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[committed] Use ACQ_REL __atomic_fetch_add in atomicity.h (PR libstdc++/51798)


Hi!

As reported by David, PowerPC* has been using (contrary to documentation)
__ATOMIC_ACQ_REL semantics (i.e. lwsync insn before and isync after)
__sync_fetch_and_add intrinsics, which is heavily used in libstdc++.
As this worked before, and there is no need for the strong consistency
among the __exchange_and_add and __atomic_add users, this patch
changes those to use __atomic_fetch_add with __ATOMIC_ACQ_REL, which
generates on powerpc* the same insn sequence as before, while
__sync_fetch_and_add now generates the more expensive sync insn before and
isync after.

Bootstrapped/regtested on x86_64-linux, i686-linux, s390-linux, s390x-linux,
powerpc64-linux 32-bit and 64-bit, approved in the PR by Benjamin, committed
to trunk.

2012-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/51798
	* config/cpu/generic/atomicity_builtins/atomicity.h
	(__exchange_and_add, __atomic_add): Use __atomic_fetch_add
	with __ATOMIC_ACQ_REL semantics instead of __sync_fetch_and_add.
	* include/ext/atomicity.h (__exchange_and_add, __atomic_add):
	Likewise.

--- libstdc++-v3/config/cpu/generic/atomicity_builtins/atomicity.h.jj	2011-01-31 14:11:47.000000000 +0100
+++ libstdc++-v3/config/cpu/generic/atomicity_builtins/atomicity.h	2012-01-27 12:10:10.395649890 +0100
@@ -1,7 +1,7 @@
 // Low-level functions for atomic operations: version for CPUs providing
 // atomic builtins -*- C++ -*-
 
-// Copyright (C) 2006, 2009, 2010 Free Software Foundation, Inc.
+// Copyright (C) 2006, 2009, 2010, 2012 Free Software Foundation, Inc.
 //
 // This file is part of the GNU ISO C++ Library.  This library is free
 // software; you can redistribute it and/or modify it under the
@@ -33,12 +33,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   _Atomic_word 
   __attribute__ ((__unused__))
   __exchange_and_add(volatile _Atomic_word* __mem, int __val) throw ()
-  { return __sync_fetch_and_add(__mem, __val); }
+  { return __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 
   void
   __attribute__ ((__unused__))
   __atomic_add(volatile _Atomic_word* __mem, int __val) throw ()
-  { __sync_fetch_and_add(__mem, __val); }
+  { __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
--- libstdc++-v3/include/ext/atomicity.h.jj	2011-12-01 11:45:00.000000000 +0100
+++ libstdc++-v3/include/ext/atomicity.h	2012-01-27 12:11:09.286306232 +0100
@@ -1,6 +1,6 @@
 // Support for atomic operations -*- C++ -*-
 
-// Copyright (C) 2004, 2005, 2006, 2008, 2009, 2010, 2011
+// Copyright (C) 2004, 2005, 2006, 2008, 2009, 2010, 2011, 2012
 // Free Software Foundation, Inc.
 //
 // This file is part of the GNU ISO C++ Library.  This library is free
@@ -45,11 +45,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #ifdef _GLIBCXX_ATOMIC_BUILTINS
   static inline _Atomic_word 
   __exchange_and_add(volatile _Atomic_word* __mem, int __val)
-  { return __sync_fetch_and_add(__mem, __val); }
+  { return __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 
   static inline void
   __atomic_add(volatile _Atomic_word* __mem, int __val)
-  { __sync_fetch_and_add(__mem, __val); }
+  { __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 #else
   _Atomic_word
   __attribute__ ((__unused__))

	Jakub


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