Adding upstream version 115.7.0esr.upstream/115.7.0esr

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

28 files changed, 7330 insertions, 0 deletions

diff --git a/mfbt/double-conversion/GIT-INFO b/mfbt/double-conversion/GIT-INFO
new file mode 100644
index 0000000000..70016edd4a
--- /dev/null
+++ b/mfbt/double-conversion/GIT-INFO
@@ -0,0 +1,5 @@
+commit 4f7a25d8ced8c7cf6eee6fd09d6788eaa23c9afe
+Author: Florian Loitsch <florian@loitsch.com>
+Date:   Thu May 18 14:20:37 2023 +0200
+
+    Prepare v3.2.0 release. (#196)
diff --git a/mfbt/double-conversion/add-mfbt-api-markers.patch b/mfbt/double-conversion/add-mfbt-api-markers.patch
new file mode 100644
index 0000000000..d8e96cea43
--- /dev/null
+++ b/mfbt/double-conversion/add-mfbt-api-markers.patch
@@ -0,0 +1,207 @@
+diff --git a/mfbt/double-conversion/double-conversion/double-to-string.h b/mfbt/double-conversion/double-conversion/double-to-string.h
+--- a/mfbt/double-conversion/double-conversion/double-to-string.h
++++ b/mfbt/double-conversion/double-conversion/double-to-string.h
+@@ -23,16 +23,17 @@
+ // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ 
+ #ifndef DOUBLE_CONVERSION_DOUBLE_TO_STRING_H_
+ #define DOUBLE_CONVERSION_DOUBLE_TO_STRING_H_
+ 
++#include "mozilla/Types.h"
+ #include "utils.h"
+ 
+ namespace double_conversion {
+ 
+ class DoubleToStringConverter {
+  public:
+   // When calling ToFixed with a double > 10^kMaxFixedDigitsBeforePoint
+   // or a requested_digits parameter > kMaxFixedDigitsAfterPoint then the
+@@ -192,17 +193,17 @@ class DoubleToStringConverter {
+   //
+   // Flags: UNIQUE_ZERO and EMIT_POSITIVE_EXPONENT_SIGN.
+   // Special values: "Infinity" and "NaN".
+   // Lower case 'e' for exponential values.
+   // decimal_in_shortest_low: -6
+   // decimal_in_shortest_high: 21
+   // max_leading_padding_zeroes_in_precision_mode: 6
+   // max_trailing_padding_zeroes_in_precision_mode: 0
+-  static const DoubleToStringConverter& EcmaScriptConverter();
++  static MFBT_API const DoubleToStringConverter& EcmaScriptConverter();
+ 
+   // Computes the shortest string of digits that correctly represent the input
+   // number. Depending on decimal_in_shortest_low and decimal_in_shortest_high
+   // (see constructor) it then either returns a decimal representation, or an
+   // exponential representation.
+   // Example with decimal_in_shortest_low = -6,
+   //              decimal_in_shortest_high = 21,
+   //              EMIT_POSITIVE_EXPONENT_SIGN activated, and
+@@ -277,17 +278,17 @@ class DoubleToStringConverter {
+   //     been provided to the constructor,
+   //   - 'value' > 10^kMaxFixedDigitsBeforePoint, or
+   //   - 'requested_digits' > kMaxFixedDigitsAfterPoint.
+   // The last two conditions imply that the result for non-special values never
+   // contains more than
+   //  1 + kMaxFixedDigitsBeforePoint + 1 + kMaxFixedDigitsAfterPoint characters
+   // (one additional character for the sign, and one for the decimal point).
+   // In addition, the buffer must be able to hold the trailing '\0' character.
+-  bool ToFixed(double value,
++  MFBT_API bool ToFixed(double value,
+                int requested_digits,
+                StringBuilder* result_builder) const;
+ 
+   // Computes a representation in exponential format with requested_digits
+   // after the decimal point. The last emitted digit is rounded.
+   // If requested_digits equals -1, then the shortest exponential representation
+   // is computed.
+   //
+@@ -311,17 +312,17 @@ class DoubleToStringConverter {
+   //     been provided to the constructor,
+   //   - 'requested_digits' > kMaxExponentialDigits.
+   //
+   // The last condition implies that the result never contains more than
+   // kMaxExponentialDigits + 8 characters (the sign, the digit before the
+   // decimal point, the decimal point, the exponent character, the
+   // exponent's sign, and at most 3 exponent digits).
+   // In addition, the buffer must be able to hold the trailing '\0' character.
+-  bool ToExponential(double value,
++  MFBT_API bool ToExponential(double value,
+                      int requested_digits,
+                      StringBuilder* result_builder) const;
+ 
+ 
+   // Computes 'precision' leading digits of the given 'value' and returns them
+   // either in exponential or decimal format, depending on
+   // max_{leading|trailing}_padding_zeroes_in_precision_mode (given to the
+   // constructor).
+@@ -352,17 +353,17 @@ class DoubleToStringConverter {
+   //     been provided to the constructor,
+   //   - precision < kMinPericisionDigits
+   //   - precision > kMaxPrecisionDigits
+   //
+   // The last condition implies that the result never contains more than
+   // kMaxPrecisionDigits + 7 characters (the sign, the decimal point, the
+   // exponent character, the exponent's sign, and at most 3 exponent digits).
+   // In addition, the buffer must be able to hold the trailing '\0' character.
+-  bool ToPrecision(double value,
++  MFBT_API bool ToPrecision(double value,
+                    int precision,
+                    StringBuilder* result_builder) const;
+ 
+   enum DtoaMode {
+     // Produce the shortest correct representation.
+     // For example the output of 0.299999999999999988897 is (the less accurate
+     // but correct) 0.3.
+     SHORTEST,
+@@ -414,44 +415,44 @@ class DoubleToStringConverter {
+   // DoubleToAscii expects the given buffer to be big enough to hold all
+   // digits and a terminating null-character. In SHORTEST-mode it expects a
+   // buffer of at least kBase10MaximalLength + 1. In all other modes the
+   // requested_digits parameter and the padding-zeroes limit the size of the
+   // output. Don't forget the decimal point, the exponent character and the
+   // terminating null-character when computing the maximal output size.
+   // The given length is only used in debug mode to ensure the buffer is big
+   // enough.
+-  static void DoubleToAscii(double v,
++  static MFBT_API void DoubleToAscii(double v,
+                             DtoaMode mode,
+                             int requested_digits,
+                             char* buffer,
+                             int buffer_length,
+                             bool* sign,
+                             int* length,
+                             int* point);
+ 
+  private:
+   // Implementation for ToShortest and ToShortestSingle.
+-  bool ToShortestIeeeNumber(double value,
++  MFBT_API bool ToShortestIeeeNumber(double value,
+                             StringBuilder* result_builder,
+                             DtoaMode mode) const;
+ 
+   // If the value is a special value (NaN or Infinity) constructs the
+   // corresponding string using the configured infinity/nan-symbol.
+   // If either of them is NULL or the value is not special then the
+   // function returns false.
+-  bool HandleSpecialValues(double value, StringBuilder* result_builder) const;
++  MFBT_API bool HandleSpecialValues(double value, StringBuilder* result_builder) const;
+   // Constructs an exponential representation (i.e. 1.234e56).
+   // The given exponent assumes a decimal point after the first decimal digit.
+-  void CreateExponentialRepresentation(const char* decimal_digits,
++  MFBT_API void CreateExponentialRepresentation(const char* decimal_digits,
+                                        int length,
+                                        int exponent,
+                                        StringBuilder* result_builder) const;
+   // Creates a decimal representation (i.e 1234.5678).
+-  void CreateDecimalRepresentation(const char* decimal_digits,
++  MFBT_API void CreateDecimalRepresentation(const char* decimal_digits,
+                                    int length,
+                                    int decimal_point,
+                                    int digits_after_point,
+                                    StringBuilder* result_builder) const;
+ 
+   const int flags_;
+   const char* const infinity_symbol_;
+   const char* const nan_symbol_;
+diff --git a/mfbt/double-conversion/double-conversion/string-to-double.h b/mfbt/double-conversion/double-conversion/string-to-double.h
+--- a/mfbt/double-conversion/double-conversion/string-to-double.h
++++ b/mfbt/double-conversion/double-conversion/string-to-double.h
+@@ -23,16 +23,17 @@
+ // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ 
+ #ifndef DOUBLE_CONVERSION_STRING_TO_DOUBLE_H_
+ #define DOUBLE_CONVERSION_STRING_TO_DOUBLE_H_
+ 
++#include "mozilla/Types.h"
+ #include "utils.h"
+ 
+ namespace double_conversion {
+ 
+ class StringToDoubleConverter {
+  public:
+   // Enumeration for allowing octals and ignoring junk when converting
+   // strings to numbers.
+@@ -178,34 +179,34 @@ class StringToDoubleConverter {
+         separator_(separator) {
+   }
+ 
+   // Performs the conversion.
+   // The output parameter 'processed_characters_count' is set to the number
+   // of characters that have been processed to read the number.
+   // Spaces than are processed with ALLOW_{LEADING|TRAILING}_SPACES are included
+   // in the 'processed_characters_count'. Trailing junk is never included.
+-  double StringToDouble(const char* buffer,
++  MFBT_API double StringToDouble(const char* buffer,
+                         int length,
+                         int* processed_characters_count) const;
+ 
+   // Same as StringToDouble above but for 16 bit characters.
+-  double StringToDouble(const uc16* buffer,
++  MFBT_API double StringToDouble(const uc16* buffer,
+                         int length,
+                         int* processed_characters_count) const;
+ 
+   // Same as StringToDouble but reads a float.
+   // Note that this is not equivalent to static_cast<float>(StringToDouble(...))
+   // due to potential double-rounding.
+-  float StringToFloat(const char* buffer,
++  MFBT_API float StringToFloat(const char* buffer,
+                       int length,
+                       int* processed_characters_count) const;
+ 
+   // Same as StringToFloat above but for 16 bit characters.
+-  float StringToFloat(const uc16* buffer,
++  MFBT_API float StringToFloat(const uc16* buffer,
+                       int length,
+                       int* processed_characters_count) const;
+ 
+   // Same as StringToDouble for T = double, and StringToFloat for T = float.
+   template <typename T>
+   T StringTo(const char* buffer,
+              int length,
+              int* processed_characters_count) const;
diff --git a/mfbt/double-conversion/debug-only-functions.patch b/mfbt/double-conversion/debug-only-functions.patch
new file mode 100644
index 0000000000..3988eaad56
--- /dev/null
+++ b/mfbt/double-conversion/debug-only-functions.patch
@@ -0,0 +1,39 @@
+diff --git a/mfbt/double-conversion/double-conversion/strtod.cc b/mfbt/double-conversion/double-conversion/strtod.cc
+--- a/mfbt/double-conversion/double-conversion/strtod.cc
++++ b/mfbt/double-conversion/double-conversion/strtod.cc
+@@ -436,16 +436,17 @@ static bool ComputeGuess(Vector<const ch
+     return true;
+   }
+   if (*guess == Double::Infinity()) {
+     return true;
+   }
+   return false;
+ }
+ 
++#ifdef DEBUG
+ static bool IsDigit(const char d) {
+   return ('0' <= d) && (d <= '9');
+ }
+ 
+ static bool IsNonZeroDigit(const char d) {
+   return ('1' <= d) && (d <= '9');
+ }
+ 
+@@ -457,16 +458,17 @@ static bool IsNonZeroDigit(const char d)
+ static bool AssertTrimmedDigits(const Vector<const char>& buffer) {
+   for(int i = 0; i < buffer.length(); ++i) {
+     if(!IsDigit(buffer[i])) {
+       return false;
+     }
+   }
+   return (buffer.length() == 0) || (IsNonZeroDigit(buffer[0]) && IsNonZeroDigit(buffer[buffer.length()-1]));
+ }
++#endif
+ 
+ double StrtodTrimmed(Vector<const char> trimmed, int exponent) {
+   DOUBLE_CONVERSION_ASSERT(trimmed.length() <= kMaxSignificantDecimalDigits);
+   DOUBLE_CONVERSION_ASSERT(AssertTrimmedDigits(trimmed));
+   double guess;
+   const bool is_correct = ComputeGuess(trimmed, exponent, &guess);
+   if (is_correct) {
+     return guess;
diff --git a/mfbt/double-conversion/double-conversion/LICENSE b/mfbt/double-conversion/double-conversion/LICENSE
new file mode 100644
index 0000000000..933718a9ef
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/LICENSE
@@ -0,0 +1,26 @@
+Copyright 2006-2011, the V8 project authors. All rights reserved.
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/mfbt/double-conversion/double-conversion/README.md b/mfbt/double-conversion/double-conversion/README.md
new file mode 100644
index 0000000000..e5d9a4e682
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/README.md
@@ -0,0 +1,55 @@
+https://github.com/google/double-conversion
+
+This project (double-conversion) provides binary-decimal and decimal-binary
+routines for IEEE doubles.
+
+The library consists of efficient conversion routines that have been extracted
+from the V8 JavaScript engine. The code has been refactored and improved so that
+it can be used more easily in other projects.
+
+There is extensive documentation in `double-conversion/string-to-double.h` and
+`double-conversion/double-to-string.h`. Other examples can be found in
+`test/cctest/test-conversions.cc`.
+
+
+Building
+========
+
+This library can be built with [scons][0] or [cmake][1].
+The checked-in Makefile simply forwards to scons, and provides a
+shortcut to run all tests:
+
+    make
+    make test
+
+Scons
+-----
+
+The easiest way to install this library is to use `scons`. It builds
+the static and shared library, and is set up to install those at the
+correct locations:
+
+    scons install
+
+Use the `DESTDIR` option to change the target directory:
+
+    scons DESTDIR=alternative_directory install
+
+Cmake
+-----
+
+To use cmake run `cmake .` in the root directory. This overwrites the
+existing Makefile.
+
+Use `-DBUILD_SHARED_LIBS=ON` to enable the compilation of shared libraries.
+Note that this disables static libraries. There is currently no way to
+build both libraries at the same time with cmake.
+
+Use `-DBUILD_TESTING=ON` to build the test executable.
+
+    cmake . -DBUILD_TESTING=ON
+    make
+    test/cctest/cctest
+
+[0]: http://www.scons.org/
+[1]: https://cmake.org/
diff --git a/mfbt/double-conversion/double-conversion/bignum-dtoa.cc b/mfbt/double-conversion/double-conversion/bignum-dtoa.cc
new file mode 100644
index 0000000000..15123e6a63
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/bignum-dtoa.cc
@@ -0,0 +1,641 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <cmath>
+
+#include "bignum-dtoa.h"
+
+#include "bignum.h"
+#include "ieee.h"
+
+namespace double_conversion {
+
+static int NormalizedExponent(uint64_t significand, int exponent) {
+  DOUBLE_CONVERSION_ASSERT(significand != 0);
+  while ((significand & Double::kHiddenBit) == 0) {
+    significand = significand << 1;
+    exponent = exponent - 1;
+  }
+  return exponent;
+}
+
+
+// Forward declarations:
+// Returns an estimation of k such that 10^(k-1) <= v < 10^k.
+static int EstimatePower(int exponent);
+// Computes v / 10^estimated_power exactly, as a ratio of two bignums, numerator
+// and denominator.
+static void InitialScaledStartValues(uint64_t significand,
+                                     int exponent,
+                                     bool lower_boundary_is_closer,
+                                     int estimated_power,
+                                     bool need_boundary_deltas,
+                                     Bignum* numerator,
+                                     Bignum* denominator,
+                                     Bignum* delta_minus,
+                                     Bignum* delta_plus);
+// Multiplies numerator/denominator so that its values lies in the range 1-10.
+// Returns decimal_point s.t.
+//  v = numerator'/denominator' * 10^(decimal_point-1)
+//     where numerator' and denominator' are the values of numerator and
+//     denominator after the call to this function.
+static void FixupMultiply10(int estimated_power, bool is_even,
+                            int* decimal_point,
+                            Bignum* numerator, Bignum* denominator,
+                            Bignum* delta_minus, Bignum* delta_plus);
+// Generates digits from the left to the right and stops when the generated
+// digits yield the shortest decimal representation of v.
+static void GenerateShortestDigits(Bignum* numerator, Bignum* denominator,
+                                   Bignum* delta_minus, Bignum* delta_plus,
+                                   bool is_even,
+                                   Vector<char> buffer, int* length);
+// Generates 'requested_digits' after the decimal point.
+static void BignumToFixed(int requested_digits, int* decimal_point,
+                          Bignum* numerator, Bignum* denominator,
+                          Vector<char> buffer, int* length);
+// Generates 'count' digits of numerator/denominator.
+// Once 'count' digits have been produced rounds the result depending on the
+// remainder (remainders of exactly .5 round upwards). Might update the
+// decimal_point when rounding up (for example for 0.9999).
+static void GenerateCountedDigits(int count, int* decimal_point,
+                                  Bignum* numerator, Bignum* denominator,
+                                  Vector<char> buffer, int* length);
+
+
+void BignumDtoa(double v, BignumDtoaMode mode, int requested_digits,
+                Vector<char> buffer, int* length, int* decimal_point) {
+  DOUBLE_CONVERSION_ASSERT(v > 0);
+  DOUBLE_CONVERSION_ASSERT(!Double(v).IsSpecial());
+  uint64_t significand;
+  int exponent;
+  bool lower_boundary_is_closer;
+  if (mode == BIGNUM_DTOA_SHORTEST_SINGLE) {
+    float f = static_cast<float>(v);
+    DOUBLE_CONVERSION_ASSERT(f == v);
+    significand = Single(f).Significand();
+    exponent = Single(f).Exponent();
+    lower_boundary_is_closer = Single(f).LowerBoundaryIsCloser();
+  } else {
+    significand = Double(v).Significand();
+    exponent = Double(v).Exponent();
+    lower_boundary_is_closer = Double(v).LowerBoundaryIsCloser();
+  }
+  bool need_boundary_deltas =
+      (mode == BIGNUM_DTOA_SHORTEST || mode == BIGNUM_DTOA_SHORTEST_SINGLE);
+
+  bool is_even = (significand & 1) == 0;
+  int normalized_exponent = NormalizedExponent(significand, exponent);
+  // estimated_power might be too low by 1.
+  int estimated_power = EstimatePower(normalized_exponent);
+
+  // Shortcut for Fixed.
+  // The requested digits correspond to the digits after the point. If the
+  // number is much too small, then there is no need in trying to get any
+  // digits.
+  if (mode == BIGNUM_DTOA_FIXED && -estimated_power - 1 > requested_digits) {
+    buffer[0] = '\0';
+    *length = 0;
+    // Set decimal-point to -requested_digits. This is what Gay does.
+    // Note that it should not have any effect anyways since the string is
+    // empty.
+    *decimal_point = -requested_digits;
+    return;
+  }
+
+  Bignum numerator;
+  Bignum denominator;
+  Bignum delta_minus;
+  Bignum delta_plus;
+  // Make sure the bignum can grow large enough. The smallest double equals
+  // 4e-324. In this case the denominator needs fewer than 324*4 binary digits.
+  // The maximum double is 1.7976931348623157e308 which needs fewer than
+  // 308*4 binary digits.
+  DOUBLE_CONVERSION_ASSERT(Bignum::kMaxSignificantBits >= 324*4);
+  InitialScaledStartValues(significand, exponent, lower_boundary_is_closer,
+                           estimated_power, need_boundary_deltas,
+                           &numerator, &denominator,
+                           &delta_minus, &delta_plus);
+  // We now have v = (numerator / denominator) * 10^estimated_power.
+  FixupMultiply10(estimated_power, is_even, decimal_point,
+                  &numerator, &denominator,
+                  &delta_minus, &delta_plus);
+  // We now have v = (numerator / denominator) * 10^(decimal_point-1), and
+  //  1 <= (numerator + delta_plus) / denominator < 10
+  switch (mode) {
+    case BIGNUM_DTOA_SHORTEST:
+    case BIGNUM_DTOA_SHORTEST_SINGLE:
+      GenerateShortestDigits(&numerator, &denominator,
+                             &delta_minus, &delta_plus,
+                             is_even, buffer, length);
+      break;
+    case BIGNUM_DTOA_FIXED:
+      BignumToFixed(requested_digits, decimal_point,
+                    &numerator, &denominator,
+                    buffer, length);
+      break;
+    case BIGNUM_DTOA_PRECISION:
+      GenerateCountedDigits(requested_digits, decimal_point,
+                            &numerator, &denominator,
+                            buffer, length);
+      break;
+    default:
+      DOUBLE_CONVERSION_UNREACHABLE();
+  }
+  buffer[*length] = '\0';
+}
+
+
+// The procedure starts generating digits from the left to the right and stops
+// when the generated digits yield the shortest decimal representation of v. A
+// decimal representation of v is a number lying closer to v than to any other
+// double, so it converts to v when read.
+//
+// This is true if d, the decimal representation, is between m- and m+, the
+// upper and lower boundaries. d must be strictly between them if !is_even.
+//           m- := (numerator - delta_minus) / denominator
+//           m+ := (numerator + delta_plus) / denominator
+//
+// Precondition: 0 <= (numerator+delta_plus) / denominator < 10.
+//   If 1 <= (numerator+delta_plus) / denominator < 10 then no leading 0 digit
+//   will be produced. This should be the standard precondition.
+static void GenerateShortestDigits(Bignum* numerator, Bignum* denominator,
+                                   Bignum* delta_minus, Bignum* delta_plus,
+                                   bool is_even,
+                                   Vector<char> buffer, int* length) {
+  // Small optimization: if delta_minus and delta_plus are the same just reuse
+  // one of the two bignums.
+  if (Bignum::Equal(*delta_minus, *delta_plus)) {
+    delta_plus = delta_minus;
+  }
+  *length = 0;
+  for (;;) {
+    uint16_t digit;
+    digit = numerator->DivideModuloIntBignum(*denominator);
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);  // digit is a uint16_t and therefore always positive.
+    // digit = numerator / denominator (integer division).
+    // numerator = numerator % denominator.
+    buffer[(*length)++] = static_cast<char>(digit + '0');
+
+    // Can we stop already?
+    // If the remainder of the division is less than the distance to the lower
+    // boundary we can stop. In this case we simply round down (discarding the
+    // remainder).
+    // Similarly we test if we can round up (using the upper boundary).
+    bool in_delta_room_minus;
+    bool in_delta_room_plus;
+    if (is_even) {
+      in_delta_room_minus = Bignum::LessEqual(*numerator, *delta_minus);
+    } else {
+      in_delta_room_minus = Bignum::Less(*numerator, *delta_minus);
+    }
+    if (is_even) {
+      in_delta_room_plus =
+          Bignum::PlusCompare(*numerator, *delta_plus, *denominator) >= 0;
+    } else {
+      in_delta_room_plus =
+          Bignum::PlusCompare(*numerator, *delta_plus, *denominator) > 0;
+    }
+    if (!in_delta_room_minus && !in_delta_room_plus) {
+      // Prepare for next iteration.
+      numerator->Times10();
+      delta_minus->Times10();
+      // We optimized delta_plus to be equal to delta_minus (if they share the
+      // same value). So don't multiply delta_plus if they point to the same
+      // object.
+      if (delta_minus != delta_plus) {
+        delta_plus->Times10();
+      }
+    } else if (in_delta_room_minus && in_delta_room_plus) {
+      // Let's see if 2*numerator < denominator.
+      // If yes, then the next digit would be < 5 and we can round down.
+      int compare = Bignum::PlusCompare(*numerator, *numerator, *denominator);
+      if (compare < 0) {
+        // Remaining digits are less than .5. -> Round down (== do nothing).
+      } else if (compare > 0) {
+        // Remaining digits are more than .5 of denominator. -> Round up.
+        // Note that the last digit could not be a '9' as otherwise the whole
+        // loop would have stopped earlier.
+        // We still have an assert here in case the preconditions were not
+        // satisfied.
+        DOUBLE_CONVERSION_ASSERT(buffer[(*length) - 1] != '9');
+        buffer[(*length) - 1]++;
+      } else {
+        // Halfway case.
+        // TODO(floitsch): need a way to solve half-way cases.
+        //   For now let's round towards even (since this is what Gay seems to
+        //   do).
+
+        if ((buffer[(*length) - 1] - '0') % 2 == 0) {
+          // Round down => Do nothing.
+        } else {
+          DOUBLE_CONVERSION_ASSERT(buffer[(*length) - 1] != '9');
+          buffer[(*length) - 1]++;
+        }
+      }
+      return;
+    } else if (in_delta_room_minus) {
+      // Round down (== do nothing).
+      return;
+    } else {  // in_delta_room_plus
+      // Round up.
+      // Note again that the last digit could not be '9' since this would have
+      // stopped the loop earlier.
+      // We still have an DOUBLE_CONVERSION_ASSERT here, in case the preconditions were not
+      // satisfied.
+      DOUBLE_CONVERSION_ASSERT(buffer[(*length) -1] != '9');
+      buffer[(*length) - 1]++;
+      return;
+    }
+  }
+}
+
+
+// Let v = numerator / denominator < 10.
+// Then we generate 'count' digits of d = x.xxxxx... (without the decimal point)
+// from left to right. Once 'count' digits have been produced we decide whether
+// to round up or down. Remainders of exactly .5 round upwards. Numbers such
+// as 9.999999 propagate a carry all the way, and change the
+// exponent (decimal_point), when rounding upwards.
+static void GenerateCountedDigits(int count, int* decimal_point,
+                                  Bignum* numerator, Bignum* denominator,
+                                  Vector<char> buffer, int* length) {
+  DOUBLE_CONVERSION_ASSERT(count >= 0);
+  for (int i = 0; i < count - 1; ++i) {
+    uint16_t digit;
+    digit = numerator->DivideModuloIntBignum(*denominator);
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);  // digit is a uint16_t and therefore always positive.
+    // digit = numerator / denominator (integer division).
+    // numerator = numerator % denominator.
+    buffer[i] = static_cast<char>(digit + '0');
+    // Prepare for next iteration.
+    numerator->Times10();
+  }
+  // Generate the last digit.
+  uint16_t digit;
+  digit = numerator->DivideModuloIntBignum(*denominator);
+  if (Bignum::PlusCompare(*numerator, *numerator, *denominator) >= 0) {
+    digit++;
+  }
+  DOUBLE_CONVERSION_ASSERT(digit <= 10);
+  buffer[count - 1] = static_cast<char>(digit + '0');
+  // Correct bad digits (in case we had a sequence of '9's). Propagate the
+  // carry until we hat a non-'9' or til we reach the first digit.
+  for (int i = count - 1; i > 0; --i) {
+    if (buffer[i] != '0' + 10) break;
+    buffer[i] = '0';
+    buffer[i - 1]++;
+  }
+  if (buffer[0] == '0' + 10) {
+    // Propagate a carry past the top place.
+    buffer[0] = '1';
+    (*decimal_point)++;
+  }
+  *length = count;
+}
+
+
+// Generates 'requested_digits' after the decimal point. It might omit
+// trailing '0's. If the input number is too small then no digits at all are
+// generated (ex.: 2 fixed digits for 0.00001).
+//
+// Input verifies:  1 <= (numerator + delta) / denominator < 10.
+static void BignumToFixed(int requested_digits, int* decimal_point,
+                          Bignum* numerator, Bignum* denominator,
+                          Vector<char> buffer, int* length) {
+  // Note that we have to look at more than just the requested_digits, since
+  // a number could be rounded up. Example: v=0.5 with requested_digits=0.
+  // Even though the power of v equals 0 we can't just stop here.
+  if (-(*decimal_point) > requested_digits) {
+    // The number is definitively too small.
+    // Ex: 0.001 with requested_digits == 1.
+    // Set decimal-point to -requested_digits. This is what Gay does.
+    // Note that it should not have any effect anyways since the string is
+    // empty.
+    *decimal_point = -requested_digits;
+    *length = 0;
+    return;
+  } else if (-(*decimal_point) == requested_digits) {
+    // We only need to verify if the number rounds down or up.
+    // Ex: 0.04 and 0.06 with requested_digits == 1.
+    DOUBLE_CONVERSION_ASSERT(*decimal_point == -requested_digits);
+    // Initially the fraction lies in range (1, 10]. Multiply the denominator
+    // by 10 so that we can compare more easily.
+    denominator->Times10();
+    if (Bignum::PlusCompare(*numerator, *numerator, *denominator) >= 0) {
+      // If the fraction is >= 0.5 then we have to include the rounded
+      // digit.
+      buffer[0] = '1';
+      *length = 1;
+      (*decimal_point)++;
+    } else {
+      // Note that we caught most of similar cases earlier.
+      *length = 0;
+    }
+    return;
+  } else {
+    // The requested digits correspond to the digits after the point.
+    // The variable 'needed_digits' includes the digits before the point.
+    int needed_digits = (*decimal_point) + requested_digits;
+    GenerateCountedDigits(needed_digits, decimal_point,
+                          numerator, denominator,
+                          buffer, length);
+  }
+}
+
+
+// Returns an estimation of k such that 10^(k-1) <= v < 10^k where
+// v = f * 2^exponent and 2^52 <= f < 2^53.
+// v is hence a normalized double with the given exponent. The output is an
+// approximation for the exponent of the decimal approximation .digits * 10^k.
+//
+// The result might undershoot by 1 in which case 10^k <= v < 10^k+1.
+// Note: this property holds for v's upper boundary m+ too.
+//    10^k <= m+ < 10^k+1.
+//   (see explanation below).
+//
+// Examples:
+//  EstimatePower(0)   => 16
+//  EstimatePower(-52) => 0
+//
+// Note: e >= 0 => EstimatedPower(e) > 0. No similar claim can be made for e<0.
+static int EstimatePower(int exponent) {
+  // This function estimates log10 of v where v = f*2^e (with e == exponent).
+  // Note that 10^floor(log10(v)) <= v, but v <= 10^ceil(log10(v)).
+  // Note that f is bounded by its container size. Let p = 53 (the double's
+  // significand size). Then 2^(p-1) <= f < 2^p.
+  //
+  // Given that log10(v) == log2(v)/log2(10) and e+(len(f)-1) is quite close
+  // to log2(v) the function is simplified to (e+(len(f)-1)/log2(10)).
+  // The computed number undershoots by less than 0.631 (when we compute log3
+  // and not log10).
+  //
+  // Optimization: since we only need an approximated result this computation
+  // can be performed on 64 bit integers. On x86/x64 architecture the speedup is
+  // not really measurable, though.
+  //
+  // Since we want to avoid overshooting we decrement by 1e10 so that
+  // floating-point imprecisions don't affect us.
+  //
+  // Explanation for v's boundary m+: the computation takes advantage of
+  // the fact that 2^(p-1) <= f < 2^p. Boundaries still satisfy this requirement
+  // (even for denormals where the delta can be much more important).
+
+  const double k1Log10 = 0.30102999566398114;  // 1/lg(10)
+
+  // For doubles len(f) == 53 (don't forget the hidden bit).
+  const int kSignificandSize = Double::kSignificandSize;
+  double estimate = ceil((exponent + kSignificandSize - 1) * k1Log10 - 1e-10);
+  return static_cast<int>(estimate);
+}
+
+
+// See comments for InitialScaledStartValues.
+static void InitialScaledStartValuesPositiveExponent(
+    uint64_t significand, int exponent,
+    int estimated_power, bool need_boundary_deltas,
+    Bignum* numerator, Bignum* denominator,
+    Bignum* delta_minus, Bignum* delta_plus) {
+  // A positive exponent implies a positive power.
+  DOUBLE_CONVERSION_ASSERT(estimated_power >= 0);
+  // Since the estimated_power is positive we simply multiply the denominator
+  // by 10^estimated_power.
+
+  // numerator = v.
+  numerator->AssignUInt64(significand);
+  numerator->ShiftLeft(exponent);
+  // denominator = 10^estimated_power.
+  denominator->AssignPowerUInt16(10, estimated_power);
+
+  if (need_boundary_deltas) {
+    // Introduce a common denominator so that the deltas to the boundaries are
+    // integers.
+    denominator->ShiftLeft(1);
+    numerator->ShiftLeft(1);
+    // Let v = f * 2^e, then m+ - v = 1/2 * 2^e; With the common
+    // denominator (of 2) delta_plus equals 2^e.
+    delta_plus->AssignUInt16(1);
+    delta_plus->ShiftLeft(exponent);
+    // Same for delta_minus. The adjustments if f == 2^p-1 are done later.
+    delta_minus->AssignUInt16(1);
+    delta_minus->ShiftLeft(exponent);
+  }
+}
+
+
+// See comments for InitialScaledStartValues
+static void InitialScaledStartValuesNegativeExponentPositivePower(
+    uint64_t significand, int exponent,
+    int estimated_power, bool need_boundary_deltas,
+    Bignum* numerator, Bignum* denominator,
+    Bignum* delta_minus, Bignum* delta_plus) {
+  // v = f * 2^e with e < 0, and with estimated_power >= 0.
+  // This means that e is close to 0 (have a look at how estimated_power is
+  // computed).
+
+  // numerator = significand
+  //  since v = significand * 2^exponent this is equivalent to
+  //  numerator = v * / 2^-exponent
+  numerator->AssignUInt64(significand);
+  // denominator = 10^estimated_power * 2^-exponent (with exponent < 0)
+  denominator->AssignPowerUInt16(10, estimated_power);
+  denominator->ShiftLeft(-exponent);
+
+  if (need_boundary_deltas) {
+    // Introduce a common denominator so that the deltas to the boundaries are
+    // integers.
+    denominator->ShiftLeft(1);
+    numerator->ShiftLeft(1);
+    // Let v = f * 2^e, then m+ - v = 1/2 * 2^e; With the common
+    // denominator (of 2) delta_plus equals 2^e.
+    // Given that the denominator already includes v's exponent the distance
+    // to the boundaries is simply 1.
+    delta_plus->AssignUInt16(1);
+    // Same for delta_minus. The adjustments if f == 2^p-1 are done later.
+    delta_minus->AssignUInt16(1);
+  }
+}
+
+
+// See comments for InitialScaledStartValues
+static void InitialScaledStartValuesNegativeExponentNegativePower(
+    uint64_t significand, int exponent,
+    int estimated_power, bool need_boundary_deltas,
+    Bignum* numerator, Bignum* denominator,
+    Bignum* delta_minus, Bignum* delta_plus) {
+  // Instead of multiplying the denominator with 10^estimated_power we
+  // multiply all values (numerator and deltas) by 10^-estimated_power.
+
+  // Use numerator as temporary container for power_ten.
+  Bignum* power_ten = numerator;
+  power_ten->AssignPowerUInt16(10, -estimated_power);
+
+  if (need_boundary_deltas) {
+    // Since power_ten == numerator we must make a copy of 10^estimated_power
+    // before we complete the computation of the numerator.
+    // delta_plus = delta_minus = 10^estimated_power
+    delta_plus->AssignBignum(*power_ten);
+    delta_minus->AssignBignum(*power_ten);
+  }
+
+  // numerator = significand * 2 * 10^-estimated_power
+  //  since v = significand * 2^exponent this is equivalent to
+  // numerator = v * 10^-estimated_power * 2 * 2^-exponent.
+  // Remember: numerator has been abused as power_ten. So no need to assign it
+  //  to itself.
+  DOUBLE_CONVERSION_ASSERT(numerator == power_ten);
+  numerator->MultiplyByUInt64(significand);
+
+  // denominator = 2 * 2^-exponent with exponent < 0.
+  denominator->AssignUInt16(1);
+  denominator->ShiftLeft(-exponent);
+
+  if (need_boundary_deltas) {
+    // Introduce a common denominator so that the deltas to the boundaries are
+    // integers.
+    numerator->ShiftLeft(1);
+    denominator->ShiftLeft(1);
+    // With this shift the boundaries have their correct value, since
+    // delta_plus = 10^-estimated_power, and
+    // delta_minus = 10^-estimated_power.
