = Map[WordIdx];
+    uintptr_t New = Old | (1ULL << BitIdx);
+    Map[WordIdx] = New;
+    return New != Old;
+  }
+
+  ATTRIBUTE_NO_SANITIZE_ALL
+  inline bool AddValueModPrime(uintptr_t Value) {
+    return AddValue(Value % kMapPrimeMod);
+  }
+
+  inline bool Get(uintptr_t Idx) {
+    assert(Idx < kMapSizeInBits);
+    uintptr_t WordIdx = Idx / kBitsInWord;
+    uintptr_t BitIdx = Idx % kBitsInWord;
+    return Map[WordIdx] & (1ULL << BitIdx);
+  }
+
+  size_t SizeInBits() const { return kMapSizeInBits; }
+
+  template <class Callback>
+  ATTRIBUTE_NO_SANITIZE_ALL
+  void ForEach(Callback CB) const {
+    for (size_t i = 0; i < kMapSizeInWords; i++)
+      if (uintptr_t M = Map[i])
+        for (size_t j = 0; j < sizeof(M) * 8; j++)
+          if (M & ((uintptr_t)1 << j))
+            CB(i * sizeof(M) * 8 + j);
+  }
+
+ private:
+  ATTRIBUTE_ALIGNED(512) uintptr_t Map[kMapSizeInWords];
+};
+
+}  // namespace fuzzer
+
+#endif  // LLVM_FUZZER_VALUE_BIT_MAP_H
diff --git a/tools/fuzzing/libfuzzer/LICENSE.TXT b/tools/fuzzing/libfuzzer/LICENSE.TXT
new file mode 100644
index 0000000000..555c8bb952
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/LICENSE.TXT
@@ -0,0 +1,68 @@
+==============================================================================
+LLVM Release License
+==============================================================================
+University of Illinois/NCSA
+Open Source License
+
+Copyright (c) 2003-2016 University of Illinois at Urbana-Champaign.
+All rights reserved.
+
+Developed by:
+
+    LLVM Team
+
+    University of Illinois at Urbana-Champaign
+
+    http://llvm.org
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal with
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+    * Redistributions of source code must retain the above copyright notice,
+      this list of conditions and the following disclaimers.
+
+    * Redistributions in binary form must reproduce the above copyright notice,
+      this list of conditions and the following disclaimers in the
+      documentation and/or other materials provided with the distribution.
+
+    * Neither the names of the LLVM Team, University of Illinois at
+      Urbana-Champaign, nor the names of its contributors may be used to
+      endorse or promote products derived from this Software without specific
+      prior written permission.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE
+SOFTWARE.
+
+==============================================================================
+Copyrights and Licenses for Third Party Software Distributed with LLVM:
+==============================================================================
+The LLVM software contains code written by third parties.  Such software will
+have its own individual LICENSE.TXT file in the directory in which it appears.
+This file will describe the copyrights, license, and restrictions which apply
+to that code.
+
+The disclaimer of warranty in the University of Illinois Open Source License
+applies to all code in the LLVM Distribution, and nothing in any of the
+other licenses gives permission to use the names of the LLVM Team or the
+University of Illinois to endorse or promote products derived from this
+Software.
+
+The following pieces of software have additional or alternate copyrights,
+licenses, and/or restrictions:
+
+Program             Directory
+-------             ---------
+Google Test         llvm/utils/unittest/googletest
+OpenBSD regex       llvm/lib/Support/{reg*, COPYRIGHT.regex}
+pyyaml tests        llvm/test/YAMLParser/{*.data, LICENSE.TXT}
+ARM contributions   llvm/lib/Target/ARM/LICENSE.TXT
+md5 contributions   llvm/lib/Support/MD5.cpp llvm/include/llvm/Support/MD5.h
diff --git a/tools/fuzzing/libfuzzer/clone_libfuzzer.sh b/tools/fuzzing/libfuzzer/clone_libfuzzer.sh
new file mode 100755
index 0000000000..ca01df2978
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/clone_libfuzzer.sh
@@ -0,0 +1,36 @@
+#!/bin/bash -e
+
+# Optionally get revision from cmd line
+[ $1 ] && REVISION=$1 || REVISION=76d07503f0c69f6632e6d8d4736e2a4cb4055a92
+
+mkdir tmp
+git clone --single-branch --no-checkout --shallow-since "2020-07-01" https://github.com/llvm/llvm-project tmp
+
+(cd tmp && git reset --hard $REVISION)
+
+# libFuzzer source files
+CPPS=($(ls tmp/compiler-rt/lib/fuzzer/*.cpp | sort -r))
+CPPS=(${CPPS[@]##*/})
+CPPS=(${CPPS[@]##FuzzerMain*}) # ignored
+CPPS=(${CPPS[@]##FuzzerInterceptors*}) # ignored
+
+# Update SOURCES entries
+sed -e "/^SOURCES/,/^]/ {/'/d}" -i moz.build
+for CPP in ${CPPS[@]}; do sed -e "/^SOURCES/ a \\    '${CPP}'," -i moz.build; done
+
+# Remove previous files
+rm *.{cpp,h,def}
+
+# Copy files
+cp tmp/compiler-rt/lib/fuzzer/*.{cpp,h,def} .
+
+# Apply local patches
+for patch in patches/*.patch
+do
+  patch -p4 < $patch
+done
+
+# Remove the temporary directory
+rm -Rf tmp/
+
+echo "Updated libFuzzer to ${REVISION}"
diff --git a/tools/fuzzing/libfuzzer/moz.build b/tools/fuzzing/libfuzzer/moz.build
new file mode 100644
index 0000000000..a04169df57
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/moz.build
@@ -0,0 +1,55 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library('fuzzer')
+
+EXPORTS += [
+    'FuzzerDefs.h',
+    'FuzzerExtFunctions.def',
+    'FuzzerExtFunctions.h',
+]
+
+SOURCES += [
+    'FuzzerCrossOver.cpp',
+    'FuzzerDataFlowTrace.cpp',
+    'FuzzerDriver.cpp',
+    'FuzzerExtFunctionsDlsym.cpp',
+    'FuzzerExtFunctionsWeak.cpp',
+    'FuzzerExtFunctionsWindows.cpp',
+    'FuzzerExtraCounters.cpp',
+    'FuzzerFork.cpp',
+    'FuzzerIO.cpp',
+    'FuzzerIOPosix.cpp',
+    'FuzzerIOWindows.cpp',
+    'FuzzerLoop.cpp',
+    'FuzzerMerge.cpp',
+    'FuzzerMutate.cpp',
+    'FuzzerSHA1.cpp',
+    'FuzzerTracePC.cpp',
+    'FuzzerUtil.cpp',
+    'FuzzerUtilDarwin.cpp',
+    'FuzzerUtilFuchsia.cpp',
+    'FuzzerUtilLinux.cpp',
+    'FuzzerUtilPosix.cpp',
+    'FuzzerUtilWindows.cpp',
+]
+
+if CONFIG['CC_TYPE'] == 'clang':
+    CXXFLAGS += ['-Wno-unreachable-code-return']
+
+# According to the LLVM docs, LibFuzzer isn't supposed to be built with any
+# sanitizer flags and in fact, building it with ASan coverage currently causes
+# Clang 3.9+ to crash, so we filter out all sanitizer-related flags here.
