2 * Copyright 2017-present Facebook, Inc.
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
8 * http://www.apache.org/licenses/LICENSE-2.0
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.
21 #include <folly/CPortability.h>
22 #include <folly/Portability.h>
25 * include or backport:
29 #if __cpp_lib_launder >= 201606
33 /* using override */ using std::launder;
42 * Approximate backport from C++17 of std::launder. It should be `constexpr`
43 * but that can't be done without specific support from the compiler.
46 FOLLY_NODISCARD inline T* launder(T* in) noexcept {
47 #if FOLLY_HAS_BUILTIN(__builtin_launder) || __GNUC__ >= 7
48 // The builtin has no unwanted side-effects.
49 return __builtin_launder(in);
51 // This inline assembler block declares that `in` is an input and an output,
52 // so the compiler has to assume that it has been changed inside the block.
53 __asm__("" : "+r"(in));
56 // MSVC does not currently have optimizations around const members of structs.
57 // _ReadWriteBarrier() will prevent compiler reordering memory accesses.
62 false, "folly::launder is not implemented for this environment");
66 /* The standard explicitly forbids laundering these */
67 void launder(void*) = delete;
68 void launder(void const*) = delete;
69 void launder(void volatile*) = delete;
70 void launder(void const volatile*) = delete;
71 template <typename T, typename... Args>
72 void launder(T (*)(Args...)) = delete;