projects / thirdparty / gcc.git / blame
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1 | ------------------------------------------------------------------------------ |
| 2 | -- -- | |
| 3 | -- GNAT RUN-TIME COMPONENTS -- | |
| 4 | -- -- | |
| 5 | -- G N A T . B R A N C H _ P R E D I C T I O N -- | |
| 6 | -- -- | |
| 7 | -- S p e c -- | |
| 8 | -- -- | |
| 4b490c1e | 9 | -- Copyright (C) 2019-2020, AdaCore -- |
| 9ead6ee5 EB |
10 | -- -- |
| 11 | -- GNAT is free software; you can redistribute it and/or modify it under -- | |
| 12 | -- terms of the GNU General Public License as published by the Free Soft- -- | |
| 13 | -- ware Foundation; either version 3, or (at your option) any later ver- -- | |
| 14 | -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- | |
| 15 | -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- | |
| 16 | -- or FITNESS FOR A PARTICULAR PURPOSE. -- | |
| 17 | -- -- | |
| 18 | -- As a special exception under Section 7 of GPL version 3, you are granted -- | |
| 19 | -- additional permissions described in the GCC Runtime Library Exception, -- | |
| 20 | -- version 3.1, as published by the Free Software Foundation. -- | |
| 21 | -- -- | |
| 22 | -- You should have received a copy of the GNU General Public License and -- | |
| 23 | -- a copy of the GCC Runtime Library Exception along with this program; -- | |
| 24 | -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see -- | |
| 25 | -- <http://www.gnu.org/licenses/>. -- | |
| 26 | -- -- | |
| 27 | -- GNAT was originally developed by the GNAT team at New York University. -- | |
| 28 | -- Extensive contributions were provided by Ada Core Technologies Inc. -- | |
| 29 | -- -- | |
| 30 | ------------------------------------------------------------------------------ | |
| 31 | ||
| 32 | -- This package provides routines giving hints to the branch predictor of the | |
| 33 | -- code generator. These hints are useful when optimization is enabled and the | |
| 34 | -- branch probability heuristics are used (which is the default), but they are | |
| 35 | -- overridden when profile feedback-directed optimization is used instead. | |
| 36 | ||
| 37 | -- The canonical pattern is to use them as the condition of an If statement: | |
| 38 | -- | |
| 39 | -- if Likely (X > 0) then | |
| 40 | -- Do_Something; | |
| 41 | -- end if; | |
| 42 | -- | |
| 43 | -- when it is not obvious that one outcome of the condition is more likely | |
| 44 | -- than the other, or else to reverse the prediction made by the heuristics | |
| 45 | -- in very peculiar cases. In the other cases, it is better not to use them, | |
| 46 | -- because predicting how programs actually perform is notoriously hard. | |
| 47 | ||
| 48 | package GNAT.Branch_Prediction is | |
| 49 | pragma Pure; | |
| 50 | ||
| 51 | function Expect (Condition : Boolean; Outcome : Boolean) return Boolean; | |
| 52 | pragma Import (Intrinsic, Expect, "__builtin_expect"); | |
| 53 | -- This function returns the value of its first parameter Condition and | |
| 54 | -- tells the branch predictor that this value is expected to be Outcome. | |
| 55 | ||
| 56 | function Likely (Condition : Boolean) return Boolean; | |
| 57 | pragma Import (Intrinsic, Likely, "__builtin_likely"); | |
| 58 | -- This function returns the value of its parameter Condition and tells | |
| 59 | -- the branch predictor that this value is expected to be True. Calling | |
| 60 | -- it is strictly equivalent to calling Expect with Outcome set to True. | |
| 61 | ||
| 62 | function Unlikely (Condition : Boolean) return Boolean; | |
| 63 | pragma Import (Intrinsic, Unlikely, "__builtin_unlikely"); | |
| 64 | -- This function returns the value of its parameter Condition and tells | |
| 65 | -- the branch predictor that this value is expected to be False. Calling | |
| 66 | -- it is strictly equivalent to calling Expect with Outcome set to False. | |
| 67 | ||
| 68 | end GNAT.Branch_Prediction; |