+for flags_var in ('OS_CFLAGS', 'OS_CXXFLAGS'):
+    COMPILE_FLAGS[flags_var] = [
+        f for f in COMPILE_FLAGS.get(flags_var, [])
+        if not f.startswith('-fsanitize')
+    ]
+
+LINK_FLAGS['OS'] = [
+    f for f in LINK_FLAGS.get('OS', [])
+    if not f.startswith('-fsanitize')
+]
diff --git a/tools/fuzzing/libfuzzer/patches/10-ef-runtime.patch b/tools/fuzzing/libfuzzer/patches/10-ef-runtime.patch
new file mode 100644
index 0000000000..34562e27a5
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/patches/10-ef-runtime.patch
@@ -0,0 +1,31 @@
+# HG changeset patch
+# User Christian Holler <choller@mozilla.com>
+# Date 1596126054 -7200
+#      Thu Jul 30 18:20:54 2020 +0200
+# Node ID 8a2a26b33d516c43c366b2f24d731d27d9843349
+# Parent  997c4109edd112695097fd8c55cbacd976cab24a
+[libFuzzer] Allow external functions to be defined at runtime
+
+diff --git a/tools/fuzzing/libfuzzer/FuzzerDriver.cpp b/tools/fuzzing/libfuzzer/FuzzerDriver.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerDriver.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerDriver.cpp
+@@ -608,17 +608,18 @@ static Vector<SizedFile> ReadCorpora(con
+       SizedFiles.push_back({File, Size});
+   return SizedFiles;
+ }
+ 
+ int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
+   using namespace fuzzer;
+   assert(argc && argv && "Argument pointers cannot be nullptr");
+   std::string Argv0((*argv)[0]);
+-  EF = new ExternalFunctions();
++  if (!EF)
++    EF = new ExternalFunctions();
+   if (EF->LLVMFuzzerInitialize)
+     EF->LLVMFuzzerInitialize(argc, argv);
+   if (EF->__msan_scoped_disable_interceptor_checks)
+     EF->__msan_scoped_disable_interceptor_checks();
+   const Vector<std::string> Args(*argv, *argv + *argc);
+   assert(!Args.empty());
+   ProgName = new std::string(Args[0]);
+   if (Argv0 != *ProgName) {
diff --git a/tools/fuzzing/libfuzzer/patches/11-callback-rv.patch b/tools/fuzzing/libfuzzer/patches/11-callback-rv.patch
new file mode 100644
index 0000000000..650444442c
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/patches/11-callback-rv.patch
@@ -0,0 +1,132 @@
+# HG changeset patch
+# User Christian Holler <choller@mozilla.com>
+# Date 1596126448 -7200
+#      Thu Jul 30 18:27:28 2020 +0200
+# Node ID ea198a0331a6db043cb5978512226977514104db
+# Parent  8a2a26b33d516c43c366b2f24d731d27d9843349
+[libFuzzer] Change libFuzzer callback contract to allow positive return values
+
+diff --git a/tools/fuzzing/libfuzzer/FuzzerInternal.h b/tools/fuzzing/libfuzzer/FuzzerInternal.h
+--- a/tools/fuzzing/libfuzzer/FuzzerInternal.h
++++ b/tools/fuzzing/libfuzzer/FuzzerInternal.h
+@@ -60,17 +60,17 @@ public:
+ 
+   static void StaticAlarmCallback();
+   static void StaticCrashSignalCallback();
+   static void StaticExitCallback();
+   static void StaticInterruptCallback();
+   static void StaticFileSizeExceedCallback();
+   static void StaticGracefulExitCallback();
+ 
+-  void ExecuteCallback(const uint8_t *Data, size_t Size);
++  int ExecuteCallback(const uint8_t *Data, size_t Size);
+   bool RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile = false,
+               InputInfo *II = nullptr, bool *FoundUniqFeatures = nullptr);
+ 
+   // Merge Corpora[1:] into Corpora[0].
+   void Merge(const Vector<std::string> &Corpora);
+   void CrashResistantMergeInternalStep(const std::string &ControlFilePath);
+   MutationDispatcher &GetMD() { return MD; }
+   void PrintFinalStats();
+diff --git a/tools/fuzzing/libfuzzer/FuzzerLoop.cpp b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
+@@ -463,17 +463,19 @@ static void RenameFeatureSetFile(const s
+              DirPlusFile(FeaturesDir, NewFile));
+ }
+ 
+ bool Fuzzer::RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile,
+                     InputInfo *II, bool *FoundUniqFeatures) {
+   if (!Size)
+     return false;
+ 
+-  ExecuteCallback(Data, Size);
++  if (ExecuteCallback(Data, Size) > 0) {
++    return false;
++  }
+ 
+   UniqFeatureSetTmp.clear();
+   size_t FoundUniqFeaturesOfII = 0;
+   size_t NumUpdatesBefore = Corpus.NumFeatureUpdates();
+   TPC.CollectFeatures([&](size_t Feature) {
+     if (Corpus.AddFeature(Feature, Size, Options.Shrink))
+       UniqFeatureSetTmp.push_back(Feature);
+     if (Options.Entropic)
+@@ -530,48 +532,49 @@ static bool LooseMemeq(const uint8_t *A,
+   const size_t Limit = 64;
+   if (Size <= 64)
+     return !memcmp(A, B, Size);
+   // Compare first and last Limit/2 bytes.
+   return !memcmp(A, B, Limit / 2) &&
+          !memcmp(A + Size - Limit / 2, B + Size - Limit / 2, Limit / 2);
+ }
+ 
+-void Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {
++int Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {
+   TPC.RecordInitialStack();
+   TotalNumberOfRuns++;
+   assert(InFuzzingThread());
+   // We copy the contents of Unit into a separate heap buffer
+   // so that we reliably find buffer overflows in it.
+   uint8_t *DataCopy = new uint8_t[Size];
+   memcpy(DataCopy, Data, Size);
+   if (EF->__msan_unpoison)
+     EF->__msan_unpoison(DataCopy, Size);
+   if (EF->__msan_unpoison_param)
+     EF->__msan_unpoison_param(2);
+   if (CurrentUnitData && CurrentUnitData != Data)
+     memcpy(CurrentUnitData, Data, Size);
+   CurrentUnitSize = Size;
++  int Res = 0;
+   {
+     ScopedEnableMsanInterceptorChecks S;
+     AllocTracer.Start(Options.TraceMalloc);
+     UnitStartTime = system_clock::now();
+     TPC.ResetMaps();
+     RunningUserCallback = true;
+-    int Res = CB(DataCopy, Size);
++    Res = CB(DataCopy, Size);
+     RunningUserCallback = false;
+     UnitStopTime = system_clock::now();
+-    (void)Res;
+-    assert(Res == 0);
++    assert(Res >= 0);
+     HasMoreMallocsThanFrees = AllocTracer.Stop();
+   }
+   if (!LooseMemeq(DataCopy, Data, Size))
+     CrashOnOverwrittenData();
+   CurrentUnitSize = 0;
+   delete[] DataCopy;
++  return Res;
+ }
+ 
+ std::string Fuzzer::WriteToOutputCorpus(const Unit &U) {
+   if (Options.OnlyASCII)
+     assert(IsASCII(U));
+   if (Options.OutputCorpus.empty())
+     return "";
+   std::string Path = DirPlusFile(Options.OutputCorpus, Hash(U));
+diff --git a/tools/fuzzing/libfuzzer/FuzzerMerge.cpp b/tools/fuzzing/libfuzzer/FuzzerMerge.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerMerge.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerMerge.cpp
+@@ -223,17 +223,19 @@ void Fuzzer::CrashResistantMergeInternal
+       U.shrink_to_fit();
+     }
+ 
+     // Write the pre-run marker.
+     OF << "STARTED " << i << " " << U.size() << "\n";
+     OF.flush();  // Flush is important since Command::Execute may crash.
+     // Run.
+     TPC.ResetMaps();
+-    ExecuteCallback(U.data(), U.size());
++    if (ExecuteCallback(U.data(), U.size()) > 0) {
++      continue;
++    }
+     // Collect coverage. We are iterating over the files in this order:
+     // * First, files in the initial corpus ordered by size, smallest first.
+     // * Then, all other files, smallest first.
+     // So it makes no sense to record all features for all files, instead we
+     // only record features that were not seen before.
+     Set<size_t> UniqFeatures;
+     TPC.CollectFeatures([&](size_t Feature) {
+       if (AllFeatures.insert(Feature).second)
diff --git a/tools/fuzzing/libfuzzer/patches/12-custom-mutator-fail.patch b/tools/fuzzing/libfuzzer/patches/12-custom-mutator-fail.patch
new file mode 100644
index 0000000000..2457c1f046
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/patches/12-custom-mutator-fail.patch
@@ -0,0 +1,53 @@
+# HG changeset patch
+# User Christian Holler <choller@mozilla.com>
+# Date 1596126768 -7200
+#      Thu Jul 30 18:32:48 2020 +0200
+# Node ID 64e7d096fa77a62b71a306b2c5383b8f75ac4945
+# Parent  ea198a0331a6db043cb5978512226977514104db
+[libFuzzer] Allow custom mutators to fail
+
+diff --git a/tools/fuzzing/libfuzzer/FuzzerLoop.cpp b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerLoop.cpp
+@@ -690,16 +690,20 @@ void Fuzzer::MutateAndTestOne() {
+     if (II.HasFocusFunction && !II.DataFlowTraceForFocusFunction.empty() &&
+         Size <= CurrentMaxMutationLen)
+       NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
+                                   II.DataFlowTraceForFocusFunction);
+ 
+     // If MutateWithMask either failed or wasn't called, call default Mutate.
