From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 12340 invoked by alias); 10 Jun 2016 07:11:32 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 10189 invoked by uid 89); 10 Jun 2016 07:11:32 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-3.3 required=5.0 tests=BAYES_00,RP_MATCHES_RCVD,SPF_PASS autolearn=ham version=3.3.2 spammy=family X-HELO: mx2.suse.de Received: from mx2.suse.de (HELO mx2.suse.de) (195.135.220.15) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (CAMELLIA256-SHA encrypted) ESMTPS; Fri, 10 Jun 2016 07:11:21 +0000 Received: from relay1.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 98627ACEC; Fri, 10 Jun 2016 07:11:18 +0000 (UTC) Date: Fri, 10 Jun 2016 07:11:00 -0000 From: Richard Biener To: Jason Merrill cc: Jakub Jelinek , gcc-patches List Subject: Re: [PATCH] Fold x/x to 1, 0/x to 0 and 0%x to 0 consistently In-Reply-To: Message-ID: References: <20160609071553.GM7387@tucnak.redhat.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-SW-Source: 2016-06/txt/msg00771.txt.bz2 On Thu, 9 Jun 2016, Jason Merrill wrote: > On Thu, Jun 9, 2016 at 3:39 AM, Richard Biener wrote: > > On Thu, 9 Jun 2016, Jakub Jelinek wrote: > > > >> On Thu, Jun 09, 2016 at 08:50:15AM +0200, Richard Biener wrote: > >> > On Wed, 8 Jun 2016, Jason Merrill wrote: > >> > > >> > > On Wed, Jun 8, 2016 at 11:16 AM, Marc Glisse wrote: > >> > > > On Wed, 8 Jun 2016, Richard Biener wrote: > >> > > > > >> > > >> The following works around PR70992 but the issue came up repeatedly > >> > > >> that we are not very consistent in preserving the undefined behavior > >> > > >> of division or modulo by zero. Ok - the only inconsistency is > >> > > >> that we fold 0 % x to 0 but not 0 % 0 (with literal zero). > >> > > >> > >> > > >> After folding is now no longer done early in the C family FEs the > >> > > >> number of diagnostic regressions with the patch below is two. > >> > > >> > >> > > >> FAIL: g++.dg/cpp1y/constexpr-sfinae.C -std=c++14 (test for excess errors) > >> > > > >> > > Yep. We don't want to fold away undefined behavior in a constexpr > >> > > function, since constexpr evaluation wants to detect undefined > >> > > behavior and treat the expression as non-constant in that case. > >> > > >> > Hmm. So 0 / x is not constant because x might be zero but 0 * x is > >> > constant because it can never invoke undefined behavior? Does this mean > >> > that 0 << n is not const because n might be too large (I suppose > >> > 0 << 12000 is not const already)? Is 0 * (-x) const? x might be INT_MIN. > >> > >> E.g. for the shifts the C++ FE has cxx_eval_check_shift_p which should > >> optionally warn and/or set *non_constant_p. 0 * (-INT_MIN) would be > >> non-constant too, etc. > >> constexpr int foo (int x) { return -x; } > >> constexpr int bar (int x) { return 0 * (-x); } > >> constexpr int a = foo (-__INT_MAX__ - 1); > >> constexpr int b = bar (-__INT_MAX__ - 1); > >> shows that we don't diagnose the latter though, most likely because > >> constexpr evaluation is done on the folded tree. > > I don't think there's any folding before constexpr evaluation in this > case, since this doesn't involve a call. > > >> So, either whatever cp_fold does (which uses fold* underneath) should avoid > >> folding such cases, or the constexpr evaluation should be done on a copy > >> made before folding. Then cp_fold doesn't have to prohibit optimizations > >> and it is a matter of constexpr.c routines to detect all the undefined > >> behavior. After all, I think doing constexpr evaluation on unfolded trees > >> will have also the advantage of better diagnostic locations. > > > > Yes, I think constexpr diagnostic / detection should be done on > > unfolded trees (not sure why we'd need to a copy here, just do folding > > later?). If cp_fold already avoids folding 0 * (-x) then it should > > simply avoid folding 0 / x or 0 % x as well (for the particular issue > > this thread started on). > > The issue is with constexpr functions, which get delayed folding like > any other function before they are used in constant expression > evaluation. The copy would be to preserve the unfolded trees through > cp_fold_function. I've been thinking about doing this anyway; this > may be the motivation to go ahead with it. Or delay the folding until genericization - that is, after all parsing is complete. Not sure what this would do to memory usage or compile-time when we keep all unfolded bodies (or even re-use them in template instantiation). Richard.