Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

1 files changed, 618 insertions, 0 deletions

diff --git a/mozglue/baseprofiler/public/ModuloBuffer.h b/mozglue/baseprofiler/public/ModuloBuffer.h
new file mode 100644
index 0000000000..80e765279e
--- /dev/null
+++ b/mozglue/baseprofiler/public/ModuloBuffer.h
@@ -0,0 +1,618 @@
+/* -*- Mode: C++; tab-width: 2; 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 ModuloBuffer_h
+#define ModuloBuffer_h
+
+#include "mozilla/leb128iterator.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/PowerOfTwo.h"
+#include "mozilla/ProfileBufferEntrySerialization.h"
+#include "mozilla/UniquePtr.h"
+
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <type_traits>
+
+namespace mozilla {
+
+// The ModuloBuffer class is a circular buffer that holds raw byte values, with
+// data-read/write helpers.
+//
+// OffsetT: Type of the internal offset into the buffer of bytes, it should be
+// large enough to access all bytes of the buffer. It will also be used as
+// Length (in bytes) of the buffer and of any subset. Default uint32_t
+// IndexT: Type of the external index, it should be large enough that overflows
+// should not happen during the lifetime of the ModuloBuffer.
+//
+// The basic usage is to create an iterator-like object with `ReaderAt(Index)`
+// or `WriterAt(Index)`, and use it to read/write data blobs. Iterators
+// automatically manage the wrap-around (through "Modulo", which is effectively
+// an AND-masking with the PowerOfTwo buffer size.)
+//
+// There is zero safety: No thread safety, no checks that iterators may be
+// overwriting data that's still to be read, etc. It's up to the caller to add
+// adequate checks.
+// The intended use is as an underlying buffer for a safer container.
+template <typename OffsetT = uint32_t, typename IndexT = uint64_t>
+class ModuloBuffer {
+ public:
+  using Byte = uint8_t;
+  static_assert(sizeof(Byte) == 1, "ModuloBuffer::Byte must be 1 byte");
+  using Offset = OffsetT;
+  static_assert(!std::numeric_limits<Offset>::is_signed,
+                "ModuloBuffer::Offset must be an unsigned integral type");
+  using Length = Offset;
+  using Index = IndexT;
+  static_assert(!std::numeric_limits<Index>::is_signed,
+                "ModuloBuffer::Index must be an unsigned integral type");
+  static_assert(sizeof(Index) >= sizeof(Offset),
+                "ModuloBuffer::Index size must >= Offset");
+
+  // Create a buffer of the given length.
+  explicit ModuloBuffer(PowerOfTwo<Length> aLength)
+      : mMask(aLength.Mask()),
+        mBuffer(WrapNotNull(new Byte[aLength.Value()])),
+        mBufferDeleter([](Byte* aBuffer) { delete[] aBuffer; }) {}
+
+  // Take ownership of an existing buffer. Existing contents is ignored.
+  // Done by extracting the raw pointer from UniquePtr<Byte[]>, and adding
+  // an equivalent `delete[]` in `mBufferDeleter`.
+  ModuloBuffer(UniquePtr<Byte[]> aExistingBuffer, PowerOfTwo<Length> aLength)
+      : mMask(aLength.Mask()),
+        mBuffer(WrapNotNull(aExistingBuffer.release())),
+        mBufferDeleter([](Byte* aBuffer) { delete[] aBuffer; }) {}
+
+  // Use an externally-owned buffer. Existing contents is ignored.
+  ModuloBuffer(Byte* aExternalBuffer, PowerOfTwo<Length> aLength)
+      : mMask(aLength.Mask()), mBuffer(WrapNotNull(aExternalBuffer)) {}
+
+  // Disallow copying, as we may uniquely own the resource.
+  ModuloBuffer(const ModuloBuffer& aOther) = delete;
+  ModuloBuffer& operator=(const ModuloBuffer& aOther) = delete;
+
+  // Allow move-construction. Stealing ownership if the original had it.
+  // This effectively prevents copy construction, and all assignments; needed so
+  // that a ModuloBuffer may be initialized from a separate construction.
+  // The moved-from ModuloBuffer still points at the resource but doesn't own
+  // it, so it won't try to free it; but accesses are not guaranteed, so it
+  // should not be used anymore.
+  ModuloBuffer(ModuloBuffer&& aOther)
+      : mMask(std::move(aOther.mMask)),
+        mBuffer(std::move(aOther.mBuffer)),
+        mBufferDeleter(std::move(aOther.mBufferDeleter)) {
+    // The above move leaves `aOther.mBufferDeleter` in a valid state but with
+    // an unspecified value, so it could theoretically still contain the
+    // original function, which would be bad because we don't want aOther to
+    // delete the resource that `this` now owns.
