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 | |
23 | namespace folly { |
24 | |
25 | namespace detail { |
26 | |
27 | extern 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 | |
41 | FOLLY_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 | |