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------------------------------------------------------------------------------
-- --
-- GNAT RUN-TIME COMPONENTS --
-- --
-- G N A T . B R A N C H _ P R E D I C T I O N --
-- --
-- S p e c --
-- --
-- Copyright (C) 2019-2022, AdaCore --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 3, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. --
-- --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception, --
-- version 3.1, as published by the Free Software Foundation. --
-- --
-- You should have received a copy of the GNU General Public License and --
-- a copy of the GCC Runtime Library Exception along with this program; --
-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
-- <http://www.gnu.org/licenses/>. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- This package provides routines giving hints to the branch predictor of the
-- code generator. These hints are useful when optimization is enabled and the
-- branch probability heuristics are used (which is the default), but they are
-- overridden when profile feedback-directed optimization is used instead.
-- The canonical pattern is to use them as the condition of an If statement:
--
-- if Likely (X > 0) then
-- Do_Something;
-- end if;
--
-- when it is not obvious that one outcome of the condition is more likely
-- than the other, or else to reverse the prediction made by the heuristics
-- in very peculiar cases. In the other cases, it is better not to use them,
-- because predicting how programs actually perform is notoriously hard.
package GNAT.Branch_Prediction is
pragma Pure;
function Expect (Condition : Boolean; Outcome : Boolean) return Boolean;
pragma Import (Intrinsic, Expect, "__builtin_expect");
-- This function returns the value of its first parameter Condition and
-- tells the branch predictor that this value is expected to be Outcome.
function Likely (Condition : Boolean) return Boolean;
pragma Import (Intrinsic, Likely, "__builtin_likely");
-- This function returns the value of its parameter Condition and tells
-- the branch predictor that this value is expected to be True. Calling
-- it is strictly equivalent to calling Expect with Outcome set to True.
function Unlikely (Condition : Boolean) return Boolean;
pragma Import (Intrinsic, Unlikely, "__builtin_unlikely");
-- This function returns the value of its parameter Condition and tells
-- the branch predictor that this value is expected to be False. Calling
-- it is strictly equivalent to calling Expect with Outcome set to False.
end GNAT.Branch_Prediction;
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