78 lines
2.2 KiB
C
78 lines
2.2 KiB
C
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/*
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* Copyright 2017-present Facebook, Inc.
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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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#pragma once
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#include <new>
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#include <folly/CPortability.h>
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#include <folly/Portability.h>
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/***
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* include or backport:
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* * std::launder
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*/
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// Note: libc++ 6+ adds std::launder but does not define __cpp_lib_launder
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#if __cpp_lib_launder >= 201606 || (_MSC_VER && _HAS_LAUNDER) || \
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(_LIBCPP_VERSION >= 6000 && __cplusplus >= 201703L)
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namespace folly {
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/* using override */ using std::launder;
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} // namespace folly
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#else
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namespace folly {
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/**
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* Approximate backport from C++17 of std::launder. It should be `constexpr`
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* but that can't be done without specific support from the compiler.
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*/
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template <typename T>
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FOLLY_NODISCARD inline T* launder(T* in) noexcept {
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#if FOLLY_HAS_BUILTIN(__builtin_launder) || __GNUC__ >= 7
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// The builtin has no unwanted side-effects.
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return __builtin_launder(in);
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#elif __GNUC__
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// This inline assembler block declares that `in` is an input and an output,
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// so the compiler has to assume that it has been changed inside the block.
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__asm__("" : "+r"(in));
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return in;
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#elif defined(_WIN32)
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// MSVC does not currently have optimizations around const members of structs.
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// _ReadWriteBarrier() will prevent compiler reordering memory accesses.
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_ReadWriteBarrier();
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return in;
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#else
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static_assert(
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false, "folly::launder is not implemented for this environment");
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#endif
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}
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/* The standard explicitly forbids laundering these */
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void launder(void*) = delete;
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void launder(void const*) = delete;
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void launder(void volatile*) = delete;
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void launder(void const volatile*) = delete;
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template <typename T, typename... Args>
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void launder(T (*)(Args...)) = delete;
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} // namespace folly
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#endif
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