+    // These assignments have been done earlier.
+    // The adjustments if f == 2^p-1 (lower boundary is closer) are done later.
+  }
+}
+
+
+// Let v = significand * 2^exponent.
+// Computes v / 10^estimated_power exactly, as a ratio of two bignums, numerator
+// and denominator. The functions GenerateShortestDigits and
+// GenerateCountedDigits will then convert this ratio to its decimal
+// representation d, with the required accuracy.
+// Then d * 10^estimated_power is the representation of v.
+// (Note: the fraction and the estimated_power might get adjusted before
+// generating the decimal representation.)
+//
+// The initial start values consist of:
+//  - a scaled numerator: s.t. numerator/denominator == v / 10^estimated_power.
+//  - a scaled (common) denominator.
+//  optionally (used by GenerateShortestDigits to decide if it has the shortest
+//  decimal converting back to v):
+//  - v - m-: the distance to the lower boundary.
+//  - m+ - v: the distance to the upper boundary.
+//
+// v, m+, m-, and therefore v - m- and m+ - v all share the same denominator.
+//
+// Let ep == estimated_power, then the returned values will satisfy:
+//  v / 10^ep = numerator / denominator.
+//  v's boundaries m- and m+:
+//    m- / 10^ep == v / 10^ep - delta_minus / denominator
+//    m+ / 10^ep == v / 10^ep + delta_plus / denominator
+//  Or in other words:
+//    m- == v - delta_minus * 10^ep / denominator;
+//    m+ == v + delta_plus * 10^ep / denominator;
+//
+// Since 10^(k-1) <= v < 10^k    (with k == estimated_power)
+//  or       10^k <= v < 10^(k+1)
+//  we then have 0.1 <= numerator/denominator < 1
+//           or    1 <= numerator/denominator < 10
+//
+// It is then easy to kickstart the digit-generation routine.
+//
+// The boundary-deltas are only filled if the mode equals BIGNUM_DTOA_SHORTEST
+// or BIGNUM_DTOA_SHORTEST_SINGLE.
+
+static void InitialScaledStartValues(uint64_t significand,
+                                     int exponent,
+                                     bool lower_boundary_is_closer,
+                                     int estimated_power,
+                                     bool need_boundary_deltas,
+                                     Bignum* numerator,
+                                     Bignum* denominator,
+                                     Bignum* delta_minus,
+                                     Bignum* delta_plus) {
+  if (exponent >= 0) {
+    InitialScaledStartValuesPositiveExponent(
+        significand, exponent, estimated_power, need_boundary_deltas,
+        numerator, denominator, delta_minus, delta_plus);
+  } else if (estimated_power >= 0) {
+    InitialScaledStartValuesNegativeExponentPositivePower(
+        significand, exponent, estimated_power, need_boundary_deltas,
+        numerator, denominator, delta_minus, delta_plus);
+  } else {
+    InitialScaledStartValuesNegativeExponentNegativePower(
+        significand, exponent, estimated_power, need_boundary_deltas,
+        numerator, denominator, delta_minus, delta_plus);
+  }
+
+  if (need_boundary_deltas && lower_boundary_is_closer) {
+    // The lower boundary is closer at half the distance of "normal" numbers.
+    // Increase the common denominator and adapt all but the delta_minus.
+    denominator->ShiftLeft(1);  // *2
+    numerator->ShiftLeft(1);    // *2
+    delta_plus->ShiftLeft(1);   // *2
+  }
+}
+
+
+// This routine multiplies numerator/denominator so that its values lies in the
+// range 1-10. That is after a call to this function we have:
+//    1 <= (numerator + delta_plus) /denominator < 10.
+// Let numerator the input before modification and numerator' the argument
+// after modification, then the output-parameter decimal_point is such that
+//  numerator / denominator * 10^estimated_power ==
+//    numerator' / denominator' * 10^(decimal_point - 1)
+// In some cases estimated_power was too low, and this is already the case. We
+// then simply adjust the power so that 10^(k-1) <= v < 10^k (with k ==
+// estimated_power) but do not touch the numerator or denominator.
+// Otherwise the routine multiplies the numerator and the deltas by 10.
+static void FixupMultiply10(int estimated_power, bool is_even,
+                            int* decimal_point,
+                            Bignum* numerator, Bignum* denominator,
+                            Bignum* delta_minus, Bignum* delta_plus) {
+  bool in_range;
+  if (is_even) {
+    // For IEEE doubles half-way cases (in decimal system numbers ending with 5)
+    // are rounded to the closest floating-point number with even significand.
+    in_range = Bignum::PlusCompare(*numerator, *delta_plus, *denominator) >= 0;
+  } else {
+    in_range = Bignum::PlusCompare(*numerator, *delta_plus, *denominator) > 0;
+  }
+  if (in_range) {
+    // Since numerator + delta_plus >= denominator we already have
+    // 1 <= numerator/denominator < 10. Simply update the estimated_power.
+    *decimal_point = estimated_power + 1;
+  } else {
+    *decimal_point = estimated_power;
+    numerator->Times10();
+    if (Bignum::Equal(*delta_minus, *delta_plus)) {
+      delta_minus->Times10();
+      delta_plus->AssignBignum(*delta_minus);
+    } else {
+      delta_minus->Times10();
+      delta_plus->Times10();
+    }
+  }
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/bignum-dtoa.h b/mfbt/double-conversion/double-conversion/bignum-dtoa.h
new file mode 100644
index 0000000000..34b961992d
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/bignum-dtoa.h
@@ -0,0 +1,84 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_BIGNUM_DTOA_H_
+#define DOUBLE_CONVERSION_BIGNUM_DTOA_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+enum BignumDtoaMode {
+  // Return the shortest correct representation.
+  // For example the output of 0.299999999999999988897 is (the less accurate but
+  // correct) 0.3.
+  BIGNUM_DTOA_SHORTEST,
+  // Same as BIGNUM_DTOA_SHORTEST but for single-precision floats.
+  BIGNUM_DTOA_SHORTEST_SINGLE,
+  // Return a fixed number of digits after the decimal point.
+  // For instance fixed(0.1, 4) becomes 0.1000
+  // If the input number is big, the output will be big.
+  BIGNUM_DTOA_FIXED,
+  // Return a fixed number of digits, no matter what the exponent is.
+  BIGNUM_DTOA_PRECISION
+};
+
+// Converts the given double 'v' to ascii.
+// The result should be interpreted as buffer * 10^(point-length).
+// The buffer will be null-terminated.
+//
+// The input v must be > 0 and different from NaN, and Infinity.
+//
+// The output depends on the given mode:
+//  - SHORTEST: produce the least amount of digits for which the internal
+//   identity requirement is still satisfied. If the digits are printed
+//   (together with the correct exponent) then reading this number will give
+//   'v' again. The buffer will choose the representation that is closest to
+//   'v'. If there are two at the same distance, than the number is round up.
+//   In this mode the 'requested_digits' parameter is ignored.
+//  - FIXED: produces digits necessary to print a given number with
+//   'requested_digits' digits after the decimal point. The produced digits
+//   might be too short in which case the caller has to fill the gaps with '0's.
+//   Example: toFixed(0.001, 5) is allowed to return buffer="1", point=-2.
+//   Halfway cases are rounded up. The call toFixed(0.15, 2) thus returns
+//     buffer="2", point=0.
+//   Note: the length of the returned buffer has no meaning wrt the significance
+//   of its digits. That is, just because it contains '0's does not mean that
+//   any other digit would not satisfy the internal identity requirement.
+//  - PRECISION: produces 'requested_digits' where the first digit is not '0'.
+//   Even though the length of produced digits usually equals
+//   'requested_digits', the function is allowed to return fewer digits, in
+//   which case the caller has to fill the missing digits with '0's.
+//   Halfway cases are again rounded up.
+// 'BignumDtoa' expects the given buffer to be big enough to hold all digits
+// and a terminating null-character.
+void BignumDtoa(double v, BignumDtoaMode mode, int requested_digits,
+                Vector<char> buffer, int* length, int* point);
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_BIGNUM_DTOA_H_
diff --git a/mfbt/double-conversion/double-conversion/bignum.cc b/mfbt/double-conversion/double-conversion/bignum.cc
new file mode 100644
index 0000000000..5c74d70d3d
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/bignum.cc
@@ -0,0 +1,797 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <algorithm>
+#include <cstring>
+
+#include "bignum.h"
+#include "utils.h"
+
+namespace double_conversion {
+
+Bignum::Chunk& Bignum::RawBigit(const int index) {
+  DOUBLE_CONVERSION_ASSERT(static_cast<unsigned>(index) < kBigitCapacity);
+  return bigits_buffer_[index];
+}
+
+
+const Bignum::Chunk& Bignum::RawBigit(const int index) const {
+  DOUBLE_CONVERSION_ASSERT(static_cast<unsigned>(index) < kBigitCapacity);
+  return bigits_buffer_[index];
+}
+
+
+template<typename S>
+static int BitSize(const S value) {
+  (void) value;  // Mark variable as used.
+  return 8 * sizeof(value);
+}
+
+// Guaranteed to lie in one Bigit.
+void Bignum::AssignUInt16(const uint16_t value) {
+  DOUBLE_CONVERSION_ASSERT(kBigitSize >= BitSize(value));
+  Zero();
+  if (value > 0) {
+    RawBigit(0) = value;
+    used_bigits_ = 1;
+  }
+}
+
+
+void Bignum::AssignUInt64(uint64_t value) {
+  Zero();
+  for(int i = 0; value > 0; ++i) {
+    RawBigit(i) = value & kBigitMask;
+    value >>= kBigitSize;
+    ++used_bigits_;
+  }
+}
+
+
+void Bignum::AssignBignum(const Bignum& other) {
+  exponent_ = other.exponent_;
+  for (int i = 0; i < other.used_bigits_; ++i) {
+    RawBigit(i) = other.RawBigit(i);
+  }
+  used_bigits_ = other.used_bigits_;
+}
+
+
+static uint64_t ReadUInt64(const Vector<const char> buffer,
+                           const int from,
+                           const int digits_to_read) {
+  uint64_t result = 0;
+  for (int i = from; i < from + digits_to_read; ++i) {
+    const int digit = buffer[i] - '0';
+    DOUBLE_CONVERSION_ASSERT(0 <= digit && digit <= 9);
+    result = result * 10 + digit;
+  }
+  return result;
+}
+
+
+void Bignum::AssignDecimalString(const Vector<const char> value) {
+  // 2^64 = 18446744073709551616 > 10^19
+  static const int kMaxUint64DecimalDigits = 19;
+  Zero();
+  int length = value.length();
+  unsigned pos = 0;
+  // Let's just say that each digit needs 4 bits.
+  while (length >= kMaxUint64DecimalDigits) {
+    const uint64_t digits = ReadUInt64(value, pos, kMaxUint64DecimalDigits);
+    pos += kMaxUint64DecimalDigits;
+    length -= kMaxUint64DecimalDigits;
+    MultiplyByPowerOfTen(kMaxUint64DecimalDigits);
+    AddUInt64(digits);
+  }
+  const uint64_t digits = ReadUInt64(value, pos, length);
+  MultiplyByPowerOfTen(length);
+  AddUInt64(digits);
+  Clamp();
+}
+
+
+static uint64_t HexCharValue(const int c) {
+  if ('0' <= c && c <= '9') {
+    return c - '0';
+  }
+  if ('a' <= c && c <= 'f') {
+    return 10 + c - 'a';
+  }
+  DOUBLE_CONVERSION_ASSERT('A' <= c && c <= 'F');
+  return 10 + c - 'A';
+}
+
+
+// Unlike AssignDecimalString(), this function is "only" used
+// for unit-tests and therefore not performance critical.
+void Bignum::AssignHexString(Vector<const char> value) {
+  Zero();
+  // Required capacity could be reduced by ignoring leading zeros.
+  EnsureCapacity(((value.length() * 4) + kBigitSize - 1) / kBigitSize);
+  DOUBLE_CONVERSION_ASSERT(sizeof(uint64_t) * 8 >= kBigitSize + 4);  // TODO: static_assert
+  // Accumulates converted hex digits until at least kBigitSize bits.
+  // Works with non-factor-of-four kBigitSizes.
+  uint64_t tmp = 0;
+  for (int cnt = 0; !value.is_empty(); value.pop_back()) {
+    tmp |= (HexCharValue(value.last()) << cnt);
+    if ((cnt += 4) >= kBigitSize) {
+      RawBigit(used_bigits_++) = (tmp & kBigitMask);
+      cnt -= kBigitSize;
+      tmp >>= kBigitSize;
+    }
+  }
+  if (tmp > 0) {
+    DOUBLE_CONVERSION_ASSERT(tmp <= kBigitMask);
+    RawBigit(used_bigits_++) = static_cast<Bignum::Chunk>(tmp & kBigitMask);
+  }
+  Clamp();
+}
+
+
+void Bignum::AddUInt64(const uint64_t operand) {
+  if (operand == 0) {
+    return;
+  }
+  Bignum other;
+  other.AssignUInt64(operand);
+  AddBignum(other);
+}
+
+
+void Bignum::AddBignum(const Bignum& other) {
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+  DOUBLE_CONVERSION_ASSERT(other.IsClamped());
+
+  // If this has a greater exponent than other append zero-bigits to this.
+  // After this call exponent_ <= other.exponent_.
+  Align(other);
+
+  // There are two possibilities:
+  //   aaaaaaaaaaa 0000  (where the 0s represent a's exponent)
+  //     bbbbb 00000000
+  //   ----------------
+  //   ccccccccccc 0000
+  // or
+  //    aaaaaaaaaa 0000
+  //  bbbbbbbbb 0000000
+  //  -----------------
+  //  cccccccccccc 0000
+  // In both cases we might need a carry bigit.
+
+  EnsureCapacity(1 + (std::max)(BigitLength(), other.BigitLength()) - exponent_);
+  Chunk carry = 0;
+  int bigit_pos = other.exponent_ - exponent_;
+  DOUBLE_CONVERSION_ASSERT(bigit_pos >= 0);
+  for (int i = used_bigits_; i < bigit_pos; ++i) {
+    RawBigit(i) = 0;
+  }
+  for (int i = 0; i < other.used_bigits_; ++i) {
+    const Chunk my = (bigit_pos < used_bigits_) ? RawBigit(bigit_pos) : 0;
+    const Chunk sum = my + other.RawBigit(i) + carry;
+    RawBigit(bigit_pos) = sum & kBigitMask;
+    carry = sum >> kBigitSize;
+    ++bigit_pos;
+  }
+  while (carry != 0) {
+    const Chunk my = (bigit_pos < used_bigits_) ? RawBigit(bigit_pos) : 0;
+    const Chunk sum = my + carry;
+    RawBigit(bigit_pos) = sum & kBigitMask;
+    carry = sum >> kBigitSize;
+    ++bigit_pos;
+  }
+  used_bigits_ = static_cast<int16_t>(std::max(bigit_pos, static_cast<int>(used_bigits_)));
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+}
+
+
+void Bignum::SubtractBignum(const Bignum& other) {
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+  DOUBLE_CONVERSION_ASSERT(other.IsClamped());
+  // We require this to be bigger than other.
+  DOUBLE_CONVERSION_ASSERT(LessEqual(other, *this));
+
+  Align(other);
+
+  const int offset = other.exponent_ - exponent_;
+  Chunk borrow = 0;
+  int i;
+  for (i = 0; i < other.used_bigits_; ++i) {
+    DOUBLE_CONVERSION_ASSERT((borrow == 0) || (borrow == 1));
+    const Chunk difference = RawBigit(i + offset) - other.RawBigit(i) - borrow;
+    RawBigit(i + offset) = difference & kBigitMask;
+    borrow = difference >> (kChunkSize - 1);
+  }
+  while (borrow != 0) {
+    const Chunk difference = RawBigit(i + offset) - borrow;
+    RawBigit(i + offset) = difference & kBigitMask;
+    borrow = difference >> (kChunkSize - 1);
+    ++i;
+  }
+  Clamp();
+}
+
+
+void Bignum::ShiftLeft(const int shift_amount) {
+  if (used_bigits_ == 0) {
+    return;
+  }
+  exponent_ += static_cast<int16_t>(shift_amount / kBigitSize);
+  const int local_shift = shift_amount % kBigitSize;
+  EnsureCapacity(used_bigits_ + 1);
+  BigitsShiftLeft(local_shift);
+}
+
+
+void Bignum::MultiplyByUInt32(const uint32_t factor) {
+  if (factor == 1) {
+    return;
+  }
+  if (factor == 0) {
+    Zero();
+    return;
+  }
+  if (used_bigits_ == 0) {
+    return;
+  }
+  // The product of a bigit with the factor is of size kBigitSize + 32.
+  // Assert that this number + 1 (for the carry) fits into double chunk.
+  DOUBLE_CONVERSION_ASSERT(kDoubleChunkSize >= kBigitSize + 32 + 1);
+  DoubleChunk carry = 0;
+  for (int i = 0; i < used_bigits_; ++i) {
+    const DoubleChunk product = static_cast<DoubleChunk>(factor) * RawBigit(i) + carry;
+    RawBigit(i) = static_cast<Chunk>(product & kBigitMask);
+    carry = (product >> kBigitSize);
+  }
+  while (carry != 0) {
+    EnsureCapacity(used_bigits_ + 1);
+    RawBigit(used_bigits_) = carry & kBigitMask;
+    used_bigits_++;
+    carry >>= kBigitSize;
+  }
+}
+
+
+void Bignum::MultiplyByUInt64(const uint64_t factor) {
+  if (factor == 1) {
+    return;
+  }
+  if (factor == 0) {
+    Zero();
+    return;
+  }
+  if (used_bigits_ == 0) {
+    return;
+  }
+  DOUBLE_CONVERSION_ASSERT(kBigitSize < 32);
+  uint64_t carry = 0;
+  const uint64_t low = factor & 0xFFFFFFFF;
+  const uint64_t high = factor >> 32;
+  for (int i = 0; i < used_bigits_; ++i) {
+    const uint64_t product_low = low * RawBigit(i);
+    const uint64_t product_high = high * RawBigit(i);
+    const uint64_t tmp = (carry & kBigitMask) + product_low;
+    RawBigit(i) = tmp & kBigitMask;
+    carry = (carry >> kBigitSize) + (tmp >> kBigitSize) +
+        (product_high << (32 - kBigitSize));
+  }
+  while (carry != 0) {
+    EnsureCapacity(used_bigits_ + 1);
+    RawBigit(used_bigits_) = carry & kBigitMask;
+    used_bigits_++;
+    carry >>= kBigitSize;
+  }
+}
+
+
+void Bignum::MultiplyByPowerOfTen(const int exponent) {
+  static const uint64_t kFive27 = DOUBLE_CONVERSION_UINT64_2PART_C(0x6765c793, fa10079d);
+  static const uint16_t kFive1 = 5;
+  static const uint16_t kFive2 = kFive1 * 5;
+  static const uint16_t kFive3 = kFive2 * 5;
+  static const uint16_t kFive4 = kFive3 * 5;
+  static const uint16_t kFive5 = kFive4 * 5;
+  static const uint16_t kFive6 = kFive5 * 5;
+  static const uint32_t kFive7 = kFive6 * 5;
+  static const uint32_t kFive8 = kFive7 * 5;
+  static const uint32_t kFive9 = kFive8 * 5;
+  static const uint32_t kFive10 = kFive9 * 5;
+  static const uint32_t kFive11 = kFive10 * 5;
+  static const uint32_t kFive12 = kFive11 * 5;
+  static const uint32_t kFive13 = kFive12 * 5;
+  static const uint32_t kFive1_to_12[] =
+      { kFive1, kFive2, kFive3, kFive4, kFive5, kFive6,
+        kFive7, kFive8, kFive9, kFive10, kFive11, kFive12 };
+
+  DOUBLE_CONVERSION_ASSERT(exponent >= 0);
+
+  if (exponent == 0) {
+    return;
+  }
+  if (used_bigits_ == 0) {
+    return;
+  }
+  // We shift by exponent at the end just before returning.
+  int remaining_exponent = exponent;
+  while (remaining_exponent >= 27) {
+    MultiplyByUInt64(kFive27);
+    remaining_exponent -= 27;
+  }
+  while (remaining_exponent >= 13) {
+    MultiplyByUInt32(kFive13);
+    remaining_exponent -= 13;
+  }
+  if (remaining_exponent > 0) {
+    MultiplyByUInt32(kFive1_to_12[remaining_exponent - 1]);
+  }
+  ShiftLeft(exponent);
+}
+
+
+void Bignum::Square() {
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+  const int product_length = 2 * used_bigits_;
+  EnsureCapacity(product_length);
+
+  // Comba multiplication: compute each column separately.
+  // Example: r = a2a1a0 * b2b1b0.
+  //    r =  1    * a0b0 +
+  //        10    * (a1b0 + a0b1) +
+  //        100   * (a2b0 + a1b1 + a0b2) +
+  //        1000  * (a2b1 + a1b2) +
+  //        10000 * a2b2
+  //
+  // In the worst case we have to accumulate nb-digits products of digit*digit.
+  //
+  // Assert that the additional number of bits in a DoubleChunk are enough to
+  // sum up used_digits of Bigit*Bigit.
+  if ((1 << (2 * (kChunkSize - kBigitSize))) <= used_bigits_) {
+    DOUBLE_CONVERSION_UNIMPLEMENTED();
+  }
+  DoubleChunk accumulator = 0;
+  // First shift the digits so we don't overwrite them.
+  const int copy_offset = used_bigits_;
+  for (int i = 0; i < used_bigits_; ++i) {
+    RawBigit(copy_offset + i) = RawBigit(i);
+  }
+  // We have two loops to avoid some 'if's in the loop.
+  for (int i = 0; i < used_bigits_; ++i) {
+    // Process temporary digit i with power i.
+    // The sum of the two indices must be equal to i.
+    int bigit_index1 = i;
+    int bigit_index2 = 0;
+    // Sum all of the sub-products.
+    while (bigit_index1 >= 0) {
+      const Chunk chunk1 = RawBigit(copy_offset + bigit_index1);
+      const Chunk chunk2 = RawBigit(copy_offset + bigit_index2);
+      accumulator += static_cast<DoubleChunk>(chunk1) * chunk2;
+      bigit_index1--;
+      bigit_index2++;
+    }
+    RawBigit(i) = static_cast<Chunk>(accumulator) & kBigitMask;
+    accumulator >>= kBigitSize;
+  }
+  for (int i = used_bigits_; i < product_length; ++i) {
+    int bigit_index1 = used_bigits_ - 1;
+    int bigit_index2 = i - bigit_index1;
+    // Invariant: sum of both indices is again equal to i.
+    // Inner loop runs 0 times on last iteration, emptying accumulator.
+    while (bigit_index2 < used_bigits_) {
+      const Chunk chunk1 = RawBigit(copy_offset + bigit_index1);
+      const Chunk chunk2 = RawBigit(copy_offset + bigit_index2);
+      accumulator += static_cast<DoubleChunk>(chunk1) * chunk2;
+      bigit_index1--;
+      bigit_index2++;
+    }
+    // The overwritten RawBigit(i) will never be read in further loop iterations,
+    // because bigit_index1 and bigit_index2 are always greater
+    // than i - used_bigits_.
+    RawBigit(i) = static_cast<Chunk>(accumulator) & kBigitMask;
+    accumulator >>= kBigitSize;
+  }
+  // Since the result was guaranteed to lie inside the number the
+  // accumulator must be 0 now.
+  DOUBLE_CONVERSION_ASSERT(accumulator == 0);
+
+  // Don't forget to update the used_digits and the exponent.
+  used_bigits_ = static_cast<int16_t>(product_length);
+  exponent_ *= 2;
+  Clamp();
+}
+
+
+void Bignum::AssignPowerUInt16(uint16_t base, const int power_exponent) {
+  DOUBLE_CONVERSION_ASSERT(base != 0);
+  DOUBLE_CONVERSION_ASSERT(power_exponent >= 0);
+  if (power_exponent == 0) {
+    AssignUInt16(1);
+    return;
+  }
+  Zero();
+  int shifts = 0;
+  // We expect base to be in range 2-32, and most often to be 10.
+  // It does not make much sense to implement different algorithms for counting
+  // the bits.
+  while ((base & 1) == 0) {
+    base >>= 1;
+    shifts++;
+  }
+  int bit_size = 0;
+  int tmp_base = base;
+  while (tmp_base != 0) {
+    tmp_base >>= 1;
+    bit_size++;
+  }
+  const int final_size = bit_size * power_exponent;
+  // 1 extra bigit for the shifting, and one for rounded final_size.
+  EnsureCapacity(final_size / kBigitSize + 2);
+
+  // Left to Right exponentiation.
+  int mask = 1;
+  while (power_exponent >= mask) mask <<= 1;
+
+  // The mask is now pointing to the bit above the most significant 1-bit of
+  // power_exponent.
+  // Get rid of first 1-bit;
+  mask >>= 2;
+  uint64_t this_value = base;
+
+  bool delayed_multiplication = false;
+  const uint64_t max_32bits = 0xFFFFFFFF;
+  while (mask != 0 && this_value <= max_32bits) {
+    this_value = this_value * this_value;
+    // Verify that there is enough space in this_value to perform the
+    // multiplication.  The first bit_size bits must be 0.
+    if ((power_exponent & mask) != 0) {
+      DOUBLE_CONVERSION_ASSERT(bit_size > 0);
+      const uint64_t base_bits_mask =
+        ~((static_cast<uint64_t>(1) << (64 - bit_size)) - 1);
+      const bool high_bits_zero = (this_value & base_bits_mask) == 0;
+      if (high_bits_zero) {
+        this_value *= base;
+      } else {
+        delayed_multiplication = true;
+      }
+    }
+    mask >>= 1;
+  }
+  AssignUInt64(this_value);
+  if (delayed_multiplication) {
+    MultiplyByUInt32(base);
+  }
+
+  // Now do the same thing as a bignum.
+  while (mask != 0) {
+    Square();
+    if ((power_exponent & mask) != 0) {
+      MultiplyByUInt32(base);
+    }
+    mask >>= 1;
+  }
+
+  // And finally add the saved shifts.
+  ShiftLeft(shifts * power_exponent);
+}
+
+
+// Precondition: this/other < 16bit.
+uint16_t Bignum::DivideModuloIntBignum(const Bignum& other) {
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+  DOUBLE_CONVERSION_ASSERT(other.IsClamped());
+  DOUBLE_CONVERSION_ASSERT(other.used_bigits_ > 0);
+
+  // Easy case: if we have less digits than the divisor than the result is 0.
+  // Note: this handles the case where this == 0, too.
+  if (BigitLength() < other.BigitLength()) {
+    return 0;
+  }
+
+  Align(other);
+
+  uint16_t result = 0;
+
+  // Start by removing multiples of 'other' until both numbers have the same
+  // number of digits.
+  while (BigitLength() > other.BigitLength()) {
+    // This naive approach is extremely inefficient if `this` divided by other
+    // is big. This function is implemented for doubleToString where
+    // the result should be small (less than 10).
+    DOUBLE_CONVERSION_ASSERT(other.RawBigit(other.used_bigits_ - 1) >= ((1 << kBigitSize) / 16));
+    DOUBLE_CONVERSION_ASSERT(RawBigit(used_bigits_ - 1) < 0x10000);
+    // Remove the multiples of the first digit.
+    // Example this = 23 and other equals 9. -> Remove 2 multiples.
+    result += static_cast<uint16_t>(RawBigit(used_bigits_ - 1));
+    SubtractTimes(other, RawBigit(used_bigits_ - 1));
+  }
+
+  DOUBLE_CONVERSION_ASSERT(BigitLength() == other.BigitLength());
+
+  // Both bignums are at the same length now.
+  // Since other has more than 0 digits we know that the access to
+  // RawBigit(used_bigits_ - 1) is safe.
+  const Chunk this_bigit = RawBigit(used_bigits_ - 1);
+  const Chunk other_bigit = other.RawBigit(other.used_bigits_ - 1);
+
+  if (other.used_bigits_ == 1) {
+    // Shortcut for easy (and common) case.
+    int quotient = this_bigit / other_bigit;
+    RawBigit(used_bigits_ - 1) = this_bigit - other_bigit * quotient;
+    DOUBLE_CONVERSION_ASSERT(quotient < 0x10000);
+    result += static_cast<uint16_t>(quotient);
+    Clamp();
+    return result;
+  }
+
+  const int division_estimate = this_bigit / (other_bigit + 1);
+  DOUBLE_CONVERSION_ASSERT(division_estimate < 0x10000);
+  result += static_cast<uint16_t>(division_estimate);
+  SubtractTimes(other, division_estimate);
+
+  if (other_bigit * (division_estimate + 1) > this_bigit) {
+    // No need to even try to subtract. Even if other's remaining digits were 0
+    // another subtraction would be too much.
+    return result;
+  }
+
+  while (LessEqual(other, *this)) {
+    SubtractBignum(other);
+    result++;
+  }
+  return result;
+}
+
+
+template<typename S>
+static int SizeInHexChars(S number) {
+  DOUBLE_CONVERSION_ASSERT(number > 0);
+  int result = 0;
+  while (number != 0) {
+    number >>= 4;
+    result++;
+  }
+  return result;
+}
+
+
+static char HexCharOfValue(const int value) {
+  DOUBLE_CONVERSION_ASSERT(0 <= value && value <= 16);
+  if (value < 10) {
+    return static_cast<char>(value + '0');
+  }
+  return static_cast<char>(value - 10 + 'A');
+}
+
+
+bool Bignum::ToHexString(char* buffer, const int buffer_size) const {
+  DOUBLE_CONVERSION_ASSERT(IsClamped());
+  // Each bigit must be printable as separate hex-character.
+  DOUBLE_CONVERSION_ASSERT(kBigitSize % 4 == 0);
+  static const int kHexCharsPerBigit = kBigitSize / 4;
+
+  if (used_bigits_ == 0) {
+    if (buffer_size < 2) {
+      return false;
+    }
+    buffer[0] = '0';
+    buffer[1] = '\0';
+    return true;
+  }
+  // We add 1 for the terminating '\0' character.
+  const int needed_chars = (BigitLength() - 1) * kHexCharsPerBigit +
+    SizeInHexChars(RawBigit(used_bigits_ - 1)) + 1;
+  if (needed_chars > buffer_size) {
+    return false;
+  }
+  int string_index = needed_chars - 1;
+  buffer[string_index--] = '\0';
+  for (int i = 0; i < exponent_; ++i) {
+    for (int j = 0; j < kHexCharsPerBigit; ++j) {
+      buffer[string_index--] = '0';
+    }
+  }
+  for (int i = 0; i < used_bigits_ - 1; ++i) {
+    Chunk current_bigit = RawBigit(i);
+    for (int j = 0; j < kHexCharsPerBigit; ++j) {
+      buffer[string_index--] = HexCharOfValue(current_bigit & 0xF);
+      current_bigit >>= 4;
+    }
+  }
+  // And finally the last bigit.
+  Chunk most_significant_bigit = RawBigit(used_bigits_ - 1);
+  while (most_significant_bigit != 0) {
+    buffer[string_index--] = HexCharOfValue(most_significant_bigit & 0xF);
+    most_significant_bigit >>= 4;
+  }
+  return true;
+}
+
+
+Bignum::Chunk Bignum::BigitOrZero(const int index) const {
+  if (index >= BigitLength()) {
+    return 0;
+  }
+  if (index < exponent_) {
+    return 0;
+  }
+  return RawBigit(index - exponent_);
+}
+
+
+int Bignum::Compare(const Bignum& a, const Bignum& b) {
+  DOUBLE_CONVERSION_ASSERT(a.IsClamped());
+  DOUBLE_CONVERSION_ASSERT(b.IsClamped());
+  const int bigit_length_a = a.BigitLength();
+  const int bigit_length_b = b.BigitLength();
+  if (bigit_length_a < bigit_length_b) {
+    return -1;
+  }
+  if (bigit_length_a > bigit_length_b) {
+    return +1;
+  }
+  for (int i = bigit_length_a - 1; i >= (std::min)(a.exponent_, b.exponent_); --i) {
+    const Chunk bigit_a = a.BigitOrZero(i);
+    const Chunk bigit_b = b.BigitOrZero(i);
+    if (bigit_a < bigit_b) {
+      return -1;
+    }
+    if (bigit_a > bigit_b) {
+      return +1;
+    }
+    // Otherwise they are equal up to this digit. Try the next digit.
+  }
+  return 0;
+}
+
+
+int Bignum::PlusCompare(const Bignum& a, const Bignum& b, const Bignum& c) {
+  DOUBLE_CONVERSION_ASSERT(a.IsClamped());
+  DOUBLE_CONVERSION_ASSERT(b.IsClamped());
+  DOUBLE_CONVERSION_ASSERT(c.IsClamped());
+  if (a.BigitLength() < b.BigitLength()) {
+    return PlusCompare(b, a, c);
+  }
+  if (a.BigitLength() + 1 < c.BigitLength()) {
+    return -1;
+  }
+  if (a.BigitLength() > c.BigitLength()) {
+    return +1;
+  }
+  // The exponent encodes 0-bigits. So if there are more 0-digits in 'a' than
+  // 'b' has digits, then the bigit-length of 'a'+'b' must be equal to the one
+  // of 'a'.
+  if (a.exponent_ >= b.BigitLength() && a.BigitLength() < c.BigitLength()) {
+    return -1;
+  }
+
+  Chunk borrow = 0;
+  // Starting at min_exponent all digits are == 0. So no need to compare them.
+  const int min_exponent = (std::min)((std::min)(a.exponent_, b.exponent_), c.exponent_);
+  for (int i = c.BigitLength() - 1; i >= min_exponent; --i) {
+    const Chunk chunk_a = a.BigitOrZero(i);
+    const Chunk chunk_b = b.BigitOrZero(i);
+    const Chunk chunk_c = c.BigitOrZero(i);
+    const Chunk sum = chunk_a + chunk_b;
+    if (sum > chunk_c + borrow) {
+      return +1;
+    } else {
+      borrow = chunk_c + borrow - sum;
+      if (borrow > 1) {
+        return -1;
+      }
+      borrow <<= kBigitSize;
+    }
+  }
+  if (borrow == 0) {
+    return 0;
+  }
+  return -1;
+}
+
+
+void Bignum::Clamp() {
+  while (used_bigits_ > 0 && RawBigit(used_bigits_ - 1) == 0) {
+    used_bigits_--;
+  }
+  if (used_bigits_ == 0) {
+    // Zero.
+    exponent_ = 0;
+  }
+}
+
+
+void Bignum::Align(const Bignum& other) {
+  if (exponent_ > other.exponent_) {
+    // If "X" represents a "hidden" bigit (by the exponent) then we are in the
+    // following case (a == this, b == other):
+    // a:  aaaaaaXXXX   or a:   aaaaaXXX
+    // b:     bbbbbbX      b: bbbbbbbbXX
+    // We replace some of the hidden digits (X) of a with 0 digits.