+     if (!NewSize)
+       NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
++
++    if (!NewSize)
++      continue;
++
+     assert(NewSize > 0 && "Mutator returned empty unit");
+     assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");
+     Size = NewSize;
+     II.NumExecutedMutations++;
+     Corpus.IncrementNumExecutedMutations();
+ 
+     bool FoundUniqFeatures = false;
+     bool NewCov = RunOne(CurrentUnitData, Size, /*MayDeleteFile=*/true, &II,
+@@ -850,17 +854,19 @@ void Fuzzer::Loop(Vector<SizedFile> &Cor
+ void Fuzzer::MinimizeCrashLoop(const Unit &U) {
+   if (U.size() <= 1)
+     return;
+   while (!TimedOut() && TotalNumberOfRuns < Options.MaxNumberOfRuns) {
+     MD.StartMutationSequence();
+     memcpy(CurrentUnitData, U.data(), U.size());
+     for (int i = 0; i < Options.MutateDepth; i++) {
+       size_t NewSize = MD.Mutate(CurrentUnitData, U.size(), MaxMutationLen);
+-      assert(NewSize > 0 && NewSize <= MaxMutationLen);
++      assert(NewSize <= MaxMutationLen);
++      if (!NewSize)
++        continue;
+       ExecuteCallback(CurrentUnitData, NewSize);
+       PrintPulseAndReportSlowInput(CurrentUnitData, NewSize);
+       TryDetectingAMemoryLeak(CurrentUnitData, NewSize,
+                               /*DuringInitialCorpusExecution*/ false);
+     }
+   }
+ }
+ 
diff --git a/tools/fuzzing/libfuzzer/patches/13-unused-write.patch b/tools/fuzzing/libfuzzer/patches/13-unused-write.patch
new file mode 100644
index 0000000000..f8ef7a5ac9
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/patches/13-unused-write.patch
@@ -0,0 +1,88 @@
+# HG changeset patch
+# User Christian Holler <choller@mozilla.com>
+# Date 1596126946 -7200
+#      Thu Jul 30 18:35:46 2020 +0200
+# Node ID 6c779ec81530b6784a714063af66085681ab7318
+# Parent  64e7d096fa77a62b71a306b2c5383b8f75ac4945
+[libFuzzer] Suppress warnings about unused return values
+
+diff --git a/tools/fuzzing/libfuzzer/FuzzerIO.cpp b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerIO.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
+@@ -3,16 +3,17 @@
+ // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ // See https://llvm.org/LICENSE.txt for license information.
+ // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ //
+ //===----------------------------------------------------------------------===//
+ // IO functions.
+ //===----------------------------------------------------------------------===//
+ 
++#include "mozilla/Unused.h"
+ #include "FuzzerDefs.h"
+ #include "FuzzerExtFunctions.h"
+ #include "FuzzerIO.h"
+ #include "FuzzerUtil.h"
+ #include <algorithm>
+ #include <cstdarg>
+ #include <fstream>
+ #include <iterator>
+@@ -68,17 +69,17 @@ void WriteToFile(const std::string &Data
+   WriteToFile(reinterpret_cast<const uint8_t *>(Data.c_str()), Data.size(),
+               Path);
+ }
+ 
+ void WriteToFile(const uint8_t *Data, size_t Size, const std::string &Path) {
+   // Use raw C interface because this function may be called from a sig handler.
+   FILE *Out = fopen(Path.c_str(), "wb");
+   if (!Out) return;
+-  fwrite(Data, sizeof(Data[0]), Size, Out);
++  mozilla::Unused << fwrite(Data, sizeof(Data[0]), Size, Out);
+   fclose(Out);
+ }
+ 
+ void ReadDirToVectorOfUnits(const char *Path, Vector<Unit> *V,
+                             long *Epoch, size_t MaxSize, bool ExitOnError) {
+   long E = Epoch ? *Epoch : 0;
+   Vector<std::string> Files;
+   ListFilesInDirRecursive(Path, Epoch, &Files, /*TopDir*/true);
+diff --git a/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
+--- a/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
++++ b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
+@@ -2,16 +2,17 @@
+ //
+ // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ // See https://llvm.org/LICENSE.txt for license information.
+ // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ //
+ //===----------------------------------------------------------------------===//
+ // IO functions implementation using Posix API.
+ //===----------------------------------------------------------------------===//
++#include "mozilla/Unused.h"
+ #include "FuzzerPlatform.h"
+ #if LIBFUZZER_POSIX || LIBFUZZER_FUCHSIA
+ 
+ #include "FuzzerExtFunctions.h"
+ #include "FuzzerIO.h"
+ #include <cstdarg>
+ #include <cstdio>
+ #include <dirent.h>
+@@ -150,17 +151,17 @@ bool IsInterestingCoverageFile(const std
+   if (FileName.find("/usr/include/") != std::string::npos)
+     return false;
+   if (FileName == "<null>")
+     return false;
+   return true;
+ }
+ 
+ void RawPrint(const char *Str) {
+-  write(2, Str, strlen(Str));
++  mozilla::Unused << write(2, Str, strlen(Str));
+ }
+ 
+ void MkDir(const std::string &Path) {
+   mkdir(Path.c_str(), 0700);
+ }
+ 
+ void RmDir(const std::string &Path) {
+   rmdir(Path.c_str());
diff --git a/tools/fuzzing/libfuzzer/patches/14-explicit-allocator.patch b/tools/fuzzing/libfuzzer/patches/14-explicit-allocator.patch
new file mode 100644
index 0000000000..8e02273193
--- /dev/null
+++ b/tools/fuzzing/libfuzzer/patches/14-explicit-allocator.patch
@@ -0,0 +1,30 @@
+# HG changeset patch
+# User Christian Holler <choller@mozilla.com>
+# Date 1596126981 -7200
+#      Thu Jul 30 18:36:21 2020 +0200
+# Node ID 069dfa3715b1d30905ff0ea1c0f66db88ce146f9
+# Parent  6c779ec81530b6784a714063af66085681ab7318
+[libFuzzer] Make fuzzer_allocator explicit
+
+diff --git a/tools/fuzzing/libfuzzer/FuzzerDefs.h b/tools/fuzzing/libfuzzer/FuzzerDefs.h
+--- a/tools/fuzzing/libfuzzer/FuzzerDefs.h
++++ b/tools/fuzzing/libfuzzer/FuzzerDefs.h
+@@ -41,17 +41,17 @@ extern ExternalFunctions *EF;
+ // We are using a custom allocator to give a different symbol name to STL
+ // containers in order to avoid ODR violations.
+ template<typename T>
+   class fuzzer_allocator: public std::allocator<T> {
+     public:
+       fuzzer_allocator() = default;
+ 
+       template<class U>
+-      fuzzer_allocator(const fuzzer_allocator<U>&) {}
++      explicit fuzzer_allocator(const fuzzer_allocator<U>&) {}
+ 
+       template<class Other>
+       struct rebind { typedef fuzzer_allocator<Other> other;  };
+   };
+ 
+ template<typename T>
+ using Vector = std::vector<T, fuzzer_allocator<T>>;
+ 
diff --git a/tools/fuzzing/messagemanager/MessageManagerFuzzer.cpp b/tools/fuzzing/messagemanager/MessageManagerFuzzer.cpp
new file mode 100644
index 0000000000..bc9ac6b6fb
--- /dev/null
+++ b/tools/fuzzing/messagemanager/MessageManagerFuzzer.cpp
@@ -0,0 +1,323 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <climits>
+#include <cmath>
+#include "FuzzingTraits.h"
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/CharacterEncoding.h"
+#include "prenv.h"
+#include "MessageManagerFuzzer.h"
+#include "mozilla/ErrorResult.h"
+#include "nsComponentManagerUtils.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsFrameMessageManager.h"
+#include "nsJSUtils.h"
+#include "nsXULAppAPI.h"
+#include "nsNetCID.h"
+#include "nsString.h"
+#include "nsUnicharUtils.h"
+#include "nsIFile.h"
+#include "nsIFileStreams.h"
+#include "nsILineInputStream.h"
+#include "nsLocalFile.h"
+#include "nsTArray.h"
+
+#ifdef IsLoggingEnabled
+// This is defined in the Windows SDK urlmon.h
+#  undef IsLoggingEnabled
+#endif
+
+#define MESSAGEMANAGER_FUZZER_DEFAULT_MUTATION_PROBABILITY 2
+#define MSGMGR_FUZZER_LOG(fmt, args...)                        \
+  if (MessageManagerFuzzer::IsLoggingEnabled()) {              \
+    printf_stderr("[MessageManagerFuzzer] " fmt "\n", ##args); \
+  }
+
+namespace mozilla {
+namespace dom {
+
+using namespace fuzzing;
+using namespace ipc;
+
+/* static */
+void MessageManagerFuzzer::ReadFile(const char* path,
+                                    nsTArray<nsCString>& aArray) {
+  nsCOMPtr<nsIFile> file;
+  nsresult rv =
+      NS_NewLocalFile(NS_ConvertUTF8toUTF16(path), true, getter_AddRefs(file));
+  NS_ENSURE_SUCCESS_VOID(rv);
+
+  bool exists = false;
+  rv = file->Exists(&exists);
+  if (NS_FAILED(rv) || !exists) {
+    return;
+  }
+
+  nsCOMPtr<nsIFileInputStream> fileStream(
+      do_CreateInstance(NS_LOCALFILEINPUTSTREAM_CONTRACTID, &rv));
+  NS_ENSURE_SUCCESS_VOID(rv);
+
+  rv = fileStream->Init(file, -1, -1, false);
+  NS_ENSURE_SUCCESS_VOID(rv);
+
+  nsCOMPtr<nsILineInputStream> lineStream(do_QueryInterface(fileStream, &rv));
+  NS_ENSURE_SUCCESS_VOID(rv);
+
+  nsAutoCString line;
+  bool more = true;
+  do {
+    rv = lineStream->ReadLine(line, &more);
+    NS_ENSURE_SUCCESS_VOID(rv);
+    aArray.AppendElement(line);
+  } while (more);
+}
+
+/* static */
+bool MessageManagerFuzzer::IsMessageNameBlacklisted(
+    const nsAString& aMessageName) {
+  static bool sFileLoaded = false;
+  static nsTArray<nsCString> valuesInFile;
+
+  if (!sFileLoaded) {
+    ReadFile(PR_GetEnv("MESSAGEMANAGER_FUZZER_BLACKLIST"), valuesInFile);
+    sFileLoaded = true;
+  }
+
+  if (valuesInFile.Length() == 0) {
+    return false;
+  }
+
+  return valuesInFile.Contains(NS_ConvertUTF16toUTF8(aMessageName).get());
+}
+
+/* static */
+nsCString MessageManagerFuzzer::GetFuzzValueFromFile() {
+  static bool sFileLoaded = false;
+  static nsTArray<nsCString> valuesInFile;
+
+  if (!sFileLoaded) {
+    ReadFile(PR_GetEnv("MESSAGEMANAGER_FUZZER_STRINGSFILE"), valuesInFile);
+    sFileLoaded = true;
+  }
+
+  // If something goes wrong with importing the file we return an empty string.
