1/*
2 * Copyright 2017-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <new>
20
21#include <folly/CPortability.h>
22#include <folly/Portability.h>
23
24/***
25 * include or backport:
26 * * std::launder
27 */
28
29// Note: libc++ 6+ adds std::launder but does not define __cpp_lib_launder
30#if __cpp_lib_launder >= 201606 || (_MSC_VER && _HAS_LAUNDER) || \
31 (_LIBCPP_VERSION >= 6000 && __cplusplus >= 201703L)
32
33namespace folly {
34
35/* using override */ using std::launder;
36
37} // namespace folly
38
39#else
40
41namespace folly {
42
43/**
44 * Approximate backport from C++17 of std::launder. It should be `constexpr`
45 * but that can't be done without specific support from the compiler.
46 */
47template <typename T>
48FOLLY_NODISCARD inline T* launder(T* in) noexcept {
49#if FOLLY_HAS_BUILTIN(__builtin_launder) || __GNUC__ >= 7
50 // The builtin has no unwanted side-effects.
51 return __builtin_launder(in);
52#elif __GNUC__
53 // This inline assembler block declares that `in` is an input and an output,
54 // so the compiler has to assume that it has been changed inside the block.
55 __asm__("" : "+r"(in));
56 return in;
57#elif defined(_WIN32)
58 // MSVC does not currently have optimizations around const members of structs.
59 // _ReadWriteBarrier() will prevent compiler reordering memory accesses.
60 _ReadWriteBarrier();
61 return in;
62#else
63 static_assert(
64 false, "folly::launder is not implemented for this environment");
65#endif
66}
67
68/* The standard explicitly forbids laundering these */
69void launder(void*) = delete;
70void launder(void const*) = delete;
71void launder(void volatile*) = delete;
72void launder(void const volatile*) = delete;
73template <typename T, typename... Args>
74void launder(T (*)(Args...)) = delete;
75} // namespace folly
76
77#endif
78