+    // a:  aaaaaa000X   or a:   aaaaa0XX
+    const int zero_bigits = exponent_ - other.exponent_;
+    EnsureCapacity(used_bigits_ + zero_bigits);
+    for (int i = used_bigits_ - 1; i >= 0; --i) {
+      RawBigit(i + zero_bigits) = RawBigit(i);
+    }
+    for (int i = 0; i < zero_bigits; ++i) {
+      RawBigit(i) = 0;
+    }
+    used_bigits_ += static_cast<int16_t>(zero_bigits);
+    exponent_ -= static_cast<int16_t>(zero_bigits);
+
+    DOUBLE_CONVERSION_ASSERT(used_bigits_ >= 0);
+    DOUBLE_CONVERSION_ASSERT(exponent_ >= 0);
+  }
+}
+
+
+void Bignum::BigitsShiftLeft(const int shift_amount) {
+  DOUBLE_CONVERSION_ASSERT(shift_amount < kBigitSize);
+  DOUBLE_CONVERSION_ASSERT(shift_amount >= 0);
+  Chunk carry = 0;
+  for (int i = 0; i < used_bigits_; ++i) {
+    const Chunk new_carry = RawBigit(i) >> (kBigitSize - shift_amount);
+    RawBigit(i) = ((RawBigit(i) << shift_amount) + carry) & kBigitMask;
+    carry = new_carry;
+  }
+  if (carry != 0) {
+    RawBigit(used_bigits_) = carry;
+    used_bigits_++;
+  }
+}
+
+
+void Bignum::SubtractTimes(const Bignum& other, const int factor) {
+  DOUBLE_CONVERSION_ASSERT(exponent_ <= other.exponent_);
+  if (factor < 3) {
+    for (int i = 0; i < factor; ++i) {
+      SubtractBignum(other);
+    }
+    return;
+  }
+  Chunk borrow = 0;
+  const int exponent_diff = other.exponent_ - exponent_;
+  for (int i = 0; i < other.used_bigits_; ++i) {
+    const DoubleChunk product = static_cast<DoubleChunk>(factor) * other.RawBigit(i);
+    const DoubleChunk remove = borrow + product;
+    const Chunk difference = RawBigit(i + exponent_diff) - (remove & kBigitMask);
+    RawBigit(i + exponent_diff) = difference & kBigitMask;
+    borrow = static_cast<Chunk>((difference >> (kChunkSize - 1)) +
+                                (remove >> kBigitSize));
+  }
+  for (int i = other.used_bigits_ + exponent_diff; i < used_bigits_; ++i) {
+    if (borrow == 0) {
+      return;
+    }
+    const Chunk difference = RawBigit(i) - borrow;
+    RawBigit(i) = difference & kBigitMask;
+    borrow = difference >> (kChunkSize - 1);
+  }
+  Clamp();
+}
+
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/bignum.h b/mfbt/double-conversion/double-conversion/bignum.h
new file mode 100644
index 0000000000..14d1ca86fc
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/bignum.h
@@ -0,0 +1,152 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_BIGNUM_H_
+#define DOUBLE_CONVERSION_BIGNUM_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+class Bignum {
+ public:
+  // 3584 = 128 * 28. We can represent 2^3584 > 10^1000 accurately.
+  // This bignum can encode much bigger numbers, since it contains an
+  // exponent.
+  static const int kMaxSignificantBits = 3584;
+
+  Bignum() : used_bigits_(0), exponent_(0) {}
+
+  void AssignUInt16(const uint16_t value);
+  void AssignUInt64(uint64_t value);
+  void AssignBignum(const Bignum& other);
+
+  void AssignDecimalString(const Vector<const char> value);
+  void AssignHexString(const Vector<const char> value);
+
+  void AssignPowerUInt16(uint16_t base, const int exponent);
+
+  void AddUInt64(const uint64_t operand);
+  void AddBignum(const Bignum& other);
+  // Precondition: this >= other.
+  void SubtractBignum(const Bignum& other);
+
+  void Square();
+  void ShiftLeft(const int shift_amount);
+  void MultiplyByUInt32(const uint32_t factor);
+  void MultiplyByUInt64(const uint64_t factor);
+  void MultiplyByPowerOfTen(const int exponent);
+  void Times10() { return MultiplyByUInt32(10); }
+  // Pseudocode:
+  //  int result = this / other;
+  //  this = this % other;
+  // In the worst case this function is in O(this/other).
+  uint16_t DivideModuloIntBignum(const Bignum& other);
+
+  bool ToHexString(char* buffer, const int buffer_size) const;
+
+  // Returns
+  //  -1 if a < b,
+  //   0 if a == b, and
+  //  +1 if a > b.
+  static int Compare(const Bignum& a, const Bignum& b);
+  static bool Equal(const Bignum& a, const Bignum& b) {
+    return Compare(a, b) == 0;
+  }
+  static bool LessEqual(const Bignum& a, const Bignum& b) {
+    return Compare(a, b) <= 0;
+  }
+  static bool Less(const Bignum& a, const Bignum& b) {
+    return Compare(a, b) < 0;
+  }
+  // Returns Compare(a + b, c);
+  static int PlusCompare(const Bignum& a, const Bignum& b, const Bignum& c);
+  // Returns a + b == c
+  static bool PlusEqual(const Bignum& a, const Bignum& b, const Bignum& c) {
+    return PlusCompare(a, b, c) == 0;
+  }
+  // Returns a + b <= c
+  static bool PlusLessEqual(const Bignum& a, const Bignum& b, const Bignum& c) {
+    return PlusCompare(a, b, c) <= 0;
+  }
+  // Returns a + b < c
+  static bool PlusLess(const Bignum& a, const Bignum& b, const Bignum& c) {
+    return PlusCompare(a, b, c) < 0;
+  }
+ private:
+  typedef uint32_t Chunk;
+  typedef uint64_t DoubleChunk;
+
+  static const int kChunkSize = sizeof(Chunk) * 8;
+  static const int kDoubleChunkSize = sizeof(DoubleChunk) * 8;
+  // With bigit size of 28 we loose some bits, but a double still fits easily
+  // into two chunks, and more importantly we can use the Comba multiplication.
+  static const int kBigitSize = 28;
+  static const Chunk kBigitMask = (1 << kBigitSize) - 1;
+  // Every instance allocates kBigitLength chunks on the stack. Bignums cannot
+  // grow. There are no checks if the stack-allocated space is sufficient.
+  static const int kBigitCapacity = kMaxSignificantBits / kBigitSize;
+
+  static void EnsureCapacity(const int size) {
+    if (size > kBigitCapacity) {
+      DOUBLE_CONVERSION_UNREACHABLE();
+    }
+  }
+  void Align(const Bignum& other);
+  void Clamp();
+  bool IsClamped() const {
+    return used_bigits_ == 0 || RawBigit(used_bigits_ - 1) != 0;
+  }
+  void Zero() {
+    used_bigits_ = 0;
+    exponent_ = 0;
+  }
+  // Requires this to have enough capacity (no tests done).
+  // Updates used_bigits_ if necessary.
+  // shift_amount must be < kBigitSize.
+  void BigitsShiftLeft(const int shift_amount);
+  // BigitLength includes the "hidden" bigits encoded in the exponent.
+  int BigitLength() const { return used_bigits_ + exponent_; }
+  Chunk& RawBigit(const int index);
+  const Chunk& RawBigit(const int index) const;
+  Chunk BigitOrZero(const int index) const;
+  void SubtractTimes(const Bignum& other, const int factor);
+
+  // The Bignum's value is value(bigits_buffer_) * 2^(exponent_ * kBigitSize),
+  // where the value of the buffer consists of the lower kBigitSize bits of
+  // the first used_bigits_ Chunks in bigits_buffer_, first chunk has lowest
+  // significant bits.
+  int16_t used_bigits_;
+  int16_t exponent_;
+  Chunk bigits_buffer_[kBigitCapacity];
+
+  DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN(Bignum);
+};
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_BIGNUM_H_
diff --git a/mfbt/double-conversion/double-conversion/cached-powers.cc b/mfbt/double-conversion/double-conversion/cached-powers.cc
new file mode 100644
index 0000000000..56bdfc9d63
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/cached-powers.cc
@@ -0,0 +1,175 @@
+// Copyright 2006-2008 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <climits>
+#include <cmath>
+#include <cstdarg>
+
+#include "utils.h"
+
+#include "cached-powers.h"
+
+namespace double_conversion {
+
+namespace PowersOfTenCache {
+
+struct CachedPower {
+  uint64_t significand;
+  int16_t binary_exponent;
+  int16_t decimal_exponent;
+};
+
+static const CachedPower kCachedPowers[] = {
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xfa8fd5a0, 081c0288), -1220, -348},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xbaaee17f, a23ebf76), -1193, -340},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8b16fb20, 3055ac76), -1166, -332},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xcf42894a, 5dce35ea), -1140, -324},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9a6bb0aa, 55653b2d), -1113, -316},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xe61acf03, 3d1a45df), -1087, -308},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xab70fe17, c79ac6ca), -1060, -300},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xff77b1fc, bebcdc4f), -1034, -292},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xbe5691ef, 416bd60c), -1007, -284},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8dd01fad, 907ffc3c), -980, -276},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd3515c28, 31559a83), -954, -268},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9d71ac8f, ada6c9b5), -927, -260},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xea9c2277, 23ee8bcb), -901, -252},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xaecc4991, 4078536d), -874, -244},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x823c1279, 5db6ce57), -847, -236},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc2109436, 4dfb5637), -821, -228},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9096ea6f, 3848984f), -794, -220},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd77485cb, 25823ac7), -768, -212},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa086cfcd, 97bf97f4), -741, -204},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xef340a98, 172aace5), -715, -196},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb23867fb, 2a35b28e), -688, -188},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x84c8d4df, d2c63f3b), -661, -180},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc5dd4427, 1ad3cdba), -635, -172},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x936b9fce, bb25c996), -608, -164},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xdbac6c24, 7d62a584), -582, -156},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa3ab6658, 0d5fdaf6), -555, -148},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xf3e2f893, dec3f126), -529, -140},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb5b5ada8, aaff80b8), -502, -132},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x87625f05, 6c7c4a8b), -475, -124},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc9bcff60, 34c13053), -449, -116},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x964e858c, 91ba2655), -422, -108},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xdff97724, 70297ebd), -396, -100},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa6dfbd9f, b8e5b88f), -369, -92},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xf8a95fcf, 88747d94), -343, -84},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb9447093, 8fa89bcf), -316, -76},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8a08f0f8, bf0f156b), -289, -68},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xcdb02555, 653131b6), -263, -60},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x993fe2c6, d07b7fac), -236, -52},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xe45c10c4, 2a2b3b06), -210, -44},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xaa242499, 697392d3), -183, -36},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xfd87b5f2, 8300ca0e), -157, -28},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xbce50864, 92111aeb), -130, -20},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8cbccc09, 6f5088cc), -103, -12},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd1b71758, e219652c), -77, -4},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9c400000, 00000000), -50, 4},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xe8d4a510, 00000000), -24, 12},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xad78ebc5, ac620000), 3, 20},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x813f3978, f8940984), 30, 28},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc097ce7b, c90715b3), 56, 36},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8f7e32ce, 7bea5c70), 83, 44},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd5d238a4, abe98068), 109, 52},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9f4f2726, 179a2245), 136, 60},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xed63a231, d4c4fb27), 162, 68},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb0de6538, 8cc8ada8), 189, 76},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x83c7088e, 1aab65db), 216, 84},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc45d1df9, 42711d9a), 242, 92},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x924d692c, a61be758), 269, 100},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xda01ee64, 1a708dea), 295, 108},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa26da399, 9aef774a), 322, 116},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xf209787b, b47d6b85), 348, 124},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb454e4a1, 79dd1877), 375, 132},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x865b8692, 5b9bc5c2), 402, 140},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xc83553c5, c8965d3d), 428, 148},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x952ab45c, fa97a0b3), 455, 156},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xde469fbd, 99a05fe3), 481, 164},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa59bc234, db398c25), 508, 172},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xf6c69a72, a3989f5c), 534, 180},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xb7dcbf53, 54e9bece), 561, 188},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x88fcf317, f22241e2), 588, 196},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xcc20ce9b, d35c78a5), 614, 204},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x98165af3, 7b2153df), 641, 212},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xe2a0b5dc, 971f303a), 667, 220},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xa8d9d153, 5ce3b396), 694, 228},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xfb9b7cd9, a4a7443c), 720, 236},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xbb764c4c, a7a44410), 747, 244},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8bab8eef, b6409c1a), 774, 252},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd01fef10, a657842c), 800, 260},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9b10a4e5, e9913129), 827, 268},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xe7109bfb, a19c0c9d), 853, 276},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xac2820d9, 623bf429), 880, 284},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x80444b5e, 7aa7cf85), 907, 292},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xbf21e440, 03acdd2d), 933, 300},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x8e679c2f, 5e44ff8f), 960, 308},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xd433179d, 9c8cb841), 986, 316},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0x9e19db92, b4e31ba9), 1013, 324},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xeb96bf6e, badf77d9), 1039, 332},
+  {DOUBLE_CONVERSION_UINT64_2PART_C(0xaf87023b, 9bf0ee6b), 1066, 340},
+};
+
+static const int kCachedPowersOffset = 348;  // -1 * the first decimal_exponent.
+static const double kD_1_LOG2_10 = 0.30102999566398114;  //  1 / lg(10)
+
+void GetCachedPowerForBinaryExponentRange(
+    int min_exponent,
+    int max_exponent,
+    DiyFp* power,
+    int* decimal_exponent) {
+  int kQ = DiyFp::kSignificandSize;
+  double k = ceil((min_exponent + kQ - 1) * kD_1_LOG2_10);
+  int foo = kCachedPowersOffset;
+  int index =
+      (foo + static_cast<int>(k) - 1) / kDecimalExponentDistance + 1;
+  DOUBLE_CONVERSION_ASSERT(0 <= index && index < static_cast<int>(DOUBLE_CONVERSION_ARRAY_SIZE(kCachedPowers)));
+  CachedPower cached_power = kCachedPowers[index];
+  DOUBLE_CONVERSION_ASSERT(min_exponent <= cached_power.binary_exponent);
+  (void) max_exponent;  // Mark variable as used.
+  DOUBLE_CONVERSION_ASSERT(cached_power.binary_exponent <= max_exponent);
+  *decimal_exponent = cached_power.decimal_exponent;
+  *power = DiyFp(cached_power.significand, cached_power.binary_exponent);
+}
+
+
+void GetCachedPowerForDecimalExponent(int requested_exponent,
+                                      DiyFp* power,
+                                      int* found_exponent) {
+  DOUBLE_CONVERSION_ASSERT(kMinDecimalExponent <= requested_exponent);
+  DOUBLE_CONVERSION_ASSERT(requested_exponent < kMaxDecimalExponent + kDecimalExponentDistance);
+  int index =
+      (requested_exponent + kCachedPowersOffset) / kDecimalExponentDistance;
+  CachedPower cached_power = kCachedPowers[index];
+  *power = DiyFp(cached_power.significand, cached_power.binary_exponent);
+  *found_exponent = cached_power.decimal_exponent;
+  DOUBLE_CONVERSION_ASSERT(*found_exponent <= requested_exponent);
+  DOUBLE_CONVERSION_ASSERT(requested_exponent < *found_exponent + kDecimalExponentDistance);
+}
+
+}  // namespace PowersOfTenCache
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/cached-powers.h b/mfbt/double-conversion/double-conversion/cached-powers.h
new file mode 100644
index 0000000000..f38c26d201
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/cached-powers.h
@@ -0,0 +1,64 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_CACHED_POWERS_H_
+#define DOUBLE_CONVERSION_CACHED_POWERS_H_
+
+#include "diy-fp.h"
+
+namespace double_conversion {
+
+namespace PowersOfTenCache {
+
+  // Not all powers of ten are cached. The decimal exponent of two neighboring
+  // cached numbers will differ by kDecimalExponentDistance.
+  static const int kDecimalExponentDistance = 8;
+
+  static const int kMinDecimalExponent = -348;
+  static const int kMaxDecimalExponent = 340;
+
+  // Returns a cached power-of-ten with a binary exponent in the range
+  // [min_exponent; max_exponent] (boundaries included).
+  void GetCachedPowerForBinaryExponentRange(int min_exponent,
+                                            int max_exponent,
+                                            DiyFp* power,
+                                            int* decimal_exponent);
+
+  // Returns a cached power of ten x ~= 10^k such that
+  //   k <= decimal_exponent < k + kCachedPowersDecimalDistance.
+  // The given decimal_exponent must satisfy
+  //   kMinDecimalExponent <= requested_exponent, and
+  //   requested_exponent < kMaxDecimalExponent + kDecimalExponentDistance.
+  void GetCachedPowerForDecimalExponent(int requested_exponent,
+                                        DiyFp* power,
+                                        int* found_exponent);
+
+}  // namespace PowersOfTenCache
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_CACHED_POWERS_H_
diff --git a/mfbt/double-conversion/double-conversion/diy-fp.h b/mfbt/double-conversion/double-conversion/diy-fp.h
new file mode 100644
index 0000000000..a2200c4ded
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/diy-fp.h
@@ -0,0 +1,137 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_DIY_FP_H_
+#define DOUBLE_CONVERSION_DIY_FP_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+// This "Do It Yourself Floating Point" class implements a floating-point number
+// with a uint64 significand and an int exponent. Normalized DiyFp numbers will
+// have the most significant bit of the significand set.
+// Multiplication and Subtraction do not normalize their results.
+// DiyFp store only non-negative numbers and are not designed to contain special
+// doubles (NaN and Infinity).
+class DiyFp {
+ public:
+  static const int kSignificandSize = 64;
+
+  DiyFp() : f_(0), e_(0) {}
+  DiyFp(const uint64_t significand, const int32_t exponent) : f_(significand), e_(exponent) {}
+
+  // this -= other.
+  // The exponents of both numbers must be the same and the significand of this
+  // must be greater or equal than the significand of other.
+  // The result will not be normalized.
+  void Subtract(const DiyFp& other) {
+    DOUBLE_CONVERSION_ASSERT(e_ == other.e_);
+    DOUBLE_CONVERSION_ASSERT(f_ >= other.f_);
+    f_ -= other.f_;
+  }
+
+  // Returns a - b.
+  // The exponents of both numbers must be the same and a must be greater
+  // or equal than b. The result will not be normalized.
+  static DiyFp Minus(const DiyFp& a, const DiyFp& b) {
+    DiyFp result = a;
+    result.Subtract(b);
+    return result;
+  }
+
+  // this *= other.
+  void Multiply(const DiyFp& other) {
+    // Simply "emulates" a 128 bit multiplication.
+    // However: the resulting number only contains 64 bits. The least
+    // significant 64 bits are only used for rounding the most significant 64
+    // bits.
+    const uint64_t kM32 = 0xFFFFFFFFU;
+    const uint64_t a = f_ >> 32;
+    const uint64_t b = f_ & kM32;
+    const uint64_t c = other.f_ >> 32;
+    const uint64_t d = other.f_ & kM32;
+    const uint64_t ac = a * c;
+    const uint64_t bc = b * c;
+    const uint64_t ad = a * d;
+    const uint64_t bd = b * d;
+    // By adding 1U << 31 to tmp we round the final result.
+    // Halfway cases will be rounded up.
+    const uint64_t tmp = (bd >> 32) + (ad & kM32) + (bc & kM32) + (1U << 31);
+    e_ += other.e_ + 64;
+    f_ = ac + (ad >> 32) + (bc >> 32) + (tmp >> 32);
+  }
+
+  // returns a * b;
+  static DiyFp Times(const DiyFp& a, const DiyFp& b) {
+    DiyFp result = a;
+    result.Multiply(b);
+    return result;
+  }
+
+  void Normalize() {
+    DOUBLE_CONVERSION_ASSERT(f_ != 0);
+    uint64_t significand = f_;
+    int32_t exponent = e_;
+
+    // This method is mainly called for normalizing boundaries. In general,
+    // boundaries need to be shifted by 10 bits, and we optimize for this case.
+    const uint64_t k10MSBits = DOUBLE_CONVERSION_UINT64_2PART_C(0xFFC00000, 00000000);
+    while ((significand & k10MSBits) == 0) {
+      significand <<= 10;
+      exponent -= 10;
+    }
+    while ((significand & kUint64MSB) == 0) {
+      significand <<= 1;
+      exponent--;
+    }
+    f_ = significand;
+    e_ = exponent;
+  }
+
+  static DiyFp Normalize(const DiyFp& a) {
+    DiyFp result = a;
+    result.Normalize();
+    return result;
+  }
+
+  uint64_t f() const { return f_; }
+  int32_t e() const { return e_; }
+
+  void set_f(uint64_t new_value) { f_ = new_value; }
+  void set_e(int32_t new_value) { e_ = new_value; }
+
+ private:
+  static const uint64_t kUint64MSB = DOUBLE_CONVERSION_UINT64_2PART_C(0x80000000, 00000000);
+
+  uint64_t f_;
+  int32_t e_;
+};
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_DIY_FP_H_
diff --git a/mfbt/double-conversion/double-conversion/double-conversion.h b/mfbt/double-conversion/double-conversion/double-conversion.h
new file mode 100644
index 0000000000..6e8884d84c
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/double-conversion.h
@@ -0,0 +1,34 @@
+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_DOUBLE_CONVERSION_H_
+#define DOUBLE_CONVERSION_DOUBLE_CONVERSION_H_
+
+#include "string-to-double.h"
+#include "double-to-string.h"
+
+#endif  // DOUBLE_CONVERSION_DOUBLE_CONVERSION_H_
diff --git a/mfbt/double-conversion/double-conversion/double-to-string.cc b/mfbt/double-conversion/double-conversion/double-to-string.cc
new file mode 100644
index 0000000000..cf4550f391
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/double-to-string.cc
@@ -0,0 +1,443 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <algorithm>
+#include <climits>
+#include <cmath>
+
+#include "double-to-string.h"
+
+#include "bignum-dtoa.h"
+#include "fast-dtoa.h"
+#include "fixed-dtoa.h"
+#include "ieee.h"
+#include "utils.h"
+
+namespace double_conversion {
+
+const DoubleToStringConverter& DoubleToStringConverter::EcmaScriptConverter() {
+  int flags = UNIQUE_ZERO | EMIT_POSITIVE_EXPONENT_SIGN;
+  static DoubleToStringConverter converter(flags,
+                                           "Infinity",
+                                           "NaN",
+                                           'e',
+                                           -6, 21,
+                                           6, 0);
+  return converter;
+}
+
+
+bool DoubleToStringConverter::HandleSpecialValues(
+    double value,
+    StringBuilder* result_builder) const {
+  Double double_inspect(value);
+  if (double_inspect.IsInfinite()) {
+    if (infinity_symbol_ == DOUBLE_CONVERSION_NULLPTR) return false;
+    if (value < 0) {
+      result_builder->AddCharacter('-');
+    }
+    result_builder->AddString(infinity_symbol_);
+    return true;
+  }
+  if (double_inspect.IsNan()) {
+    if (nan_symbol_ == DOUBLE_CONVERSION_NULLPTR) return false;
+    result_builder->AddString(nan_symbol_);
+    return true;
+  }
+  return false;
+}
+
+
+void DoubleToStringConverter::CreateExponentialRepresentation(
+    const char* decimal_digits,
+    int length,
+    int exponent,
+    StringBuilder* result_builder) const {
+  DOUBLE_CONVERSION_ASSERT(length != 0);
+  result_builder->AddCharacter(decimal_digits[0]);
+  if (length == 1) {
+    if ((flags_ & EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL) != 0) {
+      result_builder->AddCharacter('.');
+      if ((flags_ & EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL) != 0) {
+          result_builder->AddCharacter('0');
+      }
+    }
+  } else {
+    result_builder->AddCharacter('.');
+    result_builder->AddSubstring(&decimal_digits[1], length-1);
+  }
+  result_builder->AddCharacter(exponent_character_);
+  if (exponent < 0) {
+    result_builder->AddCharacter('-');
+    exponent = -exponent;
+  } else {
+    if ((flags_ & EMIT_POSITIVE_EXPONENT_SIGN) != 0) {
+      result_builder->AddCharacter('+');
+    }
+  }
+  DOUBLE_CONVERSION_ASSERT(exponent < 1e4);
+  // Changing this constant requires updating the comment of DoubleToStringConverter constructor
+  const int kMaxExponentLength = 5;
+  char buffer[kMaxExponentLength + 1];
+  buffer[kMaxExponentLength] = '\0';
+  int first_char_pos = kMaxExponentLength;
+  if (exponent == 0) {
+    buffer[--first_char_pos] = '0';
+  } else {
+    while (exponent > 0) {
+      buffer[--first_char_pos] = '0' + (exponent % 10);
+      exponent /= 10;
+    }
+  }
+  // Add prefix '0' to make exponent width >= min(min_exponent_with_, kMaxExponentLength)
+  // For example: convert 1e+9 -> 1e+09, if min_exponent_with_ is set to 2
+  while(kMaxExponentLength - first_char_pos < std::min(min_exponent_width_, kMaxExponentLength)) {
+    buffer[--first_char_pos] = '0';
+  }
+  result_builder->AddSubstring(&buffer[first_char_pos],
+                               kMaxExponentLength - first_char_pos);
+}
+
+
+void DoubleToStringConverter::CreateDecimalRepresentation(
+    const char* decimal_digits,
+    int length,
+    int decimal_point,
+    int digits_after_point,
+    StringBuilder* result_builder) const {
+  // Create a representation that is padded with zeros if needed.
+  if (decimal_point <= 0) {
+      // "0.00000decimal_rep" or "0.000decimal_rep00".
+    result_builder->AddCharacter('0');
+    if (digits_after_point > 0) {
+      result_builder->AddCharacter('.');
+      result_builder->AddPadding('0', -decimal_point);
+      DOUBLE_CONVERSION_ASSERT(length <= digits_after_point - (-decimal_point));
+      result_builder->AddSubstring(decimal_digits, length);
+      int remaining_digits = digits_after_point - (-decimal_point) - length;
+      result_builder->AddPadding('0', remaining_digits);
+    }
+  } else if (decimal_point >= length) {
+    // "decimal_rep0000.00000" or "decimal_rep.0000".
+    result_builder->AddSubstring(decimal_digits, length);
+    result_builder->AddPadding('0', decimal_point - length);
+    if (digits_after_point > 0) {
+      result_builder->AddCharacter('.');
+      result_builder->AddPadding('0', digits_after_point);
+    }
+  } else {
+    // "decima.l_rep000".
+    DOUBLE_CONVERSION_ASSERT(digits_after_point > 0);
+    result_builder->AddSubstring(decimal_digits, decimal_point);
+    result_builder->AddCharacter('.');
+    DOUBLE_CONVERSION_ASSERT(length - decimal_point <= digits_after_point);
+    result_builder->AddSubstring(&decimal_digits[decimal_point],
+                                 length - decimal_point);
+    int remaining_digits = digits_after_point - (length - decimal_point);
+    result_builder->AddPadding('0', remaining_digits);
+  }
+  if (digits_after_point == 0) {
+    if ((flags_ & EMIT_TRAILING_DECIMAL_POINT) != 0) {
+      result_builder->AddCharacter('.');
+    }
+    if ((flags_ & EMIT_TRAILING_ZERO_AFTER_POINT) != 0) {
+      result_builder->AddCharacter('0');
+    }
+  }
+}
+
+
+bool DoubleToStringConverter::ToShortestIeeeNumber(
+    double value,
+    StringBuilder* result_builder,
+    DoubleToStringConverter::DtoaMode mode) const {
+  DOUBLE_CONVERSION_ASSERT(mode == SHORTEST || mode == SHORTEST_SINGLE);
+  if (Double(value).IsSpecial()) {
+    return HandleSpecialValues(value, result_builder);
+  }
+
+  int decimal_point;
+  bool sign;
+  const int kDecimalRepCapacity = kBase10MaximalLength + 1;
+  char decimal_rep[kDecimalRepCapacity];
+  int decimal_rep_length;
+
+  DoubleToAscii(value, mode, 0, decimal_rep, kDecimalRepCapacity,
+                &sign, &decimal_rep_length, &decimal_point);
+
+  bool unique_zero = (flags_ & UNIQUE_ZERO) != 0;
+  if (sign && (value != 0.0 || !unique_zero)) {
+    result_builder->AddCharacter('-');
+  }
+
+  int exponent = decimal_point - 1;
+  if ((decimal_in_shortest_low_ <= exponent) &&
+      (exponent < decimal_in_shortest_high_)) {
+    CreateDecimalRepresentation(decimal_rep, decimal_rep_length,
+                                decimal_point,
+                                (std::max)(0, decimal_rep_length - decimal_point),
+                                result_builder);
+  } else {
+    CreateExponentialRepresentation(decimal_rep, decimal_rep_length, exponent,
+                                    result_builder);
+  }
+  return true;
+}
+
+
+bool DoubleToStringConverter::ToFixed(double value,
+                                      int requested_digits,
+                                      StringBuilder* result_builder) const {
+  if (Double(value).IsSpecial()) {
+    return HandleSpecialValues(value, result_builder);
+  }
+
+  if (requested_digits > kMaxFixedDigitsAfterPoint) return false;
+
+  // Find a sufficiently precise decimal representation of n.
+  int decimal_point;
+  bool sign;
+  // Add space for the '\0' byte.
+  const int kDecimalRepCapacity =
+      kMaxFixedDigitsBeforePoint + kMaxFixedDigitsAfterPoint + 1;
+  char decimal_rep[kDecimalRepCapacity];
+  int decimal_rep_length;
+  DoubleToAscii(value, FIXED, requested_digits,
+                decimal_rep, kDecimalRepCapacity,
+                &sign, &decimal_rep_length, &decimal_point);
+
+  bool unique_zero = ((flags_ & UNIQUE_ZERO) != 0);
+  if (sign && (value != 0.0 || !unique_zero)) {
+    result_builder->AddCharacter('-');
+  }
+
+  CreateDecimalRepresentation(decimal_rep, decimal_rep_length, decimal_point,
+                              requested_digits, result_builder);
+  return true;
+}
+
+
+bool DoubleToStringConverter::ToExponential(
+    double value,
+    int requested_digits,
+    StringBuilder* result_builder) const {
+  if (Double(value).IsSpecial()) {
+    return HandleSpecialValues(value, result_builder);
+  }
+
+  if (requested_digits < -1) return false;
+  if (requested_digits > kMaxExponentialDigits) return false;
+
+  int decimal_point;
+  bool sign;
+  // Add space for digit before the decimal point and the '\0' character.
+  const int kDecimalRepCapacity = kMaxExponentialDigits + 2;
+  DOUBLE_CONVERSION_ASSERT(kDecimalRepCapacity > kBase10MaximalLength);
+  char decimal_rep[kDecimalRepCapacity];
+#ifndef NDEBUG
+  // Problem: there is an assert in StringBuilder::AddSubstring() that
+  // will pass this buffer to strlen(), and this buffer is not generally
+  // null-terminated.
+  memset(decimal_rep, 0, sizeof(decimal_rep));
+#endif
+  int decimal_rep_length;
+
+  if (requested_digits == -1) {
+    DoubleToAscii(value, SHORTEST, 0,
+                  decimal_rep, kDecimalRepCapacity,
+                  &sign, &decimal_rep_length, &decimal_point);
+  } else {
+    DoubleToAscii(value, PRECISION, requested_digits + 1,
+                  decimal_rep, kDecimalRepCapacity,
+                  &sign, &decimal_rep_length, &decimal_point);
+    DOUBLE_CONVERSION_ASSERT(decimal_rep_length <= requested_digits + 1);
+
+    for (int i = decimal_rep_length; i < requested_digits + 1; ++i) {
+      decimal_rep[i] = '0';
+    }
+    decimal_rep_length = requested_digits + 1;
+  }
+
+  bool unique_zero = ((flags_ & UNIQUE_ZERO) != 0);
+  if (sign && (value != 0.0 || !unique_zero)) {
+    result_builder->AddCharacter('-');
+  }
+
+  int exponent = decimal_point - 1;
+  CreateExponentialRepresentation(decimal_rep,
+                                  decimal_rep_length,
+                                  exponent,
+                                  result_builder);
+  return true;
+}
+
+
+bool DoubleToStringConverter::ToPrecision(double value,
+                                          int precision,
+                                          StringBuilder* result_builder) const {
+  if (Double(value).IsSpecial()) {
+    return HandleSpecialValues(value, result_builder);
+  }
+
+  if (precision < kMinPrecisionDigits || precision > kMaxPrecisionDigits) {
+    return false;
+  }
+
+  // Find a sufficiently precise decimal representation of n.
+  int decimal_point;
+  bool sign;
+  // Add one for the terminating null character.
+  const int kDecimalRepCapacity = kMaxPrecisionDigits + 1;
+  char decimal_rep[kDecimalRepCapacity];
+  int decimal_rep_length;
+
+  DoubleToAscii(value, PRECISION, precision,
+                decimal_rep, kDecimalRepCapacity,
+                &sign, &decimal_rep_length, &decimal_point);
+  DOUBLE_CONVERSION_ASSERT(decimal_rep_length <= precision);
+
+  bool unique_zero = ((flags_ & UNIQUE_ZERO) != 0);
+  if (sign && (value != 0.0 || !unique_zero)) {
+    result_builder->AddCharacter('-');
+  }
+
+  // The exponent if we print the number as x.xxeyyy. That is with the
+  // decimal point after the first digit.
+  int exponent = decimal_point - 1;
+
+  int extra_zero = ((flags_ & EMIT_TRAILING_ZERO_AFTER_POINT) != 0) ? 1 : 0;
+  bool as_exponential =
+      (-decimal_point + 1 > max_leading_padding_zeroes_in_precision_mode_) ||
+      (decimal_point - precision + extra_zero >
+       max_trailing_padding_zeroes_in_precision_mode_);
+  if ((flags_ & NO_TRAILING_ZERO) != 0) {
+    // Truncate trailing zeros that occur after the decimal point (if exponential,
+    // that is everything after the first digit).
+    int stop = as_exponential ? 1 : std::max(1, decimal_point);
+    while (decimal_rep_length > stop && decimal_rep[decimal_rep_length - 1] == '0') {
+      --decimal_rep_length;
+    }
+    // Clamp precision to avoid the code below re-adding the zeros.
+    precision = std::min(precision, decimal_rep_length);
+  }
+  if (as_exponential) {
+    // Fill buffer to contain 'precision' digits.
+    // Usually the buffer is already at the correct length, but 'DoubleToAscii'
+    // is allowed to return less characters.
+    for (int i = decimal_rep_length; i < precision; ++i) {
+      decimal_rep[i] = '0';
+    }
+
+    CreateExponentialRepresentation(decimal_rep,
+                                    precision,
+                                    exponent,
+                                    result_builder);
+  } else {
+    CreateDecimalRepresentation(decimal_rep, decimal_rep_length, decimal_point,
+                                (std::max)(0, precision - decimal_point),
+                                result_builder);
+  }
+  return true;
+}
+
+
+static BignumDtoaMode DtoaToBignumDtoaMode(
+    DoubleToStringConverter::DtoaMode dtoa_mode) {
+  switch (dtoa_mode) {
+    case DoubleToStringConverter::SHORTEST:  return BIGNUM_DTOA_SHORTEST;
+    case DoubleToStringConverter::SHORTEST_SINGLE:
+        return BIGNUM_DTOA_SHORTEST_SINGLE;
+    case DoubleToStringConverter::FIXED:     return BIGNUM_DTOA_FIXED;
+    case DoubleToStringConverter::PRECISION: return BIGNUM_DTOA_PRECISION;
+    default:
+      DOUBLE_CONVERSION_UNREACHABLE();
+  }
+}
+
+
+void DoubleToStringConverter::DoubleToAscii(double v,
+                                            DtoaMode mode,
+                                            int requested_digits,
+                                            char* buffer,
+                                            int buffer_length,
+                                            bool* sign,
+                                            int* length,
+                                            int* point) {
+  Vector<char> vector(buffer, buffer_length);
+  DOUBLE_CONVERSION_ASSERT(!Double(v).IsSpecial());
+  DOUBLE_CONVERSION_ASSERT(mode == SHORTEST || mode == SHORTEST_SINGLE || requested_digits >= 0);
+
+  if (Double(v).Sign() < 0) {
+    *sign = true;
+    v = -v;
+  } else {
+    *sign = false;
+  }
+
+  if (mode == PRECISION && requested_digits == 0) {
+    vector[0] = '\0';
+    *length = 0;
+    return;
+  }
+
+  if (v == 0) {
+    vector[0] = '0';
+    vector[1] = '\0';
+    *length = 1;
+    *point = 1;
+    return;
+  }
+
+  bool fast_worked;
+  switch (mode) {
+    case SHORTEST:
+      fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST, 0, vector, length, point);
+      break;
+    case SHORTEST_SINGLE:
+      fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST_SINGLE, 0,
+                             vector, length, point);
+      break;
+    case FIXED:
+      fast_worked = FastFixedDtoa(v, requested_digits, vector, length, point);
+      break;
+    case PRECISION:
+      fast_worked = FastDtoa(v, FAST_DTOA_PRECISION, requested_digits,
+                             vector, length, point);
+      break;
+    default:
+      fast_worked = false;
+      DOUBLE_CONVERSION_UNREACHABLE();
+  }
+  if (fast_worked) return;
+
+  // If the fast dtoa didn't succeed use the slower bignum version.
