The Tor Browser: mfbt/double-conversion/fixed-dtoa.cc@97036ab72558 (annotated)

mfbt/double-conversion/fixed-dtoa.cc@97036ab72558 (annotated)

mfbt/double-conversion/fixed-dtoa.cc

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 // 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 <math.h>
 #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;
     ASSERT((accumulator >> 32) == 0);
   }
   void Shift(int shift_amount) {
     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) {
     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] = '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, int requested_length,
                                     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) {
   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.
     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);
       buffer[*length] = '0' + digit;
       (*length)++;
       fractionals -= static_cast<uint64_t>(digit) << point;
     }
     // If the first bit after the point is set we have to round up.
     if (((fractionals >> (point - 1)) & 1) == 1) {
       RoundUp(buffer, length, decimal_point);
     }
   } else {  // We need 128 bits.
     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);
       buffer[*length] = '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 = 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, divisor_power, 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.
     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. Mimick
     // Gay's dtoa and and set it to -fractional_count.
     *decimal_point = -fractional_count;
   }
   return true;
 }
 }  // namespace double_conversion

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mfbt/double-conversion/fixed-dtoa.cc@97036ab72558 (annotated)

mfbt/double-conversion/fixed-dtoa.cc