78 lines
2.2 KiB
C++
78 lines
2.2 KiB
C++
/*
|
|
* 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
|