+  if (valuesInFile.Length() == 0) {
+    return nsCString();
+  }
+
+  unsigned randIdx = RandomIntegerRange<unsigned>(0, valuesInFile.Length());
+  return valuesInFile.ElementAt(randIdx);
+}
+
+/* static */
+void MessageManagerFuzzer::MutateObject(JSContext* aCx, JS::HandleValue aValue,
+                                        unsigned short int aRecursionCounter) {
+  JS::Rooted<JSObject*> object(aCx, &aValue.toObject());
+  JS::Rooted<JS::IdVector> ids(aCx, JS::IdVector(aCx));
+
+  if (!JS_Enumerate(aCx, object, &ids)) {
+    return;
+  }
+
+  for (size_t i = 0, n = ids.length(); i < n; i++) {
+    // Retrieve Property name.
+    nsAutoJSString propName;
+    if (!propName.init(aCx, ids[i])) {
+      continue;
+    }
+    MSGMGR_FUZZER_LOG("%*s- Property: %s", aRecursionCounter * 4, "",
+                      NS_ConvertUTF16toUTF8(propName).get());
+
+    // The likelihood when a value gets fuzzed of this object.
+    if (!FuzzingTraits::Sometimes(DefaultMutationProbability())) {
+      continue;
+    }
+
+    // Retrieve Property value.
+    JS::RootedValue propertyValue(aCx);
+    JS_GetPropertyById(aCx, object, ids[i], &propertyValue);
+
+    JS::RootedValue newPropValue(aCx);
+    MutateValue(aCx, propertyValue, &newPropValue, aRecursionCounter);
+
+    JS_SetPropertyById(aCx, object, ids[i], newPropValue);
+  }
+}
+
+/* static */
+bool MessageManagerFuzzer::MutateValue(JSContext* aCx, JS::HandleValue aValue,
+                                       JS::MutableHandleValue aOutMutationValue,
+                                       unsigned short int aRecursionCounter) {
+  if (aValue.isInt32()) {
+    if (FuzzingTraits::Sometimes(DefaultMutationProbability() * 2)) {
+      aOutMutationValue.set(JS::Int32Value(RandomNumericLimit<int>()));
+    } else {
+      aOutMutationValue.set(JS::Int32Value(RandomInteger<int>()));
+    }
+    MSGMGR_FUZZER_LOG("%*s! Mutated value of type |int32|: '%d' to '%d'",
+                      aRecursionCounter * 4, "", aValue.toInt32(),
+                      aOutMutationValue.toInt32());
+    return true;
+  }
+
+  if (aValue.isDouble()) {
+    aOutMutationValue.set(JS::DoubleValue(RandomFloatingPoint<double>()));
+    MSGMGR_FUZZER_LOG("%*s! Mutated value of type |double|: '%f' to '%f'",
+                      aRecursionCounter * 4, "", aValue.toDouble(),
+                      aOutMutationValue.toDouble());
+    return true;
+  }
+
+  if (aValue.isBoolean()) {
+    aOutMutationValue.set(JS::BooleanValue(bool(RandomIntegerRange(0, 2))));
+    MSGMGR_FUZZER_LOG("%*s! Mutated value of type |boolean|: '%d' to '%d'",
+                      aRecursionCounter * 4, "", aValue.toBoolean(),
+                      aOutMutationValue.toBoolean());
+    return true;
+  }
+
+  if (aValue.isString()) {
+    nsCString x = GetFuzzValueFromFile();
+    if (x.IsEmpty()) {
+      return false;
+    }
+    JSString* str = JS_NewStringCopyZ(aCx, x.get());
+    aOutMutationValue.set(JS::StringValue(str));
+    JS::RootedString rootedValue(aCx, aValue.toString());
+    JS::UniqueChars valueChars = JS_EncodeStringToUTF8(aCx, rootedValue);
+    MSGMGR_FUZZER_LOG("%*s! Mutated value of type |string|: '%s' to '%s'",
+                      aRecursionCounter * 4, "", valueChars.get(), x.get());
+    return true;
+  }
+
+  if (aValue.isObject()) {
+    aRecursionCounter++;
+    MSGMGR_FUZZER_LOG("%*s<Enumerating found object>", aRecursionCounter * 4,
+                      "");
+    MutateObject(aCx, aValue, aRecursionCounter);
+    aOutMutationValue.set(aValue);
+    return true;
+  }
+
+  return false;
+}
+
+/* static */
+bool MessageManagerFuzzer::Mutate(JSContext* aCx, const nsAString& aMessageName,
+                                  ipc::StructuredCloneData* aData,
+                                  const JS::Value& aTransfer) {
+  MSGMGR_FUZZER_LOG("Message: %s in process: %d",
+                    NS_ConvertUTF16toUTF8(aMessageName).get(),
+                    XRE_GetProcessType());
+
+  unsigned short int aRecursionCounter = 0;
+  ErrorResult rv;
+  JS::RootedValue t(aCx, aTransfer);
+
+  /* Read original StructuredCloneData. */
+  JS::RootedValue scdContent(aCx);
+  aData->Read(aCx, &scdContent, rv);
+  if (NS_WARN_IF(rv.Failed())) {
+    rv.SuppressException();
+    JS_ClearPendingException(aCx);
+    return false;
+  }
+
+  JS::RootedValue scdMutationContent(aCx);
+  bool isMutated =
+      MutateValue(aCx, scdContent, &scdMutationContent, aRecursionCounter);
+
+  /* Write mutated StructuredCloneData. */
+  ipc::StructuredCloneData mutatedStructuredCloneData;
+  mutatedStructuredCloneData.Write(aCx, scdMutationContent, t,
+                                   JS::CloneDataPolicy(), rv);
+  if (NS_WARN_IF(rv.Failed())) {
+    rv.SuppressException();
+    JS_ClearPendingException(aCx);
+    return false;
+  }
+
+  // See: https://bugzilla.mozilla.org/show_bug.cgi?id=1346040
+  aData->Copy(mutatedStructuredCloneData);
+
+  /* Mutated and successfully written to StructuredCloneData object. */
+  if (isMutated) {
+    JS::RootedString str(aCx, JS_ValueToSource(aCx, scdMutationContent));
+    JS::UniqueChars strChars = JS_EncodeStringToUTF8(aCx, str);
+    MSGMGR_FUZZER_LOG("Mutated '%s' Message: %s",
+                      NS_ConvertUTF16toUTF8(aMessageName).get(),
+                      strChars.get());
+  }
+
+  return true;
+}
+
+/* static */
+unsigned int MessageManagerFuzzer::DefaultMutationProbability() {
+  static unsigned long sPropValue =
+      MESSAGEMANAGER_FUZZER_DEFAULT_MUTATION_PROBABILITY;
+  static bool sInitialized = false;
+
+  if (sInitialized) {
+    return sPropValue;
+  }
+  sInitialized = true;
+
+  // Defines the likelihood of fuzzing a message.
