[{"id":3684764,"web_url":"http://patchwork.ozlabs.org/comment/3684764/","msgid":"<8d188ded-0e3a-426d-b3ef-6357cc98440f@redhat.com>","list_archive_url":null,"date":"2026-04-30T14:49:56","subject":"Re: [PATCH] c, c++: Introduce -Wconstant-logical-operand warning\n [PR125081]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/30/26 4:16 AM, Jakub Jelinek wrote:\n> Hi!\n> \n> Given the recent (data->flags && ff_genericize) vs.\n> (data->flags & ff_genericize) typo, I've looked at warning in similar\n> cases.\n> We don't warn for cases like that at all, clang/clang++ has\n> -Wconstant-logical-operand warning enabled by default.\n> Their behavior is:\n> 1) only warns for rhs of &&/|| (why?)\n> 2) don't warn if rhs is bool\n> 3) for C++ warn if rhs is constant or folds into constant,\n> for C warn if rhs is constant or folds into constant and\n> that constant is not 0 or 1\n> 4) I think it doesn't warn if rhs comes from a macro\n> The following patch implements similar warning with similar wording,\n> just provides the value of the constant, but\n> 1) warns for lhs and rhs\n> 2) doesn't warn if either lhs or rhs is bool\n> 3) doesn't warn if lhs or rhs is or folds to constant 0 or 1\n> 4) doesn't care if it comes from a macro or not\n> I think 64 && x is similarly suspicious to x && 64 and both\n> are likely to be meant 64 & x or x & 64.\n\nAgreed.\n\n> I think having\n> && 1 or && 0 is common even in C++, people don't always write\n> && true or && false etc. and don't see why C++ would be different\n> in that from C, I think people sometimes write\n> if (1\n> #ifdef ABC\n> && ABC\n> #endif\n> #ifdef DEF\n> && DEF\n> #endif\n> && 1)\n> and similar (or similarly with 0/true/false or ||).\n\nHmm, I often write things like this, but only locally/temporarily to \ndisable a change for testing; I wouldn't mind a warning about it. But \nwe wouldn't want to warn if it comes from a macro or enumerator, and I \nsuppose it's unlikely to be an accident, so not warning also seems fine.\n\n> And the warning\n> is only enabled in -Wall, not by default. This won't warn about\n> the ff_genericize case, because that is actually data->flags && 1\n> and I think that isn't that suspicious.\n\nI think it would still be good to warn if the original operand has enum \ntype with flag_enum attribute (and use that attribute on more enums).\n\nJason","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=VSxyio3J;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=VSxyio3J","sourceware.org; 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Myers\"\n , Marek Polacek ,\n Patrick Palka ","Cc":"gcc-patches@gcc.gnu.org","References":"","From":"Jason Merrill ","In-Reply-To":"","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"qPAz5G_Nhow9pcI8EY5sf8YDCcanKx2ra6R8JuQ40cs_1777560600","X-Mimecast-Originator":"redhat.com","Content-Language":"en-US","Content-Type":"text/plain; charset=UTF-8; format=flowed","Content-Transfer-Encoding":"7bit","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"}},{"id":3684775,"web_url":"http://patchwork.ozlabs.org/comment/3684775/","msgid":"","list_archive_url":null,"date":"2026-04-30T15:07:15","subject":"Re: [PATCH] c, c++: Introduce -Wconstant-logical-operand warning\n [PR125081]","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/people/671/","name":"Jakub Jelinek","email":"jakub@redhat.com"},"content":"On Thu, Apr 30, 2026 at 10:49:56AM -0400, Jason Merrill wrote:\n> On 4/30/26 4:16 AM, Jakub Jelinek wrote:\n> > Given the recent (data->flags && ff_genericize) vs.\n> > (data->flags & ff_genericize) typo, I've looked at warning in similar\n> > cases.