[Ada] Improve support for tuning branch probability heuristics

This adds a new GNAT.Branch_Prediction package to make it possible to
tune the branch probability heuristics more finely.  This package
contains the equivalent of __builtin_expect in C/C++ plus a couple of
specializations.

The following program gives a summary of the usage:

package Q is

  I : Integer;
  pragma Volatile (I);

end Q;

with GNAT.Branch_Prediction; use GNAT.Branch_Prediction;
with Text_IO; use Text_IO;
with Q; use Q;

procedure P is
begin
  if Unlikely (I = 0) then
    Put_Line ("Zero was passed");
    return;
  end if;

  if Likely (I > 0) then
    Put_Line ("A positive number was passed");
  else
    Put_Line ("A negative number was passed");
  end if;

  if Expect ((I rem 2) = 0, False) then
    Put_Line ("An even number was passed");
  else
    Put_Line ("An odd number was passed");
  end if;
end;

2019-07-10  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add g-brapre.
	* libgnat/g-brapre.ads: New package specification.
	* doc/gnat_rm/the_gnat_library.rst: Document it.
	* gnat_rm.texi: Regenerate.

From-SVN: r273340

gcc/ada/ChangeLog

+2019-07-10  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add g-brapre.
+	* libgnat/g-brapre.ads: New package specification.
+	* doc/gnat_rm/the_gnat_library.rst: Document it.
+	* gnat_rm.texi: Regenerate.
+
 2019-07-10  Yannick Moy  <moy@adacore.com>
 
 	* osint-c.adb (Set_File_Name): Always add extension for multiple

gcc/ada/Makefile.rtl

@@ -387,6 +387,7 @@ GNATRTL_NONTASKING_OBJS= \
   g-arrspl$(objext) \
   g-awk$(objext) \
   g-binenv$(objext) \
+  g-brapre$(objext) \
   g-bubsor$(objext) \
   g-busora$(objext) \
   g-busorg$(objext) \

gcc/ada/doc/gnat_rm/the_gnat_library.rst

@@ -722,6 +722,17 @@ Provides access to key=value associations captured at bind time.
 These associations can be specified using the :switch:`-V` binder command
 line switch.
 
+.. _`GNAT.Branch_Prediction_(g-brapre.ads)`:
+
+``GNAT.Branch_Prediction`` (:file:`g-brapre.ads`)
+=================================================
+
+.. index:: GNAT.Branch_Prediction (g-brapre.ads)
+
+.. index:: Branch Prediction
+
+Provides routines giving hints to the branch predictor of the code generator.
+
 .. _`GNAT.Bounded_Buffers_(g-boubuf.ads)`:
 
 ``GNAT.Bounded_Buffers`` (:file:`g-boubuf.ads`)

gcc/ada/libgnat/g-brapre.ads

+------------------------------------------------------------------------------
+--                                                                          --
+--                         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, 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;