From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 6280 invoked by alias); 26 Jan 2012 20:40:48 -0000 Received: (qmail 6271 invoked by uid 22791); 26 Jan 2012 20:40:47 -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; Thu, 26 Jan 2012 20:40:35 +0000 From: "redi at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/51798] [4.7 regression] libstdc++ atomicity performance regression due to __sync_fetch_and_add Date: Thu, 26 Jan 2012 22:08:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: redi at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Priority: P1 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: 4.7.0 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: 2012-01/txt/msg03065.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51798 --- Comment #16 from Jonathan Wakely 2012-01-26 20:39:40 UTC --- Apologies for my phrasing. What I meant is that it's not a matter of consistency. Localising all uses of __sync_bool_compare_and_swap by adding an unnecessary additional layer of indirection in atomicity.h would not have been appropriate. GCC already abstracts the required CAS operation with __sync_bool_compare_and_swap, so that is used directly when needed. One of the main reasons for the wrappers in atomicity.h is to fall back to non-atomic ops in non-threaded code, rather than just to localize the use of intrinisics. Having a fall back for non-threaded code isn't necessary for the places where __sync_bool_compare_and_swap is used in shared_ptr and parallel mode. Our shared_ptr has a partial specialization for the non-threaded case, which doesn't use CAS, and using parallel mode in non-threaded code doesn't make sense. So __sync_bool_compare_and_swap is used directly. "It's own atomicity.h" doesn't include a CAS operation, and I see no benefit to adding CAS to that file.