1/*
2 * Copyright 2015-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 <cstdlib>
20
21#include <folly/Portability.h>
22
23namespace folly {
24
25namespace detail {
26
27extern void assume_check(bool cond);
28
29} // namespace detail
30
31/**
32 * Inform the compiler that the argument can be assumed true. It is
33 * undefined behavior if the argument is not actually true, so use
34 * with care.
35 *
36 * Implemented as a function instead of a macro because
37 * __builtin_assume does not evaluate its argument at runtime, so it
38 * cannot be used with expressions that have side-effects.
39 */
40
41FOLLY_ALWAYS_INLINE void assume(bool cond) {
42 if (kIsDebug) {
43 detail::assume_check(cond);
44 } else {
45#if defined(__clang__) // Must go first because Clang also defines __GNUC__.
46 __builtin_assume(cond);
47#elif defined(__GNUC__)
48 if (!cond) {
49 __builtin_unreachable();
50 }
51#elif defined(_MSC_VER)
52 __assume(cond);
53#else
54 // Do nothing.
55#endif
56 }
57}
58
59[[noreturn]] FOLLY_ALWAYS_INLINE void assume_unreachable() {
60 assume(false);
61 // Do a bit more to get the compiler to understand
62 // that this function really will never return.
63#if defined(__GNUC__)
64 __builtin_unreachable();
65#elif defined(_MSC_VER)
66 __assume(0);
67#else
68 // Well, it's better than nothing.
69 std::abort();
70#endif
71}
72
73} // namespace folly
74