1// Copyright 2016 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_BIT_CAST_H_
6#define BASE_BIT_CAST_H_
7
8#include <string.h>
9#include <type_traits>
10
11#include "base/compiler_specific.h"
12#include "build/build_config.h"
13
14// bit_cast<Dest,Source> is a template function that implements the equivalent
15// of "*reinterpret_cast<Dest*>(&source)".  We need this in very low-level
16// functions like the protobuf library and fast math support.
17//
18//   float f = 3.14159265358979;
19//   int i = bit_cast<int32_t>(f);
20//   // i = 0x40490fdb
21//
22// The classical address-casting method is:
23//
24//   // WRONG
25//   float f = 3.14159265358979;            // WRONG
26//   int i = * reinterpret_cast<int*>(&f);  // WRONG
27//
28// The address-casting method actually produces undefined behavior according to
29// the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15.
30// (This did not substantially change in C++11.)  Roughly, this section says: if
31// an object in memory has one type, and a program accesses it with a different
32// type, then the result is undefined behavior for most values of "different
33// type".
34//
35// This is true for any cast syntax, either *(int*)&f or
36// *reinterpret_cast<int*>(&f).  And it is particularly true for conversions
37// between integral lvalues and floating-point lvalues.
38//
39// The purpose of this paragraph is to allow optimizing compilers to assume that
40// expressions with different types refer to different memory.  Compilers are
41// known to take advantage of this.  So a non-conforming program quietly
42// produces wildly incorrect output.
43//
44// The problem is not the use of reinterpret_cast.  The problem is type punning:
45// holding an object in memory of one type and reading its bits back using a
46// different type.
47//
48// The C++ standard is more subtle and complex than this, but that is the basic
49// idea.
50//
51// Anyways ...
52//
53// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
54// example in section 3.9 .  Also, of course, bit_cast<> wraps up the nasty
55// logic in one place.
56//
57// Fortunately memcpy() is very fast.  In optimized mode, compilers replace
58// calls to memcpy() with inline object code when the size argument is a
59// compile-time constant.  On a 32-bit system, memcpy(d,s,4) compiles to one
60// load and one store, and memcpy(d,s,8) compiles to two loads and two stores.
61
62template <class Dest, class Source>
63inline Dest bit_cast(const Source& source) {
64  static_assert(sizeof(Dest) == sizeof(Source),
65                "bit_cast requires source and destination to be the same size");
66
67#if (__GNUC__ > 5 || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1) || \
68     (defined(__clang__) && defined(_LIBCPP_VERSION)))
69  // GCC 5.1 contains the first libstdc++ with is_trivially_copyable.
70  // Assume libc++ Just Works: is_trivially_copyable added on May 13th 2011.
71  // However, with libc++ when GCC is the compiler the trait is buggy, see
72  // crbug.com/607158, so fall back to the less strict variant for non-clang.
73  static_assert(std::is_trivially_copyable<Dest>::value,
74                "non-trivially-copyable bit_cast is undefined");
75  static_assert(std::is_trivially_copyable<Source>::value,
76                "non-trivially-copyable bit_cast is undefined");
77#elif HAS_FEATURE(is_trivially_copyable)
78  // The compiler supports an equivalent intrinsic.
79  static_assert(__is_trivially_copyable(Dest),
80                "non-trivially-copyable bit_cast is undefined");
81  static_assert(__is_trivially_copyable(Source),
82                "non-trivially-copyable bit_cast is undefined");
83#elif COMPILER_GCC
84  // Fallback to compiler intrinsic on GCC and clang (which pretends to be
85  // GCC). This isn't quite the same as is_trivially_copyable but it'll do for
86  // our purpose.
87  static_assert(__has_trivial_copy(Dest),
88                "non-trivially-copyable bit_cast is undefined");
89  static_assert(__has_trivial_copy(Source),
90                "non-trivially-copyable bit_cast is undefined");
91#else
92  // Do nothing, let the bots handle it.
93#endif
94
95  Dest dest;
96  memcpy(&dest, &source, sizeof(dest));
97  return dest;
98}
99
100#endif  // BASE_BIT_CAST_H_
101