+  BignumDtoaMode bignum_mode = DtoaToBignumDtoaMode(mode);
+  BignumDtoa(v, bignum_mode, requested_digits, vector, length, point);
+  vector[*length] = '\0';
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/double-to-string.h b/mfbt/double-conversion/double-conversion/double-to-string.h
new file mode 100644
index 0000000000..cf2e926d8f
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/double-to-string.h
@@ -0,0 +1,471 @@
+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_DOUBLE_TO_STRING_H_
+#define DOUBLE_CONVERSION_DOUBLE_TO_STRING_H_
+
+#include "mozilla/Types.h"
+#include "utils.h"
+
+namespace double_conversion {
+
+class DoubleToStringConverter {
+ public:
+  // When calling ToFixed with a double > 10^kMaxFixedDigitsBeforePoint
+  // or a requested_digits parameter > kMaxFixedDigitsAfterPoint then the
+  // function returns false.
+  static const int kMaxFixedDigitsBeforePoint = 308;
+  static const int kMaxFixedDigitsAfterPoint = 100;
+
+  // When calling ToExponential with a requested_digits
+  // parameter > kMaxExponentialDigits then the function returns false.
+  static const int kMaxExponentialDigits = 120;
+
+  // When calling ToPrecision with a requested_digits
+  // parameter < kMinPrecisionDigits or requested_digits > kMaxPrecisionDigits
+  // then the function returns false.
+  static const int kMinPrecisionDigits = 1;
+  static const int kMaxPrecisionDigits = 120;
+
+  // The maximal number of digits that are needed to emit a double in base 10.
+  // A higher precision can be achieved by using more digits, but the shortest
+  // accurate representation of any double will never use more digits than
+  // kBase10MaximalLength.
+  // Note that DoubleToAscii null-terminates its input. So the given buffer
+  // should be at least kBase10MaximalLength + 1 characters long.
+  static const int kBase10MaximalLength = 17;
+
+  // The maximal number of digits that are needed to emit a single in base 10.
+  // A higher precision can be achieved by using more digits, but the shortest
+  // accurate representation of any single will never use more digits than
+  // kBase10MaximalLengthSingle.
+  static const int kBase10MaximalLengthSingle = 9;
+
+  // The length of the longest string that 'ToShortest' can produce when the
+  // converter is instantiated with EcmaScript defaults (see
+  // 'EcmaScriptConverter')
+  // This value does not include the trailing '\0' character.
+  // This amount of characters is needed for negative values that hit the
+  // 'decimal_in_shortest_low' limit. For example: "-0.0000033333333333333333"
+  static const int kMaxCharsEcmaScriptShortest = 25;
+
+  enum Flags {
+    NO_FLAGS = 0,
+    EMIT_POSITIVE_EXPONENT_SIGN = 1,
+    EMIT_TRAILING_DECIMAL_POINT = 2,
+    EMIT_TRAILING_ZERO_AFTER_POINT = 4,
+    UNIQUE_ZERO = 8,
+    NO_TRAILING_ZERO = 16,
+    EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL = 32,
+    EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL = 64
+  };
+
+  // Flags should be a bit-or combination of the possible Flags-enum.
+  //  - NO_FLAGS: no special flags.
+  //  - EMIT_POSITIVE_EXPONENT_SIGN: when the number is converted into exponent
+  //    form, emits a '+' for positive exponents. Example: 1.2e+2.
+  //  - EMIT_TRAILING_DECIMAL_POINT: when the input number is an integer and is
+  //    converted into decimal format then a trailing decimal point is appended.
+  //    Example: 2345.0 is converted to "2345.".
+  //  - EMIT_TRAILING_ZERO_AFTER_POINT: in addition to a trailing decimal point
+  //    emits a trailing '0'-character. This flag requires the
+  //    EMIT_TRAILING_DECIMAL_POINT flag.
+  //    Example: 2345.0 is converted to "2345.0".
+  //  - UNIQUE_ZERO: "-0.0" is converted to "0.0".
+  //  - NO_TRAILING_ZERO: Trailing zeros are removed from the fractional portion
+  //    of the result in precision mode. Matches printf's %g.
+  //    When EMIT_TRAILING_ZERO_AFTER_POINT is also given, one trailing zero is
+  //    preserved.
+  //  - EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL: when the input number has
+  //    exactly one significant digit and is converted into exponent form then a
+  //    trailing decimal point is appended to the significand in shortest mode
+  //    or in precision mode with one requested digit.
+  //  - EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL: in addition to a trailing
+  //    decimal point emits a trailing '0'-character. This flag requires the
+  //    EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL flag.
+  //
+  // Infinity symbol and nan_symbol provide the string representation for these
+  // special values. If the string is NULL and the special value is encountered
+  // then the conversion functions return false.
+  //
+  // The exponent_character is used in exponential representations. It is
+  // usually 'e' or 'E'.
+  //
+  // When converting to the shortest representation the converter will
+  // represent input numbers in decimal format if they are in the interval
+  // [10^decimal_in_shortest_low; 10^decimal_in_shortest_high[
+  //    (lower boundary included, greater boundary excluded).
+  // Example: with decimal_in_shortest_low = -6 and
+  //               decimal_in_shortest_high = 21:
+  //   ToShortest(0.000001)  -> "0.000001"
+  //   ToShortest(0.0000001) -> "1e-7"
+  //   ToShortest(111111111111111111111.0)  -> "111111111111111110000"
+  //   ToShortest(100000000000000000000.0)  -> "100000000000000000000"
+  //   ToShortest(1111111111111111111111.0) -> "1.1111111111111111e+21"
+  //
+  // When converting to precision mode the converter may add
+  // max_leading_padding_zeroes before returning the number in exponential
+  // format.
+  // Example with max_leading_padding_zeroes_in_precision_mode = 6.
+  //   ToPrecision(0.0000012345, 2) -> "0.0000012"
+  //   ToPrecision(0.00000012345, 2) -> "1.2e-7"
+  // Similarly the converter may add up to
+  // max_trailing_padding_zeroes_in_precision_mode in precision mode to avoid
+  // returning an exponential representation. A zero added by the
+  // EMIT_TRAILING_ZERO_AFTER_POINT flag is counted for this limit.
+  // Examples for max_trailing_padding_zeroes_in_precision_mode = 1:
+  //   ToPrecision(230.0, 2) -> "230"
+  //   ToPrecision(230.0, 2) -> "230."  with EMIT_TRAILING_DECIMAL_POINT.
+  //   ToPrecision(230.0, 2) -> "2.3e2" with EMIT_TRAILING_ZERO_AFTER_POINT.
+  //
+  // When converting numbers with exactly one significant digit to exponent
+  // form in shortest mode or in precision mode with one requested digit, the
+  // EMIT_TRAILING_DECIMAL_POINT and EMIT_TRAILING_ZERO_AFTER_POINT flags have
+  // no effect. Use the EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL flag to
+  // append a decimal point in this case and the
+  // EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL flag to also append a
+  // '0'-character in this case.
+  // Example with decimal_in_shortest_low = 0:
+  //   ToShortest(0.0009) -> "9e-4"
+  //     with EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL deactivated.
+  //   ToShortest(0.0009) -> "9.e-4"
+  //     with EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL activated.
+  //   ToShortest(0.0009) -> "9.0e-4"
+  //     with EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL activated and
+  //     EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL activated.
+  //
+  // The min_exponent_width is used for exponential representations.
+  // The converter adds leading '0's to the exponent until the exponent
+  // is at least min_exponent_width digits long.
+  // The min_exponent_width is clamped to 5.
+  // As such, the exponent may never have more than 5 digits in total.
+  DoubleToStringConverter(int flags,
+                          const char* infinity_symbol,
+                          const char* nan_symbol,
+                          char exponent_character,
+                          int decimal_in_shortest_low,
+                          int decimal_in_shortest_high,
+                          int max_leading_padding_zeroes_in_precision_mode,
+                          int max_trailing_padding_zeroes_in_precision_mode,
+                          int min_exponent_width = 0)
+      : flags_(flags),
+        infinity_symbol_(infinity_symbol),
+        nan_symbol_(nan_symbol),
+        exponent_character_(exponent_character),
+        decimal_in_shortest_low_(decimal_in_shortest_low),
+        decimal_in_shortest_high_(decimal_in_shortest_high),
+        max_leading_padding_zeroes_in_precision_mode_(
+            max_leading_padding_zeroes_in_precision_mode),
+        max_trailing_padding_zeroes_in_precision_mode_(
+            max_trailing_padding_zeroes_in_precision_mode),
+        min_exponent_width_(min_exponent_width) {
+    // When 'trailing zero after the point' is set, then 'trailing point'
+    // must be set too.
+    DOUBLE_CONVERSION_ASSERT(((flags & EMIT_TRAILING_DECIMAL_POINT) != 0) ||
+        !((flags & EMIT_TRAILING_ZERO_AFTER_POINT) != 0));
+  }
+
+  // Returns a converter following the EcmaScript specification.
+  //
+  // Flags: UNIQUE_ZERO and EMIT_POSITIVE_EXPONENT_SIGN.
+  // Special values: "Infinity" and "NaN".
+  // Lower case 'e' for exponential values.
+  // decimal_in_shortest_low: -6
+  // decimal_in_shortest_high: 21
+  // max_leading_padding_zeroes_in_precision_mode: 6
+  // max_trailing_padding_zeroes_in_precision_mode: 0
+  static MFBT_API const DoubleToStringConverter& EcmaScriptConverter();
+
+  // Computes the shortest string of digits that correctly represent the input
+  // number. Depending on decimal_in_shortest_low and decimal_in_shortest_high
+  // (see constructor) it then either returns a decimal representation, or an
+  // exponential representation.
+  // Example with decimal_in_shortest_low = -6,
+  //              decimal_in_shortest_high = 21,
+  //              EMIT_POSITIVE_EXPONENT_SIGN activated, and
+  //              EMIT_TRAILING_DECIMAL_POINT deactivated:
+  //   ToShortest(0.000001)  -> "0.000001"
+  //   ToShortest(0.0000001) -> "1e-7"
+  //   ToShortest(111111111111111111111.0)  -> "111111111111111110000"
+  //   ToShortest(100000000000000000000.0)  -> "100000000000000000000"
+  //   ToShortest(1111111111111111111111.0) -> "1.1111111111111111e+21"
+  //
+  // Note: the conversion may round the output if the returned string
+  // is accurate enough to uniquely identify the input-number.
+  // For example the most precise representation of the double 9e59 equals
+  // "899999999999999918767229449717619953810131273674690656206848", but
+  // the converter will return the shorter (but still correct) "9e59".
+  //
+  // Returns true if the conversion succeeds. The conversion always succeeds
+  // except when the input value is special and no infinity_symbol or
+  // nan_symbol has been given to the constructor.
+  //
+  // The length of the longest result is the maximum of the length of the
+  // following string representations (each with possible examples):
+  // - NaN and negative infinity: "NaN", "-Infinity", "-inf".
+  // - -10^(decimal_in_shortest_high - 1):
+  //      "-100000000000000000000", "-1000000000000000.0"
+  // - the longest string in range [0; -10^decimal_in_shortest_low]. Generally,
+  //   this string is 3 + kBase10MaximalLength - decimal_in_shortest_low.
+  //   (Sign, '0', decimal point, padding zeroes for decimal_in_shortest_low,
+  //   and the significant digits).
+  //      "-0.0000033333333333333333", "-0.0012345678901234567"
+  // - the longest exponential representation. (A negative number with
+  //   kBase10MaximalLength significant digits).
+  //      "-1.7976931348623157e+308", "-1.7976931348623157E308"
+  // In addition, the buffer must be able to hold the trailing '\0' character.
+  bool ToShortest(double value, StringBuilder* result_builder) const {
+    return ToShortestIeeeNumber(value, result_builder, SHORTEST);
+  }
+
+  // Same as ToShortest, but for single-precision floats.
+  bool ToShortestSingle(float value, StringBuilder* result_builder) const {
+    return ToShortestIeeeNumber(value, result_builder, SHORTEST_SINGLE);
+  }
+
+
+  // Computes a decimal representation with a fixed number of digits after the
+  // decimal point. The last emitted digit is rounded.
+  //
+  // Examples:
+  //   ToFixed(3.12, 1) -> "3.1"
+  //   ToFixed(3.1415, 3) -> "3.142"
+  //   ToFixed(1234.56789, 4) -> "1234.5679"
+  //   ToFixed(1.23, 5) -> "1.23000"
+  //   ToFixed(0.1, 4) -> "0.1000"
+  //   ToFixed(1e30, 2) -> "1000000000000000019884624838656.00"
+  //   ToFixed(0.1, 30) -> "0.100000000000000005551115123126"
+  //   ToFixed(0.1, 17) -> "0.10000000000000001"
+  //
+  // If requested_digits equals 0, then the tail of the result depends on
+  // the EMIT_TRAILING_DECIMAL_POINT and EMIT_TRAILING_ZERO_AFTER_POINT.
+  // Examples, for requested_digits == 0,
+  //   let EMIT_TRAILING_DECIMAL_POINT and EMIT_TRAILING_ZERO_AFTER_POINT be
+  //    - false and false: then 123.45 -> 123
+  //                             0.678 -> 1
+  //    - true and false: then 123.45 -> 123.
+  //                            0.678 -> 1.
+  //    - true and true: then 123.45 -> 123.0
+  //                           0.678 -> 1.0
+  //
+  // Returns true if the conversion succeeds. The conversion always succeeds
+  // except for the following cases:
+  //   - the input value is special and no infinity_symbol or nan_symbol has
+  //     been provided to the constructor,
+  //   - 'value' > 10^kMaxFixedDigitsBeforePoint, or
+  //   - 'requested_digits' > kMaxFixedDigitsAfterPoint.
+  // The last two conditions imply that the result for non-special values never
+  // contains more than
+  //  1 + kMaxFixedDigitsBeforePoint + 1 + kMaxFixedDigitsAfterPoint characters
+  // (one additional character for the sign, and one for the decimal point).
+  // In addition, the buffer must be able to hold the trailing '\0' character.
+  MFBT_API bool ToFixed(double value,
+               int requested_digits,
+               StringBuilder* result_builder) const;
+
+  // Computes a representation in exponential format with requested_digits
+  // after the decimal point. The last emitted digit is rounded.
+  // If requested_digits equals -1, then the shortest exponential representation
+  // is computed.
+  //
+  // Examples with EMIT_POSITIVE_EXPONENT_SIGN deactivated, and
+  //               exponent_character set to 'e'.
+  //   ToExponential(3.12, 1) -> "3.1e0"
+  //   ToExponential(5.0, 3) -> "5.000e0"
+  //   ToExponential(0.001, 2) -> "1.00e-3"
+  //   ToExponential(3.1415, -1) -> "3.1415e0"
+  //   ToExponential(3.1415, 4) -> "3.1415e0"
+  //   ToExponential(3.1415, 3) -> "3.142e0"
+  //   ToExponential(123456789000000, 3) -> "1.235e14"
+  //   ToExponential(1000000000000000019884624838656.0, -1) -> "1e30"
+  //   ToExponential(1000000000000000019884624838656.0, 32) ->
+  //                     "1.00000000000000001988462483865600e30"
+  //   ToExponential(1234, 0) -> "1e3"
+  //
+  // Returns true if the conversion succeeds. The conversion always succeeds
+  // except for the following cases:
+  //   - the input value is special and no infinity_symbol or nan_symbol has
+  //     been provided to the constructor,
+  //   - 'requested_digits' > kMaxExponentialDigits.
+  //
+  // The last condition implies that the result never contains more than
+  // kMaxExponentialDigits + 8 characters (the sign, the digit before the
+  // decimal point, the decimal point, the exponent character, the
+  // exponent's sign, and at most 3 exponent digits).
+  // In addition, the buffer must be able to hold the trailing '\0' character.
+  MFBT_API bool ToExponential(double value,
+                     int requested_digits,
+                     StringBuilder* result_builder) const;
+
+
+  // Computes 'precision' leading digits of the given 'value' and returns them
+  // either in exponential or decimal format, depending on
+  // max_{leading|trailing}_padding_zeroes_in_precision_mode (given to the
+  // constructor).
+  // The last computed digit is rounded.
+  //
+  // Example with max_leading_padding_zeroes_in_precision_mode = 6.
+  //   ToPrecision(0.0000012345, 2) -> "0.0000012"
+  //   ToPrecision(0.00000012345, 2) -> "1.2e-7"
+  // Similarly the converter may add up to
+  // max_trailing_padding_zeroes_in_precision_mode in precision mode to avoid
+  // returning an exponential representation. A zero added by the
+  // EMIT_TRAILING_ZERO_AFTER_POINT flag is counted for this limit.
+  // Examples for max_trailing_padding_zeroes_in_precision_mode = 1:
+  //   ToPrecision(230.0, 2) -> "230"
+  //   ToPrecision(230.0, 2) -> "230."  with EMIT_TRAILING_DECIMAL_POINT.
+  //   ToPrecision(230.0, 2) -> "2.3e2" with EMIT_TRAILING_ZERO_AFTER_POINT.
+  // Examples for max_trailing_padding_zeroes_in_precision_mode = 3, and no
+  //    EMIT_TRAILING_ZERO_AFTER_POINT:
+  //   ToPrecision(123450.0, 6) -> "123450"
+  //   ToPrecision(123450.0, 5) -> "123450"
+  //   ToPrecision(123450.0, 4) -> "123500"
+  //   ToPrecision(123450.0, 3) -> "123000"
+  //   ToPrecision(123450.0, 2) -> "1.2e5"
+  //
+  // Returns true if the conversion succeeds. The conversion always succeeds
+  // except for the following cases:
+  //   - the input value is special and no infinity_symbol or nan_symbol has
+  //     been provided to the constructor,
+  //   - precision < kMinPericisionDigits
+  //   - precision > kMaxPrecisionDigits
+  //
+  // The last condition implies that the result never contains more than
+  // kMaxPrecisionDigits + 7 characters (the sign, the decimal point, the
+  // exponent character, the exponent's sign, and at most 3 exponent digits).
+  // In addition, the buffer must be able to hold the trailing '\0' character.
+  MFBT_API bool ToPrecision(double value,
+                   int precision,
+                   StringBuilder* result_builder) const;
+
+  enum DtoaMode {
+    // Produce the shortest correct representation.
+    // For example the output of 0.299999999999999988897 is (the less accurate
+    // but correct) 0.3.
+    SHORTEST,
+    // Same as SHORTEST, but for single-precision floats.
+    SHORTEST_SINGLE,
+    // Produce a fixed number of digits after the decimal point.
+    // For instance fixed(0.1, 4) becomes 0.1000
+    // If the input number is big, the output will be big.
+    FIXED,
+    // Fixed number of digits (independent of the decimal point).
+    PRECISION
+  };
+
+  // Converts the given double 'v' to digit characters. 'v' must not be NaN,
+  // +Infinity, or -Infinity. In SHORTEST_SINGLE-mode this restriction also
+  // applies to 'v' after it has been casted to a single-precision float. That
+  // is, in this mode static_cast<float>(v) must not be NaN, +Infinity or
+  // -Infinity.
+  //
+  // The result should be interpreted as buffer * 10^(point-length).
+  //
+  // The digits are written to the buffer in the platform's charset, which is
+  // often UTF-8 (with ASCII-range digits) but may be another charset, such
+  // as EBCDIC.
+  //
+  // The output depends on the given mode:
+  //  - SHORTEST: produce the least amount of digits for which the internal
+  //   identity requirement is still satisfied. If the digits are printed
+  //   (together with the correct exponent) then reading this number will give
+  //   'v' again. The buffer will choose the representation that is closest to
+  //   'v'. If there are two at the same distance, than the one farther away
+  //   from 0 is chosen (halfway cases - ending with 5 - are rounded up).
+  //   In this mode the 'requested_digits' parameter is ignored.
+  //  - SHORTEST_SINGLE: same as SHORTEST but with single-precision.
+  //  - FIXED: produces digits necessary to print a given number with
+  //   'requested_digits' digits after the decimal point. The produced digits
+  //   might be too short in which case the caller has to fill the remainder
+  //   with '0's.
+  //   Example: toFixed(0.001, 5) is allowed to return buffer="1", point=-2.
+  //   Halfway cases are rounded towards +/-Infinity (away from 0). The call
+  //   toFixed(0.15, 2) thus returns buffer="2", point=0.
+  //   The returned buffer may contain digits that would be truncated from the
+  //   shortest representation of the input.
+  //  - PRECISION: produces 'requested_digits' where the first digit is not '0'.
+  //   Even though the length of produced digits usually equals
+  //   'requested_digits', the function is allowed to return fewer digits, in
+  //   which case the caller has to fill the missing digits with '0's.
+  //   Halfway cases are again rounded away from 0.
+  // DoubleToAscii expects the given buffer to be big enough to hold all
+  // digits and a terminating null-character. In SHORTEST-mode it expects a
+  // buffer of at least kBase10MaximalLength + 1. In all other modes the
+  // requested_digits parameter and the padding-zeroes limit the size of the
+  // output. Don't forget the decimal point, the exponent character and the
+  // terminating null-character when computing the maximal output size.
+  // The given length is only used in debug mode to ensure the buffer is big
+  // enough.
+  static MFBT_API void DoubleToAscii(double v,
+                            DtoaMode mode,
+                            int requested_digits,
+                            char* buffer,
+                            int buffer_length,
+                            bool* sign,
+                            int* length,
+                            int* point);
+
+ private:
+  // Implementation for ToShortest and ToShortestSingle.
+  MFBT_API bool ToShortestIeeeNumber(double value,
+                            StringBuilder* result_builder,
+                            DtoaMode mode) const;
+
+  // If the value is a special value (NaN or Infinity) constructs the
+  // corresponding string using the configured infinity/nan-symbol.
+  // If either of them is NULL or the value is not special then the
+  // function returns false.
+  MFBT_API bool HandleSpecialValues(double value, StringBuilder* result_builder) const;
+  // Constructs an exponential representation (i.e. 1.234e56).
+  // The given exponent assumes a decimal point after the first decimal digit.
+  MFBT_API void CreateExponentialRepresentation(const char* decimal_digits,
+                                       int length,
+                                       int exponent,
+                                       StringBuilder* result_builder) const;
+  // Creates a decimal representation (i.e 1234.5678).
+  MFBT_API void CreateDecimalRepresentation(const char* decimal_digits,
+                                   int length,
+                                   int decimal_point,
+                                   int digits_after_point,
+                                   StringBuilder* result_builder) const;
+
+  const int flags_;
+  const char* const infinity_symbol_;
+  const char* const nan_symbol_;
+  const char exponent_character_;
+  const int decimal_in_shortest_low_;
+  const int decimal_in_shortest_high_;
+  const int max_leading_padding_zeroes_in_precision_mode_;
+  const int max_trailing_padding_zeroes_in_precision_mode_;
+  const int min_exponent_width_;
+
+  DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS(DoubleToStringConverter);
+};
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_DOUBLE_TO_STRING_H_
diff --git a/mfbt/double-conversion/double-conversion/fast-dtoa.cc b/mfbt/double-conversion/double-conversion/fast-dtoa.cc
new file mode 100644
index 0000000000..d7a23984df
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/fast-dtoa.cc
@@ -0,0 +1,665 @@
+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "fast-dtoa.h"
+
+#include "cached-powers.h"
+#include "diy-fp.h"
+#include "ieee.h"
+
+namespace double_conversion {
+
+// The minimal and maximal target exponent define the range of w's binary
+// exponent, where 'w' is the result of multiplying the input by a cached power
+// of ten.
+//
+// A different range might be chosen on a different platform, to optimize digit
+// generation, but a smaller range requires more powers of ten to be cached.
+static const int kMinimalTargetExponent = -60;
+static const int kMaximalTargetExponent = -32;
+
+
+// Adjusts the last digit of the generated number, and screens out generated
+// solutions that may be inaccurate. A solution may be inaccurate if it is
+// outside the safe interval, or if we cannot prove that it is closer to the
+// input than a neighboring representation of the same length.
+//
+// Input: * buffer containing the digits of too_high / 10^kappa
+//        * the buffer's length
+//        * distance_too_high_w == (too_high - w).f() * unit
+//        * unsafe_interval == (too_high - too_low).f() * unit
+//        * rest = (too_high - buffer * 10^kappa).f() * unit
+//        * ten_kappa = 10^kappa * unit
+//        * unit = the common multiplier
+// Output: returns true if the buffer is guaranteed to contain the closest
+//    representable number to the input.
+//  Modifies the generated digits in the buffer to approach (round towards) w.
+static bool RoundWeed(Vector<char> buffer,
+                      int length,
+                      uint64_t distance_too_high_w,
+                      uint64_t unsafe_interval,
+                      uint64_t rest,
+                      uint64_t ten_kappa,
+                      uint64_t unit) {
+  uint64_t small_distance = distance_too_high_w - unit;
+  uint64_t big_distance = distance_too_high_w + unit;
+  // Let w_low  = too_high - big_distance, and
+  //     w_high = too_high - small_distance.
+  // Note: w_low < w < w_high
+  //
+  // The real w (* unit) must lie somewhere inside the interval
+  // ]w_low; w_high[ (often written as "(w_low; w_high)")
+
+  // Basically the buffer currently contains a number in the unsafe interval
+  // ]too_low; too_high[ with too_low < w < too_high
+  //
+  //  too_high - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  //                     ^v 1 unit            ^      ^                 ^      ^
+  //  boundary_high ---------------------     .      .                 .      .
+  //                     ^v 1 unit            .      .                 .      .
+  //   - - - - - - - - - - - - - - - - - - -  +  - - + - - - - - -     .      .
+  //                                          .      .         ^       .      .
+  //                                          .  big_distance  .       .      .
+  //                                          .      .         .       .    rest
+  //                              small_distance     .         .       .      .
+  //                                          v      .         .       .      .
+  //  w_high - - - - - - - - - - - - - - - - - -     .         .       .      .
+  //                     ^v 1 unit                   .         .       .      .
+  //  w ----------------------------------------     .         .       .      .
+  //                     ^v 1 unit                   v         .       .      .
+  //  w_low  - - - - - - - - - - - - - - - - - - - - -         .       .      .
+  //                                                           .       .      v
+  //  buffer --------------------------------------------------+-------+--------
+  //                                                           .       .
+  //                                                  safe_interval    .
+  //                                                           v       .
+  //   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -     .
+  //                     ^v 1 unit                                     .
+  //  boundary_low -------------------------                     unsafe_interval
+  //                     ^v 1 unit                                     v
+  //  too_low  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  //
+  //
+  // Note that the value of buffer could lie anywhere inside the range too_low
+  // to too_high.
+  //
+  // boundary_low, boundary_high and w are approximations of the real boundaries
+  // and v (the input number). They are guaranteed to be precise up to one unit.
+  // In fact the error is guaranteed to be strictly less than one unit.
+  //
+  // Anything that lies outside the unsafe interval is guaranteed not to round
+  // to v when read again.
+  // Anything that lies inside the safe interval is guaranteed to round to v
+  // when read again.
+  // If the number inside the buffer lies inside the unsafe interval but not
+  // inside the safe interval then we simply do not know and bail out (returning
+  // false).
+  //
+  // Similarly we have to take into account the imprecision of 'w' when finding
+  // the closest representation of 'w'. If we have two potential
+  // representations, and one is closer to both w_low and w_high, then we know
+  // it is closer to the actual value v.
+  //
+  // By generating the digits of too_high we got the largest (closest to
+  // too_high) buffer that is still in the unsafe interval. In the case where
+  // w_high < buffer < too_high we try to decrement the buffer.
+  // This way the buffer approaches (rounds towards) w.
+  // There are 3 conditions that stop the decrementation process:
+  //   1) the buffer is already below w_high
+  //   2) decrementing the buffer would make it leave the unsafe interval
+  //   3) decrementing the buffer would yield a number below w_high and farther
+  //      away than the current number. In other words:
+  //              (buffer{-1} < w_high) && w_high - buffer{-1} > buffer - w_high
+  // Instead of using the buffer directly we use its distance to too_high.
+  // Conceptually rest ~= too_high - buffer
+  // We need to do the following tests in this order to avoid over- and
+  // underflows.
+  DOUBLE_CONVERSION_ASSERT(rest <= unsafe_interval);
+  while (rest < small_distance &&  // Negated condition 1
+         unsafe_interval - rest >= ten_kappa &&  // Negated condition 2
+         (rest + ten_kappa < small_distance ||  // buffer{-1} > w_high
+          small_distance - rest >= rest + ten_kappa - small_distance)) {
+    buffer[length - 1]--;
+    rest += ten_kappa;
+  }
+
+  // We have approached w+ as much as possible. We now test if approaching w-
+  // would require changing the buffer. If yes, then we have two possible
+  // representations close to w, but we cannot decide which one is closer.
+  if (rest < big_distance &&
+      unsafe_interval - rest >= ten_kappa &&
+      (rest + ten_kappa < big_distance ||
+       big_distance - rest > rest + ten_kappa - big_distance)) {
+    return false;
+  }
+
+  // Weeding test.
+  //   The safe interval is [too_low + 2 ulp; too_high - 2 ulp]
+  //   Since too_low = too_high - unsafe_interval this is equivalent to
+  //      [too_high - unsafe_interval + 4 ulp; too_high - 2 ulp]
+  //   Conceptually we have: rest ~= too_high - buffer
+  return (2 * unit <= rest) && (rest <= unsafe_interval - 4 * unit);
+}
+
+
+// Rounds the buffer upwards if the result is closer to v by possibly adding
+// 1 to the buffer. If the precision of the calculation is not sufficient to
+// round correctly, return false.
+// The rounding might shift the whole buffer in which case the kappa is
+// adjusted. For example "99", kappa = 3 might become "10", kappa = 4.
+//
+// If 2*rest > ten_kappa then the buffer needs to be round up.
+// rest can have an error of +/- 1 unit. This function accounts for the
+// imprecision and returns false, if the rounding direction cannot be
+// unambiguously determined.
+//
+// Precondition: rest < ten_kappa.
+static bool RoundWeedCounted(Vector<char> buffer,
+                             int length,
+                             uint64_t rest,
+                             uint64_t ten_kappa,
+                             uint64_t unit,
+                             int* kappa) {
+  DOUBLE_CONVERSION_ASSERT(rest < ten_kappa);
+  // The following tests are done in a specific order to avoid overflows. They
+  // will work correctly with any uint64 values of rest < ten_kappa and unit.
+  //
+  // If the unit is too big, then we don't know which way to round. For example
+  // a unit of 50 means that the real number lies within rest +/- 50. If
+  // 10^kappa == 40 then there is no way to tell which way to round.
+  if (unit >= ten_kappa) return false;
+  // Even if unit is just half the size of 10^kappa we are already completely
+  // lost. (And after the previous test we know that the expression will not
+  // over/underflow.)
+  if (ten_kappa - unit <= unit) return false;
+  // If 2 * (rest + unit) <= 10^kappa we can safely round down.
+  if ((ten_kappa - rest > rest) && (ten_kappa - 2 * rest >= 2 * unit)) {
+    return true;
+  }
+  // If 2 * (rest - unit) >= 10^kappa, then we can safely round up.
+  if ((rest > unit) && (ten_kappa - (rest - unit) <= (rest - unit))) {
+    // Increment the last digit recursively until we find a non '9' digit.
+    buffer[length - 1]++;
+    for (int i = length - 1; i > 0; --i) {
+      if (buffer[i] != '0' + 10) break;
+      buffer[i] = '0';
+      buffer[i - 1]++;
+    }
+    // If the first digit is now '0'+ 10 we had a buffer with all '9's. With the
+    // exception of the first digit all digits are now '0'. Simply switch the
+    // first digit to '1' and adjust the kappa. Example: "99" becomes "10" and
+    // the power (the kappa) is increased.
+    if (buffer[0] == '0' + 10) {
+      buffer[0] = '1';
+      (*kappa) += 1;
+    }
+    return true;
+  }
+  return false;
+}
+
+// Returns the biggest power of ten that is less than or equal to the given
+// number. We furthermore receive the maximum number of bits 'number' has.
+//
+// Returns power == 10^(exponent_plus_one-1) such that
+//    power <= number < power * 10.
+// If number_bits == 0 then 0^(0-1) is returned.
+// The number of bits must be <= 32.
+// Precondition: number < (1 << (number_bits + 1)).
+
+// Inspired by the method for finding an integer log base 10 from here:
+// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog10
+static unsigned int const kSmallPowersOfTen[] =
+    {0, 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000,
+     1000000000};
+
+static void BiggestPowerTen(uint32_t number,
+                            int number_bits,
+                            uint32_t* power,
+                            int* exponent_plus_one) {
+  DOUBLE_CONVERSION_ASSERT(number < (1u << (number_bits + 1)));
+  // 1233/4096 is approximately 1/lg(10).
+  int exponent_plus_one_guess = ((number_bits + 1) * 1233 >> 12);
+  // We increment to skip over the first entry in the kPowersOf10 table.
+  // Note: kPowersOf10[i] == 10^(i-1).
+  exponent_plus_one_guess++;
+  // We don't have any guarantees that 2^number_bits <= number.
+  if (number < kSmallPowersOfTen[exponent_plus_one_guess]) {
+    exponent_plus_one_guess--;
+  }
+  *power = kSmallPowersOfTen[exponent_plus_one_guess];
+  *exponent_plus_one = exponent_plus_one_guess;
+}
+
+// Generates the digits of input number w.
+// w is a floating-point number (DiyFp), consisting of a significand and an
+// exponent. Its exponent is bounded by kMinimalTargetExponent and
+// kMaximalTargetExponent.
+//       Hence -60 <= w.e() <= -32.
+//
+// Returns false if it fails, in which case the generated digits in the buffer
+// should not be used.
+// Preconditions:
+//  * low, w and high are correct up to 1 ulp (unit in the last place). That
+//    is, their error must be less than a unit of their last digits.
+//  * low.e() == w.e() == high.e()
+//  * low < w < high, and taking into account their error: low~ <= high~
+//  * kMinimalTargetExponent <= w.e() <= kMaximalTargetExponent
+// Postconditions: returns false if procedure fails.
+//   otherwise:
+//     * buffer is not null-terminated, but len contains the number of digits.
+//     * buffer contains the shortest possible decimal digit-sequence
+//       such that LOW < buffer * 10^kappa < HIGH, where LOW and HIGH are the
+//       correct values of low and high (without their error).
+//     * if more than one decimal representation gives the minimal number of
+//       decimal digits then the one closest to W (where W is the correct value
+//       of w) is chosen.
+// Remark: this procedure takes into account the imprecision of its input
+//   numbers. If the precision is not enough to guarantee all the postconditions
+//   then false is returned. This usually happens rarely (~0.5%).