+  const char* probability =
+      PR_GetEnv("MESSAGEMANAGER_FUZZER_MUTATION_PROBABILITY");
+  if (probability) {
+    long n = std::strtol(probability, nullptr, 10);
+    if (n != 0) {
+      sPropValue = n;
+      return sPropValue;
+    }
+  }
+
+  return sPropValue;
+}
+
+/* static */
+bool MessageManagerFuzzer::IsLoggingEnabled() {
+  static bool sInitialized = false;
+  static bool sIsLoggingEnabled = false;
+
+  if (!sInitialized) {
+    sIsLoggingEnabled = !!PR_GetEnv("MESSAGEMANAGER_FUZZER_ENABLE_LOGGING");
+    sInitialized = true;
+  }
+
+  return sIsLoggingEnabled;
+}
+
+/* static */
+bool MessageManagerFuzzer::IsEnabled() {
+  return !!PR_GetEnv("MESSAGEMANAGER_FUZZER_ENABLE") && XRE_IsContentProcess();
+}
+
+/* static */
+void MessageManagerFuzzer::TryMutate(JSContext* aCx,
+                                     const nsAString& aMessageName,
+                                     ipc::StructuredCloneData* aData,
+                                     const JS::Value& aTransfer) {
+  if (!IsEnabled()) {
+    return;
+  }
+
+  if (IsMessageNameBlacklisted(aMessageName)) {
+    MSGMGR_FUZZER_LOG("Blacklisted message: %s",
+                      NS_ConvertUTF16toUTF8(aMessageName).get());
+    return;
+  }
+
+  Mutate(aCx, aMessageName, aData, aTransfer);
+}
+
+}  // namespace dom
+}  // namespace mozilla
diff --git a/tools/fuzzing/messagemanager/MessageManagerFuzzer.h b/tools/fuzzing/messagemanager/MessageManagerFuzzer.h
new file mode 100644
index 0000000000..95854371a3
--- /dev/null
+++ b/tools/fuzzing/messagemanager/MessageManagerFuzzer.h
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_MessageManagerFuzzer_h__
+#define mozilla_dom_MessageManagerFuzzer_h__
+
+#include "jspubtd.h"
+#include "nsAString.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace dom {
+
+namespace ipc {
+class StructuredCloneData;
+}
+
+/*
+Exposed environment variables:
+MESSAGEMANAGER_FUZZER_ENABLE=1
+MESSAGEMANAGER_FUZZER_ENABLE_LOGGING=1 (optional)
+MESSAGEMANAGER_FUZZER_MUTATION_PROBABILITY=2 (optional)
+MESSAGEMANAGER_FUZZER_STRINGSFILE=<path> (optional)
+MESSAGEMANAGER_FUZZER_BLACKLIST=<path> (optional)
+*/
+
+#ifdef IsLoggingEnabled
+// This is defined in the Windows SDK urlmon.h
+#  undef IsLoggingEnabled
+#endif
+
+class MessageManagerFuzzer {
+ public:
+  static void TryMutate(JSContext* aCx, const nsAString& aMessageName,
+                        ipc::StructuredCloneData* aData,
+                        const JS::Value& aTransfer);
+
+ private:
+  static void ReadFile(const char* path, nsTArray<nsCString>& aArray);
+  static nsCString GetFuzzValueFromFile();
+  static bool IsMessageNameBlacklisted(const nsAString& aMessageName);
+  static bool Mutate(JSContext* aCx, const nsAString& aMessageName,
+                     ipc::StructuredCloneData* aData,
+                     const JS::Value& aTransfer);
+  static void Mutate(JSContext* aCx, JS::Rooted<JS::Value>& aMutation);
+  static void MutateObject(JSContext* aCx, JS::HandleValue aValue,
+                           unsigned short int aRecursionCounter);
+  static bool MutateValue(JSContext* aCx, JS::HandleValue aValue,
+                          JS::MutableHandleValue aOutMutationValue,
+                          unsigned short int aRecursionCounter);
+  static unsigned int DefaultMutationProbability();
+  static nsAutoString ReadJSON(JSContext* aCx, const JS::Value& aJSON);
+  static bool IsEnabled();
+  static bool IsLoggingEnabled();
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/messagemanager/moz.build b/tools/fuzzing/messagemanager/moz.build
new file mode 100644
index 0000000000..9fc49d7b73
--- /dev/null
+++ b/tools/fuzzing/messagemanager/moz.build
@@ -0,0 +1,11 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+SOURCES += ["MessageManagerFuzzer.cpp"]
+
+EXPORTS += ["MessageManagerFuzzer.h"]
+
+FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/moz.build b/tools/fuzzing/moz.build
new file mode 100644
index 0000000000..6678f6fb32
--- /dev/null
+++ b/tools/fuzzing/moz.build
@@ -0,0 +1,29 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DIRS += [
+    "interface",
+    "registry",
+]
+
+if not CONFIG["JS_STANDALONE"]:
+    DIRS += [
+        "common",
+        "faulty",
+        "messagemanager",
+        "shmem",
+        "ipc",
+    ]
+
+    if CONFIG["LIBFUZZER"]:
+        DIRS += [
+            "rust",
+        ]
+
+if CONFIG["LIBFUZZER"]:
+    DIRS += [
+        "libfuzzer",
+    ]
diff --git a/tools/fuzzing/registry/FuzzerRegistry.cpp b/tools/fuzzing/registry/FuzzerRegistry.cpp
new file mode 100644
index 0000000000..ed4b0c7fa8
--- /dev/null
+++ b/tools/fuzzing/registry/FuzzerRegistry.cpp
@@ -0,0 +1,26 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * * This Source Code Form is subject to the terms of the Mozilla Public
+ * * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FuzzerRegistry.h"
+
+namespace mozilla {
+
+FuzzerRegistry& FuzzerRegistry::getInstance() {
+  static FuzzerRegistry instance;
+  return instance;
+}
+
+void FuzzerRegistry::registerModule(std::string moduleName,
+                                    FuzzerInitFunc initFunc,
+                                    FuzzerTestingFunc testingFunc) {
+  moduleMap.insert(std::pair<std::string, FuzzerFunctions>(
+      moduleName, FuzzerFunctions(initFunc, testingFunc)));
+}
+
+FuzzerFunctions FuzzerRegistry::getModuleFunctions(std::string& moduleName) {
+  return moduleMap[moduleName];
+}
+
+}  // namespace mozilla
diff --git a/tools/fuzzing/registry/FuzzerRegistry.h b/tools/fuzzing/registry/FuzzerRegistry.h
new file mode 100644
index 0000000000..5976ddc5b6
--- /dev/null
+++ b/tools/fuzzing/registry/FuzzerRegistry.h
@@ -0,0 +1,44 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * * This Source Code Form is subject to the terms of the Mozilla Public
+ * * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef _FuzzerRegistry_h__
+#define _FuzzerRegistry_h__
+
+#include <cstdint>
+#include <map>
+#include <string>
+#include <utility>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Types.h"
+
+typedef int (*FuzzerInitFunc)(int*, char***);
+typedef int (*FuzzerTestingFunc)(const uint8_t*, size_t);
+
+typedef int (*LibFuzzerDriver)(int*, char***, FuzzerTestingFunc);
+
+namespace mozilla {
+
+typedef std::pair<FuzzerInitFunc, FuzzerTestingFunc> FuzzerFunctions;
+
+class FuzzerRegistry {
+ public:
+  MOZ_EXPORT static FuzzerRegistry& getInstance();
+  MOZ_EXPORT void registerModule(std::string moduleName,
+                                 FuzzerInitFunc initFunc,
+                                 FuzzerTestingFunc testingFunc);
+  MOZ_EXPORT FuzzerFunctions getModuleFunctions(std::string& moduleName);
+
+  FuzzerRegistry(FuzzerRegistry const&) = delete;
+  void operator=(FuzzerRegistry const&) = delete;
+
+ private:
