The Tor Browser: mfbt/FloatingPoint.h@97036ab72558 (annotated)

mfbt/FloatingPoint.h@97036ab72558 (annotated)

mfbt/FloatingPoint.h

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.

 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 /* Various predicates and operations on IEEE-754 floating point types. */
 #ifndef mozilla_FloatingPoint_h
 #define mozilla_FloatingPoint_h
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Casting.h"
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/Types.h"
 #include <stdint.h>
 namespace mozilla {
 /*
  * It's reasonable to ask why we have this header at all.  Don't isnan,
  * copysign, the built-in comparison operators, and the like solve these
  * problems?  Unfortunately, they don't.  We've found that various compilers
  * (MSVC, MSVC when compiling with PGO, and GCC on OS X, at least) miscompile
  * the standard methods in various situations, so we can't use them.  Some of
  * these compilers even have problems compiling seemingly reasonable bitwise
  * algorithms!  But with some care we've found algorithms that seem to not
  * trigger those compiler bugs.
  *
  * For the aforementioned reasons, be very wary of making changes to any of
  * these algorithms.  If you must make changes, keep a careful eye out for
  * compiler bustage, particularly PGO-specific bustage.
  */
 struct FloatTypeTraits
 {
     typedef uint32_t Bits;
     static const unsigned ExponentBias = 127;
     static const unsigned ExponentShift = 23;
     static const Bits SignBit         = 0x80000000UL;
     static const Bits ExponentBits    = 0x7F800000UL;
     static const Bits SignificandBits = 0x007FFFFFUL;
 };
 struct DoubleTypeTraits
 {
     typedef uint64_t Bits;
     static const unsigned ExponentBias = 1023;
     static const unsigned ExponentShift = 52;
     static const Bits SignBit         = 0x8000000000000000ULL;
     static const Bits ExponentBits    = 0x7ff0000000000000ULL;
     static const Bits SignificandBits = 0x000fffffffffffffULL;
 };
 template<typename T> struct SelectTrait;
 template<> struct SelectTrait<float> : public FloatTypeTraits {};
 template<> struct SelectTrait<double> : public DoubleTypeTraits {};
 /*
  *  This struct contains details regarding the encoding of floating-point
  *  numbers that can be useful for direct bit manipulation. As of now, the
  *  template parameter has to be float or double.
  *
  *  The nested typedef |Bits| is the unsigned integral type with the same size
  *  as T: uint32_t for float and uint64_t for double (static assertions
  *  double-check these assumptions).
  *
  *  ExponentBias is the offset that is subtracted from the exponent when
  *  computing the value, i.e. one plus the opposite of the mininum possible
  *  exponent.
  *  ExponentShift is the shift that one needs to apply to retrieve the exponent
  *  component of the value.
  *
  *  SignBit contains a bits mask. Bit-and-ing with this mask will result in
  *  obtaining the sign bit.
  *  ExponentBits contains the mask needed for obtaining the exponent bits and
  *  SignificandBits contains the mask needed for obtaining the significand bits.
  *
  *  Full details of how floating point number formats are encoded are beyond the
  *  scope of this comment. For more information, see
  *  http://en.wikipedia.org/wiki/IEEE_floating_point
  *  http://en.wikipedia.org/wiki/Floating_point#IEEE_754:_floating_point_in_modern_computers
  */
 template<typename T>
 struct FloatingPoint : public SelectTrait<T>
 {
     typedef SelectTrait<T> Base;
     typedef typename Base::Bits Bits;
     static_assert((Base::SignBit & Base::ExponentBits) == 0,
                   "sign bit shouldn't overlap exponent bits");
     static_assert((Base::SignBit & Base::SignificandBits) == 0,
                   "sign bit shouldn't overlap significand bits");
     static_assert((Base::ExponentBits & Base::SignificandBits) == 0,
                   "exponent bits shouldn't overlap significand bits");
     static_assert((Base::SignBit | Base::ExponentBits | Base::SignificandBits) ==
                   ~Bits(0),
                   "all bits accounted for");
     /*
      * These implementations assume float/double are 32/64-bit single/double format
      * number types compatible with the IEEE-754 standard.  C++ don't require this
      * to be the case.  But we required this in implementations of these algorithms
      * that preceded this header, so we shouldn't break anything if we keep doing so.
      */
