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+------------------------------------------------------------------------------
+-- --
+-- GNAT RUN-TIME COMPONENTS --
+-- --
+-- S Y S T E M . A T O M I C _ C O U N T E R S --
+-- --
+-- B o d y --
+-- --
+-- Copyright (C) 2011, 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 implements Atomic_Counter operatiobns for platforms where
+-- GCC supports __sync_add_and_fetch_4 and __sync_sub_and_fetch_4 builtins.
+
+package body System.Atomic_Counters is
+
+ procedure Sync_Add_And_Fetch
+ (Ptr : access Unsigned_32;
+ Value : Unsigned_32);
+ pragma Import (Intrinsic, Sync_Add_And_Fetch, "__sync_add_and_fetch_4");
+
+ function Sync_Sub_And_Fetch
+ (Ptr : access Unsigned_32;
+ Value : Unsigned_32) return Unsigned_32;
+ pragma Import (Intrinsic, Sync_Sub_And_Fetch, "__sync_sub_and_fetch_4");
+
+ ---------------
+ -- Decrement --
+ ---------------
+
+ function Decrement (Item : in out Atomic_Counter) return Boolean is
+ begin
+ -- Note: the use of Unrestricted_Access here is required because we
+ -- are obtaining an access-to-volatile pointer to a non-volatile object.
+ -- This is not allowed for [Unchecked_]Access, but is safe in this case
+ -- because we know that no aliases are being created.
+
+ return Sync_Sub_And_Fetch (Item.Value'Unrestricted_Access, 1) = 0;
+ end Decrement;
+
+ ---------------
+ -- Increment --
+ ---------------
+
+ procedure Increment (Item : in out Atomic_Counter) is
+ begin
+ -- Note: the use of Unrestricted_Access here is required because we
+ -- are obtaining an access-to-volatile pointer to a non-volatile object.
+ -- This is not allowed for [Unchecked_]Access, but is safe in this case
+ -- because we know that no aliases are being created.
+
+ Sync_Add_And_Fetch (Item.Value'Unrestricted_Access, 1);
+ end Increment;
+
+ ------------
+ -- Is_One --
+ ------------
+
+ function Is_One (Item : Atomic_Counter) return Boolean is
+ begin
+ return Item.Value = 1;
+ end Is_One;
+
+end System.Atomic_Counters;