From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) by sourceware.org (Postfix) with ESMTPS id 552573858D28; Mon, 12 Jun 2023 02:41:05 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 552573858D28 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=linux.ibm.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=linux.ibm.com Received: from pps.filterd (m0353725.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.17.1.19/8.17.1.19) with ESMTP id 35C2HuPl003231; Mon, 12 Jun 2023 02:41:03 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=from : to : cc : subject : references : date : in-reply-to : message-id : mime-version : content-type; s=pp1; bh=zY7X5bQu40hCQMjaz6ZZHr4gJEWfOx+fQ2Bn2NC2e9M=; b=UK2Xrsx0ilYWQqxGm05++mgZaGCzQdX/IOcJM6LsbmNgcm2qnfobQFYY9kYbj9psvJZ4 o62L2MOIb4oUQ4QOyfJhSH3WJXjgNjHrc4y69nyt8l3MzCs9rl3jHwguRpTTiwCwXimr 9zh7Q3kcKHLonc0/KA2QgXf1jCGW5vwOo5v36ZNZCEPrCx6NBvwYCoe5q1ODtfKPFob+ wYh5sK9Wz/saI1THYYaI+AnLxo13hAYXto5AzVtEDQwSZXgAfQGkvejRnJzJKz0ZtXBO oDRvFIUOlGUPXfCrWbH6+Q/5BDSrPsWt6gPEz56rA+pvHVL2Ho8Bf9eNLHMzdtZGOW0H KQ== Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 3r5tev0902-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 12 Jun 2023 02:41:02 +0000 Received: from m0353725.ppops.net (m0353725.ppops.net [127.0.0.1]) by pps.reinject (8.17.1.5/8.17.1.5) with ESMTP id 35C2dMsZ024040; Mon, 12 Jun 2023 02:41:02 GMT Received: from ppma04dal.us.ibm.com (7a.29.35a9.ip4.static.sl-reverse.com [169.53.41.122]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 3r5tev08yu-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 12 Jun 2023 02:41:02 +0000 Received: from pps.filterd (ppma04dal.us.ibm.com [127.0.0.1]) by ppma04dal.us.ibm.com (8.17.1.19/8.17.1.19) with ESMTP id 35C2062h011868; Mon, 12 Jun 2023 02:41:01 GMT Received: from smtprelay05.wdc07v.mail.ibm.com ([9.208.129.117]) by ppma04dal.us.ibm.com (PPS) with ESMTPS id 3r4gt5dmuj-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 12 Jun 2023 02:41:01 +0000 Received: from smtpav03.wdc07v.mail.ibm.com (smtpav03.wdc07v.mail.ibm.com [10.39.53.230]) by smtprelay05.wdc07v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 35C2f0UQ65798514 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 12 Jun 2023 02:41:00 GMT Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1989D58054; Mon, 12 Jun 2023 02:41:00 +0000 (GMT) Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A933C5805C; Mon, 12 Jun 2023 02:40:59 +0000 (GMT) Received: from ltcden2-lp1.aus.stglabs.ibm.com (unknown [9.3.90.43]) by smtpav03.wdc07v.mail.ibm.com (Postfix) with ESMTPS; Mon, 12 Jun 2023 02:40:59 +0000 (GMT) From: Jiufu Guo To: Segher Boessenkool Cc: gcc-patches@gcc.gnu.org, rguenther@suse.de, jeffreyalaw@gmail.com, richard.sandiford@arm.com, bergner@linux.ibm.com, linkw@gcc.gnu.org Subject: Re: [PATCH V2] Optimize '(X - N * M) / N' to 'X / N - M' if valid References: <20230607082111.2773414-1-guojiufu@linux.ibm.com> <20230609202614.GR19790@gate.crashing.org> Date: Mon, 12 Jun 2023 10:40:56 +0800 In-Reply-To: <20230609202614.GR19790@gate.crashing.org> (Segher Boessenkool's message of "Fri, 9 Jun 2023 15:26:14 -0500") Message-ID: <7nlegpe72f.fsf@ltcden2-lp1.aus.stglabs.ibm.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: Gtyry6GMWNYtdV7-mBgg3SXIJX-B-na4 X-Proofpoint-GUID: Qb-zb3uTbqKjvtqkyujkKMrs750cbK8i X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.254,Aquarius:18.0.957,Hydra:6.0.573,FMLib:17.11.176.26 definitions=2023-06-11_18,2023-06-09_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 lowpriorityscore=0 spamscore=0 malwarescore=0 bulkscore=0 suspectscore=0 impostorscore=0 mlxlogscore=336 adultscore=0 phishscore=0 priorityscore=1501 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2305260000 definitions=main-2306120020 X-Spam-Status: No, score=-5.5 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_EF,RCVD_IN_MSPIKE_H5,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: Hi, Thanks for your comments! Segher Boessenkool writes: > Hi! > > On Wed, Jun 07, 2023 at 04:21:11PM +0800, Jiufu Guo wrote: >> This patch tries to optimize "(X - N * M) / N" to "X / N - M". >> For C code, "/" towards zero (trunc_div), and "X - N * M" maybe >> wrap/overflow/underflow. So, it is valid that "X - N * M" does >> not cross zero and does not wrap/overflow/underflow. > > Is it ever valid semi-generally when N does not divide X? It is valid only if there is no wrap/overflow/underflow, and the sign of "X" and "X-N*M" are the same. Under this condition, N,M and X can be any value. > > Say X=5, N=2, M=3. Then the original expression evaluates to 0, but the > new one to -1. Whenever one of the divisions rounds up and the other > rounds down you have this problem. You are right. Since '/' is always towards zero, so, 'X' and 'X-N*M' should have the same sign bit. Otherwise, one rounds up, the other rounds down, then the transform is invalid. BR, Jeff (Jiufu Guo) > > > Segher