+//
+// Say, for the sake of example, that
+//   w.e() == -48, and w.f() == 0x1234567890abcdef
+// w's value can be computed by w.f() * 2^w.e()
+// We can obtain w's integral digits by simply shifting w.f() by -w.e().
+//  -> w's integral part is 0x1234
+//  w's fractional part is therefore 0x567890abcdef.
+// Printing w's integral part is easy (simply print 0x1234 in decimal).
+// In order to print its fraction we repeatedly multiply the fraction by 10 and
+// get each digit. Example the first digit after the point would be computed by
+//   (0x567890abcdef * 10) >> 48. -> 3
+// The whole thing becomes slightly more complicated because we want to stop
+// once we have enough digits. That is, once the digits inside the buffer
+// represent 'w' we can stop. Everything inside the interval low - high
+// represents w. However we have to pay attention to low, high and w's
+// imprecision.
+static bool DigitGen(DiyFp low,
+                     DiyFp w,
+                     DiyFp high,
+                     Vector<char> buffer,
+                     int* length,
+                     int* kappa) {
+  DOUBLE_CONVERSION_ASSERT(low.e() == w.e() && w.e() == high.e());
+  DOUBLE_CONVERSION_ASSERT(low.f() + 1 <= high.f() - 1);
+  DOUBLE_CONVERSION_ASSERT(kMinimalTargetExponent <= w.e() && w.e() <= kMaximalTargetExponent);
+  // low, w and high are imprecise, but by less than one ulp (unit in the last
+  // place).
+  // If we remove (resp. add) 1 ulp from low (resp. high) we are certain that
+  // the new numbers are outside of the interval we want the final
+  // representation to lie in.
+  // Inversely adding (resp. removing) 1 ulp from low (resp. high) would yield
+  // numbers that are certain to lie in the interval. We will use this fact
+  // later on.
+  // We will now start by generating the digits within the uncertain
+  // interval. Later we will weed out representations that lie outside the safe
+  // interval and thus _might_ lie outside the correct interval.
+  uint64_t unit = 1;
+  DiyFp too_low = DiyFp(low.f() - unit, low.e());
+  DiyFp too_high = DiyFp(high.f() + unit, high.e());
+  // too_low and too_high are guaranteed to lie outside the interval we want the
+  // generated number in.
+  DiyFp unsafe_interval = DiyFp::Minus(too_high, too_low);
+  // We now cut the input number into two parts: the integral digits and the
+  // fractionals. We will not write any decimal separator though, but adapt
+  // kappa instead.
+  // Reminder: we are currently computing the digits (stored inside the buffer)
+  // such that:   too_low < buffer * 10^kappa < too_high
+  // We use too_high for the digit_generation and stop as soon as possible.
+  // If we stop early we effectively round down.
+  DiyFp one = DiyFp(static_cast<uint64_t>(1) << -w.e(), w.e());
+  // Division by one is a shift.
+  uint32_t integrals = static_cast<uint32_t>(too_high.f() >> -one.e());
+  // Modulo by one is an and.
+  uint64_t fractionals = too_high.f() & (one.f() - 1);
+  uint32_t divisor;
+  int divisor_exponent_plus_one;
+  BiggestPowerTen(integrals, DiyFp::kSignificandSize - (-one.e()),
+                  &divisor, &divisor_exponent_plus_one);
+  *kappa = divisor_exponent_plus_one;
+  *length = 0;
+  // Loop invariant: buffer = too_high / 10^kappa  (integer division)
+  // The invariant holds for the first iteration: kappa has been initialized
+  // with the divisor exponent + 1. And the divisor is the biggest power of ten
+  // that is smaller than integrals.
+  while (*kappa > 0) {
+    int digit = integrals / divisor;
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);
+    buffer[*length] = static_cast<char>('0' + digit);
+    (*length)++;
+    integrals %= divisor;
+    (*kappa)--;
+    // Note that kappa now equals the exponent of the divisor and that the
+    // invariant thus holds again.
+    uint64_t rest =
+        (static_cast<uint64_t>(integrals) << -one.e()) + fractionals;
+    // Invariant: too_high = buffer * 10^kappa + DiyFp(rest, one.e())
+    // Reminder: unsafe_interval.e() == one.e()
+    if (rest < unsafe_interval.f()) {
+      // Rounding down (by not emitting the remaining digits) yields a number
+      // that lies within the unsafe interval.
+      return RoundWeed(buffer, *length, DiyFp::Minus(too_high, w).f(),
+                       unsafe_interval.f(), rest,
+                       static_cast<uint64_t>(divisor) << -one.e(), unit);
+    }
+    divisor /= 10;
+  }
+
+  // The integrals have been generated. We are at the point of the decimal
+  // separator. In the following loop we simply multiply the remaining digits by
+  // 10 and divide by one. We just need to pay attention to multiply associated
+  // data (like the interval or 'unit'), too.
+  // Note that the multiplication by 10 does not overflow, because w.e >= -60
+  // and thus one.e >= -60.
+  DOUBLE_CONVERSION_ASSERT(one.e() >= -60);
+  DOUBLE_CONVERSION_ASSERT(fractionals < one.f());
+  DOUBLE_CONVERSION_ASSERT(DOUBLE_CONVERSION_UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF) / 10 >= one.f());
+  for (;;) {
+    fractionals *= 10;
+    unit *= 10;
+    unsafe_interval.set_f(unsafe_interval.f() * 10);
+    // Integer division by one.
+    int digit = static_cast<int>(fractionals >> -one.e());
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);
+    buffer[*length] = static_cast<char>('0' + digit);
+    (*length)++;
+    fractionals &= one.f() - 1;  // Modulo by one.
+    (*kappa)--;
+    if (fractionals < unsafe_interval.f()) {
+      return RoundWeed(buffer, *length, DiyFp::Minus(too_high, w).f() * unit,
+                       unsafe_interval.f(), fractionals, one.f(), unit);
+    }
+  }
+}
+
+
+
+// Generates (at most) requested_digits digits of input number w.
+// w is a floating-point number (DiyFp), consisting of a significand and an
+// exponent. Its exponent is bounded by kMinimalTargetExponent and
+// kMaximalTargetExponent.
+//       Hence -60 <= w.e() <= -32.
+//
+// Returns false if it fails, in which case the generated digits in the buffer
+// should not be used.
+// Preconditions:
+//  * w is correct up to 1 ulp (unit in the last place). That
+//    is, its error must be strictly less than a unit of its last digit.
+//  * kMinimalTargetExponent <= w.e() <= kMaximalTargetExponent
+//
+// Postconditions: returns false if procedure fails.
+//   otherwise:
+//     * buffer is not null-terminated, but length contains the number of
+//       digits.
+//     * the representation in buffer is the most precise representation of
+//       requested_digits digits.
+//     * buffer contains at most requested_digits digits of w. If there are less
+//       than requested_digits digits then some trailing '0's have been removed.
+//     * kappa is such that
+//            w = buffer * 10^kappa + eps with |eps| < 10^kappa / 2.
+//
+// Remark: This procedure takes into account the imprecision of its input
+//   numbers. If the precision is not enough to guarantee all the postconditions
+//   then false is returned. This usually happens rarely, but the failure-rate
+//   increases with higher requested_digits.
+static bool DigitGenCounted(DiyFp w,
+                            int requested_digits,
+                            Vector<char> buffer,
+                            int* length,
+                            int* kappa) {
+  DOUBLE_CONVERSION_ASSERT(kMinimalTargetExponent <= w.e() && w.e() <= kMaximalTargetExponent);
+  DOUBLE_CONVERSION_ASSERT(kMinimalTargetExponent >= -60);
+  DOUBLE_CONVERSION_ASSERT(kMaximalTargetExponent <= -32);
+  // w is assumed to have an error less than 1 unit. Whenever w is scaled we
+  // also scale its error.
+  uint64_t w_error = 1;
+  // We cut the input number into two parts: the integral digits and the
+  // fractional digits. We don't emit any decimal separator, but adapt kappa
+  // instead. Example: instead of writing "1.2" we put "12" into the buffer and
+  // increase kappa by 1.
+  DiyFp one = DiyFp(static_cast<uint64_t>(1) << -w.e(), w.e());
+  // Division by one is a shift.
+  uint32_t integrals = static_cast<uint32_t>(w.f() >> -one.e());
+  // Modulo by one is an and.
+  uint64_t fractionals = w.f() & (one.f() - 1);
+  uint32_t divisor;
+  int divisor_exponent_plus_one;
+  BiggestPowerTen(integrals, DiyFp::kSignificandSize - (-one.e()),
+                  &divisor, &divisor_exponent_plus_one);
+  *kappa = divisor_exponent_plus_one;
+  *length = 0;
+
+  // Loop invariant: buffer = w / 10^kappa  (integer division)
+  // The invariant holds for the first iteration: kappa has been initialized
+  // with the divisor exponent + 1. And the divisor is the biggest power of ten
+  // that is smaller than 'integrals'.
+  while (*kappa > 0) {
+    int digit = integrals / divisor;
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);
+    buffer[*length] = static_cast<char>('0' + digit);
+    (*length)++;
+    requested_digits--;
+    integrals %= divisor;
+    (*kappa)--;
+    // Note that kappa now equals the exponent of the divisor and that the
+    // invariant thus holds again.
+    if (requested_digits == 0) break;
+    divisor /= 10;
+  }
+
+  if (requested_digits == 0) {
+    uint64_t rest =
+        (static_cast<uint64_t>(integrals) << -one.e()) + fractionals;
+    return RoundWeedCounted(buffer, *length, rest,
+                            static_cast<uint64_t>(divisor) << -one.e(), w_error,
+                            kappa);
+  }
+
+  // The integrals have been generated. We are at the point of the decimal
+  // separator. In the following loop we simply multiply the remaining digits by
+  // 10 and divide by one. We just need to pay attention to multiply associated
+  // data (the 'unit'), too.
+  // Note that the multiplication by 10 does not overflow, because w.e >= -60
+  // and thus one.e >= -60.
+  DOUBLE_CONVERSION_ASSERT(one.e() >= -60);
+  DOUBLE_CONVERSION_ASSERT(fractionals < one.f());
+  DOUBLE_CONVERSION_ASSERT(DOUBLE_CONVERSION_UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF) / 10 >= one.f());
+  while (requested_digits > 0 && fractionals > w_error) {
+    fractionals *= 10;
+    w_error *= 10;
+    // Integer division by one.
+    int digit = static_cast<int>(fractionals >> -one.e());
+    DOUBLE_CONVERSION_ASSERT(digit <= 9);
+    buffer[*length] = static_cast<char>('0' + digit);
+    (*length)++;
+    requested_digits--;
+    fractionals &= one.f() - 1;  // Modulo by one.
+    (*kappa)--;
+  }
+  if (requested_digits != 0) return false;
+  return RoundWeedCounted(buffer, *length, fractionals, one.f(), w_error,
+                          kappa);
+}
+
+
+// Provides a decimal representation of v.
+// Returns true if it succeeds, otherwise the result cannot be trusted.
+// There will be *length digits inside the buffer (not null-terminated).
+// If the function returns true then
+//        v == (double) (buffer * 10^decimal_exponent).
+// The digits in the buffer are the shortest representation possible: no
+// 0.09999999999999999 instead of 0.1. The shorter representation will even be
+// chosen even if the longer one would be closer to v.
+// The last digit will be closest to the actual v. That is, even if several
+// digits might correctly yield 'v' when read again, the closest will be
+// computed.
+static bool Grisu3(double v,
+                   FastDtoaMode mode,
+                   Vector<char> buffer,
+                   int* length,
+                   int* decimal_exponent) {
+  DiyFp w = Double(v).AsNormalizedDiyFp();
+  // boundary_minus and boundary_plus are the boundaries between v and its
+  // closest floating-point neighbors. Any number strictly between
+  // boundary_minus and boundary_plus will round to v when convert to a double.
+  // Grisu3 will never output representations that lie exactly on a boundary.
+  DiyFp boundary_minus, boundary_plus;
+  if (mode == FAST_DTOA_SHORTEST) {
+    Double(v).NormalizedBoundaries(&boundary_minus, &boundary_plus);
+  } else {
+    DOUBLE_CONVERSION_ASSERT(mode == FAST_DTOA_SHORTEST_SINGLE);
+    float single_v = static_cast<float>(v);
+    Single(single_v).NormalizedBoundaries(&boundary_minus, &boundary_plus);
+  }
+  DOUBLE_CONVERSION_ASSERT(boundary_plus.e() == w.e());
+  DiyFp ten_mk;  // Cached power of ten: 10^-k
+  int mk;        // -k
+  int ten_mk_minimal_binary_exponent =
+     kMinimalTargetExponent - (w.e() + DiyFp::kSignificandSize);
+  int ten_mk_maximal_binary_exponent =
+     kMaximalTargetExponent - (w.e() + DiyFp::kSignificandSize);
+  PowersOfTenCache::GetCachedPowerForBinaryExponentRange(
+      ten_mk_minimal_binary_exponent,
+      ten_mk_maximal_binary_exponent,
+      &ten_mk, &mk);
+  DOUBLE_CONVERSION_ASSERT((kMinimalTargetExponent <= w.e() + ten_mk.e() +
+          DiyFp::kSignificandSize) &&
+         (kMaximalTargetExponent >= w.e() + ten_mk.e() +
+          DiyFp::kSignificandSize));
+  // Note that ten_mk is only an approximation of 10^-k. A DiyFp only contains a
+  // 64 bit significand and ten_mk is thus only precise up to 64 bits.
+
+  // The DiyFp::Times procedure rounds its result, and ten_mk is approximated
+  // too. The variable scaled_w (as well as scaled_boundary_minus/plus) are now
+  // off by a small amount.
+  // In fact: scaled_w - w*10^k < 1ulp (unit in the last place) of scaled_w.
+  // In other words: let f = scaled_w.f() and e = scaled_w.e(), then
+  //           (f-1) * 2^e < w*10^k < (f+1) * 2^e
+  DiyFp scaled_w = DiyFp::Times(w, ten_mk);
+  DOUBLE_CONVERSION_ASSERT(scaled_w.e() ==
+         boundary_plus.e() + ten_mk.e() + DiyFp::kSignificandSize);
+  // In theory it would be possible to avoid some recomputations by computing
+  // the difference between w and boundary_minus/plus (a power of 2) and to
+  // compute scaled_boundary_minus/plus by subtracting/adding from
+  // scaled_w. However the code becomes much less readable and the speed
+  // enhancements are not terrific.
+  DiyFp scaled_boundary_minus = DiyFp::Times(boundary_minus, ten_mk);
+  DiyFp scaled_boundary_plus  = DiyFp::Times(boundary_plus,  ten_mk);
+
+  // DigitGen will generate the digits of scaled_w. Therefore we have
+  // v == (double) (scaled_w * 10^-mk).
+  // Set decimal_exponent == -mk and pass it to DigitGen. If scaled_w is not an
+  // integer than it will be updated. For instance if scaled_w == 1.23 then
+  // the buffer will be filled with "123" and the decimal_exponent will be
+  // decreased by 2.
+  int kappa;
+  bool result = DigitGen(scaled_boundary_minus, scaled_w, scaled_boundary_plus,
+                         buffer, length, &kappa);
+  *decimal_exponent = -mk + kappa;
+  return result;
+}
+
+
+// The "counted" version of grisu3 (see above) only generates requested_digits
+// number of digits. This version does not generate the shortest representation,
+// and with enough requested digits 0.1 will at some point print as 0.9999999...
+// Grisu3 is too imprecise for real halfway cases (1.5 will not work) and
+// therefore the rounding strategy for halfway cases is irrelevant.
+static bool Grisu3Counted(double v,
+                          int requested_digits,
+                          Vector<char> buffer,
+                          int* length,
+                          int* decimal_exponent) {
+  DiyFp w = Double(v).AsNormalizedDiyFp();
+  DiyFp ten_mk;  // Cached power of ten: 10^-k
+  int mk;        // -k
+  int ten_mk_minimal_binary_exponent =
+     kMinimalTargetExponent - (w.e() + DiyFp::kSignificandSize);
+  int ten_mk_maximal_binary_exponent =
+     kMaximalTargetExponent - (w.e() + DiyFp::kSignificandSize);
+  PowersOfTenCache::GetCachedPowerForBinaryExponentRange(
+      ten_mk_minimal_binary_exponent,
+      ten_mk_maximal_binary_exponent,
+      &ten_mk, &mk);
+  DOUBLE_CONVERSION_ASSERT((kMinimalTargetExponent <= w.e() + ten_mk.e() +
+          DiyFp::kSignificandSize) &&
+         (kMaximalTargetExponent >= w.e() + ten_mk.e() +
+          DiyFp::kSignificandSize));
+  // Note that ten_mk is only an approximation of 10^-k. A DiyFp only contains a
+  // 64 bit significand and ten_mk is thus only precise up to 64 bits.
+
+  // The DiyFp::Times procedure rounds its result, and ten_mk is approximated
+  // too. The variable scaled_w (as well as scaled_boundary_minus/plus) are now
+  // off by a small amount.
+  // In fact: scaled_w - w*10^k < 1ulp (unit in the last place) of scaled_w.
+  // In other words: let f = scaled_w.f() and e = scaled_w.e(), then
+  //           (f-1) * 2^e < w*10^k < (f+1) * 2^e
+  DiyFp scaled_w = DiyFp::Times(w, ten_mk);
+
+  // We now have (double) (scaled_w * 10^-mk).
+  // DigitGen will generate the first requested_digits digits of scaled_w and
+  // return together with a kappa such that scaled_w ~= buffer * 10^kappa. (It
+  // will not always be exactly the same since DigitGenCounted only produces a
+  // limited number of digits.)
+  int kappa;
+  bool result = DigitGenCounted(scaled_w, requested_digits,
+                                buffer, length, &kappa);
+  *decimal_exponent = -mk + kappa;
+  return result;
+}
+
+
+bool FastDtoa(double v,
+              FastDtoaMode mode,
+              int requested_digits,
+              Vector<char> buffer,
+              int* length,
+              int* decimal_point) {
+  DOUBLE_CONVERSION_ASSERT(v > 0);
+  DOUBLE_CONVERSION_ASSERT(!Double(v).IsSpecial());
+
+  bool result = false;
+  int decimal_exponent = 0;
+  switch (mode) {
+    case FAST_DTOA_SHORTEST:
+    case FAST_DTOA_SHORTEST_SINGLE:
+      result = Grisu3(v, mode, buffer, length, &decimal_exponent);
+      break;
+    case FAST_DTOA_PRECISION:
+      result = Grisu3Counted(v, requested_digits,
+                             buffer, length, &decimal_exponent);
+      break;
+    default:
+      DOUBLE_CONVERSION_UNREACHABLE();
+  }
+  if (result) {
+    *decimal_point = *length + decimal_exponent;
+    buffer[*length] = '\0';
+  }
+  return result;
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/fast-dtoa.h b/mfbt/double-conversion/double-conversion/fast-dtoa.h
new file mode 100644
index 0000000000..5f1e8eee5e
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/fast-dtoa.h
@@ -0,0 +1,88 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_FAST_DTOA_H_
+#define DOUBLE_CONVERSION_FAST_DTOA_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+enum FastDtoaMode {
+  // Computes the shortest representation of the given input. The returned
+  // result will be the most accurate number of this length. Longer
+  // representations might be more accurate.
+  FAST_DTOA_SHORTEST,
+  // Same as FAST_DTOA_SHORTEST but for single-precision floats.
+  FAST_DTOA_SHORTEST_SINGLE,
+  // Computes a representation where the precision (number of digits) is
+  // given as input. The precision is independent of the decimal point.
+  FAST_DTOA_PRECISION
+};
+
+// FastDtoa will produce at most kFastDtoaMaximalLength digits. This does not
+// include the terminating '\0' character.
+static const int kFastDtoaMaximalLength = 17;
+// Same for single-precision numbers.
+static const int kFastDtoaMaximalSingleLength = 9;
+
+// Provides a decimal representation of v.
+// The result should be interpreted as buffer * 10^(point - length).
+//
+// Precondition:
+//   * v must be a strictly positive finite double.
+//
+// Returns true if it succeeds, otherwise the result can not be trusted.
+// There will be *length digits inside the buffer followed by a null terminator.
+// If the function returns true and mode equals
+//   - FAST_DTOA_SHORTEST, then
+//     the parameter requested_digits is ignored.
+//     The result satisfies
+//         v == (double) (buffer * 10^(point - length)).
+//     The digits in the buffer are the shortest representation possible. E.g.
+//     if 0.099999999999 and 0.1 represent the same double then "1" is returned
+//     with point = 0.
+//     The last digit will be closest to the actual v. That is, even if several
+//     digits might correctly yield 'v' when read again, the buffer will contain
+//     the one closest to v.
+//   - FAST_DTOA_PRECISION, then
+//     the buffer contains requested_digits digits.
+//     the difference v - (buffer * 10^(point-length)) is closest to zero for
+//     all possible representations of requested_digits digits.
+//     If there are two values that are equally close, then FastDtoa returns
+//     false.
+// For both modes the buffer must be large enough to hold the result.
+bool FastDtoa(double d,
+              FastDtoaMode mode,
+              int requested_digits,
+              Vector<char> buffer,
+              int* length,
+              int* decimal_point);
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_FAST_DTOA_H_
diff --git a/mfbt/double-conversion/double-conversion/fixed-dtoa.cc b/mfbt/double-conversion/double-conversion/fixed-dtoa.cc
new file mode 100644
index 0000000000..e739b19804
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/fixed-dtoa.cc
@@ -0,0 +1,405 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <cmath>
+
+#include "fixed-dtoa.h"
+#include "ieee.h"
+
+namespace double_conversion {
+
+// Represents a 128bit type. This class should be replaced by a native type on
+// platforms that support 128bit integers.
+class UInt128 {
+ public:
+  UInt128() : high_bits_(0), low_bits_(0) { }
+  UInt128(uint64_t high, uint64_t low) : high_bits_(high), low_bits_(low) { }
+
+  void Multiply(uint32_t multiplicand) {
+    uint64_t accumulator;
+
+    accumulator = (low_bits_ & kMask32) * multiplicand;
+    uint32_t part = static_cast<uint32_t>(accumulator & kMask32);
+    accumulator >>= 32;
+    accumulator = accumulator + (low_bits_ >> 32) * multiplicand;
+    low_bits_ = (accumulator << 32) + part;
+    accumulator >>= 32;
+    accumulator = accumulator + (high_bits_ & kMask32) * multiplicand;
+    part = static_cast<uint32_t>(accumulator & kMask32);
+    accumulator >>= 32;
+    accumulator = accumulator + (high_bits_ >> 32) * multiplicand;
+    high_bits_ = (accumulator << 32) + part;
+    DOUBLE_CONVERSION_ASSERT((accumulator >> 32) == 0);
+  }
+
+  void Shift(int shift_amount) {
+    DOUBLE_CONVERSION_ASSERT(-64 <= shift_amount && shift_amount <= 64);
+    if (shift_amount == 0) {
+      return;
+    } else if (shift_amount == -64) {
+      high_bits_ = low_bits_;
+      low_bits_ = 0;
+    } else if (shift_amount == 64) {
+      low_bits_ = high_bits_;
+      high_bits_ = 0;
+    } else if (shift_amount <= 0) {
+      high_bits_ <<= -shift_amount;
+      high_bits_ += low_bits_ >> (64 + shift_amount);
+      low_bits_ <<= -shift_amount;
+    } else {
+      low_bits_ >>= shift_amount;
+      low_bits_ += high_bits_ << (64 - shift_amount);
+      high_bits_ >>= shift_amount;
+    }
+  }
+
+  // Modifies *this to *this MOD (2^power).
+  // Returns *this DIV (2^power).
+  int DivModPowerOf2(int power) {
+    if (power >= 64) {
+      int result = static_cast<int>(high_bits_ >> (power - 64));
+      high_bits_ -= static_cast<uint64_t>(result) << (power - 64);
+      return result;
+    } else {
+      uint64_t part_low = low_bits_ >> power;
+      uint64_t part_high = high_bits_ << (64 - power);
+      int result = static_cast<int>(part_low + part_high);
+      high_bits_ = 0;
+      low_bits_ -= part_low << power;
+      return result;
+    }
+  }
+
+  bool IsZero() const {
+    return high_bits_ == 0 && low_bits_ == 0;
+  }
+
+  int BitAt(int position) const {
+    if (position >= 64) {
+      return static_cast<int>(high_bits_ >> (position - 64)) & 1;
+    } else {
+      return static_cast<int>(low_bits_ >> position) & 1;
+    }
+  }
+
+ private:
+  static const uint64_t kMask32 = 0xFFFFFFFF;
+  // Value == (high_bits_ << 64) + low_bits_
+  uint64_t high_bits_;
+  uint64_t low_bits_;
+};
+
+
+static const int kDoubleSignificandSize = 53;  // Includes the hidden bit.
+
+
+static void FillDigits32FixedLength(uint32_t number, int requested_length,
+                                    Vector<char> buffer, int* length) {
+  for (int i = requested_length - 1; i >= 0; --i) {
+    buffer[(*length) + i] = '0' + number % 10;
+    number /= 10;
+  }
+  *length += requested_length;
+}
+
+
+static void FillDigits32(uint32_t number, Vector<char> buffer, int* length) {
+  int number_length = 0;
+  // We fill the digits in reverse order and exchange them afterwards.
+  while (number != 0) {
+    int digit = number % 10;
+    number /= 10;
+    buffer[(*length) + number_length] = static_cast<char>('0' + digit);
+    number_length++;
+  }
+  // Exchange the digits.
+  int i = *length;
+  int j = *length + number_length - 1;
+  while (i < j) {
+    char tmp = buffer[i];
+    buffer[i] = buffer[j];
+    buffer[j] = tmp;
+    i++;
+    j--;
+  }
+  *length += number_length;
+}
+
+
+static void FillDigits64FixedLength(uint64_t number,
+                                    Vector<char> buffer, int* length) {
+  const uint32_t kTen7 = 10000000;
+  // For efficiency cut the number into 3 uint32_t parts, and print those.
+  uint32_t part2 = static_cast<uint32_t>(number % kTen7);
+  number /= kTen7;
+  uint32_t part1 = static_cast<uint32_t>(number % kTen7);
+  uint32_t part0 = static_cast<uint32_t>(number / kTen7);
+
+  FillDigits32FixedLength(part0, 3, buffer, length);
+  FillDigits32FixedLength(part1, 7, buffer, length);
+  FillDigits32FixedLength(part2, 7, buffer, length);
+}
+
+
+static void FillDigits64(uint64_t number, Vector<char> buffer, int* length) {
+  const uint32_t kTen7 = 10000000;
+  // For efficiency cut the number into 3 uint32_t parts, and print those.
+  uint32_t part2 = static_cast<uint32_t>(number % kTen7);
+  number /= kTen7;
+  uint32_t part1 = static_cast<uint32_t>(number % kTen7);
+  uint32_t part0 = static_cast<uint32_t>(number / kTen7);
+
+  if (part0 != 0) {
+    FillDigits32(part0, buffer, length);
+    FillDigits32FixedLength(part1, 7, buffer, length);
+    FillDigits32FixedLength(part2, 7, buffer, length);
+  } else if (part1 != 0) {
+    FillDigits32(part1, buffer, length);
+    FillDigits32FixedLength(part2, 7, buffer, length);
+  } else {
+    FillDigits32(part2, buffer, length);
+  }
+}
+
+
+static void RoundUp(Vector<char> buffer, int* length, int* decimal_point) {
+  // An empty buffer represents 0.
+  if (*length == 0) {
+    buffer[0] = '1';
+    *decimal_point = 1;
+    *length = 1;
+    return;
+  }
+  // Round the last digit until we either have a digit that was not '9' or until
+  // we reached the first digit.
+  buffer[(*length) - 1]++;
+  for (int i = (*length) - 1; i > 0; --i) {
+    if (buffer[i] != '0' + 10) {
+      return;
+    }
+    buffer[i] = '0';
+    buffer[i - 1]++;
+  }
+  // If the first digit is now '0' + 10, we would need to set it to '0' and add
+  // a '1' in front. However we reach the first digit only if all following
+  // digits had been '9' before rounding up. Now all trailing digits are '0' and
+  // we simply switch the first digit to '1' and update the decimal-point
+  // (indicating that the point is now one digit to the right).
+  if (buffer[0] == '0' + 10) {
+    buffer[0] = '1';
+    (*decimal_point)++;
+  }
+}
+
+
+// The given fractionals number represents a fixed-point number with binary
+// point at bit (-exponent).
+// Preconditions:
+//   -128 <= exponent <= 0.
+//   0 <= fractionals * 2^exponent < 1
+//   The buffer holds the result.
+// The function will round its result. During the rounding-process digits not
+// generated by this function might be updated, and the decimal-point variable
+// might be updated. If this function generates the digits 99 and the buffer
+// already contained "199" (thus yielding a buffer of "19999") then a
+// rounding-up will change the contents of the buffer to "20000".
+static void FillFractionals(uint64_t fractionals, int exponent,
+                            int fractional_count, Vector<char> buffer,
+                            int* length, int* decimal_point) {
+  DOUBLE_CONVERSION_ASSERT(-128 <= exponent && exponent <= 0);
+  // 'fractionals' is a fixed-point number, with binary point at bit
+  // (-exponent). Inside the function the non-converted remainder of fractionals
+  // is a fixed-point number, with binary point at bit 'point'.
+  if (-exponent <= 64) {
+    // One 64 bit number is sufficient.
+    DOUBLE_CONVERSION_ASSERT(fractionals >> 56 == 0);
+    int point = -exponent;
+    for (int i = 0; i < fractional_count; ++i) {
+      if (fractionals == 0) break;
+      // Instead of multiplying by 10 we multiply by 5 and adjust the point
+      // location. This way the fractionals variable will not overflow.
+      // Invariant at the beginning of the loop: fractionals < 2^point.
+      // Initially we have: point <= 64 and fractionals < 2^56
+      // After each iteration the point is decremented by one.
+      // Note that 5^3 = 125 < 128 = 2^7.
+      // Therefore three iterations of this loop will not overflow fractionals
+      // (even without the subtraction at the end of the loop body). At this
+      // time point will satisfy point <= 61 and therefore fractionals < 2^point
+      // and any further multiplication of fractionals by 5 will not overflow.
+      fractionals *= 5;
+      point--;
+      int digit = static_cast<int>(fractionals >> point);
+      DOUBLE_CONVERSION_ASSERT(digit <= 9);
+      buffer[*length] = static_cast<char>('0' + digit);
+      (*length)++;
+      fractionals -= static_cast<uint64_t>(digit) << point;
+    }
+    // If the first bit after the point is set we have to round up.
+    DOUBLE_CONVERSION_ASSERT(fractionals == 0 || point - 1 >= 0);
+    if ((fractionals != 0) && ((fractionals >> (point - 1)) & 1) == 1) {
+      RoundUp(buffer, length, decimal_point);
+    }
+  } else {  // We need 128 bits.
+    DOUBLE_CONVERSION_ASSERT(64 < -exponent && -exponent <= 128);
+    UInt128 fractionals128 = UInt128(fractionals, 0);
+    fractionals128.Shift(-exponent - 64);
+    int point = 128;
+    for (int i = 0; i < fractional_count; ++i) {
+      if (fractionals128.IsZero()) break;
+      // As before: instead of multiplying by 10 we multiply by 5 and adjust the
+      // point location.
+      // This multiplication will not overflow for the same reasons as before.
+      fractionals128.Multiply(5);
+      point--;
+      int digit = fractionals128.DivModPowerOf2(point);
+      DOUBLE_CONVERSION_ASSERT(digit <= 9);
+      buffer[*length] = static_cast<char>('0' + digit);
+      (*length)++;
+    }
+    if (fractionals128.BitAt(point - 1) == 1) {
+      RoundUp(buffer, length, decimal_point);
+    }
+  }
+}
+
+
+// Removes leading and trailing zeros.
+// If leading zeros are removed then the decimal point position is adjusted.
+static void TrimZeros(Vector<char> buffer, int* length, int* decimal_point) {
+  while (*length > 0 && buffer[(*length) - 1] == '0') {
+    (*length)--;
+  }
+  int first_non_zero = 0;
+  while (first_non_zero < *length && buffer[first_non_zero] == '0') {
+    first_non_zero++;
+  }
+  if (first_non_zero != 0) {
+    for (int i = first_non_zero; i < *length; ++i) {
+      buffer[i - first_non_zero] = buffer[i];
+    }
+    *length -= first_non_zero;
+    *decimal_point -= first_non_zero;
+  }
+}
+
+
+bool FastFixedDtoa(double v,
+                   int fractional_count,
+                   Vector<char> buffer,
+                   int* length,
+                   int* decimal_point) {
+  const uint32_t kMaxUInt32 = 0xFFFFFFFF;
+  uint64_t significand = Double(v).Significand();
+  int exponent = Double(v).Exponent();
+  // v = significand * 2^exponent (with significand a 53bit integer).
+  // If the exponent is larger than 20 (i.e. we may have a 73bit number) then we
+  // don't know how to compute the representation. 2^73 ~= 9.5*10^21.
+  // If necessary this limit could probably be increased, but we don't need
+  // more.
+  if (exponent > 20) return false;
+  if (fractional_count > 20) return false;
+  *length = 0;
+  // At most kDoubleSignificandSize bits of the significand are non-zero.
+  // Given a 64 bit integer we have 11 0s followed by 53 potentially non-zero
+  // bits:  0..11*..0xxx..53*..xx
+  if (exponent + kDoubleSignificandSize > 64) {
+    // The exponent must be > 11.
+    //
+    // We know that v = significand * 2^exponent.
+    // And the exponent > 11.
+    // We simplify the task by dividing v by 10^17.
+    // The quotient delivers the first digits, and the remainder fits into a 64
+    // bit number.
+    // Dividing by 10^17 is equivalent to dividing by 5^17*2^17.
+    const uint64_t kFive17 = DOUBLE_CONVERSION_UINT64_2PART_C(0xB1, A2BC2EC5);  // 5^17
+    uint64_t divisor = kFive17;
+    int divisor_power = 17;
+    uint64_t dividend = significand;
+    uint32_t quotient;
+    uint64_t remainder;
+    // Let v = f * 2^e with f == significand and e == exponent.
+    // Then need q (quotient) and r (remainder) as follows:
+    //   v            = q * 10^17       + r
+    //   f * 2^e      = q * 10^17       + r
+    //   f * 2^e      = q * 5^17 * 2^17 + r
+    // If e > 17 then
+    //   f * 2^(e-17) = q * 5^17        + r/2^17
+    // else
+    //   f  = q * 5^17 * 2^(17-e) + r/2^e
+    if (exponent > divisor_power) {
+      // We only allow exponents of up to 20 and therefore (17 - e) <= 3
+      dividend <<= exponent - divisor_power;
+      quotient = static_cast<uint32_t>(dividend / divisor);
+      remainder = (dividend % divisor) << divisor_power;
+    } else {
+      divisor <<= divisor_power - exponent;
+      quotient = static_cast<uint32_t>(dividend / divisor);
+      remainder = (dividend % divisor) << exponent;
+    }
+    FillDigits32(quotient, buffer, length);
+    FillDigits64FixedLength(remainder, buffer, length);
+    *decimal_point = *length;
+  } else if (exponent >= 0) {
+    // 0 <= exponent <= 11
+    significand <<= exponent;
+    FillDigits64(significand, buffer, length);
+    *decimal_point = *length;
+  } else if (exponent > -kDoubleSignificandSize) {
+    // We have to cut the number.
+    uint64_t integrals = significand >> -exponent;
+    uint64_t fractionals = significand - (integrals << -exponent);
+    if (integrals > kMaxUInt32) {
+      FillDigits64(integrals, buffer, length);
+    } else {
+      FillDigits32(static_cast<uint32_t>(integrals), buffer, length);
+    }
+    *decimal_point = *length;
+    FillFractionals(fractionals, exponent, fractional_count,
+                    buffer, length, decimal_point);
+  } else if (exponent < -128) {
+    // This configuration (with at most 20 digits) means that all digits must be
+    // 0.