+  FuzzerRegistry(){};
+  std::map<std::string, FuzzerFunctions> moduleMap;
+};
+
+}  // namespace mozilla
+
+#endif  // _FuzzerRegistry_h__
diff --git a/tools/fuzzing/registry/moz.build b/tools/fuzzing/registry/moz.build
new file mode 100644
index 0000000000..4aa005a56e
--- /dev/null
+++ b/tools/fuzzing/registry/moz.build
@@ -0,0 +1,20 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("fuzzer-registry")
+
+SOURCES += [
+    "FuzzerRegistry.cpp",
+]
+
+EXPORTS += [
+    "FuzzerRegistry.h",
+]
+
+if CONFIG["JS_STANDALONE"]:
+    FINAL_LIBRARY = "js"
+else:
+    FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/rust/Cargo.toml b/tools/fuzzing/rust/Cargo.toml
new file mode 100644
index 0000000000..1779e191fd
--- /dev/null
+++ b/tools/fuzzing/rust/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "gecko-fuzz-targets"
+version = "0.1.0"
+authors = ["fuzzing@mozilla.com"]
+
+[dependencies]
+libc = "0.2"
+tempfile = "3"
+lazy_static = "1.4.0"
+rkv = { version = "0.16", features = ["with-fuzzer-no-link"] }
+lmdb-rkv = { version = "0.14", features = ["with-fuzzer-no-link"] }
diff --git a/tools/fuzzing/rust/RustFuzzingTargets.cpp b/tools/fuzzing/rust/RustFuzzingTargets.cpp
new file mode 100644
index 0000000000..7b4dd6c86c
--- /dev/null
+++ b/tools/fuzzing/rust/RustFuzzingTargets.cpp
@@ -0,0 +1,15 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FuzzingInterface.h"
+#include "RustFuzzingTargets.h"
+
+int FuzzingInitDummy(int* argc, char*** argv) { return 0; }
+
+MOZ_FUZZING_INTERFACE_RAW(FuzzingInitDummy, fuzz_rkv_db_file, RkvDbFile);
+MOZ_FUZZING_INTERFACE_RAW(FuzzingInitDummy, fuzz_rkv_db_name, RkvDbName);
+MOZ_FUZZING_INTERFACE_RAW(FuzzingInitDummy, fuzz_rkv_key_write, RkvKeyWrite);
+MOZ_FUZZING_INTERFACE_RAW(FuzzingInitDummy, fuzz_rkv_val_write, RkvValWrite);
+MOZ_FUZZING_INTERFACE_RAW(FuzzingInitDummy, fuzz_rkv_calls, RkvCalls);
diff --git a/tools/fuzzing/rust/RustFuzzingTargets.h b/tools/fuzzing/rust/RustFuzzingTargets.h
new file mode 100644
index 0000000000..8f44fb277c
--- /dev/null
+++ b/tools/fuzzing/rust/RustFuzzingTargets.h
@@ -0,0 +1,23 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Interface definitions for fuzzing rust modules
+ */
+
+#ifndef RustFuzzingTargets_h__
+#define RustFuzzingTargets_h__
+
+extern "C" {
+
+int fuzz_rkv_db_file(const uint8_t* raw_data, size_t size);
+int fuzz_rkv_db_name(const uint8_t* raw_data, size_t size);
+int fuzz_rkv_key_write(const uint8_t* raw_data, size_t size);
+int fuzz_rkv_val_write(const uint8_t* raw_data, size_t size);
+int fuzz_rkv_calls(const uint8_t* raw_data, size_t size);
+
+}  // extern "C"
+
+#endif  // RustFuzzingTargets_h__
diff --git a/tools/fuzzing/rust/moz.build b/tools/fuzzing/rust/moz.build
new file mode 100644
index 0000000000..aa6aaa29dc
--- /dev/null
+++ b/tools/fuzzing/rust/moz.build
@@ -0,0 +1,16 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("fuzzer-rust-targets")
+
+SOURCES += [
+    "RustFuzzingTargets.cpp",
+]
+
+# Add libFuzzer configuration directives
+include("/tools/fuzzing/libfuzzer-config.mozbuild")
+
+FINAL_LIBRARY = "xul-gtest"
diff --git a/tools/fuzzing/rust/src/lib.rs b/tools/fuzzing/rust/src/lib.rs
new file mode 100644
index 0000000000..1df45153c1
--- /dev/null
+++ b/tools/fuzzing/rust/src/lib.rs
@@ -0,0 +1,341 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#[macro_use]
+extern crate lazy_static;
+extern crate libc;
+extern crate lmdb;
+extern crate rkv;
+extern crate tempfile;
+
+use rkv::backend::{
+    BackendEnvironmentBuilder, SafeMode, SafeModeDatabase, SafeModeEnvironment,
+    SafeModeRoTransaction, SafeModeRwTransaction,
+};
+use std::fs;
+use std::fs::File;
+use std::io::Write;
+use std::iter;
+use std::path::Path;
+use std::sync::Arc;
+use std::thread;
+use tempfile::Builder;
+
+type Rkv = rkv::Rkv<SafeModeEnvironment>;
+type SingleStore = rkv::SingleStore<SafeModeDatabase>;
+
+fn eat_lmdb_err<T>(value: Result<T, rkv::StoreError>) -> Result<Option<T>, rkv::StoreError> {
+    match value {
+        Ok(value) => Ok(Some(value)),
+        Err(rkv::StoreError::LmdbError(_)) => Ok(None),
+        Err(err) => {
+            println!("Not a crash, but an error outside LMDB: {}", err);
+            println!("A refined fuzzing test, or changes to RKV, might be required.");
+            Err(err)
+        }
+    }
+}
+
+fn panic_with_err(err: rkv::StoreError) {
+    println!("Got error: {}", err);
+    Result::Err(err).unwrap()
+}
+
+#[no_mangle]
+pub extern "C" fn fuzz_rkv_db_file(raw_data: *const u8, size: libc::size_t) -> libc::c_int {
+    let data = unsafe { std::slice::from_raw_parts(raw_data as *const u8, size as usize) };
+
+    // First 8192 bytes are for the lock file.
+    if data.len() < 8192 {
+        return 0;
+    }
+    let (lock, db) = data.split_at(8192);
+    let mut lock_file = File::create("data.lock").unwrap();
+    lock_file.write_all(lock).unwrap();
+    let mut db_file = File::create("data.mdb").unwrap();
+    db_file.write_all(db).unwrap();
+
+    let env = Rkv::with_capacity::<SafeMode>(Path::new("."), 2).unwrap();
+    let store = env
+        .open_single("test", rkv::StoreOptions::create())
+        .unwrap();
+
+    let reader = env.read().unwrap();
+    eat_lmdb_err(store.get(&reader, &[0])).unwrap();
+
+    0
+}
+
+#[no_mangle]
+pub extern "C" fn fuzz_rkv_db_name(raw_data: *const u8, size: libc::size_t) -> libc::c_int {
+    let data = unsafe { std::slice::from_raw_parts(raw_data as *const u8, size as usize) };
+
+    let root = Builder::new().prefix("fuzz_rkv_db_name").tempdir().unwrap();
+    fs::create_dir_all(root.path()).unwrap();
+
+    let env = Rkv::new::<SafeMode>(root.path()).unwrap();
+    let name = String::from_utf8_lossy(data);
+    println!("Checking string: '{:?}'", name);
+    // Some strings are invalid database names, and are handled as store errors.
+    // Ignore those errors, but not others.
+    let store = eat_lmdb_err(env.open_single(name.as_ref(), rkv::StoreOptions::create())).unwrap();
+
+    if let Some(store) = store {
+        let reader = env.read().unwrap();
+        eat_lmdb_err(store.get(&reader, &[0])).unwrap();
+    };
+
+    0
+}
+
+#[no_mangle]
+pub extern "C" fn fuzz_rkv_key_write(raw_data: *const u8, size: libc::size_t) -> libc::c_int {
+    let data = unsafe { std::slice::from_raw_parts(raw_data as *const u8, size as usize) };
+
+    let root = Builder::new()
+        .prefix("fuzz_rkv_key_write")
+        .tempdir()
+        .unwrap();
+    fs::create_dir_all(root.path()).unwrap();
+
+    let env = Rkv::new::<SafeMode>(root.path()).unwrap();
+    let store = env
+        .open_single("test", rkv::StoreOptions::create())
+        .unwrap();
+
+    let mut writer = env.write().unwrap();
+    // Some data are invalid values, and are handled as store errors.
+    // Ignore those errors, but not others.
