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93 lines
3.4 KiB
93 lines
3.4 KiB
/*
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* Copyright (C) 2018 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef LIBTEXTCLASSIFIER_UTILS_BASE_CASTS_H_
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#define LIBTEXTCLASSIFIER_UTILS_BASE_CASTS_H_
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#include <string.h> // for memcpy
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namespace libtextclassifier3 {
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// bit_cast<Dest, Source> is a template function that implements the equivalent
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// of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level
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// functions like fast math support.
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//
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// float f = 3.14159265358979;
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// int i = bit_cast<int32>(f);
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// // i = 0x40490fdb
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//
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// The classical address-casting method is:
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//
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// // WRONG
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// float f = 3.14159265358979; // WRONG
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// int i = * reinterpret_cast<int*>(&f); // WRONG
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//
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// The address-casting method actually produces undefined behavior
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// according to ISO C++ specification section 3.10 -15 -. Roughly, this
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// section says: if an object in memory has one type, and a program
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// accesses it with a different type, then the result is undefined
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// behavior for most values of "different type".
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//
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// This is true for any cast syntax, either *(int*)&f or
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// *reinterpret_cast<int*>(&f). And it is particularly true for
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// conversions between integral lvalues and floating-point lvalues.
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//
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// The purpose of 3.10 -15- is to allow optimizing compilers to assume
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// that expressions with different types refer to different memory. gcc
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// 4.0.1 has an optimizer that takes advantage of this. So a
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// non-conforming program quietly produces wildly incorrect output.
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//
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// The problem is not the use of reinterpret_cast. The problem is type
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// punning: holding an object in memory of one type and reading its bits
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// back using a different type.
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//
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// The C++ standard is more subtle and complex than this, but that
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// is the basic idea.
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//
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// Anyways ...
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//
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// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
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// example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty
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// logic in one place.
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//
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// Fortunately memcpy() is very fast. In optimized mode, with a
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// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
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// code with the minimal amount of data movement. On a 32-bit system,
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// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
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// compiles to two loads and two stores.
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//
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// Mike Chastain tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc
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// 7.1.
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//
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// WARNING: if Dest or Source is a non-POD type, the result of the memcpy
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// is likely to surprise you.
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//
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// Props to Bill Gibbons for the compile time assertion technique and
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// Art Komninos and Igor Tandetnik for the msvc experiments.
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template <class Dest, class Source>
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inline Dest bit_cast(const Source &source) {
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static_assert(sizeof(Dest) == sizeof(Source), "Sizes do not match");
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Dest dest;
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memcpy(&dest, &source, sizeof(dest));
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return dest;
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}
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} // namespace libtextclassifier3
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#endif // LIBTEXTCLASSIFIER_UTILS_BASE_CASTS_H_
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