1 /*
2 * Copyright (C) 2017 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #ifndef LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_
18 #define LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_
19
20 #include <string.h> // for memcpy
21
22 namespace libtextclassifier {
23
24 // lang_id_bit_cast<Dest,Source> is a template function that implements the
25 // equivalent of "*reinterpret_cast<Dest*>(&source)". We need this in
26 // very low-level functions like the protobuf library and fast math
27 // support.
28 //
29 // float f = 3.14159265358979;
30 // int i = lang_id_bit_cast<int32>(f);
31 // // i = 0x40490fdb
32 //
33 // The classical address-casting method is:
34 //
35 // // WRONG
36 // float f = 3.14159265358979; // WRONG
37 // int i = * reinterpret_cast<int*>(&f); // WRONG
38 //
39 // The address-casting method actually produces undefined behavior
40 // according to ISO C++ specification section 3.10 -15 -. Roughly, this
41 // section says: if an object in memory has one type, and a program
42 // accesses it with a different type, then the result is undefined
43 // behavior for most values of "different type".
44 //
45 // This is true for any cast syntax, either *(int*)&f or
46 // *reinterpret_cast<int*>(&f). And it is particularly true for
47 // conversions between integral lvalues and floating-point lvalues.
48 //
49 // The purpose of 3.10 -15- is to allow optimizing compilers to assume
50 // that expressions with different types refer to different memory. gcc
51 // 4.0.1 has an optimizer that takes advantage of this. So a
52 // non-conforming program quietly produces wildly incorrect output.
53 //
54 // The problem is not the use of reinterpret_cast. The problem is type
55 // punning: holding an object in memory of one type and reading its bits
56 // back using a different type.
57 //
58 // The C++ standard is more subtle and complex than this, but that
59 // is the basic idea.
60 //
61 // Anyways ...
62 //
63 // lang_id_bit_cast<> calls memcpy() which is blessed by the standard,
64 // especially by the example in section 3.9 . Also, of course,
65 // lang_id_bit_cast<> wraps up the nasty logic in one place.
66 //
67 // Fortunately memcpy() is very fast. In optimized mode, with a
68 // constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
69 // code with the minimal amount of data movement. On a 32-bit system,
70 // memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
71 // compiles to two loads and two stores.
72 //
73 // I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1.
74 //
75 // WARNING: if Dest or Source is a non-POD type, the result of the memcpy
76 // is likely to surprise you.
77 //
78 // Props to Bill Gibbons for the compile time assertion technique and
79 // Art Komninos and Igor Tandetnik for the msvc experiments.
80 //
81 // -- mec 2005-10-17
82
83 template <class Dest, class Source>
bit_cast(const Source & source)84 inline Dest bit_cast(const Source &source) {
85 static_assert(sizeof(Dest) == sizeof(Source), "Sizes do not match");
86
87 Dest dest;
88 memcpy(&dest, &source, sizeof(dest));
89 return dest;
90 }
91
92 } // namespace libtextclassifier
93
94 #endif // LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_
95