80c663882a
Summary: First draft. Unit tests pass. Test Plan: unit tests attached Reviewers: heyongqiang Reviewed By: heyongqiang Differential Revision: https://reviews.facebook.net/D3969
65 lines
2.1 KiB
C++
65 lines
2.1 KiB
C++
/*
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* Licensed to the Apache Software Foundation (ASF) under one
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* or more contributor license agreements. See the NOTICE file
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* distributed with this work for additional information
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* regarding copyright ownership. The ASF licenses this file
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* to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance
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* with the License. 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,
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* software distributed under the License is distributed on an
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* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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* KIND, either express or implied. See the License for the
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* specific language governing permissions and limitations
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* under the License.
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*/
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#ifndef THRIFT_UTIL_BITWISECAST_H_
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#define THRIFT_UTIL_BITWISECAST_H_
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#include <boost/static_assert.hpp>
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// Use this to get around strict aliasing rules.
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// For example, uint64_t i = bitwise_cast<uint64_t>(returns_double());
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// The most obvious implementation is to just cast a pointer,
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// but that doesn't work.
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// For a pretty in-depth explanation of the problem, see
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// http://cellperformance.beyond3d.com/articles/2006/06/ (...)
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// understanding-strict-aliasing.html
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template <typename To, typename From>
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static inline To bitwise_cast(From from) {
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BOOST_STATIC_ASSERT(sizeof(From) == sizeof(To));
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// BAD!!! These are all broken with -O2.
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//return *reinterpret_cast<To*>(&from); // BAD!!!
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//return *static_cast<To*>(static_cast<void*>(&from)); // BAD!!!
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//return *(To*)(void*)&from; // BAD!!!
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// Super clean and partially blessed by section 3.9 of the standard.
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//unsigned char c[sizeof(from)];
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//memcpy(c, &from, sizeof(from));
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//To to;
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//memcpy(&to, c, sizeof(c));
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//return to;
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// Slightly more questionable.
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// Same code emitted by GCC.
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//To to;
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//memcpy(&to, &from, sizeof(from));
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//return to;
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// Technically undefined, but almost universally supported,
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// and the most efficient implementation.
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union {
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From f;
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To t;
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} u;
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u.f = from;
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return u.t;
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}
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#endif
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