+    if (aOther.mBufferDeleter) {
+      // `aOther` still had a non-empty deleter, reset it.
+      aOther.mBufferDeleter = nullptr;
+    }
+  }
+
+  // Disallow assignment, as we have some `const` members.
+  ModuloBuffer& operator=(ModuloBuffer&& aOther) = delete;
+
+  // Destructor, deletes the resource if we uniquely own it.
+  ~ModuloBuffer() {
+    if (mBufferDeleter) {
+      mBufferDeleter(mBuffer);
+    }
+  }
+
+  PowerOfTwo<Length> BufferLength() const {
+    return PowerOfTwo<Length>(mMask.MaskValue() + 1);
+  }
+
+  // Size of external resources.
+  // Note: `mBufferDeleter`'s potential external data (for its captures) is not
+  // included, as it's hidden in the `std::function` implementation.
+  size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    if (!mBufferDeleter) {
+      // If we don't have a buffer deleter, assume we don't own the data, so
+      // it's probably on the stack, or should be reported by its owner.
+      return 0;
+    }
+    return aMallocSizeOf(mBuffer);
+  }
+
+  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  ProfileBufferEntryReader EntryReaderFromTo(
+      Index aStart, Index aEnd, ProfileBufferBlockIndex aBlockIndex,
+      ProfileBufferBlockIndex aNextBlockIndex) const {
+    using EntrySpan = Span<const ProfileBufferEntryReader::Byte>;
+    if (aStart == aEnd) {
+      return ProfileBufferEntryReader{};
+    }
+    // Don't allow over-wrapping.
+    MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+    // Start offset in 0 .. (buffer size - 1)
+    Offset start = static_cast<Offset>(aStart) & mMask;
+    // End offset in 1 .. (buffer size)
+    Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+    if (start < end) {
+      // Segment doesn't cross buffer threshold, one span is enough.
+      return ProfileBufferEntryReader{EntrySpan(&mBuffer[start], end - start),
+                                      aBlockIndex, aNextBlockIndex};
+    }
+    // Segment crosses buffer threshold, we need one span until the end and one
+    // span restarting at the beginning of the buffer.
+    return ProfileBufferEntryReader{
+        EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+        EntrySpan(&mBuffer[0], end), aBlockIndex, aNextBlockIndex};
+  }
+
+  // Return an entry writer for the given range.
+  ProfileBufferEntryWriter EntryWriterFromTo(Index aStart, Index aEnd) const {
+    using EntrySpan = Span<ProfileBufferEntryReader::Byte>;
+    if (aStart == aEnd) {
+      return ProfileBufferEntryWriter{};
+    }
+    MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+    // Start offset in 0 .. (buffer size - 1)
+    Offset start = static_cast<Offset>(aStart) & mMask;
+    // End offset in 1 .. (buffer size)
+    Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+    if (start < end) {
+      // Segment doesn't cross buffer threshold, one span is enough.
+      return ProfileBufferEntryWriter{
+          EntrySpan(&mBuffer[start], end - start),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd)};
+    }
+    // Segment crosses buffer threshold, we need one span until the end and one
+    // span restarting at the beginning of the buffer.
+    return ProfileBufferEntryWriter{
+        EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+        EntrySpan(&mBuffer[0], end),
+        ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+        ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd)};
+  }
+
+  // Emplace an entry writer into `aMaybeEntryWriter` for the given range.
+  void EntryWriterFromTo(Maybe<ProfileBufferEntryWriter>& aMaybeEntryWriter,
+                         Index aStart, Index aEnd) const {
+    MOZ_ASSERT(aMaybeEntryWriter.isNothing(),
+               "Reference entry writer should be Nothing.");
+    using EntrySpan = Span<ProfileBufferEntryReader::Byte>;
+    if (aStart == aEnd) {
+      return;
+    }
+    MOZ_ASSERT(aEnd - aStart <= mMask.MaskValue() + 1);
+    // Start offset in 0 .. (buffer size - 1)
+    Offset start = static_cast<Offset>(aStart) & mMask;
+    // End offset in 1 .. (buffer size)
+    Offset end = (static_cast<Offset>(aEnd - 1) & mMask) + 1;
+    if (start < end) {
+      // Segment doesn't cross buffer threshold, one span is enough.