+    eat_lmdb_err(store.put(&mut writer, data, &rkv::Value::Str("val"))).unwrap();
+
+    0
+}
+
+#[no_mangle]
+pub extern "C" fn fuzz_rkv_val_write(raw_data: *const u8, size: libc::size_t) -> libc::c_int {
+    let data = unsafe { std::slice::from_raw_parts(raw_data as *const u8, size as usize) };
+
+    let root = Builder::new()
+        .prefix("fuzz_rkv_val_write")
+        .tempdir()
+        .unwrap();
+    fs::create_dir_all(root.path()).unwrap();
+
+    let env = Rkv::new::<SafeMode>(root.path()).unwrap();
+    let store = env
+        .open_single("test", rkv::StoreOptions::create())
+        .unwrap();
+
+    let mut writer = env.write().unwrap();
+    let string = String::from_utf8_lossy(data);
+    let value = rkv::Value::Str(&string);
+    store.put(&mut writer, "key", &value).unwrap();
+
+    0
+}
+
+lazy_static! {
+    static ref STATIC_DATA: Vec<String> = {
+        let sizes = vec![4, 16, 128, 512, 1024];
+        let mut data = Vec::new();
+
+        for (i, s) in sizes.into_iter().enumerate() {
+            let bytes = iter::repeat('a' as u8 + i as u8).take(s).collect();
+            data.push(String::from_utf8(bytes).unwrap());
+        }
+
+        data
+    };
+}
+
+#[no_mangle]
+pub extern "C" fn fuzz_rkv_calls(raw_data: *const u8, size: libc::size_t) -> libc::c_int {
+    let data = unsafe { std::slice::from_raw_parts(raw_data as *const u8, size as usize) };
+    let mut fuzz = data.iter().copied();
+
+    fn maybe_do<I: Iterator<Item = u8>>(fuzz: &mut I, f: impl FnOnce(&mut I) -> ()) -> Option<()> {
+        match fuzz.next().map(|byte| byte % 2) {
+            Some(0) => Some(f(fuzz)),
+            _ => None,
+        }
+    }
+
+    fn maybe_abort<I: Iterator<Item = u8>>(
+        fuzz: &mut I,
+        read: rkv::Reader<SafeModeRoTransaction>,
+    ) -> Result<(), rkv::StoreError> {
+        match fuzz.next().map(|byte| byte % 2) {
+            Some(0) => Ok(read.abort()),
+            _ => Ok(()),
+        }
+    };
+
+    fn maybe_commit<I: Iterator<Item = u8>>(
+        fuzz: &mut I,
+        write: rkv::Writer<SafeModeRwTransaction>,
+    ) -> Result<(), rkv::StoreError> {
+        match fuzz.next().map(|byte| byte % 3) {
+            Some(0) => write.commit(),
+            Some(1) => Ok(write.abort()),
+            _ => Ok(()),
+        }
+    };
+
+    fn get_static_data<'a, I: Iterator<Item = u8>>(fuzz: &mut I) -> Option<&'a String> {
+        fuzz.next().map(|byte| {
+            let data = &*STATIC_DATA;
+            let n = byte as usize;
+            data.get(n % data.len()).unwrap()
+        })
+    };
+
+    fn get_fuzz_data<I: Iterator<Item = u8> + Clone>(
+        fuzz: &mut I,
+        max_len: usize,
+    ) -> Option<Vec<u8>> {
+        fuzz.next().map(|byte| {
+            let n = byte as usize;
+            fuzz.clone().take((n * n) % max_len).collect()
+        })
+    };
+
+    fn get_any_data<I: Iterator<Item = u8> + Clone>(
+        fuzz: &mut I,
+        max_len: usize,
+    ) -> Option<Vec<u8>> {
+        match fuzz.next().map(|byte| byte % 2) {
+            Some(0) => get_static_data(fuzz).map(|v| v.clone().into_bytes()),
+            Some(1) => get_fuzz_data(fuzz, max_len),
+            _ => None,
+        }
+    };
+
+    fn store_put<I: Iterator<Item = u8> + Clone>(fuzz: &mut I, env: &Rkv, store: &SingleStore) {
+        let key = match get_any_data(fuzz, 1024) {
+            Some(key) => key,
+            None => return,
+        };
+        let value = match get_any_data(fuzz, std::usize::MAX) {
+            Some(value) => value,
+            None => return,
+        };
+
+        let mut writer = env.write().unwrap();
+        let mut full = false;
+
+        match store.put(&mut writer, key, &rkv::Value::Blob(&value)) {
+            Ok(_) => {}
+            Err(rkv::StoreError::LmdbError(lmdb::Error::BadValSize)) => {}
+            Err(rkv::StoreError::LmdbError(lmdb::Error::MapFull)) => full = true,
+            Err(err) => panic_with_err(err),
+        };
+
+        if full {
+            writer.abort();
+            store_resize(fuzz, env);
+        } else {
+            maybe_commit(fuzz, writer).unwrap();
+        }
+    }
+
+    fn store_get<I: Iterator<Item = u8> + Clone>(fuzz: &mut I, env: &Rkv, store: &SingleStore) {
+        let key = match get_any_data(fuzz, 1024) {
+            Some(key) => key,
+            None => return,
+        };
+
+        let mut reader = match env.read() {
+            Ok(reader) => reader,
+            Err(rkv::StoreError::LmdbError(lmdb::Error::ReadersFull)) => return,
+            Err(err) => return panic_with_err(err),
+        };
+
+        match store.get(&mut reader, key) {
+            Ok(_) => {}
+            Err(rkv::StoreError::LmdbError(lmdb::Error::BadValSize)) => {}
+            Err(err) => panic_with_err(err),
+        };
+
+        maybe_abort(fuzz, reader).unwrap();
+    }
+
+    fn store_delete<I: Iterator<Item = u8> + Clone>(fuzz: &mut I, env: &Rkv, store: &SingleStore) {
+        let key = match get_any_data(fuzz, 1024) {
+            Some(key) => key,
+            None => return,
+        };
+
+        let mut writer = env.write().unwrap();
+
+        match store.delete(&mut writer, key) {
+            Ok(_) => {}
+            Err(rkv::StoreError::LmdbError(lmdb::Error::BadValSize)) => {}
+            Err(rkv::StoreError::LmdbError(lmdb::Error::NotFound)) => {}
+            Err(err) => panic_with_err(err),
+        };
+
+        maybe_commit(fuzz, writer).unwrap();
+    }
+
+    fn store_resize<I: Iterator<Item = u8>>(fuzz: &mut I, env: &Rkv) {
+        let n = fuzz.next().unwrap_or(1) as usize;
+        env.set_map_size(1_048_576 * (n % 100)).unwrap() // 1,048,576 bytes, i.e. 1MiB.
+    };
+
+    let root = Builder::new().prefix("fuzz_rkv_calls").tempdir().unwrap();
+    fs::create_dir_all(root.path()).unwrap();
+
+    let mut builder: SafeMode = Rkv::environment_builder();
+    builder.set_max_dbs(1); // need at least one db
+
+    maybe_do(&mut fuzz, |fuzz| {
+        let n = fuzz.next().unwrap_or(126) as u32; // default
+        builder.set_max_readers(1 + n);
+    });
+    maybe_do(&mut fuzz, |fuzz| {
+        let n = fuzz.next().unwrap_or(0) as u32;
+        builder.set_max_dbs(1 + n);
+    });
+    maybe_do(&mut fuzz, |fuzz| {
+        let n = fuzz.next().unwrap_or(1) as usize;
+        builder.set_map_size(1_048_576 * (n % 100)); // 1,048,576 bytes, i.e. 1MiB.
+    });
+
+    let env = Rkv::from_builder(root.path(), builder).unwrap();
+    let store = env
+        .open_single("test", rkv::StoreOptions::create())
+        .unwrap();
+
+    let shared_env = Arc::new(env);
+    let shared_store = Arc::new(store);
+
+    let use_threads = fuzz.next().map(|byte| byte % 10 == 0).unwrap_or(false);
+    let max_threads = if use_threads { 16 } else { 1 };
+    let num_threads = fuzz.next().unwrap_or(0) as usize % max_threads + 1;
+    let chunk_size = fuzz.len() / num_threads;
+
+    let threads = (0..num_threads).map(|_| {
+        let env = shared_env.clone();
+        let store = shared_store.clone();
+
+        let chunk: Vec<_> = fuzz.by_ref().take(chunk_size).collect();
+        let mut fuzz = chunk.into_iter();
+
+        thread::spawn(move || loop {
+            match fuzz.next().map(|byte| byte % 4) {
+                Some(0) => store_put(&mut fuzz, &env, &store),
+                Some(1) => store_get(&mut fuzz, &env, &store),
+                Some(2) => store_delete(&mut fuzz, &env, &store),
+                Some(3) => store_resize(&mut fuzz, &env),
+                _ => break,
+            }
+        })
+    });
+
+    for handle in threads {
+        handle.join().unwrap()
+    }
+
+    0
+}
diff --git a/tools/fuzzing/shmem/SharedMemoryFuzzer.cpp b/tools/fuzzing/shmem/SharedMemoryFuzzer.cpp
new file mode 100644
index 0000000000..49a79fa975
--- /dev/null
+++ b/tools/fuzzing/shmem/SharedMemoryFuzzer.cpp
@@ -0,0 +1,122 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FuzzingMutate.h"
+#include "FuzzingTraits.h"
+#include "nsDebug.h"
+#include "prenv.h"
+#include "SharedMemoryFuzzer.h"
+
+#define SHMEM_FUZZER_DEFAULT_MUTATION_PROBABILITY 2
+#define SHMEM_FUZZER_DEFAULT_MUTATION_FACTOR 500
+#define SHMEM_FUZZER_LOG(fmt, args...)                       \
+  if (SharedMemoryFuzzer::IsLoggingEnabled()) {              \
+    printf_stderr("[SharedMemoryFuzzer] " fmt "\n", ##args); \
+  }
+
+namespace mozilla {
+namespace ipc {
+
+using namespace fuzzing;
+
+/* static */
+bool SharedMemoryFuzzer::IsLoggingEnabled() {
+  static bool sInitialized = false;
+  static bool sIsLoggingEnabled = false;
+
+  if (!sInitialized) {
+    sIsLoggingEnabled = !!PR_GetEnv("SHMEM_FUZZER_ENABLE_LOGGING");
+    sInitialized = true;
+  }
+  return sIsLoggingEnabled;
+}
+
+/* static */
+bool SharedMemoryFuzzer::IsEnabled() {
+  static bool sInitialized = false;
+  static bool sIsFuzzerEnabled = false;
+
+  if (!sInitialized) {
+    sIsFuzzerEnabled = !!PR_GetEnv("SHMEM_FUZZER_ENABLE");
+  }
+  return sIsFuzzerEnabled;
+}
+
+/* static */
+uint64_t SharedMemoryFuzzer::MutationProbability() {
+  static uint64_t sPropValue = SHMEM_FUZZER_DEFAULT_MUTATION_PROBABILITY;
+  static bool sInitialized = false;
+
+  if (sInitialized) {
+    return sPropValue;
+  }
+  sInitialized = true;
+
+  const char* probability = PR_GetEnv("SHMEM_FUZZER_MUTATION_PROBABILITY");
+  if (probability) {
+    long n = std::strtol(probability, nullptr, 10);
+    if (n != 0) {
+      sPropValue = n;
+      return sPropValue;
+    }
+  }
+  return sPropValue;
+}
+
+/* static */
+uint64_t SharedMemoryFuzzer::MutationFactor() {
+  static uint64_t sPropValue = SHMEM_FUZZER_DEFAULT_MUTATION_FACTOR;
+  static bool sInitialized = false;
+
+  if (sInitialized) {
+    return sPropValue;
+  }
+  sInitialized = true;
+
+  const char* factor = PR_GetEnv("SHMEM_FUZZER_MUTATION_FACTOR");
+  if (factor) {
+    long n = strtol(factor, nullptr, 10);
+    if (n != 0) {
+      sPropValue = n;
+      return sPropValue;
+    }
+  }
+  return sPropValue;
+}
+
+/* static */
+void* SharedMemoryFuzzer::MutateSharedMemory(void* aMemory, size_t aSize) {
+  if (!IsEnabled()) {
+    return aMemory;
+  }
+
+  if (aSize == 0) {
+    /* Shmem opened from foreign handle. */
+    SHMEM_FUZZER_LOG("shmem is of size 0.");
+    return aMemory;
+  }
+
+  if (!aMemory) {
+    /* Memory space is not mapped. */
+    SHMEM_FUZZER_LOG("shmem memory space is not mapped.");
+    return aMemory;
+  }
+
+  // The likelihood when a value gets fuzzed of this object.
