From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by sourceware.org (Postfix) with ESMTP id 4AEA13986815 for ; Mon, 21 Sep 2020 15:51:21 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 4AEA13986815 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: sourceware.org; spf=fail smtp.mailfrom=segher@kernel.crashing.org Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 08LFnHQj025276; Mon, 21 Sep 2020 10:49:18 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 08LFnHlB025274; Mon, 21 Sep 2020 10:49:17 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 21 Sep 2020 10:49:17 -0500 From: Segher Boessenkool To: Alan Modra Cc: gcc-patches@sourceware.org Subject: Re: [RS6000] rs6000_rtx_costs cost IOR Message-ID: <20200921154917.GA28786@gate.crashing.org> References: <20200915011946.3395-1-amodra@gmail.com> <20200915011946.3395-7-amodra@gmail.com> <20200917000206.GI28786@gate.crashing.org> <20200917034219.GP5452@bubble.grove.modra.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200917034219.GP5452@bubble.grove.modra.org> User-Agent: Mutt/1.4.2.3i X-Spam-Status: No, score=-7.2 required=5.0 tests=BAYES_00, JMQ_SPF_NEUTRAL, KAM_DMARC_STATUS, TXREP, T_SPF_HELO_PERMERROR, T_SPF_PERMERROR autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 21 Sep 2020 15:51:22 -0000 Hi! On Thu, Sep 17, 2020 at 01:12:19PM +0930, Alan Modra wrote: > On Wed, Sep 16, 2020 at 07:02:06PM -0500, Segher Boessenkool wrote: > > > + /* Test both regs even though the one in the mask is > > > + constrained to be equal to the output. Increasing > > > + cost may well result in rejecting an invalid insn > > > + earlier. */ > > > > Is that ever actually useful? > > Possibly not in this particular case, but I did see cases where > invalid insns were rejected early by costing non-reg sub-expressions. But does that ever change generated code? This makes the compiler a lot harder to read and understand. To the point that such micro-optimisations makes worthwhile optimisations hard or impossible to do. Segher