+      aMaybeEntryWriter.emplace(
+          EntrySpan(&mBuffer[start], end - start),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd));
+    } else {
+      // Segment crosses buffer threshold, we need one span until the end and
+      // one span restarting at the beginning of the buffer.
+      aMaybeEntryWriter.emplace(
+          EntrySpan(&mBuffer[start], mMask.MaskValue() + 1 - start),
+          EntrySpan(&mBuffer[0], end),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aStart),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(aEnd));
+    }
+  }
+
+  // All ModuloBuffer operations should be done through this iterator, which has
+  // an effectively infinite range. The underlying wrapping-around is hidden.
+  // Use `ReaderAt(Index)` or `WriterAt(Index)` to create it.
+  //
+  // `const Iterator<...>` means the iterator itself cannot change, i.e., it
+  // cannot move, and only its const methods are available. Note that these
+  // const methods may still be used to modify the buffer contents (e.g.:
+  // `operator*()`, `Poke()`).
+  //
+  // `Iterator</*IsBufferConst=*/true>` means the buffer contents cannot be
+  // modified, i.e., write operations are forbidden, but the iterator may still
+  // move if non-const itself.
+  template <bool IsBufferConst>
+  class Iterator {
+    // Alias to const- or mutable-`ModuloBuffer` depending on `IsBufferConst`.
+    using ConstOrMutableBuffer =
+        std::conditional_t<IsBufferConst, const ModuloBuffer, ModuloBuffer>;
+
+    // Implementation note about the strange enable-if's below:
+    //   `template <bool NotIBC = !IsBufferConst> enable_if_t<NotIBC>`
+    // which intuitively could be simplified to:
+    //   `enable_if_t<!IsBufferConst>`
+    // The former extra-templated syntax is in fact necessary to delay
+    // instantiation of these functions until they are actually needed.
+    //
+    // If we were just doing `enable_if_t<!IsBufferConst>`, this would only
+    // depend on the *class* (`ModuloBuffer<...>::Iterator`), which gets
+    // instantiated when a `ModuloBuffer` is created with some template
+    // arguments; at that point, all non-templated methods get instantiated, so
+    // there's no "SFINAE" happening, and `enable_if_t<...>` is actually doing
+    // `typename enable_if<...>::type` on the spot, but there is no `type` if
+    // `IsBufferConst` is true, so it just fails right away. E.g.:
+    // error: no type named 'type' in 'std::enable_if<false, void>';
+    //        'enable_if' cannot be used to disable this declaration
+    // note: in instantiation of template type alias 'enable_if_t'
+    // > std::enable_if_t<!IsBufferConst> WriteObject(const T& aObject) {
+    //       in instantiation of template class
+    //       'mozilla::ModuloBuffer<...>::Iterator<true>'
+    // > auto it = mb.ReaderAt(1);
+    //
+    // By adding another template level `template <bool NotIsBufferConst =
+    // !IsBufferConst>`, the instantiation is delayed until the function is
+    // actually invoked somewhere, e.g. `it.Poke(...);`.
+    // So at that invocation point, the compiler looks for a "Poke" name in it,
+    // and considers potential template instantiations that could work. The
+    // `enable_if_t` is *now* attempted, with `NotIsBufferConst` taking its
+    // value from `!IsBufferConst`:
+    // - If `IsBufferConst` is false, `NotIsBufferConst` is true,
+    // `enable_if<NotIsBufferConst>` does define a `type` (`void` by default),
+    // so `enable_if_t` happily becomes `void`, the function exists and may be
+    // called.
+    // - Otherwise if `IsBufferConst` is true, `NotIsBufferConst` is false,
+    // `enable_if<NotIsBufferConst>` does *not* define a `type`, therefore
+    // `enable_if_t` produces an error because there is no `type`. Now "SFINAE"
+    // happens: This "Substitution Failure Is Not An Error" (by itself)... But
+    // then, there are no other functions named "Poke" as requested in the
+    // `it.Poke(...);` call, so we are now getting an error (can't find
+    // function), as expected because `it` had `IsBufferConst`==true. (But at
+    // least the compiler waited until this invocation attempt before outputting
+    // an error.)
+    //
+    // C++ is fun!
+
+   public:
+    // These definitions are expected by std functions, to recognize this as an
+    // iterator. See https://en.cppreference.com/w/cpp/iterator/iterator_traits
+    using difference_type = Index;
+    using value_type = Byte;
+    using pointer = std::conditional_t<IsBufferConst, const Byte*, Byte*>;
+    using reference = std::conditional_t<IsBufferConst, const Byte&, Byte&>;
+    using iterator_category = std::random_access_iterator_tag;
+
+    // Can always copy/assign from the same kind of iterator.