+  if (!FuzzingTraits::Sometimes(MutationProbability())) {
+    return aMemory;
+  }
+
+  const size_t max = FuzzingTraits::Frequency(aSize, MutationFactor());
+  SHMEM_FUZZER_LOG("shmem of size: %zu / mutations: %zu", aSize, max);
+  for (size_t i = 0; i < max; i++) {
+    FuzzingMutate::ChangeBit((uint8_t*)aMemory, aSize);
+  }
+  return aMemory;
+}
+
+}  // namespace ipc
+}  // namespace mozilla
diff --git a/tools/fuzzing/shmem/SharedMemoryFuzzer.h b/tools/fuzzing/shmem/SharedMemoryFuzzer.h
new file mode 100644
index 0000000000..51bfe5ef3e
--- /dev/null
+++ b/tools/fuzzing/shmem/SharedMemoryFuzzer.h
@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_SharedMemoryFuzzer_h
+#define mozilla_dom_SharedMemoryFuzzer_h
+
+namespace mozilla {
+namespace ipc {
+
+/*
+ * Exposed environment variables:
+ * SHMEM_FUZZER_ENABLE=1
+ * SHMEM_FUZZER_ENABLE_LOGGING=1       (optional)
+ * SHMEM_FUZZER_MUTATION_PROBABILITY=2 (optional)
+ * SHMEM_FUZZER_MUTATION_FACTOR=500    (optional)
+ */
+
+class SharedMemoryFuzzer {
+ public:
+  static void* MutateSharedMemory(void* aMemory, size_t aSize);
+
+ private:
+  static uint64_t MutationProbability();
+  static uint64_t MutationFactor();
+  static bool IsEnabled();
+  static bool IsLoggingEnabled();
+};
+
+}  // namespace ipc
+}  // namespace mozilla
+
+#endif
diff --git a/tools/fuzzing/shmem/moz.build b/tools/fuzzing/shmem/moz.build
new file mode 100644
index 0000000000..ee9c549920
--- /dev/null
+++ b/tools/fuzzing/shmem/moz.build
@@ -0,0 +1,11 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += ["SharedMemoryFuzzer.cpp"]
+
+EXPORTS.mozilla.ipc += ["SharedMemoryFuzzer.h"]
+
+FINAL_LIBRARY = "xul"
diff --git a/tools/fuzzing/smoke/js.py b/tools/fuzzing/smoke/js.py
new file mode 100755
index 0000000000..c787e072e7
--- /dev/null
+++ b/tools/fuzzing/smoke/js.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+""" Hello I am a fake jsshell for testing purpose.
+Add more features!
+"""
+from __future__ import absolute_import
+import argparse
+import sys
+
+
+def run():
+    parser = argparse.ArgumentParser(description="Process some integers.")
+    parser.add_argument("-e", type=str, default=None)
+
+    parser.add_argument("--fuzzing-safe", action="store_true", default=False)
+
+    args = parser.parse_args()
+
+    if args.e is not None:
+        if "crash()" in args.e:
+            sys.exit(1)
+
+
+if __name__ == "__main__":
+    run()
diff --git a/tools/fuzzing/smoke/smoke.py b/tools/fuzzing/smoke/smoke.py
new file mode 100644
index 0000000000..b81fa53941
--- /dev/null
+++ b/tools/fuzzing/smoke/smoke.py
@@ -0,0 +1,72 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+""" Smoke test script for Fuzzing
+
+This script can be used to perform simple calls using `jsshell`
+or whatever other tools you may add.
+
+The call is done via `taskcluster/ci/fuzzing/kind.yml` and
+files contained in the `target.jsshell.zip` and `target.fuzztest.tests.tar.gz`
+build artifacts are downloaded to run things.
+
+Everything included in this directory will be added in
+`target.fuzztest.tests.tar.gz` at build time, so you can add more scripts and
+tools if you need. They will be located in `$MOZ_FETCHES_DIR` and follow the
+same directory structure than the source tree.
+"""
+from __future__ import absolute_import
+from distutils.spawn import find_executable
+import os
+import os.path
+import subprocess
+import shlex
+import sys
+
+
+def run_jsshell(command, label=None):
+    """Invokes `jsshell` with command.
+
+    This function will use the `JSSHELL` environment variable,
+    and fallback to a `js` executable if it finds one
+    """
+    shell = os.environ.get("JSSHELL")
+    if shell is None:
+        shell = find_executable("js")
+        if shell is None:
+            raise FileNotFoundError(shell)
+    else:
+        if not os.path.exists(shell) or not os.path.isfile(shell):
+            raise FileNotFoundError(shell)
+
+    if label is None:
+        label = command
+    sys.stdout.write(label)
+    cmd = [shell] + shlex.split(command)
+    sys.stdout.flush()
+    try:
+        subprocess.check_call(cmd)
+    finally:
+        sys.stdout.write("\n")
+        sys.stdout.flush()
+
+
+def smoke_test():
+    # first, let's make sure it catches crashes so we don't have false
+    # positives.
+    try:
+        run_jsshell("-e 'crash();'", "Testing for crash\n")
+    except subprocess.CalledProcessError:
+        pass
+    else:
+        raise Exception("Could not get the process to crash")
+
+    # now let's proceed with some tests
+    run_jsshell("--fuzzing-safe -e 'print(\"PASSED\")'", "Simple Fuzzing...")
+
+    # add more smoke tests here
+
+
+if __name__ == "__main__":
+    # if this calls raises an error, the job will turn red in the CI.
+    smoke_test()
diff --git a/tools/fuzzing/smoke/tests.py b/tools/fuzzing/smoke/tests.py
new file mode 100644
index 0000000000..9b49380cad
--- /dev/null
+++ b/tools/fuzzing/smoke/tests.py
@@ -0,0 +1,35 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+from __future__ import absolute_import
+import os
+import pytest
+from contextlib import contextmanager
+
+import smoke
+
+JS = os.path.join(os.path.dirname(__file__), "js.py")
+
+
+@contextmanager
+def fake_js():
+    os.environ["JSSHELL"] = JS
+    try:
+        yield
+    finally:
+        del os.environ["JSSHELL"]
+
+
+def test_run_no_jsshell():
+    with pytest.raises(FileNotFoundError):
+        smoke.run_jsshell("--fuzzing-safe -e 'print(\"PASSED\")'")
+
+
+def test_run_jsshell_set():
+    with fake_js():
+        smoke.run_jsshell("--fuzzing-safe -e 'print(\"PASSED\")'")
+
+
+def test_smoke_test():
+    with fake_js():
+        smoke.smoke_test()