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1 // Copyright 2016 The Chromium Authors. All rights reserved. | 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 | 2 // Use of this source code is governed by a BSD-style license that can be |
3 // found in the LICENSE file. | 3 // found in the LICENSE file. |
4 | 4 |
5 #ifndef BASE_BIT_CAST_H_ | 5 #ifndef BASE_BIT_CAST_H_ |
6 #define BASE_BIT_CAST_H_ | 6 #define BASE_BIT_CAST_H_ |
7 | 7 |
8 #include <string.h> | 8 #include <string.h> |
9 #include <type_traits> | 9 #include <type_traits> |
10 | 10 |
11 #include "base/compiler_specific.h" | 11 #include "base/compiler_specific.h" |
| 12 #include "base/template_util.h" |
12 #include "build/build_config.h" | 13 #include "build/build_config.h" |
13 | 14 |
14 // bit_cast<Dest,Source> is a template function that implements the equivalent | 15 // 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 // of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level |
16 // functions like the protobuf library and fast math support. | 17 // functions like the protobuf library and fast math support. |
17 // | 18 // |
18 // float f = 3.14159265358979; | 19 // float f = 3.14159265358979; |
19 // int i = bit_cast<int32_t>(f); | 20 // int i = bit_cast<int32_t>(f); |
20 // // i = 0x40490fdb | 21 // // i = 0x40490fdb |
21 // | 22 // |
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56 // | 57 // |
57 // Fortunately memcpy() is very fast. In optimized mode, compilers replace | 58 // 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 // 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 // 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 // load and one store, and memcpy(d,s,8) compiles to two loads and two stores. |
61 | 62 |
62 template <class Dest, class Source> | 63 template <class Dest, class Source> |
63 inline Dest bit_cast(const Source& source) { | 64 inline Dest bit_cast(const Source& source) { |
64 static_assert(sizeof(Dest) == sizeof(Source), | 65 static_assert(sizeof(Dest) == sizeof(Source), |
65 "bit_cast requires source and destination to be the same size"); | 66 "bit_cast requires source and destination to be the same size"); |
66 | 67 static_assert(base::is_trivially_copyable<Dest>::value, |
67 #if (__GNUC__ > 5 || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1) || \ | 68 "bit_cast requires the destination type to be copyable"); |
68 (defined(__clang__) && defined(_LIBCPP_VERSION))) | 69 static_assert(base::is_trivially_copyable<Source>::value, |
69 // GCC 5.1 contains the first libstdc++ with is_trivially_copyable. | 70 "bit_cast requires the source type to be 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 | 71 |
95 Dest dest; | 72 Dest dest; |
96 memcpy(&dest, &source, sizeof(dest)); | 73 memcpy(&dest, &source, sizeof(dest)); |
97 return dest; | 74 return dest; |
98 } | 75 } |
99 | 76 |
100 #endif // BASE_BIT_CAST_H_ | 77 #endif // BASE_BIT_CAST_H_ |
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