+    Iterator(const Iterator& aRhs) = default;
+    Iterator& operator=(const Iterator& aRhs) = default;
+
+    // Can implicitly copy an Iterator-to-mutable (reader+writer) to
+    // Iterator-to-const (reader-only), but not the reverse.
+    template <bool IsRhsBufferConst,
+              typename = std::enable_if_t<(!IsRhsBufferConst) && IsBufferConst>>
+    MOZ_IMPLICIT Iterator(const Iterator<IsRhsBufferConst>& aRhs)
+        : mModuloBuffer(aRhs.mModuloBuffer), mIndex(aRhs.mIndex) {}
+
+    // Can implicitly assign from an Iterator-to-mutable (reader+writer) to
+    // Iterator-to-const (reader-only), but not the reverse.
+    template <bool IsRhsBufferConst,
+              typename = std::enable_if_t<(!IsRhsBufferConst) && IsBufferConst>>
+    Iterator& operator=(const Iterator<IsRhsBufferConst>& aRhs) {
+      mModuloBuffer = aRhs.mModuloBuffer;
+      mIndex = aRhs.mIndex;
+      return *this;
+    }
+
+    // Current location of the iterator in the `Index` range.
+    // Note that due to wrapping, multiple indices may effectively point at the
+    // same byte in the buffer.
+    Index CurrentIndex() const { return mIndex; }
+
+    // Location comparison in the `Index` range. I.e., two `Iterator`s may look
+    // unequal, but refer to the same buffer location.
+    // Must be on the same buffer.
+    bool operator==(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex == aRhs.mIndex;
+    }
+    bool operator!=(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex != aRhs.mIndex;
+    }
+    bool operator<(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex < aRhs.mIndex;
+    }
+    bool operator<=(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex <= aRhs.mIndex;
+    }
+    bool operator>(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex > aRhs.mIndex;
+    }
+    bool operator>=(const Iterator& aRhs) const {
+      MOZ_ASSERT(mModuloBuffer == aRhs.mModuloBuffer);
+      return mIndex >= aRhs.mIndex;
+    }
+
+    // Movement in the `Index` range.
+    Iterator& operator++() {
+      ++mIndex;
+      return *this;
+    }
+    Iterator operator++(int) {
+      Iterator here(*mModuloBuffer, mIndex);
+      ++mIndex;
+      return here;
+    }
+    Iterator& operator--() {
+      --mIndex;
+      return *this;
+    }
+    Iterator operator--(int) {
+      Iterator here(*mModuloBuffer, mIndex);
+      --mIndex;
+      return here;
+    }
+    Iterator& operator+=(Length aLength) {
+      mIndex += aLength;
+      return *this;
+    }
+    Iterator operator+(Length aLength) const {
+      return Iterator(*mModuloBuffer, mIndex + aLength);
+    }
+    friend Iterator operator+(Length aLength, const Iterator& aIt) {
+      return aIt + aLength;
+    }
+    Iterator& operator-=(Length aLength) {
+      mIndex -= aLength;
+      return *this;
+    }
+    Iterator operator-(Length aLength) const {
+      return Iterator(*mModuloBuffer, mIndex - aLength);
+    }
+
+    // Distance from `aRef` to here in the `Index` range.
+    // May be negative (as 2's complement) if `aRef > *this`.
+    Index operator-(const Iterator& aRef) const {
+      MOZ_ASSERT(mModuloBuffer == aRef.mModuloBuffer);
+      return mIndex - aRef.mIndex;
+    }
+
+    // Dereference a single byte (read-only if `IsBufferConst` is true).
+    reference operator*() const {
+      return mModuloBuffer->mBuffer[OffsetInBuffer()];
+    }
+
+    // Random-access dereference.
+    reference operator[](Length aLength) const { return *(*this + aLength); }
+
+    // Write data (if `IsBufferConst` is false) but don't move iterator.
+    template <bool NotIsBufferConst = !IsBufferConst>
+    std::enable_if_t<NotIsBufferConst> Poke(const void* aSrc,
+                                            Length aLength) const {
+      // Don't allow data larger than the buffer.
+      MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+      // Offset inside the buffer (corresponding to our Index).
+      Offset offset = OffsetInBuffer();
+      // Compute remaining bytes between this offset and the end of the buffer.