     static_assert(sizeof(T) == sizeof(Bits), "Bits must be same size as T");
 };
 /** Determines whether a double is NaN. */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 IsNaN(T t)
 {
   /*
    * A float/double is NaN if all exponent bits are 1 and the significand contains at
    * least one non-zero bit.
    */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return (bits & Traits::ExponentBits) == Traits::ExponentBits &&
          (bits & Traits::SignificandBits) != 0;
 }
 /** Determines whether a float/double is +Infinity or -Infinity. */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 IsInfinite(T t)
 {
   /* Infinities have all exponent bits set to 1 and an all-0 significand. */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return (bits & ~Traits::SignBit) == Traits::ExponentBits;
 }
 /** Determines whether a float/double is not NaN or infinite. */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 IsFinite(T t)
 {
   /*
    * NaN and Infinities are the only non-finite floats/doubles, and both have all
    * exponent bits set to 1.
    */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return (bits & Traits::ExponentBits) != Traits::ExponentBits;
 }
 /**
  * Determines whether a float/double is negative.  It is an error to call this method
  * on a float/double which is NaN.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 IsNegative(T t)
 {
   MOZ_ASSERT(!IsNaN(t), "NaN does not have a sign");
   /* The sign bit is set if the double is negative. */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return (bits & Traits::SignBit) != 0;
 }
 /** Determines whether a float/double represents -0. */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 IsNegativeZero(T t)
 {
   /* Only the sign bit is set if the value is -0. */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return bits == Traits::SignBit;
 }
 /**
  * Returns the exponent portion of the float/double.
  *
  * Zero is not special-cased, so ExponentComponent(0.0) is
  * -int_fast16_t(Traits::ExponentBias).
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE int_fast16_t
 ExponentComponent(T t)
 {
   /*
    * The exponent component of a float/double is an unsigned number, biased from its
    * actual value.  Subtract the bias to retrieve the actual exponent.
    */
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   Bits bits = BitwiseCast<Bits>(t);
   return int_fast16_t((bits & Traits::ExponentBits) >> Traits::ExponentShift) -
          int_fast16_t(Traits::ExponentBias);
 }
 /** Returns +Infinity. */
 template<typename T>
 static MOZ_ALWAYS_INLINE T
 PositiveInfinity()
 {
   /*
    * Positive infinity has all exponent bits set, sign bit set to 0, and no
    * significand.
    */
   typedef FloatingPoint<T> Traits;
   return BitwiseCast<T>(Traits::ExponentBits);
 }
 /** Returns -Infinity. */
 template<typename T>
 static MOZ_ALWAYS_INLINE T
 NegativeInfinity()
 {
   /*
    * Negative infinity has all exponent bits set, sign bit set to 1, and no
    * significand.
    */
   typedef FloatingPoint<T> Traits;
   return BitwiseCast<T>(Traits::SignBit | Traits::ExponentBits);
 }
 /** Constructs a NaN value with the specified sign bit and significand bits. */
 template<typename T>
 static MOZ_ALWAYS_INLINE T
 SpecificNaN(int signbit, typename FloatingPoint<T>::Bits significand)
 {
   typedef FloatingPoint<T> Traits;
   MOZ_ASSERT(signbit == 0 || signbit == 1);
   MOZ_ASSERT((significand & ~Traits::SignificandBits) == 0);
   MOZ_ASSERT(significand & Traits::SignificandBits);
   T t = BitwiseCast<T>((signbit ? Traits::SignBit : 0) |
                        Traits::ExponentBits |
                        significand);
   MOZ_ASSERT(IsNaN(t));
   return t;
 }
 /** Computes the smallest non-zero positive float/double value. */
 template<typename T>
 static MOZ_ALWAYS_INLINE T
 MinNumberValue()
 {
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   return BitwiseCast<T>(Bits(1));
 }
 /**
  * If t is equal to some int32_t value, set *i to that value and return true;
  * otherwise return false.
  *
  * Note that negative zero is "equal" to zero here. To test whether a value can
  * be losslessly converted to int32_t and back, use NumberIsInt32 instead.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 NumberEqualsInt32(T t, int32_t* i)
 {
   /*
    * XXX Casting a floating-point value that doesn't truncate to int32_t, to
    *     int32_t, induces undefined behavior.  We should definitely fix this
    *     (bug 744965), but as apparently it "works" in practice, it's not a