+    DOUBLE_CONVERSION_ASSERT(fractional_count <= 20);
+    buffer[0] = '\0';
+    *length = 0;
+    *decimal_point = -fractional_count;
+  } else {
+    *decimal_point = 0;
+    FillFractionals(significand, exponent, fractional_count,
+                    buffer, length, decimal_point);
+  }
+  TrimZeros(buffer, length, decimal_point);
+  buffer[*length] = '\0';
+  if ((*length) == 0) {
+    // The string is empty and the decimal_point thus has no importance. Mimic
+    // Gay's dtoa and set it to -fractional_count.
+    *decimal_point = -fractional_count;
+  }
+  return true;
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/fixed-dtoa.h b/mfbt/double-conversion/double-conversion/fixed-dtoa.h
new file mode 100644
index 0000000000..3bdd08e21f
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/fixed-dtoa.h
@@ -0,0 +1,56 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_FIXED_DTOA_H_
+#define DOUBLE_CONVERSION_FIXED_DTOA_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+// Produces digits necessary to print a given number with
+// 'fractional_count' digits after the decimal point.
+// The buffer must be big enough to hold the result plus one terminating null
+// character.
+//
+// The produced digits might be too short in which case the caller has to fill
+// the gaps with '0's.
+// Example: FastFixedDtoa(0.001, 5, ...) is allowed to return buffer = "1", and
+// decimal_point = -2.
+// Halfway cases are rounded towards +/-Infinity (away from 0). The call
+// FastFixedDtoa(0.15, 2, ...) thus returns buffer = "2", decimal_point = 0.
+// The returned buffer may contain digits that would be truncated from the
+// shortest representation of the input.
+//
+// This method only works for some parameters. If it can't handle the input it
+// returns false. The output is null-terminated when the function succeeds.
+bool FastFixedDtoa(double v, int fractional_count,
+                   Vector<char> buffer, int* length, int* decimal_point);
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_FIXED_DTOA_H_
diff --git a/mfbt/double-conversion/double-conversion/ieee.h b/mfbt/double-conversion/double-conversion/ieee.h
new file mode 100644
index 0000000000..9203f4d558
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/ieee.h
@@ -0,0 +1,447 @@
+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_DOUBLE_H_
+#define DOUBLE_CONVERSION_DOUBLE_H_
+
+#include "diy-fp.h"
+
+namespace double_conversion {
+
+// We assume that doubles and uint64_t have the same endianness.
+static uint64_t double_to_uint64(double d) { return BitCast<uint64_t>(d); }
+static double uint64_to_double(uint64_t d64) { return BitCast<double>(d64); }
+static uint32_t float_to_uint32(float f) { return BitCast<uint32_t>(f); }
+static float uint32_to_float(uint32_t d32) { return BitCast<float>(d32); }
+
+// Helper functions for doubles.
+class Double {
+ public:
+  static const uint64_t kSignMask = DOUBLE_CONVERSION_UINT64_2PART_C(0x80000000, 00000000);
+  static const uint64_t kExponentMask = DOUBLE_CONVERSION_UINT64_2PART_C(0x7FF00000, 00000000);
+  static const uint64_t kSignificandMask = DOUBLE_CONVERSION_UINT64_2PART_C(0x000FFFFF, FFFFFFFF);
+  static const uint64_t kHiddenBit = DOUBLE_CONVERSION_UINT64_2PART_C(0x00100000, 00000000);
+  static const uint64_t kQuietNanBit = DOUBLE_CONVERSION_UINT64_2PART_C(0x00080000, 00000000);
+  static const int kPhysicalSignificandSize = 52;  // Excludes the hidden bit.
+  static const int kSignificandSize = 53;
+  static const int kExponentBias = 0x3FF + kPhysicalSignificandSize;
+  static const int kMaxExponent = 0x7FF - kExponentBias;
+
+  Double() : d64_(0) {}
+  explicit Double(double d) : d64_(double_to_uint64(d)) {}
+  explicit Double(uint64_t d64) : d64_(d64) {}
+  explicit Double(DiyFp diy_fp)
+    : d64_(DiyFpToUint64(diy_fp)) {}
+
+  // The value encoded by this Double must be greater or equal to +0.0.
+  // It must not be special (infinity, or NaN).
+  DiyFp AsDiyFp() const {
+    DOUBLE_CONVERSION_ASSERT(Sign() > 0);
+    DOUBLE_CONVERSION_ASSERT(!IsSpecial());
+    return DiyFp(Significand(), Exponent());
+  }
+
+  // The value encoded by this Double must be strictly greater than 0.
+  DiyFp AsNormalizedDiyFp() const {
+    DOUBLE_CONVERSION_ASSERT(value() > 0.0);
+    uint64_t f = Significand();
+    int e = Exponent();
+
+    // The current double could be a denormal.
+    while ((f & kHiddenBit) == 0) {
+      f <<= 1;
+      e--;
+    }
+    // Do the final shifts in one go.
+    f <<= DiyFp::kSignificandSize - kSignificandSize;
+    e -= DiyFp::kSignificandSize - kSignificandSize;
+    return DiyFp(f, e);
+  }
+
+  // Returns the double's bit as uint64.
+  uint64_t AsUint64() const {
+    return d64_;
+  }
+
+  // Returns the next greater double. Returns +infinity on input +infinity.
+  double NextDouble() const {
+    if (d64_ == kInfinity) return Double(kInfinity).value();
+    if (Sign() < 0 && Significand() == 0) {
+      // -0.0
+      return 0.0;
+    }
+    if (Sign() < 0) {
+      return Double(d64_ - 1).value();
+    } else {
+      return Double(d64_ + 1).value();
+    }
+  }
+
+  double PreviousDouble() const {
+    if (d64_ == (kInfinity | kSignMask)) return -Infinity();
+    if (Sign() < 0) {
+      return Double(d64_ + 1).value();
+    } else {
+      if (Significand() == 0) return -0.0;
+      return Double(d64_ - 1).value();
+    }
+  }
+
+  int Exponent() const {
+    if (IsDenormal()) return kDenormalExponent;
+
+    uint64_t d64 = AsUint64();
+    int biased_e =
+        static_cast<int>((d64 & kExponentMask) >> kPhysicalSignificandSize);
+    return biased_e - kExponentBias;
+  }
+
+  uint64_t Significand() const {
+    uint64_t d64 = AsUint64();
+    uint64_t significand = d64 & kSignificandMask;
+    if (!IsDenormal()) {
+      return significand + kHiddenBit;
+    } else {
+      return significand;
+    }
+  }
+
+  // Returns true if the double is a denormal.
+  bool IsDenormal() const {
+    uint64_t d64 = AsUint64();
+    return (d64 & kExponentMask) == 0;
+  }
+
+  // We consider denormals not to be special.
+  // Hence only Infinity and NaN are special.
+  bool IsSpecial() const {
+    uint64_t d64 = AsUint64();
+    return (d64 & kExponentMask) == kExponentMask;
+  }
+
+  bool IsNan() const {
+    uint64_t d64 = AsUint64();
+    return ((d64 & kExponentMask) == kExponentMask) &&
+        ((d64 & kSignificandMask) != 0);
+  }
+
+  bool IsQuietNan() const {
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+    return IsNan() && ((AsUint64() & kQuietNanBit) == 0);
+#else
+    return IsNan() && ((AsUint64() & kQuietNanBit) != 0);
+#endif
+  }
+
+  bool IsSignalingNan() const {
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+    return IsNan() && ((AsUint64() & kQuietNanBit) != 0);
+#else
+    return IsNan() && ((AsUint64() & kQuietNanBit) == 0);
+#endif
+  }
+
+
+  bool IsInfinite() const {
+    uint64_t d64 = AsUint64();
+    return ((d64 & kExponentMask) == kExponentMask) &&
+        ((d64 & kSignificandMask) == 0);
+  }
+
+  int Sign() const {
+    uint64_t d64 = AsUint64();
+    return (d64 & kSignMask) == 0? 1: -1;
+  }
+
+  // Precondition: the value encoded by this Double must be greater or equal
+  // than +0.0.
+  DiyFp UpperBoundary() const {
+    DOUBLE_CONVERSION_ASSERT(Sign() > 0);
+    return DiyFp(Significand() * 2 + 1, Exponent() - 1);
+  }
+
+  // Computes the two boundaries of this.
+  // The bigger boundary (m_plus) is normalized. The lower boundary has the same
+  // exponent as m_plus.
+  // Precondition: the value encoded by this Double must be greater than 0.
+  void NormalizedBoundaries(DiyFp* out_m_minus, DiyFp* out_m_plus) const {
+    DOUBLE_CONVERSION_ASSERT(value() > 0.0);
+    DiyFp v = this->AsDiyFp();
+    DiyFp m_plus = DiyFp::Normalize(DiyFp((v.f() << 1) + 1, v.e() - 1));
+    DiyFp m_minus;
+    if (LowerBoundaryIsCloser()) {
+      m_minus = DiyFp((v.f() << 2) - 1, v.e() - 2);
+    } else {
+      m_minus = DiyFp((v.f() << 1) - 1, v.e() - 1);
+    }
+    m_minus.set_f(m_minus.f() << (m_minus.e() - m_plus.e()));
+    m_minus.set_e(m_plus.e());
+    *out_m_plus = m_plus;
+    *out_m_minus = m_minus;
+  }
+
+  bool LowerBoundaryIsCloser() const {
+    // The boundary is closer if the significand is of the form f == 2^p-1 then
+    // the lower boundary is closer.
+    // Think of v = 1000e10 and v- = 9999e9.
+    // Then the boundary (== (v - v-)/2) is not just at a distance of 1e9 but
+    // at a distance of 1e8.
+    // The only exception is for the smallest normal: the largest denormal is
+    // at the same distance as its successor.
+    // Note: denormals have the same exponent as the smallest normals.
+    bool physical_significand_is_zero = ((AsUint64() & kSignificandMask) == 0);
+    return physical_significand_is_zero && (Exponent() != kDenormalExponent);
+  }
+
+  double value() const { return uint64_to_double(d64_); }
+
+  // Returns the significand size for a given order of magnitude.
+  // If v = f*2^e with 2^p-1 <= f <= 2^p then p+e is v's order of magnitude.
+  // This function returns the number of significant binary digits v will have
+  // once it's encoded into a double. In almost all cases this is equal to
+  // kSignificandSize. The only exceptions are denormals. They start with
+  // leading zeroes and their effective significand-size is hence smaller.
+  static int SignificandSizeForOrderOfMagnitude(int order) {
+    if (order >= (kDenormalExponent + kSignificandSize)) {
+      return kSignificandSize;
+    }
+    if (order <= kDenormalExponent) return 0;
+    return order - kDenormalExponent;
+  }
+
+  static double Infinity() {
+    return Double(kInfinity).value();
+  }
+
+  static double NaN() {
+    return Double(kNaN).value();
+  }
+
+ private:
+  static const int kDenormalExponent = -kExponentBias + 1;
+  static const uint64_t kInfinity = DOUBLE_CONVERSION_UINT64_2PART_C(0x7FF00000, 00000000);
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+  static const uint64_t kNaN = DOUBLE_CONVERSION_UINT64_2PART_C(0x7FF7FFFF, FFFFFFFF);
+#else
+  static const uint64_t kNaN = DOUBLE_CONVERSION_UINT64_2PART_C(0x7FF80000, 00000000);
+#endif
+
+
+  const uint64_t d64_;
+
+  static uint64_t DiyFpToUint64(DiyFp diy_fp) {
+    uint64_t significand = diy_fp.f();
+    int exponent = diy_fp.e();
+    while (significand > kHiddenBit + kSignificandMask) {
+      significand >>= 1;
+      exponent++;
+    }
+    if (exponent >= kMaxExponent) {
+      return kInfinity;
+    }
+    if (exponent < kDenormalExponent) {
+      return 0;
+    }
+    while (exponent > kDenormalExponent && (significand & kHiddenBit) == 0) {
+      significand <<= 1;
+      exponent--;
+    }
+    uint64_t biased_exponent;
+    if (exponent == kDenormalExponent && (significand & kHiddenBit) == 0) {
+      biased_exponent = 0;
+    } else {
+      biased_exponent = static_cast<uint64_t>(exponent + kExponentBias);
+    }
+    return (significand & kSignificandMask) |
+        (biased_exponent << kPhysicalSignificandSize);
+  }
+
+  DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN(Double);
+};
+
+class Single {
+ public:
+  static const uint32_t kSignMask = 0x80000000;
+  static const uint32_t kExponentMask = 0x7F800000;
+  static const uint32_t kSignificandMask = 0x007FFFFF;
+  static const uint32_t kHiddenBit = 0x00800000;
+  static const uint32_t kQuietNanBit = 0x00400000;
+  static const int kPhysicalSignificandSize = 23;  // Excludes the hidden bit.
+  static const int kSignificandSize = 24;
+
+  Single() : d32_(0) {}
+  explicit Single(float f) : d32_(float_to_uint32(f)) {}
+  explicit Single(uint32_t d32) : d32_(d32) {}
+
+  // The value encoded by this Single must be greater or equal to +0.0.
+  // It must not be special (infinity, or NaN).
+  DiyFp AsDiyFp() const {
+    DOUBLE_CONVERSION_ASSERT(Sign() > 0);
+    DOUBLE_CONVERSION_ASSERT(!IsSpecial());
+    return DiyFp(Significand(), Exponent());
+  }
+
+  // Returns the single's bit as uint64.
+  uint32_t AsUint32() const {
+    return d32_;
+  }
+
+  int Exponent() const {
+    if (IsDenormal()) return kDenormalExponent;
+
+    uint32_t d32 = AsUint32();
+    int biased_e =
+        static_cast<int>((d32 & kExponentMask) >> kPhysicalSignificandSize);
+    return biased_e - kExponentBias;
+  }
+
+  uint32_t Significand() const {
+    uint32_t d32 = AsUint32();
+    uint32_t significand = d32 & kSignificandMask;
+    if (!IsDenormal()) {
+      return significand + kHiddenBit;
+    } else {
+      return significand;
+    }
+  }
+
+  // Returns true if the single is a denormal.
+  bool IsDenormal() const {
+    uint32_t d32 = AsUint32();
+    return (d32 & kExponentMask) == 0;
+  }
+
+  // We consider denormals not to be special.
+  // Hence only Infinity and NaN are special.
+  bool IsSpecial() const {
+    uint32_t d32 = AsUint32();
+    return (d32 & kExponentMask) == kExponentMask;
+  }
+
+  bool IsNan() const {
+    uint32_t d32 = AsUint32();
+    return ((d32 & kExponentMask) == kExponentMask) &&
+        ((d32 & kSignificandMask) != 0);
+  }
+
+  bool IsQuietNan() const {
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+    return IsNan() && ((AsUint32() & kQuietNanBit) == 0);
+#else
+    return IsNan() && ((AsUint32() & kQuietNanBit) != 0);
+#endif
+  }
+
+  bool IsSignalingNan() const {
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+    return IsNan() && ((AsUint32() & kQuietNanBit) != 0);
+#else
+    return IsNan() && ((AsUint32() & kQuietNanBit) == 0);
+#endif
+  }
+
+
+  bool IsInfinite() const {
+    uint32_t d32 = AsUint32();
+    return ((d32 & kExponentMask) == kExponentMask) &&
+        ((d32 & kSignificandMask) == 0);
+  }
+
+  int Sign() const {
+    uint32_t d32 = AsUint32();
+    return (d32 & kSignMask) == 0? 1: -1;
+  }
+
+  // Computes the two boundaries of this.
+  // The bigger boundary (m_plus) is normalized. The lower boundary has the same
+  // exponent as m_plus.
+  // Precondition: the value encoded by this Single must be greater than 0.
+  void NormalizedBoundaries(DiyFp* out_m_minus, DiyFp* out_m_plus) const {
+    DOUBLE_CONVERSION_ASSERT(value() > 0.0);
+    DiyFp v = this->AsDiyFp();
+    DiyFp m_plus = DiyFp::Normalize(DiyFp((v.f() << 1) + 1, v.e() - 1));
+    DiyFp m_minus;
+    if (LowerBoundaryIsCloser()) {
+      m_minus = DiyFp((v.f() << 2) - 1, v.e() - 2);
+    } else {
+      m_minus = DiyFp((v.f() << 1) - 1, v.e() - 1);
+    }
+    m_minus.set_f(m_minus.f() << (m_minus.e() - m_plus.e()));
+    m_minus.set_e(m_plus.e());
+    *out_m_plus = m_plus;
+    *out_m_minus = m_minus;
+  }
+
+  // Precondition: the value encoded by this Single must be greater or equal
+  // than +0.0.
+  DiyFp UpperBoundary() const {
+    DOUBLE_CONVERSION_ASSERT(Sign() > 0);
+    return DiyFp(Significand() * 2 + 1, Exponent() - 1);
+  }
+
+  bool LowerBoundaryIsCloser() const {
+    // The boundary is closer if the significand is of the form f == 2^p-1 then
+    // the lower boundary is closer.
+    // Think of v = 1000e10 and v- = 9999e9.
+    // Then the boundary (== (v - v-)/2) is not just at a distance of 1e9 but
+    // at a distance of 1e8.
+    // The only exception is for the smallest normal: the largest denormal is
+    // at the same distance as its successor.
+    // Note: denormals have the same exponent as the smallest normals.
+    bool physical_significand_is_zero = ((AsUint32() & kSignificandMask) == 0);
+    return physical_significand_is_zero && (Exponent() != kDenormalExponent);
+  }
+
+  float value() const { return uint32_to_float(d32_); }
+
+  static float Infinity() {
+    return Single(kInfinity).value();
+  }
+
+  static float NaN() {
+    return Single(kNaN).value();
+  }
+
+ private:
+  static const int kExponentBias = 0x7F + kPhysicalSignificandSize;
+  static const int kDenormalExponent = -kExponentBias + 1;
+  static const int kMaxExponent = 0xFF - kExponentBias;
+  static const uint32_t kInfinity = 0x7F800000;
+#if (defined(__mips__) && !defined(__mips_nan2008)) || defined(__hppa__)
+  static const uint32_t kNaN = 0x7FBFFFFF;
+#else
+  static const uint32_t kNaN = 0x7FC00000;
+#endif
+
+  const uint32_t d32_;
+
+  DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN(Single);
+};
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_DOUBLE_H_
diff --git a/mfbt/double-conversion/double-conversion/string-to-double.cc b/mfbt/double-conversion/double-conversion/string-to-double.cc
new file mode 100644
index 0000000000..972956ca69
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/string-to-double.cc
@@ -0,0 +1,818 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <climits>
+#include <locale>
+#include <cmath>
+
+#include "string-to-double.h"
+
+#include "ieee.h"
+#include "strtod.h"
+#include "utils.h"
+
+#ifdef _MSC_VER
+#  if _MSC_VER >= 1900
+// Fix MSVC >= 2015 (_MSC_VER == 1900) warning
+// C4244: 'argument': conversion from 'const uc16' to 'char', possible loss of data
+// against Advance and friends, when instantiated with **it as char, not uc16.
+ __pragma(warning(disable: 4244))
+#  endif
+#  if _MSC_VER <= 1700 // VS2012, see IsDecimalDigitForRadix warning fix, below
+#    define VS2012_RADIXWARN
+#  endif
+#endif
+
+namespace double_conversion {
+
+namespace {
+
+inline char ToLower(char ch) {
+  static const std::ctype<char>& cType =
+      std::use_facet<std::ctype<char> >(std::locale::classic());
+  return cType.tolower(ch);
+}
+
+inline char Pass(char ch) {
+  return ch;
+}
+
+template <class Iterator, class Converter>
+static inline bool ConsumeSubStringImpl(Iterator* current,
+                                        Iterator end,
+                                        const char* substring,
+                                        Converter converter) {
+  DOUBLE_CONVERSION_ASSERT(converter(**current) == *substring);
+  for (substring++; *substring != '\0'; substring++) {
+    ++*current;
+    if (*current == end || converter(**current) != *substring) {
+      return false;
+    }
+  }
+  ++*current;
+  return true;
+}
+
+// Consumes the given substring from the iterator.
+// Returns false, if the substring does not match.
+template <class Iterator>
+static bool ConsumeSubString(Iterator* current,
+                             Iterator end,
+                             const char* substring,
+                             bool allow_case_insensitivity) {
+  if (allow_case_insensitivity) {
+    return ConsumeSubStringImpl(current, end, substring, ToLower);
+  } else {
+    return ConsumeSubStringImpl(current, end, substring, Pass);
+  }
+}
+
+// Consumes first character of the str is equal to ch
+inline bool ConsumeFirstCharacter(char ch,
+                                         const char* str,
+                                         bool case_insensitivity) {
+  return case_insensitivity ? ToLower(ch) == str[0] : ch == str[0];
+}
+}  // namespace
+
+// Maximum number of significant digits in decimal representation.
+// The longest possible double in decimal representation is
+// (2^53 - 1) * 2 ^ -1074 that is (2 ^ 53 - 1) * 5 ^ 1074 / 10 ^ 1074
+// (768 digits). If we parse a number whose first digits are equal to a
+// mean of 2 adjacent doubles (that could have up to 769 digits) the result
+// must be rounded to the bigger one unless the tail consists of zeros, so
+// we don't need to preserve all the digits.
+const int kMaxSignificantDigits = 772;
+
+
+static const char kWhitespaceTable7[] = { 32, 13, 10, 9, 11, 12 };
+static const int kWhitespaceTable7Length = DOUBLE_CONVERSION_ARRAY_SIZE(kWhitespaceTable7);
+
+
+static const uc16 kWhitespaceTable16[] = {
+  160, 8232, 8233, 5760, 6158, 8192, 8193, 8194, 8195,
+  8196, 8197, 8198, 8199, 8200, 8201, 8202, 8239, 8287, 12288, 65279
+};
+static const int kWhitespaceTable16Length = DOUBLE_CONVERSION_ARRAY_SIZE(kWhitespaceTable16);
+
+
+static bool isWhitespace(int x) {
+  if (x < 128) {
+    for (int i = 0; i < kWhitespaceTable7Length; i++) {
+      if (kWhitespaceTable7[i] == x) return true;
+    }
+  } else {
+    for (int i = 0; i < kWhitespaceTable16Length; i++) {
+      if (kWhitespaceTable16[i] == x) return true;
+    }
+  }
+  return false;
+}
+
+
+// Returns true if a nonspace found and false if the end has reached.
+template <class Iterator>
+static inline bool AdvanceToNonspace(Iterator* current, Iterator end) {
+  while (*current != end) {
+    if (!isWhitespace(**current)) return true;
+    ++*current;
+  }
+  return false;
+}
+
+
+static bool isDigit(int x, int radix) {
+  return (x >= '0' && x <= '9' && x < '0' + radix)
+      || (radix > 10 && x >= 'a' && x < 'a' + radix - 10)
+      || (radix > 10 && x >= 'A' && x < 'A' + radix - 10);
+}
+
+
+static double SignedZero(bool sign) {
+  return sign ? -0.0 : 0.0;
+}
+
+
+// Returns true if 'c' is a decimal digit that is valid for the given radix.
+//
+// The function is small and could be inlined, but VS2012 emitted a warning
+// because it constant-propagated the radix and concluded that the last
+// condition was always true. Moving it into a separate function and
+// suppressing optimisation keeps the compiler from warning.
+#ifdef VS2012_RADIXWARN
+#pragma optimize("",off)
+static bool IsDecimalDigitForRadix(int c, int radix) {
+  return '0' <= c && c <= '9' && (c - '0') < radix;
+}
+#pragma optimize("",on)
+#else
+static bool inline IsDecimalDigitForRadix(int c, int radix) {
+  return '0' <= c && c <= '9' && (c - '0') < radix;
+}
+#endif
+// Returns true if 'c' is a character digit that is valid for the given radix.
+// The 'a_character' should be 'a' or 'A'.
+//
+// The function is small and could be inlined, but VS2012 emitted a warning
+// because it constant-propagated the radix and concluded that the first
+// condition was always false. By moving it into a separate function the
+// compiler wouldn't warn anymore.
+static bool IsCharacterDigitForRadix(int c, int radix, char a_character) {
+  return radix > 10 && c >= a_character && c < a_character + radix - 10;
+}
+
+// Returns true, when the iterator is equal to end.
+template<class Iterator>
+static bool Advance (Iterator* it, uc16 separator, int base, Iterator& end) {
+  if (separator == StringToDoubleConverter::kNoSeparator) {
+    ++(*it);
+    return *it == end;
+  }
+  if (!isDigit(**it, base)) {
+    ++(*it);
+    return *it == end;
+  }
+  ++(*it);
+  if (*it == end) return true;
+  if (*it + 1 == end) return false;
+  if (**it == separator && isDigit(*(*it + 1), base)) {
+    ++(*it);
+  }
+  return *it == end;
+}
+
+// Checks whether the string in the range start-end is a hex-float string.
+// This function assumes that the leading '0x'/'0X' is already consumed.
+//
+// Hex float strings are of one of the following forms:
+//   - hex_digits+ 'p' ('+'|'-')? exponent_digits+
+//   - hex_digits* '.' hex_digits+ 'p' ('+'|'-')? exponent_digits+
+//   - hex_digits+ '.' 'p' ('+'|'-')? exponent_digits+
+template<class Iterator>
+static bool IsHexFloatString(Iterator start,
+                             Iterator end,
+                             uc16 separator,
+                             bool allow_trailing_junk) {
+  DOUBLE_CONVERSION_ASSERT(start != end);
+
+  Iterator current = start;
+
+  bool saw_digit = false;
+  while (isDigit(*current, 16)) {
+    saw_digit = true;
+    if (Advance(&current, separator, 16, end)) return false;
+  }
+  if (*current == '.') {
+    if (Advance(&current, separator, 16, end)) return false;
+    while (isDigit(*current, 16)) {
+      saw_digit = true;
+      if (Advance(&current, separator, 16, end)) return false;
+    }
+  }
+  if (!saw_digit) return false;
+  if (*current != 'p' && *current != 'P') return false;
+  if (Advance(&current, separator, 16, end)) return false;
+  if (*current == '+' || *current == '-') {
+    if (Advance(&current, separator, 16, end)) return false;
+  }
+  if (!isDigit(*current, 10)) return false;
+  if (Advance(&current, separator, 16, end)) return true;
+  while (isDigit(*current, 10)) {
+    if (Advance(&current, separator, 16, end)) return true;
+  }
+  return allow_trailing_junk || !AdvanceToNonspace(&current, end);
+}
+
+
+// Parsing integers with radix 2, 4, 8, 16, 32. Assumes current != end.
+//
+// If parse_as_hex_float is true, then the string must be a valid
+// hex-float.
+template <int radix_log_2, class Iterator>
+static double RadixStringToIeee(Iterator* current,
+                                Iterator end,
+                                bool sign,
+                                uc16 separator,
+                                bool parse_as_hex_float,
+                                bool allow_trailing_junk,
+                                double junk_string_value,
+                                bool read_as_double,
+                                bool* result_is_junk) {
+  DOUBLE_CONVERSION_ASSERT(*current != end);
+  DOUBLE_CONVERSION_ASSERT(!parse_as_hex_float ||
+      IsHexFloatString(*current, end, separator, allow_trailing_junk));
+
+  const int kDoubleSize = Double::kSignificandSize;
+  const int kSingleSize = Single::kSignificandSize;
+  const int kSignificandSize = read_as_double? kDoubleSize: kSingleSize;
+
+  *result_is_junk = true;
+
+  int64_t number = 0;
+  int exponent = 0;
+  const int radix = (1 << radix_log_2);
+  // Whether we have encountered a '.' and are parsing the decimal digits.
+  // Only relevant if parse_as_hex_float is true.
+  bool post_decimal = false;
+
+  // Skip leading 0s.
+  while (**current == '0') {
+    if (Advance(current, separator, radix, end)) {
+      *result_is_junk = false;
+      return SignedZero(sign);
+    }
+  }
+
+  while (true) {
+    int digit;
+    if (IsDecimalDigitForRadix(**current, radix)) {
+      digit = static_cast<char>(**current) - '0';
+      if (post_decimal) exponent -= radix_log_2;
+    } else if (IsCharacterDigitForRadix(**current, radix, 'a')) {
+      digit = static_cast<char>(**current) - 'a' + 10;
+      if (post_decimal) exponent -= radix_log_2;
+    } else if (IsCharacterDigitForRadix(**current, radix, 'A')) {
+      digit = static_cast<char>(**current) - 'A' + 10;
+      if (post_decimal) exponent -= radix_log_2;
+    } else if (parse_as_hex_float && **current == '.') {
+      post_decimal = true;
+      Advance(current, separator, radix, end);
+      DOUBLE_CONVERSION_ASSERT(*current != end);
+      continue;
+    } else if (parse_as_hex_float && (**current == 'p' || **current == 'P')) {
+      break;
+    } else {
+      if (allow_trailing_junk || !AdvanceToNonspace(current, end)) {
+        break;
+      } else {
+        return junk_string_value;
+      }
+    }
+
+    number = number * radix + digit;
+    int overflow = static_cast<int>(number >> kSignificandSize);
+    if (overflow != 0) {
+      // Overflow occurred. Need to determine which direction to round the
+      // result.
+      int overflow_bits_count = 1;
+      while (overflow > 1) {
+        overflow_bits_count++;
+        overflow >>= 1;
+      }
+
+      int dropped_bits_mask = ((1 << overflow_bits_count) - 1);
+      int dropped_bits = static_cast<int>(number) & dropped_bits_mask;
+      number >>= overflow_bits_count;
+      exponent += overflow_bits_count;
+
+      bool zero_tail = true;
+      for (;;) {
+        if (Advance(current, separator, radix, end)) break;
+        if (parse_as_hex_float && **current == '.') {
+          // Just run over the '.'. We are just trying to see whether there is
+          // a non-zero digit somewhere.
+          Advance(current, separator, radix, end);
+          DOUBLE_CONVERSION_ASSERT(*current != end);
+          post_decimal = true;
+        }
+        if (!isDigit(**current, radix)) break;
+        zero_tail = zero_tail && **current == '0';
+        if (!post_decimal) exponent += radix_log_2;
+      }
+
+      if (!parse_as_hex_float &&
+          !allow_trailing_junk &&
+          AdvanceToNonspace(current, end)) {
+        return junk_string_value;
+      }
+
+      int middle_value = (1 << (overflow_bits_count - 1));
+      if (dropped_bits > middle_value) {
+        number++;  // Rounding up.
+      } else if (dropped_bits == middle_value) {
+        // Rounding to even to consistency with decimals: half-way case rounds
+        // up if significant part is odd and down otherwise.
+        if ((number & 1) != 0 || !zero_tail) {
+          number++;  // Rounding up.
+        }
+      }
+
+      // Rounding up may cause overflow.
+      if ((number & ((int64_t)1 << kSignificandSize)) != 0) {
+        exponent++;
+        number >>= 1;
+      }
+      break;
+    }
+    if (Advance(current, separator, radix, end)) break;
+  }
+
+  DOUBLE_CONVERSION_ASSERT(number < ((int64_t)1 << kSignificandSize));
+  DOUBLE_CONVERSION_ASSERT(static_cast<int64_t>(static_cast<double>(number)) == number);
+
+  *result_is_junk = false;
+
+  if (parse_as_hex_float) {
+    DOUBLE_CONVERSION_ASSERT(**current == 'p' || **current == 'P');
+    Advance(current, separator, radix, end);
+    DOUBLE_CONVERSION_ASSERT(*current != end);
+    bool is_negative = false;
+    if (**current == '+') {
+      Advance(current, separator, radix, end);
+      DOUBLE_CONVERSION_ASSERT(*current != end);
+    } else if (**current == '-') {
+      is_negative = true;
+      Advance(current, separator, radix, end);
+      DOUBLE_CONVERSION_ASSERT(*current != end);
+    }
+    int written_exponent = 0;
+    while (IsDecimalDigitForRadix(**current, 10)) {
+      // No need to read exponents if they are too big. That could potentially overflow
+      // the `written_exponent` variable.
+      if (abs(written_exponent) <= 100 * Double::kMaxExponent) {
+        written_exponent = 10 * written_exponent + **current - '0';
+      }
+      if (Advance(current, separator, radix, end)) break;
+    }
+    if (is_negative) written_exponent = -written_exponent;
+    exponent += written_exponent;
+  }
+
+  if (exponent == 0 || number == 0) {
+    if (sign) {
+      if (number == 0) return -0.0;
+      number = -number;
+    }
+    return static_cast<double>(number);
+  }
+
+  DOUBLE_CONVERSION_ASSERT(number != 0);
+  double result = Double(DiyFp(number, exponent)).value();
+  return sign ? -result : result;
+}
+
+template <class Iterator>
+double StringToDoubleConverter::StringToIeee(
+    Iterator input,
+    int length,
+    bool read_as_double,
+    int* processed_characters_count) const {
+  Iterator current = input;
+  Iterator end = input + length;
+
+  *processed_characters_count = 0;
+
+  const bool allow_trailing_junk = (flags_ & ALLOW_TRAILING_JUNK) != 0;
+  const bool allow_leading_spaces = (flags_ & ALLOW_LEADING_SPACES) != 0;
+  const bool allow_trailing_spaces = (flags_ & ALLOW_TRAILING_SPACES) != 0;
+  const bool allow_spaces_after_sign = (flags_ & ALLOW_SPACES_AFTER_SIGN) != 0;
+  const bool allow_case_insensitivity = (flags_ & ALLOW_CASE_INSENSITIVITY) != 0;
+
+  // To make sure that iterator dereferencing is valid the following
+  // convention is used:
+  // 1. Each '++current' statement is followed by check for equality to 'end'.
+  // 2. If AdvanceToNonspace returned false then current == end.
+  // 3. If 'current' becomes equal to 'end' the function returns or goes to
+  // 'parsing_done'.
+  // 4. 'current' is not dereferenced after the 'parsing_done' label.
+  // 5. Code before 'parsing_done' may rely on 'current != end'.
+  if (current == end) return empty_string_value_;
+
+  if (allow_leading_spaces || allow_trailing_spaces) {
+    if (!AdvanceToNonspace(&current, end)) {
+      *processed_characters_count = static_cast<int>(current - input);
+      return empty_string_value_;
+    }
+    if (!allow_leading_spaces && (input != current)) {
+      // No leading spaces allowed, but AdvanceToNonspace moved forward.
+      return junk_string_value_;
+    }
+  }
+
+  // Exponent will be adjusted if insignificant digits of the integer part
+  // or insignificant leading zeros of the fractional part are dropped.
+  int exponent = 0;
+  int significant_digits = 0;
+  int insignificant_digits = 0;
+  bool nonzero_digit_dropped = false;
+
+  bool sign = false;
+
+  if (*current == '+' || *current == '-') {
+    sign = (*current == '-');
+    ++current;
+    Iterator next_non_space = current;
+    // Skip following spaces (if allowed).
