78 lines
3.0 KiB
C
78 lines
3.0 KiB
C
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// Copyright 2016 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_BIT_CAST_H_
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#define BASE_BIT_CAST_H_
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#include <string.h>
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#include <type_traits>
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#include "base/compiler_specific.h"
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#include "base/template_util.h"
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#include "build/build_config.h"
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// bit_cast<Dest,Source> is a template function that implements the equivalent
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// of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level
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// functions like the protobuf library and fast math support.
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//
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// float f = 3.14159265358979;
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// int i = bit_cast<int32_t>(f);
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// // i = 0x40490fdb
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//
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// The classical address-casting method is:
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//
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// // WRONG
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// float f = 3.14159265358979; // WRONG
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// int i = * reinterpret_cast<int*>(&f); // WRONG
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//
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// The address-casting method actually produces undefined behavior according to
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// the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15.
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// (This did not substantially change in C++11.) Roughly, this section says: if
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// an object in memory has one type, and a program accesses it with a different
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// type, then the result is undefined behavior for most values of "different
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// type".
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//
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// This is true for any cast syntax, either *(int*)&f or
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// *reinterpret_cast<int*>(&f). And it is particularly true for conversions
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// between integral lvalues and floating-point lvalues.
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//
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// The purpose of this paragraph is to allow optimizing compilers to assume that
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// expressions with different types refer to different memory. Compilers are
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// known to take advantage of this. So a non-conforming program quietly
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// produces wildly incorrect output.
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//
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// The problem is not the use of reinterpret_cast. The problem is type punning:
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// holding an object in memory of one type and reading its bits back using a
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// different type.
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//
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// The C++ standard is more subtle and complex than this, but that is the basic
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// idea.
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//
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// Anyways ...
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//
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// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
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// example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty
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// logic in one place.
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//
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// Fortunately memcpy() is very fast. In optimized mode, compilers replace
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// calls to memcpy() with inline object code when the size argument is a
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// compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one
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// load and one store, and memcpy(d,s,8) compiles to two loads and two stores.
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template <class Dest, class Source>
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inline Dest bit_cast(const Source& source) {
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static_assert(sizeof(Dest) == sizeof(Source),
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"bit_cast requires source and destination to be the same size");
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static_assert(base::is_trivially_copyable<Dest>::value,
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"bit_cast requires the destination type to be copyable");
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static_assert(base::is_trivially_copyable<Source>::value,
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"bit_cast requires the source type to be copyable");
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Dest dest;
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memcpy(&dest, &source, sizeof(dest));
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return dest;
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}
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#endif // BASE_BIT_CAST_H_
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