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- /*
- * Copyright 2017-present Facebook, Inc.
- *
- * 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
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
- #pragma once
- #include <new>
- #include <folly/CPortability.h>
- #include <folly/Portability.h>
- /***
- * include or backport:
- * * std::launder
- */
- // Note: libc++ 6+ adds std::launder but does not define __cpp_lib_launder
- #if __cpp_lib_launder >= 201606 || (_MSC_VER && _HAS_LAUNDER) || \
- (_LIBCPP_VERSION >= 6000 && __cplusplus >= 201703L)
- namespace folly {
- /* using override */ using std::launder;
- } // namespace folly
- #else
- namespace folly {
- /**
- * Approximate backport from C++17 of std::launder. It should be `constexpr`
- * but that can't be done without specific support from the compiler.
- */
- template <typename T>
- FOLLY_NODISCARD inline T* launder(T* in) noexcept {
- #if FOLLY_HAS_BUILTIN(__builtin_launder) || __GNUC__ >= 7
- // The builtin has no unwanted side-effects.
- return __builtin_launder(in);
- #elif __GNUC__
- // This inline assembler block declares that `in` is an input and an output,
- // so the compiler has to assume that it has been changed inside the block.
- __asm__("" : "+r"(in));
- return in;
- #elif defined(_WIN32)
- // MSVC does not currently have optimizations around const members of structs.
- // _ReadWriteBarrier() will prevent compiler reordering memory accesses.
- _ReadWriteBarrier();
- return in;
- #else
- static_assert(
- false, "folly::launder is not implemented for this environment");
- #endif
- }
- /* The standard explicitly forbids laundering these */
- void launder(void*) = delete;
- void launder(void const*) = delete;
- void launder(void volatile*) = delete;
- void launder(void const volatile*) = delete;
- template <typename T, typename... Args>
- void launder(T (*)(Args...)) = delete;
- } // namespace folly
- #endif
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