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* Copyright (C) 2017 The Android Open Source Project
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include <string.h> // for memcpy
namespace libtextclassifier2 {
// bit_cast<Dest, Source> is a template function that implements the equivalent
// of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level
// functions like fast math support.
// float f = 3.14159265358979;
// int i = bit_cast<int32>(f);
// // i = 0x40490fdb
// The classical address-casting method is:
// // WRONG
// float f = 3.14159265358979; // WRONG
// int i = * reinterpret_cast<int*>(&f); // WRONG
// The address-casting method actually produces undefined behavior
// according to ISO C++ specification section 3.10 -15 -. Roughly, this
// section says: if an object in memory has one type, and a program
// accesses it with a different type, then the result is undefined
// behavior for most values of "different type".
// This is true for any cast syntax, either *(int*)&f or
// *reinterpret_cast<int*>(&f). And it is particularly true for
// conversions between integral lvalues and floating-point lvalues.
// The purpose of 3.10 -15- is to allow optimizing compilers to assume
// that expressions with different types refer to different memory. gcc
// 4.0.1 has an optimizer that takes advantage of this. So a
// non-conforming program quietly produces wildly incorrect output.
// The problem is not the use of reinterpret_cast. The problem is type
// punning: holding an object in memory of one type and reading its bits
// back using a different type.
// The C++ standard is more subtle and complex than this, but that
// is the basic idea.
// Anyways ...
// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
// example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty
// logic in one place.
// Fortunately memcpy() is very fast. In optimized mode, with a
// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
// code with the minimal amount of data movement. On a 32-bit system,
// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
// compiles to two loads and two stores.
// Mike Chastain tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc
// 7.1.
// WARNING: if Dest or Source is a non-POD type, the result of the memcpy
// is likely to surprise you.
// Props to Bill Gibbons for the compile time assertion technique and
// Art Komninos and Igor Tandetnik for the msvc experiments.
template <class Dest, class Source>
inline Dest bit_cast(const Source &source) {
static_assert(sizeof(Dest) == sizeof(Source), "Sizes do not match");
Dest dest;
memcpy(&dest, &source, sizeof(dest));
return dest;
} // namespace libtextclassifier2