From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 19844 invoked by alias); 14 Aug 2011 01:30:50 -0000 Received: (qmail 19833 invoked by uid 22791); 14 Aug 2011 01:30:49 -0000 X-SWARE-Spam-Status: No, hits=-2.9 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00 X-Spam-Check-By: sourceware.org Received: from localhost (HELO gcc.gnu.org) (127.0.0.1) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Sun, 14 Aug 2011 01:30:36 +0000 From: "tanzhangxi at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/50065] -Os, -O2, -O3 optimization breaks LD/ST ordering on 32-bit SPARC Date: Sun, 14 Aug 2011 04:42:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: rtl-optimization X-Bugzilla-Keywords: X-Bugzilla-Severity: major X-Bugzilla-Who: tanzhangxi at gmail dot com X-Bugzilla-Status: RESOLVED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2011-08/txt/msg01272.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50065 --- Comment #4 from Zhangxi Tan 2011-08-14 01:30:33 UTC --- I don't think this is an valid optimization. There are only two memory models in SPARC32, TSO and PSO (not RMO in the 64-bit v9). Both don't allow relaxing the read->write order, i.e. 'LD remap_barrier' should always be executed before 'ST lock'. This optimization violates the memory model, therefore should be prohibited. In addition, I still(In reply to comment #2) > > instruction 2C, clrb [%g1] corresponds to inline function 'spinlock_unlock' > > *(volatile unsigned char*)lock = 0; > > > > This happens before the lock protected content 'remap_barrier++', i.e. > > > > 30: c6 00 a0 00 ld [ %g2 ], %g3 > > 34: 86 00 e0 01 inc %g3 > > 38: 81 c3 e0 08 retl > > 3c: c6 20 a0 00 st %g3, [ %g2 ] ---> use the branch delay slot > > > > This is wrong and will cause serious lock issues under a multithreading > > environment. > > On what grounds is this wrong exactly? The end of the code is equivalent to: > > volatile unsigned char lock; > int remap_barrier; > > remap_barrier++; > lock = 0; > > It is perfectly valid for an optimizing C compiler to swap the two lines. > > You want something like: > > static inline void spin_unlock(char *lock) > { > __asm__ __volatile__("stb %%g0, [%0]" : : "r" (lock) : "memory"); > }