From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 32294 invoked by alias); 6 Aug 2014 13:29:45 -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 32283 invoked by uid 89); 6 Aug 2014 13:29:44 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2 X-HELO: mail-wi0-f179.google.com Received: from mail-wi0-f179.google.com (HELO mail-wi0-f179.google.com) (209.85.212.179) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-SHA encrypted) ESMTPS; Wed, 06 Aug 2014 13:29:43 +0000 Received: by mail-wi0-f179.google.com with SMTP id f8so3186880wiw.0 for ; Wed, 06 Aug 2014 06:29:40 -0700 (PDT) MIME-Version: 1.0 X-Received: by 10.194.80.7 with SMTP id n7mr15817481wjx.8.1407331780571; Wed, 06 Aug 2014 06:29:40 -0700 (PDT) Received: by 10.194.20.69 with HTTP; Wed, 6 Aug 2014 06:29:40 -0700 (PDT) In-Reply-To: <53E22BEA.7070801@linaro.org> References: <53BA4458.30804@linaro.org> <53BFD000.1030909@linaro.org> <53C34734.2080103@linaro.org> <53DB1CE2.3080401@linaro.org> <53DBBA6B.3070507@linaro.org> <20140805142142.GW7393@tucnak.redhat.com> <53E22BEA.7070801@linaro.org> Date: Wed, 06 Aug 2014 13:29:00 -0000 Message-ID: Subject: Re: [PATCH 2/2] Enable elimination of zext/sext From: Richard Biener To: Kugan Cc: Jakub Jelinek , "gcc-patches@gcc.gnu.org" Content-Type: text/plain; charset=UTF-8 X-IsSubscribed: yes X-SW-Source: 2014-08/txt/msg00466.txt.bz2 On Wed, Aug 6, 2014 at 3:21 PM, Kugan wrote: > On 06/08/14 22:09, Richard Biener wrote: >> On Tue, Aug 5, 2014 at 4:21 PM, Jakub Jelinek wrote: >>> On Tue, Aug 05, 2014 at 04:17:41PM +0200, Richard Biener wrote: >>>> what's the semantic of setting SRP_SIGNED_AND_UNSIGNED >>>> on the subreg? That is, for the created (subreg:lhs_mode >>>> (reg: N))? >>> >>> SRP_SIGNED_AND_UNSIGNED on a subreg should mean that >>> the subreg is both zero and sign extended, which means >>> that the topmost bit of the narrower mode is known to be zero, >>> and all bits above it in the wider mode are known to be zero too. >>> SRP_SIGNED means that the topmost bit of the narrower mode is >>> either 0 or 1 and depending on that the above wider mode bits >>> are either all 0 or all 1. >>> SRP_UNSIGNED means that regardless of the topmost bit value, >>> all above wider mode bits are 0. >> >> Ok, then from the context of the patch we already know that >> either SRP_UNSIGNED or SRP_SIGNED is true which means >> that the value is sign- or zero-extended. >> >> I suppose inside promoted_for_type_p >> TYPE_MODE (TREE_TYPE (ssa)) == lhs_mode, I'm not sure >> why you pass !unsignedp as lhs_uns. > > In expand_expr_real_1, it is already known that it is promoted for > unsigned_p and we are setting SUBREG_PROMOTED_SET (temp, unsignedp). > > If we can prove that it is also promoted for !unsignedp, we can set > SUBREG_PROMOTED_SET (temp, SRP_SIGNED_AND_UNSIGNED). > > promoted_for_type_p should prove this based on the value range info. > >> >> Now, from 'ssa' alone we can't tell anything about a larger mode >> registers value if that is either zero- or sign-extended. But we >> know that those bits are properly zero-extended if unsignedp >> and properly sign-extended if !unsignedp? >> >> So what the predicate tries to prove is that sign- and zero-extending >> results in the same larger-mode value. This is true if the >> MSB of the smaller mode is not set. >> >> Let's assume that smaller mode is that of 'ssa' then the test >> is just >> >> return (!tree_int_cst_sign_bit (min) && !tree_int_cst_sign_bit (max)); >> >> no? > > hmm, is this because we will never have a call to promoted_for_type_p > with same sign (ignoring PROMOTE_MODE) for 'ssa' and the larger mode. > The case with larger mode signed and 'ssa' unsigned will not work. > Therefore larger mode unsigned and 'ssa' signed will be the only case > that we should consider. > > However, with PROMOTE_MODE, isnt that we will miss some cases with this. No, PROMOTE_MODE will still either sign- or zero-extend. If either results in zeros in the upper bits then PROMOTE_MODE doesn't matter. Richard. > Thanks, > Kugan > >