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