| /* |
| * Copyright (c) 2020 The WebRTC project authors. All Rights Reserved. |
| * |
| * Use of this source code is governed by a BSD-style license |
| * that can be found in the LICENSE file in the root of the source |
| * tree. An additional intellectual property rights grant can be found |
| * in the file PATENTS. All contributing project authors may |
| * be found in the AUTHORS file in the root of the source tree. |
| */ |
| |
| #ifndef RTC_BASE_SYSTEM_ASSUME_H_ |
| #define RTC_BASE_SYSTEM_ASSUME_H_ |
| |
| // Possibly evaluate `p`, promising the compiler that the result is true; the |
| // compiler is allowed (but not required) to use this information when |
| // optimizing the code. USE WITH CAUTION! If you promise the compiler things |
| // that aren't true, it will build a broken binary for you. |
| // |
| // As a simple example, the compiler is allowed to transform this |
| // |
| // RTC_ASSUME(x == 4); |
| // return x; |
| // |
| // into this |
| // |
| // return 4; |
| // |
| // It is even allowed to propagate the assumption "backwards in time", if it can |
| // prove that it must have held at some earlier time. For example, the compiler |
| // is allowed to transform this |
| // |
| // int Add(int x, int y) { |
| // if (x == 17) |
| // y += 1; |
| // RTC_ASSUME(x != 17); |
| // return x + y; |
| // } |
| // |
| // into this |
| // |
| // int Add(int x, int y) { |
| // return x + y; |
| // } |
| // |
| // since if `x` isn't 17 on the third line of the function body, the test of `x |
| // == 17` on the first line must fail since nothing can modify the local |
| // variable `x` in between. |
| // |
| // The intended use is to allow the compiler to optimize better. For example, |
| // here we allow the compiler to omit an instruction that ensures correct |
| // rounding of negative arguments: |
| // |
| // int DivBy2(int x) { |
| // RTC_ASSUME(x >= 0); |
| // return x / 2; |
| // } |
| // |
| // and here we allow the compiler to possibly omit a null check: |
| // |
| // void Delete(int* p) { |
| // RTC_ASSUME(p != nullptr); |
| // delete p; |
| // } |
| // |
| // clang-format off |
| #if defined(__GNUC__) |
| #define RTC_ASSUME(p) do { if (!(p)) __builtin_unreachable(); } while (0) |
| #else |
| #define RTC_ASSUME(p) do {} while (0) |
| #endif |
| // clang-format on |
| |
| #endif // RTC_BASE_SYSTEM_ASSUME_H_ |
