From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 6728 invoked by alias); 27 Sep 2011 09:29:02 -0000 Received: (qmail 6713 invoked by uid 22791); 27 Sep 2011 09:29:01 -0000 X-SWARE-Spam-Status: No, hits=-2.8 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00,TW_TM 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; Tue, 27 Sep 2011 09:28:43 +0000 From: "rguenther at suse dot de" To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/50527] inconsistent vla align Date: Tue, 27 Sep 2011 10:08:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: middle-end X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: rguenther at suse dot de X-Bugzilla-Status: UNCONFIRMED 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: 2011-09/txt/msg02016.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527 --- Comment #5 from rguenther at suse dot de 2011-09-27 09:28:42 UTC --- On Tue, 27 Sep 2011, vries at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527 > > --- Comment #3 from vries at gcc dot gnu.org 2011-09-27 09:21:12 UTC --- > > Or alternatively (given we re-compute alignment together with folding alloca), > > assign the same alignment as folding would. > > At the point that we determine the alloca alignment during propagation in > visit_stmt, we cannot predict whether that alloca will be folded (during the > same or later ccp phase). > > So the only way to achieve other alignment is to be conservative a bit longer > for vla-allocas with respect to alignment: > - keep align at 1 byte during ccp. > - if we fold during ccp, assign align calculated at folding > - after we are sure there is no more folding (at expand, or f.i. at the end of > the second ccp phase if we limit folding to the first 2 ccp phases, to take > advantage of the larger alignment in the middle-end), we assign > BIGGEST_ALIGNMENT. I think we can check if the size is constant in evaluate_stmt and compute alignment according to that. It should only change from non-constant to constant, thus properly go down the lattice during propagation. We don't want to force excessive alignment on the replacement decls as that might require re-aligning the stack which is expensive. > > The question is of course what standards say about the alignment of > > alloca (4) > > I think alloca is non-standard. But in the context of > fold_builtin_alloca_for_var, alloca is the implementation vehicle of vlas, so > the question is what the standard says about alignment of vlas. Indeed.