+    if (!AdvanceToNonspace(&next_non_space, end)) return junk_string_value_;
+    if (!allow_spaces_after_sign && (current != next_non_space)) {
+      return junk_string_value_;
+    }
+    current = next_non_space;
+  }
+
+  if (infinity_symbol_ != DOUBLE_CONVERSION_NULLPTR) {
+    if (ConsumeFirstCharacter(*current, infinity_symbol_, allow_case_insensitivity)) {
+      if (!ConsumeSubString(&current, end, infinity_symbol_, allow_case_insensitivity)) {
+        return junk_string_value_;
+      }
+
+      if (!(allow_trailing_spaces || allow_trailing_junk) && (current != end)) {
+        return junk_string_value_;
+      }
+      if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
+        return junk_string_value_;
+      }
+
+      *processed_characters_count = static_cast<int>(current - input);
+      return sign ? -Double::Infinity() : Double::Infinity();
+    }
+  }
+
+  if (nan_symbol_ != DOUBLE_CONVERSION_NULLPTR) {
+    if (ConsumeFirstCharacter(*current, nan_symbol_, allow_case_insensitivity)) {
+      if (!ConsumeSubString(&current, end, nan_symbol_, allow_case_insensitivity)) {
+        return junk_string_value_;
+      }
+
+      if (!(allow_trailing_spaces || allow_trailing_junk) && (current != end)) {
+        return junk_string_value_;
+      }
+      if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
+        return junk_string_value_;
+      }
+
+      *processed_characters_count = static_cast<int>(current - input);
+      return sign ? -Double::NaN() : Double::NaN();
+    }
+  }
+
+  bool leading_zero = false;
+  if (*current == '0') {
+    if (Advance(&current, separator_, 10, end)) {
+      *processed_characters_count = static_cast<int>(current - input);
+      return SignedZero(sign);
+    }
+
+    leading_zero = true;
+
+    // It could be hexadecimal value.
+    if (((flags_ & ALLOW_HEX) || (flags_ & ALLOW_HEX_FLOATS)) &&
+        (*current == 'x' || *current == 'X')) {
+      ++current;
+
+      if (current == end) return junk_string_value_;  // "0x"
+
+      bool parse_as_hex_float = (flags_ & ALLOW_HEX_FLOATS) &&
+                IsHexFloatString(current, end, separator_, allow_trailing_junk);
+
+      if (!parse_as_hex_float && !isDigit(*current, 16)) {
+        return junk_string_value_;
+      }
+
+      bool result_is_junk;
+      double result = RadixStringToIeee<4>(&current,
+                                           end,
+                                           sign,
+                                           separator_,
+                                           parse_as_hex_float,
+                                           allow_trailing_junk,
+                                           junk_string_value_,
+                                           read_as_double,
+                                           &result_is_junk);
+      if (!result_is_junk) {
+        if (allow_trailing_spaces) AdvanceToNonspace(&current, end);
+        *processed_characters_count = static_cast<int>(current - input);
+      }
+      return result;
+    }
+
+    // Ignore leading zeros in the integer part.
+    while (*current == '0') {
+      if (Advance(&current, separator_, 10, end)) {
+        *processed_characters_count = static_cast<int>(current - input);
+        return SignedZero(sign);
+      }
+    }
+  }
+
+  bool octal = leading_zero && (flags_ & ALLOW_OCTALS) != 0;
+
+  // The longest form of simplified number is: "-<significant digits>.1eXXX\0".
+  const int kBufferSize = kMaxSignificantDigits + 10;
+  DOUBLE_CONVERSION_STACK_UNINITIALIZED char
+      buffer[kBufferSize];  // NOLINT: size is known at compile time.
+  int buffer_pos = 0;
+
+  // Copy significant digits of the integer part (if any) to the buffer.
+  while (*current >= '0' && *current <= '9') {
+    if (significant_digits < kMaxSignificantDigits) {
+      DOUBLE_CONVERSION_ASSERT(buffer_pos < kBufferSize);
+      buffer[buffer_pos++] = static_cast<char>(*current);
+      significant_digits++;
+      // Will later check if it's an octal in the buffer.
+    } else {
+      insignificant_digits++;  // Move the digit into the exponential part.
+      nonzero_digit_dropped = nonzero_digit_dropped || *current != '0';
+    }
+    octal = octal && *current < '8';
+    if (Advance(&current, separator_, 10, end)) goto parsing_done;
+  }
+
+  if (significant_digits == 0) {
+    octal = false;
+  }
+
+  if (*current == '.') {
+    if (octal && !allow_trailing_junk) return junk_string_value_;
+    if (octal) goto parsing_done;
+
+    if (Advance(&current, separator_, 10, end)) {
+      if (significant_digits == 0 && !leading_zero) {
+        return junk_string_value_;
+      } else {
+        goto parsing_done;
+      }
+    }
+
+    if (significant_digits == 0) {
+      // octal = false;
+      // Integer part consists of 0 or is absent. Significant digits start after
+      // leading zeros (if any).
+      while (*current == '0') {
+        if (Advance(&current, separator_, 10, end)) {
+          *processed_characters_count = static_cast<int>(current - input);
+          return SignedZero(sign);
+        }
+        exponent--;  // Move this 0 into the exponent.
+      }
+    }
+
+    // There is a fractional part.
+    // We don't emit a '.', but adjust the exponent instead.
+    while (*current >= '0' && *current <= '9') {
+      if (significant_digits < kMaxSignificantDigits) {
+        DOUBLE_CONVERSION_ASSERT(buffer_pos < kBufferSize);
+        buffer[buffer_pos++] = static_cast<char>(*current);
+        significant_digits++;
+        exponent--;
+      } else {
+        // Ignore insignificant digits in the fractional part.
+        nonzero_digit_dropped = nonzero_digit_dropped || *current != '0';
+      }
+      if (Advance(&current, separator_, 10, end)) goto parsing_done;
+    }
+  }
+
+  if (!leading_zero && exponent == 0 && significant_digits == 0) {
+    // If leading_zeros is true then the string contains zeros.
+    // If exponent < 0 then string was [+-]\.0*...
+    // If significant_digits != 0 the string is not equal to 0.
+    // Otherwise there are no digits in the string.
+    return junk_string_value_;
+  }
+
+  // Parse exponential part.
+  if (*current == 'e' || *current == 'E') {
+    if (octal && !allow_trailing_junk) return junk_string_value_;
+    if (octal) goto parsing_done;
+    Iterator junk_begin = current;
+    ++current;
+    if (current == end) {
+      if (allow_trailing_junk) {
+        current = junk_begin;
+        goto parsing_done;
+      } else {
+        return junk_string_value_;
+      }
+    }
+    char exponen_sign = '+';
+    if (*current == '+' || *current == '-') {
+      exponen_sign = static_cast<char>(*current);
+      ++current;
+      if (current == end) {
+        if (allow_trailing_junk) {
+          current = junk_begin;
+          goto parsing_done;
+        } else {
+          return junk_string_value_;
+        }
+      }
+    }
+
+    if (current == end || *current < '0' || *current > '9') {
+      if (allow_trailing_junk) {
+        current = junk_begin;
+        goto parsing_done;
+      } else {
+        return junk_string_value_;
+      }
+    }
+
+    const int max_exponent = INT_MAX / 2;
+    DOUBLE_CONVERSION_ASSERT(-max_exponent / 2 <= exponent && exponent <= max_exponent / 2);
+    int num = 0;
+    do {
+      // Check overflow.
+      int digit = *current - '0';
+      if (num >= max_exponent / 10
+          && !(num == max_exponent / 10 && digit <= max_exponent % 10)) {
+        num = max_exponent;
+      } else {
+        num = num * 10 + digit;
+      }
+      ++current;
+    } while (current != end && *current >= '0' && *current <= '9');
+
+    exponent += (exponen_sign == '-' ? -num : num);
+  }
+
+  if (!(allow_trailing_spaces || allow_trailing_junk) && (current != end)) {
+    return junk_string_value_;
+  }
+  if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
+    return junk_string_value_;
+  }
+  if (allow_trailing_spaces) {
+    AdvanceToNonspace(&current, end);
+  }
+
+  parsing_done:
+  exponent += insignificant_digits;
+
+  if (octal) {
+    double result;
+    bool result_is_junk;
+    char* start = buffer;
+    result = RadixStringToIeee<3>(&start,
+                                  buffer + buffer_pos,
+                                  sign,
+                                  separator_,
+                                  false, // Don't parse as hex_float.
+                                  allow_trailing_junk,
+                                  junk_string_value_,
+                                  read_as_double,
+                                  &result_is_junk);
+    DOUBLE_CONVERSION_ASSERT(!result_is_junk);
+    *processed_characters_count = static_cast<int>(current - input);
+    return result;
+  }
+
+  if (nonzero_digit_dropped) {
+    buffer[buffer_pos++] = '1';
+    exponent--;
+  }
+
+  DOUBLE_CONVERSION_ASSERT(buffer_pos < kBufferSize);
+  buffer[buffer_pos] = '\0';
+
+  // Code above ensures there are no leading zeros and the buffer has fewer than
+  // kMaxSignificantDecimalDigits characters. Trim trailing zeros.
+  Vector<const char> chars(buffer, buffer_pos);
+  chars = TrimTrailingZeros(chars);
+  exponent += buffer_pos - chars.length();
+
+  double converted;
+  if (read_as_double) {
+    converted = StrtodTrimmed(chars, exponent);
+  } else {
+    converted = StrtofTrimmed(chars, exponent);
+  }
+  *processed_characters_count = static_cast<int>(current - input);
+  return sign? -converted: converted;
+}
+
+
+double StringToDoubleConverter::StringToDouble(
+    const char* buffer,
+    int length,
+    int* processed_characters_count) const {
+  return StringToIeee(buffer, length, true, processed_characters_count);
+}
+
+
+double StringToDoubleConverter::StringToDouble(
+    const uc16* buffer,
+    int length,
+    int* processed_characters_count) const {
+  return StringToIeee(buffer, length, true, processed_characters_count);
+}
+
+
+float StringToDoubleConverter::StringToFloat(
+    const char* buffer,
+    int length,
+    int* processed_characters_count) const {
+  return static_cast<float>(StringToIeee(buffer, length, false,
+                                         processed_characters_count));
+}
+
+
+float StringToDoubleConverter::StringToFloat(
+    const uc16* buffer,
+    int length,
+    int* processed_characters_count) const {
+  return static_cast<float>(StringToIeee(buffer, length, false,
+                                         processed_characters_count));
+}
+
+
+template<>
+double StringToDoubleConverter::StringTo<double>(
+    const char* buffer,
+    int length,
+    int* processed_characters_count) const {
+    return StringToDouble(buffer, length, processed_characters_count);
+}
+
+
+template<>
+float StringToDoubleConverter::StringTo<float>(
+    const char* buffer,
+    int length,
+    int* processed_characters_count) const {
+    return StringToFloat(buffer, length, processed_characters_count);
+}
+
+
+template<>
+double StringToDoubleConverter::StringTo<double>(
+    const uc16* buffer,
+    int length,
+    int* processed_characters_count) const {
+    return StringToDouble(buffer, length, processed_characters_count);
+}
+
+
+template<>
+float StringToDoubleConverter::StringTo<float>(
+    const uc16* buffer,
+    int length,
+    int* processed_characters_count) const {
+    return StringToFloat(buffer, length, processed_characters_count);
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/string-to-double.h b/mfbt/double-conversion/double-conversion/string-to-double.h
new file mode 100644
index 0000000000..24972da26d
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/string-to-double.h
@@ -0,0 +1,239 @@
+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_STRING_TO_DOUBLE_H_
+#define DOUBLE_CONVERSION_STRING_TO_DOUBLE_H_
+
+#include "mozilla/Types.h"
+#include "utils.h"
+
+namespace double_conversion {
+
+class StringToDoubleConverter {
+ public:
+  // Enumeration for allowing octals and ignoring junk when converting
+  // strings to numbers.
+  enum Flags {
+    NO_FLAGS = 0,
+    ALLOW_HEX = 1,
+    ALLOW_OCTALS = 2,
+    ALLOW_TRAILING_JUNK = 4,
+    ALLOW_LEADING_SPACES = 8,
+    ALLOW_TRAILING_SPACES = 16,
+    ALLOW_SPACES_AFTER_SIGN = 32,
+    ALLOW_CASE_INSENSITIVITY = 64,
+    ALLOW_CASE_INSENSIBILITY = 64,  // Deprecated
+    ALLOW_HEX_FLOATS = 128,
+  };
+
+  static const uc16 kNoSeparator = '\0';
+
+  // Flags should be a bit-or combination of the possible Flags-enum.
+  //  - NO_FLAGS: no special flags.
+  //  - ALLOW_HEX: recognizes the prefix "0x". Hex numbers may only be integers.
+  //      Ex: StringToDouble("0x1234") -> 4660.0
+  //          In StringToDouble("0x1234.56") the characters ".56" are trailing
+  //          junk. The result of the call is hence dependent on
+  //          the ALLOW_TRAILING_JUNK flag and/or the junk value.
+  //      With this flag "0x" is a junk-string. Even with ALLOW_TRAILING_JUNK,
+  //      the string will not be parsed as "0" followed by junk.
+  //
+  //  - ALLOW_OCTALS: recognizes the prefix "0" for octals:
+  //      If a sequence of octal digits starts with '0', then the number is
+  //      read as octal integer. Octal numbers may only be integers.
+  //      Ex: StringToDouble("01234") -> 668.0
+  //          StringToDouble("012349") -> 12349.0  // Not a sequence of octal
+  //                                               // digits.
+  //          In StringToDouble("01234.56") the characters ".56" are trailing
+  //          junk. The result of the call is hence dependent on
+  //          the ALLOW_TRAILING_JUNK flag and/or the junk value.
+  //          In StringToDouble("01234e56") the characters "e56" are trailing
+  //          junk, too.
+  //  - ALLOW_TRAILING_JUNK: ignore trailing characters that are not part of
+  //      a double literal.
+  //  - ALLOW_LEADING_SPACES: skip over leading whitespace, including spaces,
+  //                          new-lines, and tabs.
+  //  - ALLOW_TRAILING_SPACES: ignore trailing whitespace.
+  //  - ALLOW_SPACES_AFTER_SIGN: ignore whitespace after the sign.
+  //       Ex: StringToDouble("-   123.2") -> -123.2.
+  //           StringToDouble("+   123.2") -> 123.2
+  //  - ALLOW_CASE_INSENSITIVITY: ignore case of characters for special values:
+  //      infinity and nan.
+  //  - ALLOW_HEX_FLOATS: allows hexadecimal float literals.
+  //      This *must* start with "0x" and separate the exponent with "p".
+  //      Examples: 0x1.2p3 == 9.0
+  //                0x10.1p0 == 16.0625
+  //      ALLOW_HEX and ALLOW_HEX_FLOATS are indented.
+  //
+  // empty_string_value is returned when an empty string is given as input.
+  // If ALLOW_LEADING_SPACES or ALLOW_TRAILING_SPACES are set, then a string
+  // containing only spaces is converted to the 'empty_string_value', too.
+  //
+  // junk_string_value is returned when
+  //  a) ALLOW_TRAILING_JUNK is not set, and a junk character (a character not
+  //     part of a double-literal) is found.
+  //  b) ALLOW_TRAILING_JUNK is set, but the string does not start with a
+  //     double literal.
+  //
+  // infinity_symbol and nan_symbol are strings that are used to detect
+  // inputs that represent infinity and NaN. They can be null, in which case
+  // they are ignored.
+  // The conversion routine first reads any possible signs. Then it compares the
+  // following character of the input-string with the first character of
+  // the infinity, and nan-symbol. If either matches, the function assumes, that
+  // a match has been found, and expects the following input characters to match
+  // the remaining characters of the special-value symbol.
+  // This means that the following restrictions apply to special-value symbols:
+  //  - they must not start with signs ('+', or '-'),
+  //  - they must not have the same first character.
+  //  - they must not start with digits.
+  //
+  // If the separator character is not kNoSeparator, then that specific
+  // character is ignored when in between two valid digits of the significant.
+  // It is not allowed to appear in the exponent.
+  // It is not allowed to lead or trail the number.
+  // It is not allowed to appear twice next to each other.
+  //
+  // Examples:
+  //  flags = ALLOW_HEX | ALLOW_TRAILING_JUNK,
+  //  empty_string_value = 0.0,
+  //  junk_string_value = NaN,
+  //  infinity_symbol = "infinity",
+  //  nan_symbol = "nan":
+  //    StringToDouble("0x1234") -> 4660.0.
+  //    StringToDouble("0x1234K") -> 4660.0.
+  //    StringToDouble("") -> 0.0  // empty_string_value.
+  //    StringToDouble(" ") -> NaN  // junk_string_value.
+  //    StringToDouble(" 1") -> NaN  // junk_string_value.
+  //    StringToDouble("0x") -> NaN  // junk_string_value.
+  //    StringToDouble("-123.45") -> -123.45.
+  //    StringToDouble("--123.45") -> NaN  // junk_string_value.
+  //    StringToDouble("123e45") -> 123e45.
+  //    StringToDouble("123E45") -> 123e45.
+  //    StringToDouble("123e+45") -> 123e45.
+  //    StringToDouble("123E-45") -> 123e-45.
+  //    StringToDouble("123e") -> 123.0  // trailing junk ignored.
+  //    StringToDouble("123e-") -> 123.0  // trailing junk ignored.
+  //    StringToDouble("+NaN") -> NaN  // NaN string literal.
+  //    StringToDouble("-infinity") -> -inf.  // infinity literal.
+  //    StringToDouble("Infinity") -> NaN  // junk_string_value.
+  //
+  //  flags = ALLOW_OCTAL | ALLOW_LEADING_SPACES,
+  //  empty_string_value = 0.0,
+  //  junk_string_value = NaN,
+  //  infinity_symbol = NULL,
+  //  nan_symbol = NULL:
+  //    StringToDouble("0x1234") -> NaN  // junk_string_value.
+  //    StringToDouble("01234") -> 668.0.
+  //    StringToDouble("") -> 0.0  // empty_string_value.
+  //    StringToDouble(" ") -> 0.0  // empty_string_value.
+  //    StringToDouble(" 1") -> 1.0
+  //    StringToDouble("0x") -> NaN  // junk_string_value.
+  //    StringToDouble("0123e45") -> NaN  // junk_string_value.
+  //    StringToDouble("01239E45") -> 1239e45.
+  //    StringToDouble("-infinity") -> NaN  // junk_string_value.
+  //    StringToDouble("NaN") -> NaN  // junk_string_value.
+  //
+  //  flags = NO_FLAGS,
+  //  separator = ' ':
+  //    StringToDouble("1 2 3 4") -> 1234.0
+  //    StringToDouble("1  2") -> NaN // junk_string_value
+  //    StringToDouble("1 000 000.0") -> 1000000.0
+  //    StringToDouble("1.000 000") -> 1.0
+  //    StringToDouble("1.0e1 000") -> NaN // junk_string_value
+  StringToDoubleConverter(int flags,
+                          double empty_string_value,
+                          double junk_string_value,
+                          const char* infinity_symbol,
+                          const char* nan_symbol,
+                          uc16 separator = kNoSeparator)
+      : flags_(flags),
+        empty_string_value_(empty_string_value),
+        junk_string_value_(junk_string_value),
+        infinity_symbol_(infinity_symbol),
+        nan_symbol_(nan_symbol),
+        separator_(separator) {
+  }
+
+  // Performs the conversion.
+  // The output parameter 'processed_characters_count' is set to the number
+  // of characters that have been processed to read the number.
+  // Spaces than are processed with ALLOW_{LEADING|TRAILING}_SPACES are included
+  // in the 'processed_characters_count'. Trailing junk is never included.
+  MFBT_API double StringToDouble(const char* buffer,
+                        int length,
+                        int* processed_characters_count) const;
+
+  // Same as StringToDouble above but for 16 bit characters.
+  MFBT_API double StringToDouble(const uc16* buffer,
+                        int length,
+                        int* processed_characters_count) const;
+
+  // Same as StringToDouble but reads a float.
+  // Note that this is not equivalent to static_cast<float>(StringToDouble(...))
+  // due to potential double-rounding.
+  MFBT_API float StringToFloat(const char* buffer,
+                      int length,
+                      int* processed_characters_count) const;
+
+  // Same as StringToFloat above but for 16 bit characters.
+  MFBT_API float StringToFloat(const uc16* buffer,
+                      int length,
+                      int* processed_characters_count) const;
+
+  // Same as StringToDouble for T = double, and StringToFloat for T = float.
+  template <typename T>
+  T StringTo(const char* buffer,
+             int length,
+             int* processed_characters_count) const;
+
+  // Same as StringTo above but for 16 bit characters.
+  template <typename T>
+  T StringTo(const uc16* buffer,
+             int length,
+             int* processed_characters_count) const;
+
+ private:
+  const int flags_;
+  const double empty_string_value_;
+  const double junk_string_value_;
+  const char* const infinity_symbol_;
+  const char* const nan_symbol_;
+  const uc16 separator_;
+
+  template <class Iterator>
+  double StringToIeee(Iterator start_pointer,
+                      int length,
+                      bool read_as_double,
+                      int* processed_characters_count) const;
+
+  DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS(StringToDoubleConverter);
+};
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_STRING_TO_DOUBLE_H_
diff --git a/mfbt/double-conversion/double-conversion/strtod.cc b/mfbt/double-conversion/double-conversion/strtod.cc
new file mode 100644
index 0000000000..b9d8443c24
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/strtod.cc
@@ -0,0 +1,610 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <climits>
+#include <cstdarg>
+
+#include "bignum.h"
+#include "cached-powers.h"
+#include "ieee.h"
+#include "strtod.h"
+
+namespace double_conversion {
+
+#if defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+// 2^53 = 9007199254740992.
+// Any integer with at most 15 decimal digits will hence fit into a double
+// (which has a 53bit significand) without loss of precision.
+static const int kMaxExactDoubleIntegerDecimalDigits = 15;
+#endif // #if defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+// 2^64 = 18446744073709551616 > 10^19
+static const int kMaxUint64DecimalDigits = 19;
+
+// Max double: 1.7976931348623157 x 10^308
+// Min non-zero double: 4.9406564584124654 x 10^-324
+// Any x >= 10^309 is interpreted as +infinity.
+// Any x <= 10^-324 is interpreted as 0.
+// Note that 2.5e-324 (despite being smaller than the min double) will be read
+// as non-zero (equal to the min non-zero double).
+static const int kMaxDecimalPower = 309;
+static const int kMinDecimalPower = -324;
+
+// 2^64 = 18446744073709551616
+static const uint64_t kMaxUint64 = DOUBLE_CONVERSION_UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF);
+
+
+#if defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+static const double exact_powers_of_ten[] = {
+  1.0,  // 10^0
+  10.0,
+  100.0,
+  1000.0,
+  10000.0,
+  100000.0,
+  1000000.0,
+  10000000.0,
+  100000000.0,
+  1000000000.0,
+  10000000000.0,  // 10^10
+  100000000000.0,
+  1000000000000.0,
+  10000000000000.0,
+  100000000000000.0,
+  1000000000000000.0,
+  10000000000000000.0,
+  100000000000000000.0,
+  1000000000000000000.0,
+  10000000000000000000.0,
+  100000000000000000000.0,  // 10^20
+  1000000000000000000000.0,
+  // 10^22 = 0x21e19e0c9bab2400000 = 0x878678326eac9 * 2^22
+  10000000000000000000000.0
+};
+static const int kExactPowersOfTenSize = DOUBLE_CONVERSION_ARRAY_SIZE(exact_powers_of_ten);
+#endif // #if defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+
+// Maximum number of significant digits in the decimal representation.
+// In fact the value is 772 (see conversions.cc), but to give us some margin
+// we round up to 780.
+static const int kMaxSignificantDecimalDigits = 780;
+
+static Vector<const char> TrimLeadingZeros(Vector<const char> buffer) {
+  for (int i = 0; i < buffer.length(); i++) {
+    if (buffer[i] != '0') {
+      return buffer.SubVector(i, buffer.length());
+    }
+  }
+  return Vector<const char>(buffer.start(), 0);
+}
+
+static void CutToMaxSignificantDigits(Vector<const char> buffer,
+                                       int exponent,
+                                       char* significant_buffer,
+                                       int* significant_exponent) {
+  for (int i = 0; i < kMaxSignificantDecimalDigits - 1; ++i) {
+    significant_buffer[i] = buffer[i];
+  }
+  // The input buffer has been trimmed. Therefore the last digit must be
+  // different from '0'.
+  DOUBLE_CONVERSION_ASSERT(buffer[buffer.length() - 1] != '0');
+  // Set the last digit to be non-zero. This is sufficient to guarantee
+  // correct rounding.
+  significant_buffer[kMaxSignificantDecimalDigits - 1] = '1';
+  *significant_exponent =
+      exponent + (buffer.length() - kMaxSignificantDecimalDigits);
+}
+
+
+// Trims the buffer and cuts it to at most kMaxSignificantDecimalDigits.
+// If possible the input-buffer is reused, but if the buffer needs to be
+// modified (due to cutting), then the input needs to be copied into the
+// buffer_copy_space.
+static void TrimAndCut(Vector<const char> buffer, int exponent,
+                       char* buffer_copy_space, int space_size,
+                       Vector<const char>* trimmed, int* updated_exponent) {
+  Vector<const char> left_trimmed = TrimLeadingZeros(buffer);
+  Vector<const char> right_trimmed = TrimTrailingZeros(left_trimmed);
+  exponent += left_trimmed.length() - right_trimmed.length();
+  if (right_trimmed.length() > kMaxSignificantDecimalDigits) {
+    (void) space_size;  // Mark variable as used.
+    DOUBLE_CONVERSION_ASSERT(space_size >= kMaxSignificantDecimalDigits);
+    CutToMaxSignificantDigits(right_trimmed, exponent,
+                              buffer_copy_space, updated_exponent);
+    *trimmed = Vector<const char>(buffer_copy_space,
+                                 kMaxSignificantDecimalDigits);
+  } else {
+    *trimmed = right_trimmed;
+    *updated_exponent = exponent;
+  }
+}
+
+
+// Reads digits from the buffer and converts them to a uint64.
+// Reads in as many digits as fit into a uint64.
+// When the string starts with "1844674407370955161" no further digit is read.
+// Since 2^64 = 18446744073709551616 it would still be possible read another
+// digit if it was less or equal than 6, but this would complicate the code.
+static uint64_t ReadUint64(Vector<const char> buffer,
+                           int* number_of_read_digits) {
+  uint64_t result = 0;
+  int i = 0;
+  while (i < buffer.length() && result <= (kMaxUint64 / 10 - 1)) {
+    int digit = buffer[i++] - '0';
+    DOUBLE_CONVERSION_ASSERT(0 <= digit && digit <= 9);
+    result = 10 * result + digit;
+  }
+  *number_of_read_digits = i;
+  return result;
+}
+
+
+// Reads a DiyFp from the buffer.
+// The returned DiyFp is not necessarily normalized.
+// If remaining_decimals is zero then the returned DiyFp is accurate.
+// Otherwise it has been rounded and has error of at most 1/2 ulp.
+static void ReadDiyFp(Vector<const char> buffer,
+                      DiyFp* result,
+                      int* remaining_decimals) {
+  int read_digits;
+  uint64_t significand = ReadUint64(buffer, &read_digits);
+  if (buffer.length() == read_digits) {
+    *result = DiyFp(significand, 0);
+    *remaining_decimals = 0;
+  } else {
+    // Round the significand.
+    if (buffer[read_digits] >= '5') {
+      significand++;
+    }
+    // Compute the binary exponent.
+    int exponent = 0;
+    *result = DiyFp(significand, exponent);
+    *remaining_decimals = buffer.length() - read_digits;
+  }
+}
+
+
+static bool DoubleStrtod(Vector<const char> trimmed,
+                         int exponent,
+                         double* result) {
+#if !defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+  // Avoid "unused parameter" warnings
+  (void) trimmed;
+  (void) exponent;
+  (void) result;
+  // On x86 the floating-point stack can be 64 or 80 bits wide. If it is
+  // 80 bits wide (as is the case on Linux) then double-rounding occurs and the
+  // result is not accurate.
+  // We know that Windows32 uses 64 bits and is therefore accurate.
+  return false;
+#else
+  if (trimmed.length() <= kMaxExactDoubleIntegerDecimalDigits) {
+    int read_digits;
+    // The trimmed input fits into a double.
+    // If the 10^exponent (resp. 10^-exponent) fits into a double too then we
+    // can compute the result-double simply by multiplying (resp. dividing) the
+    // two numbers.
+    // This is possible because IEEE guarantees that floating-point operations
+    // return the best possible approximation.
+    if (exponent < 0 && -exponent < kExactPowersOfTenSize) {
+      // 10^-exponent fits into a double.
+      *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
+      DOUBLE_CONVERSION_ASSERT(read_digits == trimmed.length());
+      *result /= exact_powers_of_ten[-exponent];
+      return true;
+    }
+    if (0 <= exponent && exponent < kExactPowersOfTenSize) {
+      // 10^exponent fits into a double.
+      *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
+      DOUBLE_CONVERSION_ASSERT(read_digits == trimmed.length());
+      *result *= exact_powers_of_ten[exponent];
+      return true;
+    }
+    int remaining_digits =
+        kMaxExactDoubleIntegerDecimalDigits - trimmed.length();
+    if ((0 <= exponent) &&
+        (exponent - remaining_digits < kExactPowersOfTenSize)) {
+      // The trimmed string was short and we can multiply it with
+      // 10^remaining_digits. As a result the remaining exponent now fits
+      // into a double too.
+      *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
+      DOUBLE_CONVERSION_ASSERT(read_digits == trimmed.length());
+      *result *= exact_powers_of_ten[remaining_digits];
+      *result *= exact_powers_of_ten[exponent - remaining_digits];
+      return true;
+    }
+  }
+  return false;
+#endif
+}
+
+
+// Returns 10^exponent as an exact DiyFp.
+// The given exponent must be in the range [1; kDecimalExponentDistance[.
+static DiyFp AdjustmentPowerOfTen(int exponent) {
+  DOUBLE_CONVERSION_ASSERT(0 < exponent);
+  DOUBLE_CONVERSION_ASSERT(exponent < PowersOfTenCache::kDecimalExponentDistance);
+  // Simply hardcode the remaining powers for the given decimal exponent
+  // distance.
+  DOUBLE_CONVERSION_ASSERT(PowersOfTenCache::kDecimalExponentDistance == 8);
+  switch (exponent) {
+    case 1: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0xa0000000, 00000000), -60);
+    case 2: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0xc8000000, 00000000), -57);
+    case 3: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0xfa000000, 00000000), -54);
+    case 4: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0x9c400000, 00000000), -50);
+    case 5: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0xc3500000, 00000000), -47);
+    case 6: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0xf4240000, 00000000), -44);
+    case 7: return DiyFp(DOUBLE_CONVERSION_UINT64_2PART_C(0x98968000, 00000000), -40);
+    default:
+      DOUBLE_CONVERSION_UNREACHABLE();
+  }
+}
+
+
+// If the function returns true then the result is the correct double.
+// Otherwise it is either the correct double or the double that is just below
+// the correct double.
+static bool DiyFpStrtod(Vector<const char> buffer,
+                        int exponent,
+                        double* result) {
+  DiyFp input;
+  int remaining_decimals;
+  ReadDiyFp(buffer, &input, &remaining_decimals);
+  // Since we may have dropped some digits the input is not accurate.
+  // If remaining_decimals is different than 0 than the error is at most
+  // .5 ulp (unit in the last place).
+  // We don't want to deal with fractions and therefore keep a common
+  // denominator.
+  const int kDenominatorLog = 3;
+  const int kDenominator = 1 << kDenominatorLog;
+  // Move the remaining decimals into the exponent.
+  exponent += remaining_decimals;
+  uint64_t error = (remaining_decimals == 0 ? 0 : kDenominator / 2);
+
+  int old_e = input.e();
+  input.Normalize();
+  error <<= old_e - input.e();
+
+  DOUBLE_CONVERSION_ASSERT(exponent <= PowersOfTenCache::kMaxDecimalExponent);
+  if (exponent < PowersOfTenCache::kMinDecimalExponent) {
+    *result = 0.0;
+    return true;
+  }
+  DiyFp cached_power;
+  int cached_decimal_exponent;
+  PowersOfTenCache::GetCachedPowerForDecimalExponent(exponent,
+                                                     &cached_power,
+                                                     &cached_decimal_exponent);
+
+  if (cached_decimal_exponent != exponent) {
+    int adjustment_exponent = exponent - cached_decimal_exponent;
+    DiyFp adjustment_power = AdjustmentPowerOfTen(adjustment_exponent);
+    input.Multiply(adjustment_power);
+    if (kMaxUint64DecimalDigits - buffer.length() >= adjustment_exponent) {
+      // The product of input with the adjustment power fits into a 64 bit
+      // integer.
+      DOUBLE_CONVERSION_ASSERT(DiyFp::kSignificandSize == 64);
+    } else {
+      // The adjustment power is exact. There is hence only an error of 0.5.
+      error += kDenominator / 2;
+    }
+  }
+
+  input.Multiply(cached_power);
+  // The error introduced by a multiplication of a*b equals
+  //   error_a + error_b + error_a*error_b/2^64 + 0.5
+  // Substituting a with 'input' and b with 'cached_power' we have
+  //   error_b = 0.5  (all cached powers have an error of less than 0.5 ulp),
+  //   error_ab = 0 or 1 / kDenominator > error_a*error_b/ 2^64
+  int error_b = kDenominator / 2;
+  int error_ab = (error == 0 ? 0 : 1);  // We round up to 1.
+  int fixed_error = kDenominator / 2;
+  error += error_b + error_ab + fixed_error;
+
+  old_e = input.e();
+  input.Normalize();
+  error <<= old_e - input.e();
+
+  // See if the double's significand changes if we add/subtract the error.
+  int order_of_magnitude = DiyFp::kSignificandSize + input.e();
+  int effective_significand_size =
+      Double::SignificandSizeForOrderOfMagnitude(order_of_magnitude);
+  int precision_digits_count =
+      DiyFp::kSignificandSize - effective_significand_size;
+  if (precision_digits_count + kDenominatorLog >= DiyFp::kSignificandSize) {
+    // This can only happen for very small denormals. In this case the
+    // half-way multiplied by the denominator exceeds the range of an uint64.
+    // Simply shift everything to the right.
+    int shift_amount = (precision_digits_count + kDenominatorLog) -
+        DiyFp::kSignificandSize + 1;
+    input.set_f(input.f() >> shift_amount);
+    input.set_e(input.e() + shift_amount);
+    // We add 1 for the lost precision of error, and kDenominator for
+    // the lost precision of input.f().
+    error = (error >> shift_amount) + 1 + kDenominator;
+    precision_digits_count -= shift_amount;
+  }
+  // We use uint64_ts now. This only works if the DiyFp uses uint64_ts too.
+  DOUBLE_CONVERSION_ASSERT(DiyFp::kSignificandSize == 64);
+  DOUBLE_CONVERSION_ASSERT(precision_digits_count < 64);
+  uint64_t one64 = 1;
+  uint64_t precision_bits_mask = (one64 << precision_digits_count) - 1;
+  uint64_t precision_bits = input.f() & precision_bits_mask;
+  uint64_t half_way = one64 << (precision_digits_count - 1);
+  precision_bits *= kDenominator;
+  half_way *= kDenominator;
+  DiyFp rounded_input(input.f() >> precision_digits_count,
+                      input.e() + precision_digits_count);
+  if (precision_bits >= half_way + error) {
+    rounded_input.set_f(rounded_input.f() + 1);
+  }
+  // If the last_bits are too close to the half-way case than we are too
+  // inaccurate and round down. In this case we return false so that we can
+  // fall back to a more precise algorithm.
+
+  *result = Double(rounded_input).value();
+  if (half_way - error < precision_bits && precision_bits < half_way + error) {
+    // Too imprecise. The caller will have to fall back to a slower version.
+    // However the returned number is guaranteed to be either the correct
+    // double, or the next-lower double.
+    return false;
+  } else {
+    return true;
+  }
+}
+
+
+// Returns
+//   - -1 if buffer*10^exponent < diy_fp.
+//   -  0 if buffer*10^exponent == diy_fp.
+//   - +1 if buffer*10^exponent > diy_fp.