+      Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+      if (MOZ_LIKELY(remaining >= aLength)) {
+        // Enough space to write everything before the end.
+        memcpy(&mModuloBuffer->mBuffer[offset], aSrc, aLength);
+      } else {
+        // Not enough space. Write as much as possible before the end.
+        memcpy(&mModuloBuffer->mBuffer[offset], aSrc, remaining);
+        // And then continue from the beginning of the buffer.
+        memcpy(&mModuloBuffer->mBuffer[0],
+               static_cast<const Byte*>(aSrc) + remaining,
+               (aLength - remaining));
+      }
+    }
+
+    // Write object data (if `IsBufferConst` is false) but don't move iterator.
+    // Note that this copies bytes from the object, with the intent to read them
+    // back later. Restricted to trivially-copyable types, which support this
+    // without Undefined Behavior!
+    template <typename T, bool NotIsBufferConst = !IsBufferConst>
+    std::enable_if_t<NotIsBufferConst> PokeObject(const T& aObject) const {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "PokeObject<T> - T must be trivially copyable");
+      return Poke(&aObject, sizeof(T));
+    }
+
+    // Write data (if `IsBufferConst` is false) and move iterator ahead.
+    template <bool NotIsBufferConst = !IsBufferConst>
+    std::enable_if_t<NotIsBufferConst> Write(const void* aSrc, Length aLength) {
+      Poke(aSrc, aLength);
+      mIndex += aLength;
+    }
+
+    // Write object data (if `IsBufferConst` is false) and move iterator ahead.
+    // Note that this copies bytes from the object, with the intent to read them
+    // back later. Restricted to trivially-copyable types, which support this
+    // without Undefined Behavior!
+    template <typename T, bool NotIsBufferConst = !IsBufferConst>
+    std::enable_if_t<NotIsBufferConst> WriteObject(const T& aObject) {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "WriteObject<T> - T must be trivially copyable");
+      return Write(&aObject, sizeof(T));
+    }
+
+    // Number of bytes needed to represent `aValue` in unsigned LEB128.
+    template <typename T>
+    static unsigned ULEB128Size(T aValue) {
+      return ::mozilla::ULEB128Size(aValue);
+    }
+
+    // Write number as unsigned LEB128 (if `IsBufferConst` is false) and move
+    // iterator ahead.
+    template <typename T, bool NotIsBufferConst = !IsBufferConst>
+    std::enable_if_t<NotIsBufferConst> WriteULEB128(T aValue) {
+      ::mozilla::WriteULEB128(aValue, *this);
+    }
+
+    // Read data but don't move iterator.
+    void Peek(void* aDst, Length aLength) const {
+      // Don't allow data larger than the buffer.
+      MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+      // Offset inside the buffer (corresponding to our Index).
+      Offset offset = OffsetInBuffer();
+      // Compute remaining bytes between this offset and the end of the buffer.
+      Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+      if (MOZ_LIKELY(remaining >= aLength)) {
+        // Can read everything we need before the end of the buffer.
+        memcpy(aDst, &mModuloBuffer->mBuffer[offset], aLength);
+      } else {
+        // Read as much as possible before the end of the buffer.
+        memcpy(aDst, &mModuloBuffer->mBuffer[offset], remaining);
+        // And then continue from the beginning of the buffer.
+        memcpy(static_cast<Byte*>(aDst) + remaining, &mModuloBuffer->mBuffer[0],
+               (aLength - remaining));
+      }
+    }
+
+    // Read data into an object but don't move iterator.
+    // Note that this overwrites `aObject` with bytes from the buffer.
+    // Restricted to trivially-copyable types, which support this without
+    // Undefined Behavior!
+    template <typename T>
+    void PeekIntoObject(T& aObject) const {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "PeekIntoObject<T> - T must be trivially copyable");
+      Peek(&aObject, sizeof(T));
+    }
+
+    // Read data as an object but don't move iterator.
+    // Note that this creates an default `T` first, and then overwrites it with
+    // bytes from the buffer. Restricted to trivially-copyable types, which
+    // support this without Undefined Behavior!
+    template <typename T>
+    T PeekObject() const {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "PeekObject<T> - T must be trivially copyable");
+      T object;
+      PeekIntoObject(object);
+      return object;
+    }
+
+    // Read data and move iterator ahead.
+    void Read(void* aDst, Length aLength) {
+      Peek(aDst, aLength);
+      mIndex += aLength;
+    }
+
+    // Read data into a mutable iterator and move both iterators ahead.
+    void ReadInto(Iterator</* IsBufferConst */ false>& aDst, Length aLength) {
+      // Don't allow data larger than the buffer.