    *     pressing concern now.
    */
   return t == (*i = int32_t(t));
 }
 /**
  * If d can be converted to int32_t and back to an identical double value,
  * set *i to that value and return true; otherwise return false.
  *
  * The difference between this and NumberEqualsInt32 is that this method returns
  * false for negative zero.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 NumberIsInt32(T t, int32_t* i)
 {
   return !IsNegativeZero(t) && NumberEqualsInt32(t, i);
 }
 /**
  * Computes a NaN value.  Do not use this method if you depend upon a particular
  * NaN value being returned.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE T
 UnspecifiedNaN()
 {
   /*
    * If we can use any quiet NaN, we might as well use the all-ones NaN,
    * since it's cheap to materialize on common platforms (such as x64, where
    * this value can be represented in a 32-bit signed immediate field, allowing
    * it to be stored to memory in a single instruction).
    */
   typedef FloatingPoint<T> Traits;
   return SpecificNaN<T>(1, Traits::SignificandBits);
 }
 /**
  * Compare two doubles for equality, *without* equating -0 to +0, and equating
  * any NaN value to any other NaN value.  (The normal equality operators equate
  * -0 with +0, and they equate NaN to no other value.)
  */
 template<typename T>
 static inline bool
 NumbersAreIdentical(T t1, T t2)
 {
   typedef FloatingPoint<T> Traits;
   typedef typename Traits::Bits Bits;
   if (IsNaN(t1))
     return IsNaN(t2);
   return BitwiseCast<Bits>(t1) == BitwiseCast<Bits>(t2);
 }
 namespace detail {
 template<typename T>
 struct FuzzyEqualsEpsilon;
 template<>
 struct FuzzyEqualsEpsilon<float>
 {
   // A number near 1e-5 that is exactly representable in
   // floating point
   static const float value() { return 1.0f / (1 << 17); }
 };
 template<>
 struct FuzzyEqualsEpsilon<double>
 {
   // A number near 1e-12 that is exactly representable in
   // a double
   static const double value() { return 1.0 / (1LL << 40); }
 };
 } // namespace detail
 /**
  * Compare two floating point values for equality, modulo rounding error. That
  * is, the two values are considered equal if they are both not NaN and if they
  * are less than or equal to epsilon apart. The default value of epsilon is near
  * 1e-5.
  *
  * For most scenarios you will want to use FuzzyEqualsMultiplicative instead,
  * as it is more reasonable over the entire range of floating point numbers.
  * This additive version should only be used if you know the range of the numbers
  * you are dealing with is bounded and stays around the same order of magnitude.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 FuzzyEqualsAdditive(T val1, T val2, T epsilon = detail::FuzzyEqualsEpsilon<T>::value())
 {
   static_assert(IsFloatingPoint<T>::value, "floating point type required");
   return Abs(val1 - val2) <= epsilon;
 }
 /**
  * Compare two floating point values for equality, allowing for rounding error
  * relative to the magnitude of the values. That is, the two values are
  * considered equal if they are both not NaN and they are less than or equal to
  * some epsilon apart, where the epsilon is scaled by the smaller of the two
  * argument values.
  *
  * In most cases you will want to use this rather than FuzzyEqualsAdditive, as
  * this function effectively masks out differences in the bottom few bits of
  * the floating point numbers being compared, regardless of what order of magnitude
  * those numbers are at.
  */
 template<typename T>
 static MOZ_ALWAYS_INLINE bool
 FuzzyEqualsMultiplicative(T val1, T val2, T epsilon = detail::FuzzyEqualsEpsilon<T>::value())
 {
   static_assert(IsFloatingPoint<T>::value, "floating point type required");
   // can't use std::min because of bug 965340
   T smaller = Abs(val1) < Abs(val2) ? Abs(val1) : Abs(val2);
   return Abs(val1 - val2) <= epsilon * smaller;
 }
 /**
  * Returns true if the given value can be losslessly represented as an IEEE-754
  * single format number, false otherwise.  All NaN values are considered
  * representable (notwithstanding that the exact bit pattern of a double format
  * NaN value can't be exactly represented in single format).
  *
  * This function isn't inlined to avoid buggy optimizations by MSVC.
  */
 MOZ_WARN_UNUSED_RESULT
 extern MFBT_API bool
 IsFloat32Representable(double x);
 } /* namespace mozilla */
 #endif /* mozilla_FloatingPoint_h */

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mfbt/FloatingPoint.h@97036ab72558 (annotated)

mfbt/FloatingPoint.h