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 14B353858D32 for ; Mon, 29 May 2023 18:19:11 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 14B353858D32 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 mr3.vodafonemail.de ([145.253.228.163]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1q3hSV-0000aj-Ub for gcc@gnu.org; Mon, 29 May 2023 14:19:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nexgo.de; s=vfde-smtpout-mb-15sep; t=1685384335; bh=dM8apBkoxQRav9yHNLKOP5sQayPg2qsl4DNnZyhiWm8=; h=Message-ID:From:To:References:In-Reply-To:Subject:Date: Content-Type:X-Mailer:From; b=jWXchqfZrVKOIpC4mfqpEfe9BG7kQ3JMnSi1PwEFN4Jfy8Ajj9xIgz72Sfme9WqYy nEig/pPHlROWJ5bDwM8EqeTmxfGUPEMbRcI/8tlB6LWuAwx0pwh6YrGVr66Z47Dubq PgJQpQ7AKh2MVaVKjyeJEg68z3Q5JfgKB3b3gDPU= 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 mr3.vodafonemail.de (Postfix) with ESMTPS id 4QVP1C0NRPz1yVn; Mon, 29 May 2023 18:18:55 +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 4QVP134qbfz9sD0; Mon, 29 May 2023 18:18:44 +0000 (UTC) Message-ID: <89B619BD4A4E40A4BDE82CD91986895A@H270> From: "Stefan Kanthak" To: "Andrew Pinski" Cc: References: <23A490318B7149D88618A7CDA2CEDB14@H270> <8DB226CF451A4430A8D7D5CBFE6B3972@H270> In-Reply-To: Subject: Who cares about size? (was: Who cares about performance (or Intel's CPU errata)?) Date: Mon, 29 May 2023 20:11:53 +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: 1555 X-purgate-ID: 155817::1685384330-357F94DE-A8E69F71/0/0 Received-SPF: pass client-ip=145.253.228.163; envelope-from=stefan.kanthak@nexgo.de; helo=mr3.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: "Andrew Pinski" wrote: > On Sat, May 27, 2023 at 3:54 PM Stefan Kanthak wrote: >> Nevertheless GCC fails to optimise code properly: >> >> --- .c --- >> int ispowerof2(unsigned long long argument) { >> return __builtin_popcountll(argument) == 1; >> } >> --- EOF --- >> >> GCC 13.3 gcc -m32 -mpopcnt -O3 >> >> https://godbolt.org/z/fT7a7jP4e >> ispowerof2(unsigned long long): >> xor eax, eax >> xor edx, edx >> popcnt eax, [esp+4] >> popcnt edx, [esp+8] >> add eax, edx # eax is less than 64! >> cmp eax, 1 -> dec eax # 2 bytes shorter sete al movzx eax, al ret > > dec eax is done for -Os already. > -O2 means performance, it does not mean decrease size. But -Os is supposed to optimise for size? REALITY CHECK: GCC 13.3 gcc -m32 -mpopcnt -Os https://godbolt.org/z/41Ed6rr6r ispowerof2(unsigned long long): push ebp mov ebp, esp sub esp, 16 push [ebp+12] push [ebp+8] call __popcountdi2 add esp, 16 # superfluous! leave dec eax sete al movzx eax, al ret While -O3 generates 9 instructions in a total of 24 bytes, -Os[lowmotion] but generates 12 instructions in 29 bytes, PLUS the uncounted instructions/bytes of __popcountdi2()! This is what I call an EPIC FAILURE! Stefan