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/*
* 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_
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