+      MOZ_ASSERT(aLength <= mModuloBuffer->BufferLength().Value());
+      MOZ_ASSERT(aLength <= aDst.mModuloBuffer->BufferLength().Value());
+      // Offset inside the buffer (corresponding to our Index).
+      Offset offset = OffsetInBuffer();
+      // Compute remaining bytes between this offset and the end of the buffer.
+      Length remaining = mModuloBuffer->BufferLength().Value() - offset;
+      if (MOZ_LIKELY(remaining >= aLength)) {
+        // Can read everything we need before the end of the buffer.
+        aDst.Write(&mModuloBuffer->mBuffer[offset], aLength);
+      } else {
+        // Read as much as possible before the end of the buffer.
+        aDst.Write(&mModuloBuffer->mBuffer[offset], remaining);
+        // And then continue from the beginning of the buffer.
+        aDst.Write(&mModuloBuffer->mBuffer[0], (aLength - remaining));
+      }
+      mIndex += aLength;
+    }
+
+    // Read data into an object and move iterator ahead.
+    // Note that this overwrites `aObject` with bytes from the buffer.
+    // Restricted to trivially-copyable types, which support this without
+    // Undefined Behavior!
+    template <typename T>
+    void ReadIntoObject(T& aObject) {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "ReadIntoObject<T> - T must be trivially copyable");
+      Read(&aObject, sizeof(T));
+    }
+
+    // Read data as an object and move iterator ahead.
+    // Note that this creates an default `T` first, and then overwrites it with
+    // bytes from the buffer. Restricted to trivially-copyable types, which
+    // support this without Undefined Behavior!
+    template <typename T>
+    T ReadObject() {
+      static_assert(std::is_trivially_copyable<T>::value,
+                    "ReadObject<T> - T must be trivially copyable");
+      T object;
+      ReadIntoObject(object);
+      return object;
+    }
+
+    // Read an unsigned LEB128 number and move iterator ahead.
+    template <typename T>
+    T ReadULEB128() {
+      return ::mozilla::ReadULEB128<T>(*this);
+    }
+
+   private:
+    // Only a ModuloBuffer can instantiate its iterator.
+    friend class ModuloBuffer;
+
+    Iterator(ConstOrMutableBuffer& aBuffer, Index aIndex)
+        : mModuloBuffer(WrapNotNull(&aBuffer)), mIndex(aIndex) {}
+
+    // Convert the Iterator's mIndex into an offset inside the byte buffer.
+    Offset OffsetInBuffer() const {
+      return static_cast<Offset>(mIndex) & mModuloBuffer->mMask;
+    }
+
+    // ModuloBuffer that this Iterator operates on.
+    // Using a non-null pointer instead of a reference, to allow re-assignment
+    // of an Iterator variable.
+    NotNull<ConstOrMutableBuffer*> mModuloBuffer;
+
+    // Position of this iterator in the wider `Index` range. (Will be wrapped
+    // around as needed when actually accessing bytes from the buffer.)
+    Index mIndex;
+  };
+
+  // Shortcut to iterator to const (read-only) data.
+  using Reader = Iterator<true>;
+  // Shortcut to iterator to non-const (read/write) data.
+  using Writer = Iterator<false>;
+
+  // Create an iterator to const data at the given index.
+  Reader ReaderAt(Index aIndex) const { return Reader(*this, aIndex); }
+
+  // Create an iterator to non-const data at the given index.
+  Writer WriterAt(Index aIndex) { return Writer(*this, aIndex); }
+
+#ifdef DEBUG
+  void Dump() const {
+    Length len = BufferLength().Value();
+    if (len > 128) {
+      len = 128;
+    }
+    for (Length i = 0; i < len; ++i) {
+      printf("%02x ", mBuffer[i]);
+    }
+    printf("\n");
+  }
+#endif  // DEBUG
+
+ private:
+  // Mask used to convert an index to an offset in `mBuffer`
+  const PowerOfTwoMask<Offset> mMask;
+
+  // Buffer data. `const NotNull<...>` shows that `mBuffer is `const`, and
+  // `Byte* const` shows that the pointer cannot be changed to point at
+  // something else, but the pointed-at `Byte`s are writable.
+  const NotNull<Byte* const> mBuffer;
+
+  // Function used to release the buffer resource (if needed).
+  std::function<void(Byte*)> mBufferDeleter;
+};
+
+}  // namespace mozilla
+
+#endif  // ModuloBuffer_h