1bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi/* 2bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * Copyright (C) 2017 The Android Open Source Project 3bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * 4bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * Licensed under the Apache License, Version 2.0 (the "License"); 5bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * you may not use this file except in compliance with the License. 6bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * You may obtain a copy of the License at 7bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * 8bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * http://www.apache.org/licenses/LICENSE-2.0 9bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * 10bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * Unless required by applicable law or agreed to in writing, software 11bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * distributed under the License is distributed on an "AS IS" BASIS, 12bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * See the License for the specific language governing permissions and 14bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi * limitations under the License. 15bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi */ 16bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 17bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi#ifndef LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_ 18bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi#define LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_ 19bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 20bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi#include <string.h> // for memcpy 21bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 22bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifinamespace libtextclassifier { 23bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 24bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// lang_id_bit_cast<Dest,Source> is a template function that implements the 25bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// equivalent of "*reinterpret_cast<Dest*>(&source)". We need this in 26bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// very low-level functions like the protobuf library and fast math 27bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// support. 28bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 29bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// float f = 3.14159265358979; 30bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// int i = lang_id_bit_cast<int32>(f); 31bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// // i = 0x40490fdb 32bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 33bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// The classical address-casting method is: 34bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 35bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// // WRONG 36bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// float f = 3.14159265358979; // WRONG 37bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// int i = * reinterpret_cast<int*>(&f); // WRONG 38bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 39bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// The address-casting method actually produces undefined behavior 40bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// according to ISO C++ specification section 3.10 -15 -. Roughly, this 41bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// section says: if an object in memory has one type, and a program 42bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// accesses it with a different type, then the result is undefined 43bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// behavior for most values of "different type". 44bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 45bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// This is true for any cast syntax, either *(int*)&f or 46bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// *reinterpret_cast<int*>(&f). And it is particularly true for 47bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// conversions between integral lvalues and floating-point lvalues. 48bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 49bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// The purpose of 3.10 -15- is to allow optimizing compilers to assume 50bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// that expressions with different types refer to different memory. gcc 51bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 4.0.1 has an optimizer that takes advantage of this. So a 52bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// non-conforming program quietly produces wildly incorrect output. 53bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 54bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// The problem is not the use of reinterpret_cast. The problem is type 55bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// punning: holding an object in memory of one type and reading its bits 56bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// back using a different type. 57bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 58bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// The C++ standard is more subtle and complex than this, but that 59bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// is the basic idea. 60bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 61bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// Anyways ... 62bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 63bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// lang_id_bit_cast<> calls memcpy() which is blessed by the standard, 64bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// especially by the example in section 3.9 . Also, of course, 65bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// lang_id_bit_cast<> wraps up the nasty logic in one place. 66bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 67bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// Fortunately memcpy() is very fast. In optimized mode, with a 68bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline 69bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// code with the minimal amount of data movement. On a 32-bit system, 70bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8) 71bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// compiles to two loads and two stores. 72bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 73bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1. 74bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 75bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// WARNING: if Dest or Source is a non-POD type, the result of the memcpy 76bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// is likely to surprise you. 77bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 78bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// Props to Bill Gibbons for the compile time assertion technique and 79bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// Art Komninos and Igor Tandetnik for the msvc experiments. 80bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// 81bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi// -- mec 2005-10-17 82bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 83bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifitemplate <class Dest, class Source> 84bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifiinline Dest bit_cast(const Source &source) { 85bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi static_assert(sizeof(Dest) == sizeof(Source), "Sizes do not match"); 86bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 87bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi Dest dest; 88bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi memcpy(&dest, &source, sizeof(dest)); 89bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi return dest; 90bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi} 91bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 92bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi} // namespace libtextclassifier 93bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi 94bda09f1da39ce38a5ece4757b82a64776e53214cMatt Sharifi#endif // LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_ 95