Luis Hector Chavez | 0c4f26a | 2016-07-15 16:23:21 -0700 | [diff] [blame] | 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" |
Jay Civelli | 3a83cdd | 2017-03-22 17:31:44 -0700 | [diff] [blame] | 12 | #include "base/template_util.h" |
Luis Hector Chavez | 0c4f26a | 2016-07-15 16:23:21 -0700 | [diff] [blame] | 13 | #include "build/build_config.h" |
| 14 | |
| 15 | // bit_cast<Dest,Source> is a template function that implements the equivalent |
| 16 | // of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level |
| 17 | // functions like the protobuf library and fast math support. |
| 18 | // |
| 19 | // float f = 3.14159265358979; |
| 20 | // int i = bit_cast<int32_t>(f); |
| 21 | // // i = 0x40490fdb |
| 22 | // |
| 23 | // The classical address-casting method is: |
| 24 | // |
| 25 | // // WRONG |
| 26 | // float f = 3.14159265358979; // WRONG |
| 27 | // int i = * reinterpret_cast<int*>(&f); // WRONG |
| 28 | // |
| 29 | // The address-casting method actually produces undefined behavior according to |
| 30 | // the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15. |
| 31 | // (This did not substantially change in C++11.) Roughly, this section says: if |
| 32 | // an object in memory has one type, and a program accesses it with a different |
| 33 | // type, then the result is undefined behavior for most values of "different |
| 34 | // type". |
| 35 | // |
| 36 | // This is true for any cast syntax, either *(int*)&f or |
| 37 | // *reinterpret_cast<int*>(&f). And it is particularly true for conversions |
| 38 | // between integral lvalues and floating-point lvalues. |
| 39 | // |
| 40 | // The purpose of this paragraph is to allow optimizing compilers to assume that |
| 41 | // expressions with different types refer to different memory. Compilers are |
| 42 | // known to take advantage of this. So a non-conforming program quietly |
| 43 | // produces wildly incorrect output. |
| 44 | // |
| 45 | // The problem is not the use of reinterpret_cast. The problem is type punning: |
| 46 | // holding an object in memory of one type and reading its bits back using a |
| 47 | // different type. |
| 48 | // |
| 49 | // The C++ standard is more subtle and complex than this, but that is the basic |
| 50 | // idea. |
| 51 | // |
| 52 | // Anyways ... |
| 53 | // |
| 54 | // bit_cast<> calls memcpy() which is blessed by the standard, especially by the |
| 55 | // example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty |
| 56 | // logic in one place. |
| 57 | // |
| 58 | // Fortunately memcpy() is very fast. In optimized mode, compilers replace |
| 59 | // calls to memcpy() with inline object code when the size argument is a |
| 60 | // compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one |
| 61 | // load and one store, and memcpy(d,s,8) compiles to two loads and two stores. |
| 62 | |
| 63 | template <class Dest, class Source> |
| 64 | inline Dest bit_cast(const Source& source) { |
| 65 | static_assert(sizeof(Dest) == sizeof(Source), |
| 66 | "bit_cast requires source and destination to be the same size"); |
Jay Civelli | 3a83cdd | 2017-03-22 17:31:44 -0700 | [diff] [blame] | 67 | static_assert(base::is_trivially_copyable<Dest>::value, |
| 68 | "bit_cast requires the destination type to be copyable"); |
| 69 | static_assert(base::is_trivially_copyable<Source>::value, |
| 70 | "bit_cast requires the source type to be copyable"); |
Luis Hector Chavez | 0c4f26a | 2016-07-15 16:23:21 -0700 | [diff] [blame] | 71 | |
| 72 | Dest dest; |
| 73 | memcpy(&dest, &source, sizeof(dest)); |
| 74 | return dest; |
| 75 | } |
| 76 | |
| 77 | #endif // BASE_BIT_CAST_H_ |