From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from eggs.gnu.org (eggs.gnu.org [IPv6:2001:470:142:3::10]) by sourceware.org (Postfix) with ESMTPS id 23F473858C53 for ; Sat, 27 May 2023 21:11:37 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 23F473858C53 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=nexgo.de Authentication-Results: sourceware.org; spf=fail smtp.mailfrom=nexgo.de Received: from mr5.vodafonemail.de ([145.253.228.165]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1q31CI-0007Vs-OQ for gcc@gnu.org; Sat, 27 May 2023 17:11:36 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nexgo.de; s=vfde-smtpout-mb-15sep; t=1685221891; bh=hNIouSGIM4Q5Wev1cmP4tfRbgKzlIMCIdy3B6VCtc18=; h=Message-ID:From:To:Subject:Date:Content-Type:X-Mailer:From; b=jUXk6cTSTwNAn+1pwaRdLa86dIVtVg6IFGCvNpPMN6KbXOLmgYCghU5iVgBHU2h59 UyRW6QA8Vut0sp/qt6Kcj78K1czmMLYiVwVsTx8eEbWkfwGGPoWuVm2MzZ7+kat17/ Ry9m4Z2x3QPJWEnLXPm2IvgMRE+KllPDKR/4wm/8= Received: from smtp.vodafone.de (unknown [10.0.0.2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by mr5.vodafonemail.de (Postfix) with ESMTPS id 4QTDxH5GFxz1yD6 for ; Sat, 27 May 2023 21:11:31 +0000 (UTC) Received: from H270 (p5b38f631.dip0.t-ipconnect.de [91.56.246.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by smtp.vodafone.de (Postfix) with ESMTPSA id 4QTDxC3W98z9rxM for ; Sat, 27 May 2023 21:11:24 +0000 (UTC) Message-ID: <2A5397441B7D46C7928856D7AAF5836B@H270> From: "Stefan Kanthak" To: Subject: Another epic optimiser failure Date: Sat, 27 May 2023 23:04:11 +0200 Organization: Me, myself & IT MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Windows Mail 6.0.6002.18197 X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.24158 X-purgate-type: clean X-purgate: clean X-purgate-size: 1664 X-purgate-ID: 155817::1685221887-2A7FB4F8-E92CD32E/0/0 Received-SPF: pass client-ip=145.253.228.165; envelope-from=stefan.kanthak@nexgo.de; helo=mr5.vodafonemail.de X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9,DKIM_SIGNED=0.1,DKIM_VALID=-0.1,DKIM_VALID_AU=-0.1,DKIM_VALID_EF=-0.1,RCVD_IN_DNSWL_LOW=-0.7,SPF_HELO_NONE=0.001,SPF_PASS=-0.001,T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-Spam-Status: No, score=-2.4 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_FAIL,SPF_HELO_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: --- .c --- int ispowerof2(unsigned long long argument) { return __builtin_popcountll(argument) == 1; } --- EOF --- GCC 13.3 gcc -m32 -march=alderlake -O3 gcc -m32 -march=sapphirerapids -O3 gcc -m32 -mpopcnt -mtune=sapphirerapids -O3 https://gcc.godbolt.org/z/cToYrrYPq ispowerof2(unsigned long long): xor eax, eax # superfluous xor edx, edx # superfluous popcnt eax, [esp+4] popcnt edx, [esp+8] add eax, edx cmp eax, 1 -> dec eax sete al movzx eax, al # superfluous ret 9 instructions in 28 bytes # 6 instructions in 20 bytes OUCH: popcnt writes the WHOLE result register, there is ABSOLUTELY no need to clear it beforehand nor to clear the higher 24 bits afterwards! JFTR: before GCC zealots write nonsense: see -march= or -mtune= GCC 13.3 gcc -mpopcnt -mtune=barcelona -O3 https://gcc.godbolt.org/z/3Ks8vh7a6 ispowerof2(unsigned long long): popcnt rdi, rdi -> popcnt rax, rdi xor eax, eax # superfluous! dec edi -> dec eax sete al -> setz al ret GCC 13.3 gcc -m32 -mpopcnt -mtune=barcelona -O3 https://gcc.godbolt.org/z/s5s5KTGnv ispowerof2(unsigned long long): popcnt eax, [esp+4] popcnt edx, [esp+8] add eax, edx dec eax sete al movzx eax, al # superfluous! ret Will GCC eventually generate properly optimised code instead of bloat? Stefan