gecko-dev/mfbt/FloatingPoint.h

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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
/* 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/StandardInteger.h"
/*
* 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.
*
* Some users require that this file be C-compatible. Unfortunately, this means
* no mozilla namespace to contain everything, no detail namespace clarifying
* MozDoublePun to be an internal data structure, and so on.
*/
/*
* These implementations all assume |double| is a 64-bit double format number
* type, compatible with the IEEE-754 standard. C/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 continue doing so.
*/
MOZ_STATIC_ASSERT(sizeof(double) == sizeof(uint64_t), "double must be 64 bits");
/*
* Constant expressions in C can't refer to consts, unfortunately, so #define
* these rather than use |const uint64_t|.
*/
#define MOZ_DOUBLE_SIGN_BIT 0x8000000000000000ULL
#define MOZ_DOUBLE_EXPONENT_BITS 0x7ff0000000000000ULL
#define MOZ_DOUBLE_SIGNIFICAND_BITS 0x000fffffffffffffULL
#define MOZ_DOUBLE_EXPONENT_BIAS 1023
#define MOZ_DOUBLE_EXPONENT_SHIFT 52
MOZ_STATIC_ASSERT((MOZ_DOUBLE_SIGN_BIT & MOZ_DOUBLE_EXPONENT_BITS) == 0,
"sign bit doesn't overlap exponent bits");
MOZ_STATIC_ASSERT((MOZ_DOUBLE_SIGN_BIT & MOZ_DOUBLE_SIGNIFICAND_BITS) == 0,
"sign bit doesn't overlap significand bits");
MOZ_STATIC_ASSERT((MOZ_DOUBLE_EXPONENT_BITS & MOZ_DOUBLE_SIGNIFICAND_BITS) == 0,
"exponent bits don't overlap significand bits");
MOZ_STATIC_ASSERT((MOZ_DOUBLE_SIGN_BIT | MOZ_DOUBLE_EXPONENT_BITS | MOZ_DOUBLE_SIGNIFICAND_BITS)
== ~(uint64_t)0,
"all bits accounted for");
#ifdef __cplusplus
extern "C" {
#endif
/*
* This union is NOT a public data structure, and it is not to be used outside
* this file!
*/
union MozDoublePun {
/*
* Every way to pun the bits of a double introduces an additional layer of
* complexity, across a multitude of platforms, architectures, and ABIs.
* Use *only* uint64_t to reduce complexity. Don't add new punning here
* without discussion!
*/
uint64_t u;
double d;
};
/** Determines whether a double is NaN. */
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_NaN(double d)
{
union MozDoublePun pun;
pun.d = d;
/*
* A double is NaN if all exponent bits are 1 and the significand contains at
* least one non-zero bit.
*/
return (pun.u & MOZ_DOUBLE_EXPONENT_BITS) == MOZ_DOUBLE_EXPONENT_BITS &&
(pun.u & MOZ_DOUBLE_SIGNIFICAND_BITS) != 0;
}
/** Determines whether a double is +Infinity or -Infinity. */
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_INFINITE(double d)
{
union MozDoublePun pun;
pun.d = d;
/* Infinities have all exponent bits set to 1 and an all-0 significand. */
return (pun.u & ~MOZ_DOUBLE_SIGN_BIT) == MOZ_DOUBLE_EXPONENT_BITS;
}
/** Determines whether a double is not NaN or infinite. */
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_FINITE(double d)
{
union MozDoublePun pun;
pun.d = d;
/*
* NaN and Infinities are the only non-finite doubles, and both have all
* exponent bits set to 1.
*/
return (pun.u & MOZ_DOUBLE_EXPONENT_BITS) != MOZ_DOUBLE_EXPONENT_BITS;
}
/**
* Determines whether a double is negative. It is an error to call this method
* on a double which is NaN.
*/
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_NEGATIVE(double d)
{
union MozDoublePun pun;
pun.d = d;
MOZ_ASSERT(!MOZ_DOUBLE_IS_NaN(d), "NaN does not have a sign");
/* The sign bit is set if the double is negative. */
return (pun.u & MOZ_DOUBLE_SIGN_BIT) != 0;
}
/** Determines whether a double represents -0. */
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_NEGATIVE_ZERO(double d)
{
union MozDoublePun pun;
pun.d = d;
/* Only the sign bit is set if the double is -0. */
return pun.u == MOZ_DOUBLE_SIGN_BIT;
}
/** Returns the exponent portion of the double. */
static MOZ_ALWAYS_INLINE int_fast16_t
MOZ_DOUBLE_EXPONENT(double d)
{
union MozDoublePun pun;
pun.d = d;
/*
* The exponent component of a double is an unsigned number, biased from its
* actual value. Subtract the bias to retrieve the actual exponent.
*/
return (int_fast16_t)((pun.u & MOZ_DOUBLE_EXPONENT_BITS) >> MOZ_DOUBLE_EXPONENT_SHIFT) -
MOZ_DOUBLE_EXPONENT_BIAS;
}
/** Returns +Infinity. */
static MOZ_ALWAYS_INLINE double
MOZ_DOUBLE_POSITIVE_INFINITY()
{
union MozDoublePun pun;
/*
* Positive infinity has all exponent bits set, sign bit set to 0, and no
* significand.
*/
pun.u = MOZ_DOUBLE_EXPONENT_BITS;
return pun.d;
}
/** Returns -Infinity. */
static MOZ_ALWAYS_INLINE double
MOZ_DOUBLE_NEGATIVE_INFINITY()
{
union MozDoublePun pun;
/*
* Negative infinity has all exponent bits set, sign bit set to 1, and no
* significand.
*/
pun.u = MOZ_DOUBLE_SIGN_BIT | MOZ_DOUBLE_EXPONENT_BITS;
return pun.d;
}
/** Constructs a NaN value with the specified sign bit and significand bits. */
static MOZ_ALWAYS_INLINE double
MOZ_DOUBLE_SPECIFIC_NaN(int signbit, uint64_t significand)
{
union MozDoublePun pun;
MOZ_ASSERT(signbit == 0 || signbit == 1);
MOZ_ASSERT((significand & ~MOZ_DOUBLE_SIGNIFICAND_BITS) == 0);
MOZ_ASSERT(significand & MOZ_DOUBLE_SIGNIFICAND_BITS);
pun.u = (signbit ? MOZ_DOUBLE_SIGN_BIT : 0) |
MOZ_DOUBLE_EXPONENT_BITS |
significand;
MOZ_ASSERT(MOZ_DOUBLE_IS_NaN(pun.d));
return pun.d;
}
/**
* Computes a NaN value. Do not use this method if you depend upon a particular
* NaN value being returned.
*/
static MOZ_ALWAYS_INLINE double
MOZ_DOUBLE_NaN()
{
return MOZ_DOUBLE_SPECIFIC_NaN(0, 0xfffffffffffffULL);
}
/** Computes the smallest non-zero positive double value. */
static MOZ_ALWAYS_INLINE double
MOZ_DOUBLE_MIN_VALUE()
{
union MozDoublePun pun;
pun.u = 1;
return pun.d;
}
static MOZ_ALWAYS_INLINE int
MOZ_DOUBLE_IS_INT32(double d, int32_t* i)
{
/*
* XXX Casting a double 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 !MOZ_DOUBLE_IS_NEGATIVE_ZERO(d) && d == (*i = (int32_t)d);
}
#ifdef __cplusplus
} /* extern "C" */
#endif
#endif /* mozilla_FloatingPoint_h_ */