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
62 template <class Dest, class Source>
bit_cast(const Source & source)63 inline 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