+// Preconditions:
+//   buffer.length() + exponent <= kMaxDecimalPower + 1
+//   buffer.length() + exponent > kMinDecimalPower
+//   buffer.length() <= kMaxDecimalSignificantDigits
+static int CompareBufferWithDiyFp(Vector<const char> buffer,
+                                  int exponent,
+                                  DiyFp diy_fp) {
+  DOUBLE_CONVERSION_ASSERT(buffer.length() + exponent <= kMaxDecimalPower + 1);
+  DOUBLE_CONVERSION_ASSERT(buffer.length() + exponent > kMinDecimalPower);
+  DOUBLE_CONVERSION_ASSERT(buffer.length() <= kMaxSignificantDecimalDigits);
+  // Make sure that the Bignum will be able to hold all our numbers.
+  // Our Bignum implementation has a separate field for exponents. Shifts will
+  // consume at most one bigit (< 64 bits).
+  // ln(10) == 3.3219...
+  DOUBLE_CONVERSION_ASSERT(((kMaxDecimalPower + 1) * 333 / 100) < Bignum::kMaxSignificantBits);
+  Bignum buffer_bignum;
+  Bignum diy_fp_bignum;
+  buffer_bignum.AssignDecimalString(buffer);
+  diy_fp_bignum.AssignUInt64(diy_fp.f());
+  if (exponent >= 0) {
+    buffer_bignum.MultiplyByPowerOfTen(exponent);
+  } else {
+    diy_fp_bignum.MultiplyByPowerOfTen(-exponent);
+  }
+  if (diy_fp.e() > 0) {
+    diy_fp_bignum.ShiftLeft(diy_fp.e());
+  } else {
+    buffer_bignum.ShiftLeft(-diy_fp.e());
+  }
+  return Bignum::Compare(buffer_bignum, diy_fp_bignum);
+}
+
+
+// Returns true if the guess is the correct double.
+// Returns false, when guess is either correct or the next-lower double.
+static bool ComputeGuess(Vector<const char> trimmed, int exponent,
+                         double* guess) {
+  if (trimmed.length() == 0) {
+    *guess = 0.0;
+    return true;
+  }
+  if (exponent + trimmed.length() - 1 >= kMaxDecimalPower) {
+    *guess = Double::Infinity();
+    return true;
+  }
+  if (exponent + trimmed.length() <= kMinDecimalPower) {
+    *guess = 0.0;
+    return true;
+  }
+
+  if (DoubleStrtod(trimmed, exponent, guess) ||
+      DiyFpStrtod(trimmed, exponent, guess)) {
+    return true;
+  }
+  if (*guess == Double::Infinity()) {
+    return true;
+  }
+  return false;
+}
+
+#ifdef DEBUG
+static bool IsDigit(const char d) {
+  return ('0' <= d) && (d <= '9');
+}
+
+static bool IsNonZeroDigit(const char d) {
+  return ('1' <= d) && (d <= '9');
+}
+
+#ifdef __has_cpp_attribute
+#if __has_cpp_attribute(maybe_unused)
+[[maybe_unused]]
+#endif
+#endif
+static bool AssertTrimmedDigits(const Vector<const char>& buffer) {
+  for(int i = 0; i < buffer.length(); ++i) {
+    if(!IsDigit(buffer[i])) {
+      return false;
+    }
+  }
+  return (buffer.length() == 0) || (IsNonZeroDigit(buffer[0]) && IsNonZeroDigit(buffer[buffer.length()-1]));
+}
+#endif
+
+double StrtodTrimmed(Vector<const char> trimmed, int exponent) {
+  DOUBLE_CONVERSION_ASSERT(trimmed.length() <= kMaxSignificantDecimalDigits);
+  DOUBLE_CONVERSION_ASSERT(AssertTrimmedDigits(trimmed));
+  double guess;
+  const bool is_correct = ComputeGuess(trimmed, exponent, &guess);
+  if (is_correct) {
+    return guess;
+  }
+  DiyFp upper_boundary = Double(guess).UpperBoundary();
+  int comparison = CompareBufferWithDiyFp(trimmed, exponent, upper_boundary);
+  if (comparison < 0) {
+    return guess;
+  } else if (comparison > 0) {
+    return Double(guess).NextDouble();
+  } else if ((Double(guess).Significand() & 1) == 0) {
+    // Round towards even.
+    return guess;
+  } else {
+    return Double(guess).NextDouble();
+  }
+}
+
+double Strtod(Vector<const char> buffer, int exponent) {
+  char copy_buffer[kMaxSignificantDecimalDigits];
+  Vector<const char> trimmed;
+  int updated_exponent;
+  TrimAndCut(buffer, exponent, copy_buffer, kMaxSignificantDecimalDigits,
+             &trimmed, &updated_exponent);
+  return StrtodTrimmed(trimmed, updated_exponent);
+}
+
+static float SanitizedDoubletof(double d) {
+  DOUBLE_CONVERSION_ASSERT(d >= 0.0);
+  // ASAN has a sanitize check that disallows casting doubles to floats if
+  // they are too big.
+  // https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#available-checks
+  // The behavior should be covered by IEEE 754, but some projects use this
+  // flag, so work around it.
+  float max_finite = 3.4028234663852885981170418348451692544e+38;
+  // The half-way point between the max-finite and infinity value.
+  // Since infinity has an even significand everything equal or greater than
+  // this value should become infinity.
+  double half_max_finite_infinity =
+      3.40282356779733661637539395458142568448e+38;
+  if (d >= max_finite) {
+    if (d >= half_max_finite_infinity) {
+      return Single::Infinity();
+    } else {
+      return max_finite;
+    }
+  } else {
+    return static_cast<float>(d);
+  }
+}
+
+float Strtof(Vector<const char> buffer, int exponent) {
+  char copy_buffer[kMaxSignificantDecimalDigits];
+  Vector<const char> trimmed;
+  int updated_exponent;
+  TrimAndCut(buffer, exponent, copy_buffer, kMaxSignificantDecimalDigits,
+             &trimmed, &updated_exponent);
+  exponent = updated_exponent;
+  return StrtofTrimmed(trimmed, exponent);
+}
+
+float StrtofTrimmed(Vector<const char> trimmed, int exponent) {
+  DOUBLE_CONVERSION_ASSERT(trimmed.length() <= kMaxSignificantDecimalDigits);
+  DOUBLE_CONVERSION_ASSERT(AssertTrimmedDigits(trimmed));
+
+  double double_guess;
+  bool is_correct = ComputeGuess(trimmed, exponent, &double_guess);
+
+  float float_guess = SanitizedDoubletof(double_guess);
+  if (float_guess == double_guess) {
+    // This shortcut triggers for integer values.
+    return float_guess;
+  }
+
+  // We must catch double-rounding. Say the double has been rounded up, and is
+  // now a boundary of a float, and rounds up again. This is why we have to
+  // look at previous too.
+  // Example (in decimal numbers):
+  //    input: 12349
+  //    high-precision (4 digits): 1235
+  //    low-precision (3 digits):
+  //       when read from input: 123
+  //       when rounded from high precision: 124.
+  // To do this we simply look at the neighbors of the correct result and see
+  // if they would round to the same float. If the guess is not correct we have
+  // to look at four values (since two different doubles could be the correct
+  // double).
+
+  double double_next = Double(double_guess).NextDouble();
+  double double_previous = Double(double_guess).PreviousDouble();
+
+  float f1 = SanitizedDoubletof(double_previous);
+  float f2 = float_guess;
+  float f3 = SanitizedDoubletof(double_next);
+  float f4;
+  if (is_correct) {
+    f4 = f3;
+  } else {
+    double double_next2 = Double(double_next).NextDouble();
+    f4 = SanitizedDoubletof(double_next2);
+  }
+  (void) f2;  // Mark variable as used.
+  DOUBLE_CONVERSION_ASSERT(f1 <= f2 && f2 <= f3 && f3 <= f4);
+
+  // If the guess doesn't lie near a single-precision boundary we can simply
+  // return its float-value.
+  if (f1 == f4) {
+    return float_guess;
+  }
+
+  DOUBLE_CONVERSION_ASSERT((f1 != f2 && f2 == f3 && f3 == f4) ||
+         (f1 == f2 && f2 != f3 && f3 == f4) ||
+         (f1 == f2 && f2 == f3 && f3 != f4));
+
+  // guess and next are the two possible candidates (in the same way that
+  // double_guess was the lower candidate for a double-precision guess).
+  float guess = f1;
+  float next = f4;
+  DiyFp upper_boundary;
+  if (guess == 0.0f) {
+    float min_float = 1e-45f;
+    upper_boundary = Double(static_cast<double>(min_float) / 2).AsDiyFp();
+  } else {
+    upper_boundary = Single(guess).UpperBoundary();
+  }
+  int comparison = CompareBufferWithDiyFp(trimmed, exponent, upper_boundary);
+  if (comparison < 0) {
+    return guess;
+  } else if (comparison > 0) {
+    return next;
+  } else if ((Single(guess).Significand() & 1) == 0) {
+    // Round towards even.
+    return guess;
+  } else {
+    return next;
+  }
+}
+
+}  // namespace double_conversion
diff --git a/mfbt/double-conversion/double-conversion/strtod.h b/mfbt/double-conversion/double-conversion/strtod.h
new file mode 100644
index 0000000000..77221fb9d5
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/strtod.h
@@ -0,0 +1,64 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_STRTOD_H_
+#define DOUBLE_CONVERSION_STRTOD_H_
+
+#include "utils.h"
+
+namespace double_conversion {
+
+// The buffer must only contain digits in the range [0-9]. It must not
+// contain a dot or a sign. It must not start with '0', and must not be empty.
+double Strtod(Vector<const char> buffer, int exponent);
+
+// The buffer must only contain digits in the range [0-9]. It must not
+// contain a dot or a sign. It must not start with '0', and must not be empty.
+float Strtof(Vector<const char> buffer, int exponent);
+
+// Same as Strtod, but assumes that 'trimmed' is already trimmed, as if run
+// through TrimAndCut. That is, 'trimmed' must have no leading or trailing
+// zeros, must not be a lone zero, and must not have 'too many' digits.
+double StrtodTrimmed(Vector<const char> trimmed, int exponent);
+
+// Same as Strtof, but assumes that 'trimmed' is already trimmed, as if run
+// through TrimAndCut. That is, 'trimmed' must have no leading or trailing
+// zeros, must not be a lone zero, and must not have 'too many' digits.
+float StrtofTrimmed(Vector<const char> trimmed, int exponent);
+
+inline Vector<const char> TrimTrailingZeros(Vector<const char> buffer) {
+  for (int i = buffer.length() - 1; i >= 0; --i) {
+    if (buffer[i] != '0') {
+      return buffer.SubVector(0, i + 1);
+    }
+  }
+  return Vector<const char>(buffer.start(), 0);
+}
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_STRTOD_H_
diff --git a/mfbt/double-conversion/double-conversion/utils.h b/mfbt/double-conversion/double-conversion/utils.h
new file mode 100644
index 0000000000..629ac2b9a5
--- /dev/null
+++ b/mfbt/double-conversion/double-conversion/utils.h
@@ -0,0 +1,421 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef DOUBLE_CONVERSION_UTILS_H_
+#define DOUBLE_CONVERSION_UTILS_H_
+
+// Use DOUBLE_CONVERSION_NON_PREFIXED_MACROS to get unprefixed macros as was
+// the case in double-conversion releases prior to 3.1.6
+
+#include <cstdlib>
+#include <cstring>
+
+// For pre-C++11 compatibility
+#if __cplusplus >= 201103L
+#define DOUBLE_CONVERSION_NULLPTR nullptr
+#else
+#define DOUBLE_CONVERSION_NULLPTR NULL
+#endif
+
+#include "mozilla/Assertions.h"
+
+#ifndef DOUBLE_CONVERSION_ASSERT
+#define DOUBLE_CONVERSION_ASSERT(condition)         \
+    MOZ_ASSERT(condition)
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(ASSERT)
+#define ASSERT DOUBLE_CONVERSION_ASSERT
+#endif
+
+#ifndef DOUBLE_CONVERSION_UNIMPLEMENTED
+#define DOUBLE_CONVERSION_UNIMPLEMENTED() \
+    MOZ_CRASH("DOUBLE_CONVERSION_UNIMPLEMENTED")
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UNIMPLEMENTED)
+#define UNIMPLEMENTED DOUBLE_CONVERSION_UNIMPLEMENTED
+#endif
+
+#ifndef DOUBLE_CONVERSION_NO_RETURN
+#ifdef _MSC_VER
+#define DOUBLE_CONVERSION_NO_RETURN __declspec(noreturn)
+#else
+#define DOUBLE_CONVERSION_NO_RETURN __attribute__((noreturn))
+#endif
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(NO_RETURN)
+#define NO_RETURN DOUBLE_CONVERSION_NO_RETURN
+#endif
+
+#ifndef DOUBLE_CONVERSION_UNREACHABLE
+#ifdef _MSC_VER
+void DOUBLE_CONVERSION_NO_RETURN abort_noreturn();
+inline void abort_noreturn() { MOZ_CRASH("abort_noreturn"); }
+#define DOUBLE_CONVERSION_UNREACHABLE()   (abort_noreturn())
+#else
+#define DOUBLE_CONVERSION_UNREACHABLE()   \
+    MOZ_CRASH("DOUBLE_CONVERSION_UNREACHABLE")
+#endif
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UNREACHABLE)
+#define UNREACHABLE DOUBLE_CONVERSION_UNREACHABLE
+#endif
+
+// Not all compilers support __has_attribute and combining a check for both
+// ifdef and __has_attribute on the same preprocessor line isn't portable.
+#ifdef __has_attribute
+#   define DOUBLE_CONVERSION_HAS_ATTRIBUTE(x) __has_attribute(x)
+#else
+#   define DOUBLE_CONVERSION_HAS_ATTRIBUTE(x) 0
+#endif
+
+#ifndef DOUBLE_CONVERSION_UNUSED
+#if DOUBLE_CONVERSION_HAS_ATTRIBUTE(unused)
+#define DOUBLE_CONVERSION_UNUSED __attribute__((unused))
+#else
+#define DOUBLE_CONVERSION_UNUSED
+#endif
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UNUSED)
+#define UNUSED DOUBLE_CONVERSION_UNUSED
+#endif
+
+#if DOUBLE_CONVERSION_HAS_ATTRIBUTE(uninitialized)
+#define DOUBLE_CONVERSION_STACK_UNINITIALIZED __attribute__((uninitialized))
+#else
+#define DOUBLE_CONVERSION_STACK_UNINITIALIZED
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(STACK_UNINITIALIZED)
+#define STACK_UNINITIALIZED DOUBLE_CONVERSION_STACK_UNINITIALIZED
+#endif
+
+// Double operations detection based on target architecture.
+// Linux uses a 80bit wide floating point stack on x86. This induces double
+// rounding, which in turn leads to wrong results.
+// An easy way to test if the floating-point operations are correct is to
+// evaluate: 89255.0/1e22. If the floating-point stack is 64 bits wide then
+// the result is equal to 89255e-22.
+// The best way to test this, is to create a division-function and to compare
+// the output of the division with the expected result. (Inlining must be
+// disabled.)
+// On Linux,x86 89255e-22 != Div_double(89255.0/1e22)
+//
+// For example:
+/*
+// -- in div.c
+double Div_double(double x, double y) { return x / y; }
+
+// -- in main.c
+double Div_double(double x, double y);  // Forward declaration.
+
+int main(int argc, char** argv) {
+  return Div_double(89255.0, 1e22) == 89255e-22;
+}
+*/
+// Run as follows ./main || echo "correct"
+//
+// If it prints "correct" then the architecture should be here, in the "correct" section.
+#if defined(_M_X64) || defined(__x86_64__) || \
+    defined(__ARMEL__) || defined(__avr32__) || defined(_M_ARM) || defined(_M_ARM64) || \
+    defined(__hppa__) || defined(__ia64__) || \
+    defined(__mips__) || \
+    defined(__loongarch__) || \
+    defined(__nios2__) || defined(__ghs) || \
+    defined(__powerpc__) || defined(__ppc__) || defined(__ppc64__) || \
+    defined(_POWER) || defined(_ARCH_PPC) || defined(_ARCH_PPC64) || \
+    defined(__sparc__) || defined(__sparc) || defined(__s390__) || \
+    defined(__SH4__) || defined(__alpha__) || \
+    defined(_MIPS_ARCH_MIPS32R2) || defined(__ARMEB__) ||\
+    defined(__AARCH64EL__) || defined(__aarch64__) || defined(__AARCH64EB__) || \
+    defined(__riscv) || defined(__e2k__) || \
+    defined(__or1k__) || defined(__arc__) || defined(__ARC64__) || \
+    defined(__microblaze__) || defined(__XTENSA__) || \
+    defined(__EMSCRIPTEN__) || defined(__wasm32__)
+#define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
+#elif defined(__mc68000__) || \
+    defined(__pnacl__) || defined(__native_client__)
+#undef DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS
+#elif defined(_M_IX86) || defined(__i386__) || defined(__i386)
+#if defined(_WIN32)
+// Windows uses a 64bit wide floating point stack.
+#define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
+#else
+#undef DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS
+#endif  // _WIN32
+#else
+#error Target architecture was not detected as supported by Double-Conversion.
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(CORRECT_DOUBLE_OPERATIONS)
+#define CORRECT_DOUBLE_OPERATIONS DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS
+#endif
+
+#if defined(_WIN32) && !defined(__MINGW32__)
+
+typedef signed char int8_t;
+typedef unsigned char uint8_t;
+typedef short int16_t;  // NOLINT
+typedef unsigned short uint16_t;  // NOLINT
+typedef int int32_t;
+typedef unsigned int uint32_t;
+typedef __int64 int64_t;
+typedef unsigned __int64 uint64_t;
+// intptr_t and friends are defined in crtdefs.h through stdio.h.
+
+#else
+
+#include <stdint.h>
+
+#endif
+
+typedef uint16_t uc16;
+
+// The following macro works on both 32 and 64-bit platforms.
+// Usage: instead of writing 0x1234567890123456
+//      write DOUBLE_CONVERSION_UINT64_2PART_C(0x12345678,90123456);
+#define DOUBLE_CONVERSION_UINT64_2PART_C(a, b) (((static_cast<uint64_t>(a) << 32) + 0x##b##u))
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UINT64_2PART_C)
+#define UINT64_2PART_C DOUBLE_CONVERSION_UINT64_2PART_C
+#endif
+
+// The expression DOUBLE_CONVERSION_ARRAY_SIZE(a) is a compile-time constant of type
+// size_t which represents the number of elements of the given
+// array. You should only use DOUBLE_CONVERSION_ARRAY_SIZE on statically allocated
+// arrays.
+#ifndef DOUBLE_CONVERSION_ARRAY_SIZE
+#define DOUBLE_CONVERSION_ARRAY_SIZE(a)                                   \
+  ((sizeof(a) / sizeof(*(a))) /                         \
+  static_cast<size_t>(!(sizeof(a) % sizeof(*(a)))))
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(ARRAY_SIZE)
+#define ARRAY_SIZE DOUBLE_CONVERSION_ARRAY_SIZE
+#endif
+
+// A macro to disallow the evil copy constructor and operator= functions
+// This should be used in the private: declarations for a class
+#ifndef DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN
+#define DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN(TypeName)      \
+  TypeName(const TypeName&);                    \
+  void operator=(const TypeName&)
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(DC_DISALLOW_COPY_AND_ASSIGN)
+#define DC_DISALLOW_COPY_AND_ASSIGN DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN
+#endif
+
+// A macro to disallow all the implicit constructors, namely the
+// default constructor, copy constructor and operator= functions.
+//
+// This should be used in the private: declarations for a class
+// that wants to prevent anyone from instantiating it. This is
+// especially useful for classes containing only static methods.
+#ifndef DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS
+#define DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName) \
+  TypeName();                                    \
+  DOUBLE_CONVERSION_DISALLOW_COPY_AND_ASSIGN(TypeName)
+#endif
+#if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(DC_DISALLOW_IMPLICIT_CONSTRUCTORS)
+#define DC_DISALLOW_IMPLICIT_CONSTRUCTORS DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS
+#endif
+
+namespace double_conversion {
+
+inline int StrLength(const char* string) {
+  size_t length = strlen(string);
+  DOUBLE_CONVERSION_ASSERT(length == static_cast<size_t>(static_cast<int>(length)));
+  return static_cast<int>(length);
+}
+
+// This is a simplified version of V8's Vector class.
+template <typename T>
+class Vector {
+ public:
+  Vector() : start_(DOUBLE_CONVERSION_NULLPTR), length_(0) {}
+  Vector(T* data, int len) : start_(data), length_(len) {
+    DOUBLE_CONVERSION_ASSERT(len == 0 || (len > 0 && data != DOUBLE_CONVERSION_NULLPTR));
+  }
+
+  // Returns a vector using the same backing storage as this one,
+  // spanning from and including 'from', to but not including 'to'.
+  Vector<T> SubVector(int from, int to) {
+    DOUBLE_CONVERSION_ASSERT(to <= length_);
+    DOUBLE_CONVERSION_ASSERT(from < to);
+    DOUBLE_CONVERSION_ASSERT(0 <= from);
+    return Vector<T>(start() + from, to - from);
+  }
+
+  // Returns the length of the vector.
+  int length() const { return length_; }
+
+  // Returns whether or not the vector is empty.
+  bool is_empty() const { return length_ == 0; }
+
+  // Returns the pointer to the start of the data in the vector.
+  T* start() const { return start_; }
+
+  // Access individual vector elements - checks bounds in debug mode.
+  T& operator[](int index) const {
+    DOUBLE_CONVERSION_ASSERT(0 <= index && index < length_);
+    return start_[index];
+  }
+
+  T& first() { return start_[0]; }
+
+  T& last() { return start_[length_ - 1]; }
+
+  void pop_back() {
+    DOUBLE_CONVERSION_ASSERT(!is_empty());
+    --length_;
+  }
+
+ private:
+  T* start_;
+  int length_;
+};
+
+
+// Helper class for building result strings in a character buffer. The
+// purpose of the class is to use safe operations that checks the
+// buffer bounds on all operations in debug mode.
+class StringBuilder {
+ public:
+  StringBuilder(char* buffer, int buffer_size)
+      : buffer_(buffer, buffer_size), position_(0) { }
+
+  ~StringBuilder() { if (!is_finalized()) Finalize(); }
+
+  int size() const { return buffer_.length(); }
+
+  // Get the current position in the builder.
+  int position() const {
+    DOUBLE_CONVERSION_ASSERT(!is_finalized());
+    return position_;
+  }
+
+  // Reset the position.
+  void Reset() { position_ = 0; }
+
+  // Add a single character to the builder. It is not allowed to add
+  // 0-characters; use the Finalize() method to terminate the string
+  // instead.
+  void AddCharacter(char c) {
+    DOUBLE_CONVERSION_ASSERT(c != '\0');
+    DOUBLE_CONVERSION_ASSERT(!is_finalized() && position_ < buffer_.length());
+    buffer_[position_++] = c;
+  }
+
+  // Add an entire string to the builder. Uses strlen() internally to
+  // compute the length of the input string.
+  void AddString(const char* s) {
+    AddSubstring(s, StrLength(s));
+  }
+
+  // Add the first 'n' characters of the given string 's' to the
+  // builder. The input string must have enough characters.
+  void AddSubstring(const char* s, int n) {
+    DOUBLE_CONVERSION_ASSERT(!is_finalized() && position_ + n < buffer_.length());
+    DOUBLE_CONVERSION_ASSERT(static_cast<size_t>(n) <= strlen(s));
+    memmove(&buffer_[position_], s, static_cast<size_t>(n));
+    position_ += n;
+  }
+
+
+  // Add character padding to the builder. If count is non-positive,
+  // nothing is added to the builder.
+  void AddPadding(char c, int count) {
+    for (int i = 0; i < count; i++) {
+      AddCharacter(c);
+    }
+  }
+
+  // Finalize the string by 0-terminating it and returning the buffer.
+  char* Finalize() {
+    DOUBLE_CONVERSION_ASSERT(!is_finalized() && position_ < buffer_.length());
+    buffer_[position_] = '\0';
+    // Make sure nobody managed to add a 0-character to the
+    // buffer while building the string.
+    DOUBLE_CONVERSION_ASSERT(strlen(buffer_.start()) == static_cast<size_t>(position_));
+    position_ = -1;
+    DOUBLE_CONVERSION_ASSERT(is_finalized());
+    return buffer_.start();
+  }
+
+ private:
+  Vector<char> buffer_;
+  int position_;
+
+  bool is_finalized() const { return position_ < 0; }
+
+  DOUBLE_CONVERSION_DISALLOW_IMPLICIT_CONSTRUCTORS(StringBuilder);
+};
+
+// The type-based aliasing rule allows the compiler to assume that pointers of
+// different types (for some definition of different) never alias each other.
+// Thus the following code does not work:
+//
+// float f = foo();
+// int fbits = *(int*)(&f);
+//
+// The compiler 'knows' that the int pointer can't refer to f since the types
+// don't match, so the compiler may cache f in a register, leaving random data
+// in fbits.  Using C++ style casts makes no difference, however a pointer to
+// char data is assumed to alias any other pointer.  This is the 'memcpy
+// exception'.
+//
+// Bit_cast uses the memcpy exception to move the bits from a variable of one
+// type of a variable of another type.  Of course the end result is likely to
+// be implementation dependent.  Most compilers (gcc-4.2 and MSVC 2005)
+// will completely optimize BitCast away.
+//
+// There is an additional use for BitCast.
+// Recent gccs will warn when they see casts that may result in breakage due to
+// the type-based aliasing rule.  If you have checked that there is no breakage
+// you can use BitCast to cast one pointer type to another.  This confuses gcc
+// enough that it can no longer see that you have cast one pointer type to
+// another thus avoiding the warning.
+template <class Dest, class Source>
+Dest BitCast(const Source& source) {
+  // Compile time assertion: sizeof(Dest) == sizeof(Source)
+  // A compile error here means your Dest and Source have different sizes.
+#if __cplusplus >= 201103L
+  static_assert(sizeof(Dest) == sizeof(Source),
+                "source and destination size mismatch");
+#else
+  DOUBLE_CONVERSION_UNUSED
+  typedef char VerifySizesAreEqual[sizeof(Dest) == sizeof(Source) ? 1 : -1];
+#endif
+
+  Dest dest;
+  memmove(&dest, &source, sizeof(dest));
+  return dest;
+}
+
+template <class Dest, class Source>
+Dest BitCast(Source* source) {
+  return BitCast<Dest>(reinterpret_cast<uintptr_t>(source));
+}
+
+}  // namespace double_conversion
+
+#endif  // DOUBLE_CONVERSION_UTILS_H_
diff --git a/mfbt/double-conversion/to-fixed-dbl-max.patch b/mfbt/double-conversion/to-fixed-dbl-max.patch
new file mode 100644
index 0000000000..3b1cbf0b45
--- /dev/null
+++ b/mfbt/double-conversion/to-fixed-dbl-max.patch
@@ -0,0 +1,51 @@
+diff --git a/mfbt/double-conversion/double-conversion/double-to-string.cc b/mfbt/double-conversion/double-conversion/double-to-string.cc
+--- a/mfbt/double-conversion/double-conversion/double-to-string.cc
++++ b/mfbt/double-conversion/double-conversion/double-to-string.cc
+@@ -207,25 +207,21 @@ bool DoubleToStringConverter::ToShortest
+   }
+   return true;
+ }
+ 
+ 
+ bool DoubleToStringConverter::ToFixed(double value,
+                                       int requested_digits,
+                                       StringBuilder* result_builder) const {
+-  DOUBLE_CONVERSION_ASSERT(kMaxFixedDigitsBeforePoint == 60);
+-  const double kFirstNonFixed = 1e60;
+-
+   if (Double(value).IsSpecial()) {
+     return HandleSpecialValues(value, result_builder);
+   }
+ 
+   if (requested_digits > kMaxFixedDigitsAfterPoint) return false;
+-  if (value >= kFirstNonFixed || value <= -kFirstNonFixed) return false;
+ 
+   // Find a sufficiently precise decimal representation of n.
+   int decimal_point;
+   bool sign;
+   // Add space for the '\0' byte.
+   const int kDecimalRepCapacity =
+       kMaxFixedDigitsBeforePoint + kMaxFixedDigitsAfterPoint + 1;
+   char decimal_rep[kDecimalRepCapacity];
+diff --git a/mfbt/double-conversion/double-conversion/double-to-string.h b/mfbt/double-conversion/double-conversion/double-to-string.h
+--- a/mfbt/double-conversion/double-conversion/double-to-string.h
++++ b/mfbt/double-conversion/double-conversion/double-to-string.h
+@@ -33,17 +33,17 @@
+ 
+ namespace double_conversion {
+ 
+ class DoubleToStringConverter {
+  public:
+   // When calling ToFixed with a double > 10^kMaxFixedDigitsBeforePoint
+   // or a requested_digits parameter > kMaxFixedDigitsAfterPoint then the
+   // function returns false.
+-  static const int kMaxFixedDigitsBeforePoint = 60;
++  static const int kMaxFixedDigitsBeforePoint = 308;
+   static const int kMaxFixedDigitsAfterPoint = 100;
+ 
+   // When calling ToExponential with a requested_digits
+   // parameter > kMaxExponentialDigits then the function returns false.
+   static const int kMaxExponentialDigits = 120;
+ 
+   // When calling ToPrecision with a requested_digits
+   // parameter < kMinPrecisionDigits or requested_digits > kMaxPrecisionDigits
diff --git a/mfbt/double-conversion/update.sh b/mfbt/double-conversion/update.sh
new file mode 100755
index 0000000000..9e83081abe
--- /dev/null
+++ b/mfbt/double-conversion/update.sh
@@ -0,0 +1,76 @@
+#!/bin/bash
+
+# Usage: ./update.sh [<git-rev-to-use>]
+#
+# Copies the needed files from a directory containing the original
+# double-conversion source that we need.  If no revision is specified, the tip
+# revision is used.  See GIT-INFO for the last revision used.
+
+set -e
+
+LOCAL_PATCHES=""
+
+LOCAL_PATCHES="$LOCAL_PATCHES add-mfbt-api-markers.patch"
+LOCAL_PATCHES="$LOCAL_PATCHES use-mozilla-assertions.patch"
+LOCAL_PATCHES="$LOCAL_PATCHES debug-only-functions.patch"
+LOCAL_PATCHES="$LOCAL_PATCHES to-fixed-dbl-max.patch"
+
+TMPDIR=`mktemp -d`
+LOCAL_CLONE="$TMPDIR/new-double-conversion"
+
+git clone https://github.com/google/double-conversion.git "$LOCAL_CLONE"
+
+# If a particular revision was requested, check it out.
+if [ "$1" !=  "" ]; then
+  git -C "$LOCAL_CLONE" checkout "$1"
+fi
+
+# First clear out everything already present.
+DEST=./double-conversion
+mv "$DEST" "$TMPDIR"/old-double-conversion
+mkdir "$DEST"
+
+# Copy over critical files.
+cp "$LOCAL_CLONE/LICENSE" "$DEST/"
+cp "$LOCAL_CLONE/README.md" "$DEST/"
+
+# Includes
+for header in "$LOCAL_CLONE/double-conversion/"*.h; do
+  cp "$header" "$DEST/"
+done
+
+# Source
+for ccfile in "$LOCAL_CLONE/double-conversion/"*.cc; do
+  cp "$ccfile" "$DEST/"
+done
+
+# Now apply our local patches.
+for patch in $LOCAL_PATCHES; do
+  patch --directory "$DEST" --strip 4 < "$patch"
+
+  # Out-of-date patches may spew *.{orig,rej} when applied.  Report an error if
+  # any such file is found, and roll the source directory back to its previous
+  # state in such case.
+  detritus_files=`find "$DEST" -name '*.orig' -o -name '*.rej'`
+  if [ "$detritus_files" != "" ]; then
+    echo "ERROR: Local patch $patch created these detritus files when applied:"
+    echo ""
+    echo "  $detritus_files"
+    echo ""
+    echo "Please fix $patch before running $0."
+
+    rm -rf "$DEST"
+    mv "$TMPDIR"/source "$DEST"
+
+    exit 1
+  fi
+done
+
+# Update Mercurial file status.
+hg addremove "$DEST"
+
+# Note the revision used in this update.
+git -C "$LOCAL_CLONE" show -s > ./GIT-INFO
+
+# Delete the tmpdir.
+rm -rf "$TMPDIR"
diff --git a/mfbt/double-conversion/use-mozilla-assertions.patch b/mfbt/double-conversion/use-mozilla-assertions.patch
new file mode 100644
index 0000000000..4a7574f9b5
--- /dev/null
+++ b/mfbt/double-conversion/use-mozilla-assertions.patch
@@ -0,0 +1,60 @@
+diff --git a/mfbt/double-conversion/double-conversion/utils.h b/mfbt/double-conversion/double-conversion/utils.h
+--- a/mfbt/double-conversion/double-conversion/utils.h
++++ b/mfbt/double-conversion/double-conversion/utils.h
+@@ -36,27 +36,29 @@
+ 
+ // For pre-C++11 compatibility
+ #if __cplusplus >= 201103L
+ #define DOUBLE_CONVERSION_NULLPTR nullptr
+ #else
+ #define DOUBLE_CONVERSION_NULLPTR NULL
+ #endif
+ 
+-#include <cassert>
++#include "mozilla/Assertions.h"
++
+ #ifndef DOUBLE_CONVERSION_ASSERT
+ #define DOUBLE_CONVERSION_ASSERT(condition)         \
+-    assert(condition)
++    MOZ_ASSERT(condition)
+ #endif
+ #if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(ASSERT)
+ #define ASSERT DOUBLE_CONVERSION_ASSERT
+ #endif
+ 
+ #ifndef DOUBLE_CONVERSION_UNIMPLEMENTED
+-#define DOUBLE_CONVERSION_UNIMPLEMENTED() (abort())
++#define DOUBLE_CONVERSION_UNIMPLEMENTED() \
++    MOZ_CRASH("DOUBLE_CONVERSION_UNIMPLEMENTED")
+ #endif
+ #if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UNIMPLEMENTED)
+ #define UNIMPLEMENTED DOUBLE_CONVERSION_UNIMPLEMENTED
+ #endif
+ 
+ #ifndef DOUBLE_CONVERSION_NO_RETURN
+ #ifdef _MSC_VER
+ #define DOUBLE_CONVERSION_NO_RETURN __declspec(noreturn)
+@@ -66,20 +68,21 @@
+ #endif
+ #if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(NO_RETURN)
+ #define NO_RETURN DOUBLE_CONVERSION_NO_RETURN
+ #endif
+ 
+ #ifndef DOUBLE_CONVERSION_UNREACHABLE
+ #ifdef _MSC_VER
+ void DOUBLE_CONVERSION_NO_RETURN abort_noreturn();
+-inline void abort_noreturn() { abort(); }
++inline void abort_noreturn() { MOZ_CRASH("abort_noreturn"); }
+ #define DOUBLE_CONVERSION_UNREACHABLE()   (abort_noreturn())
+ #else
+-#define DOUBLE_CONVERSION_UNREACHABLE()   (abort())
++#define DOUBLE_CONVERSION_UNREACHABLE()   \
++    MOZ_CRASH("DOUBLE_CONVERSION_UNREACHABLE")
+ #endif
+ #endif
+ #if defined(DOUBLE_CONVERSION_NON_PREFIXED_MACROS) && !defined(UNREACHABLE)
+ #define UNREACHABLE DOUBLE_CONVERSION_UNREACHABLE
+ #endif
+ 
+ // Not all compilers support __has_attribute and combining a check for both
+ // ifdef and __has_attribute on the same preprocessor line isn't portable.