\n> > We don't warn for cases like that at all, clang/clang++ has\n> > -Wconstant-logical-operand warning enabled by default.\n> > Their behavior is:\n> > 1) only warns for rhs of &&/|| (why?)\n> > 2) don't warn if rhs is bool\n> > 3) for C++ warn if rhs is constant or folds into constant,\n> > for C warn if rhs is constant or folds into constant and\n> > that constant is not 0 or 1\n> > 4) I think it doesn't warn if rhs comes from a macro\n> > I think having\n> > && 1 or && 0 is common even in C++, people don't always write\n> > && true or && false etc. and don't see why C++ would be different\n> > in that from C, I think people sometimes write\n> > if (1\n> > #ifdef ABC\n> > && ABC\n> > #endif\n> > #ifdef DEF\n> > && DEF\n> > #endif\n> > && 1)\n> > and similar (or similarly with 0/true/false or ||).\n> \n> Hmm, I often write things like this, but only locally/temporarily to disable\n> a change for testing; I wouldn't mind a warning about it. But we wouldn't\n> want to warn if it comes from a macro or enumerator, and I suppose it's\n> unlikely to be an accident, so not warning also seems fine.\n> \n> > And the warning\n> > is only enabled in -Wall, not by default. This won't warn about\n> > the ff_genericize case, because that is actually data->flags && 1\n> > and I think that isn't that suspicious.\n> \n> I think it would still be good to warn if the original operand has enum type\n> with flag_enum attribute (and use that attribute on more enums).\n\nI can certainly implement that. Or just warn also whenever the constant is\n1 with some enum type originally, I don't see why people actually would write\nenum { A, B }; if (x && B) unless it is a poor man's pre-C11 replacement of bool (so\nsay typedef enum BOOL { FALSE, TRUE } BOOL;\nand using if (x && TRUE)). Or warn for constant 1 if it has enum type and\nthe enum type contains more enumerators than just 0/1. One can always get\nrid of the warning by using && ENUM != 0 instead of just && ENUM and make\nthe code even more readable that way.\n\n\tJakub","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=IMp9TUOY;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=IMp9TUOY","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; spf=pass smtp.mailfrom=redhat.com","server2.sourceware.org;\n arc=none smtp.remote-ip=170.10.133.124"],"Received":["from vm01.sourceware.org (vm01.sourceware.org [38.145.34.32])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1) server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g5yGP4fSkz1xqf\n\tfor ; Fri, 01 May 2026 01:07:56 +1000 (AEST)","from vm01.sourceware.org (localhost [127.0.0.1])\n\tby sourceware.org (Postfix) with ESMTP id BDA774371D66\n\tfor ; Thu, 30 Apr 2026 15:07:53 +0000 (GMT)","from us-smtp-delivery-124.mimecast.com\n (us-smtp-delivery-124.mimecast.com [170.10.133.124])\n by sourceware.org (Postfix) with ESMTP id 7005D436F7CF\n for ; Thu, 30 Apr 2026 15:07:25 +0000 (GMT)","from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com\n (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by\n relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3,\n cipher=TLS_AES_256_GCM_SHA384) id us-mta-360-V8A9NMLvPXSDlbhH_ZFfLg-1; Thu,\n 30 Apr 2026 11:07:20 -0400","from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com\n (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111])\n (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest\n SHA256)\n (No client certificate requested)\n by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS\n id B25EC18005A9\n for ; Thu, 30 Apr 2026 15:07:19 +0000 (UTC)","from tucnak.zalov.cz (unknown [10.44.34.21])\n by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with\n ESMTPS\n id E4339180045E; Thu, 30 Apr 2026 15:07:18 +0000 (UTC)","from tucnak.zalov.cz (localhost [127.0.0.1])\n by tucnak.zalov.cz (8.18.1/8.18.1) with ESMTPS id 63UF7G1i3802023\n (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT);\n Thu, 30 Apr 2026 17:07:16 +0200","(from jakub@localhost)\n by tucnak.zalov.cz (8.18.1/8.18.1/Submit) id 63UF7FVY3802022;\n Thu, 30 Apr 2026 17:07:15 +0200"],"DKIM-Filter":["OpenDKIM Filter v2.11.0 sourceware.org BDA774371D66","OpenDKIM Filter v2.11.0 sourceware.org 7005D436F7CF"],"DMARC-Filter":"OpenDMARC Filter v1.4.2 sourceware.org 7005D436F7CF","ARC-Filter":"OpenARC Filter v1.0.0 sourceware.org 7005D436F7CF","ARC-Seal":"i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1777561645; cv=none;\n b=tACPbxn5mPAup7U/IenmMx/PZhmb+92rLn/zceWqan1mxQglTFL6+BN6y57gPpkpY2SSPwM7nleReBJJiLPcIJ2n3nZC11m6VdULUc90KKJUumD7wAaT6OR28KkwOJ2UG0tz/m6OSeFrrQ/ctENu50zWGnYbZx4qi8JzU/qdtas=","ARC-Message-Signature":"i=1; a=rsa-sha256; d=sourceware.org; s=key;\n t=1777561645; c=relaxed/simple;\n bh=TZ3Wn0qzXmtrya8FqzFt2LnpGmiGGZJ4HY/bB2PwloY=;\n h=DKIM-Signature:Date:From:To:Subject:Message-ID:MIME-Version;\n b=Izy1KR80oYrnTaMerCh5zwzPaVXu3APbAd4Xj2PgdCxZUlJ83a1U3BkP19MoPQcZ0l8t+tT0dV3h3Aj7bStNNelCsaJckLDeNMUw/9f9+F8Pt7GZ8Dx6vc5CFmU/YRJrDDuAQ/0ZbJiywKUwXAlsJxGcJ+aqSvFyy66iNbaIXgY=","ARC-Authentication-Results":"i=1; server2.sourceware.org","DKIM-Signature":"v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com;\n s=mimecast20190719; t=1777561645;\n h=from:from:reply-to:reply-to:subject:subject:date:date:\n message-id:message-id:to:to:cc:cc:mime-version:mime-version:\n content-type:content-type:in-reply-to:in-reply-to: references:references;\n bh=wEwTb1+i0u3fyV8uFJk+bwhtxVAHDKbub8SKuVtq5wg=;\n b=IMp9TUOYrkL3jXDqTW+PIyyjZfz5SKW0QWRty+1hzrJ4ecEp5YfBOkYwnFoPTBAcF271Lz\n cI6isduNB9L7roKuhWmol6vt70sl1f6gg7JB8xVOwuwbLeBjKHZPpy8K2R+63zXCb0ZKxi\n k3M59Axdu0IzX0CqI5v7WzflsU6nFyU=","X-MC-Unique":"V8A9NMLvPXSDlbhH_ZFfLg-1","X-Mimecast-MFC-AGG-ID":"V8A9NMLvPXSDlbhH_ZFfLg_1777561640","Date":"Thu, 30 Apr 2026 17:07:15 +0200","From":"Jakub Jelinek ","To":"Jason Merrill ","Cc":"\"Joseph S. Myers\" ,\n Marek Polacek , Patrick Palka ,\n gcc-patches@gcc.gnu.org","Subject":"Re: [PATCH] c, c++: Introduce -Wconstant-logical-operand warning\n [PR125081]","Message-ID":"","References":"\n <8d188ded-0e3a-426d-b3ef-6357cc98440f@redhat.com>","MIME-Version":"1.0","In-Reply-To":"<8d188ded-0e3a-426d-b3ef-6357cc98440f@redhat.com>","X-Scanned-By":"MIMEDefang 3.4.1 on 10.30.177.111","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"wY9yDb5zwJA2dqSOLPrMiyRkd2EhgFy8uylmbce-J2w_1777561640","X-Mimecast-Originator":"redhat.com","Content-Type":"text/plain; charset=us-ascii","Content-Disposition":"inline","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"Jakub Jelinek ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"}},{"id":3684885,"web_url":"http://patchwork.ozlabs.org/comment/3684885/","msgid":"<9acfb25a-8cb0-45f1-8059-d959d8e88b2a@redhat.com>","list_archive_url":null,"date":"2026-04-30T18:08:46","subject":"Re: [PATCH] c, c++: Introduce -Wconstant-logical-operand warning\n [PR125081]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/30/26 11:07 AM, Jakub Jelinek wrote:\n> On Thu, Apr 30, 2026 at 10:49:56AM -0400, Jason Merrill wrote:\n>> On 4/30/26 4:16 AM, Jakub Jelinek wrote:\n>>> Given the recent (data->flags && ff_genericize) vs.\n>>> (data->flags & ff_genericize) typo, I've looked at warning in similar\n>>> cases.\n>>> We don't warn for cases like that at all, clang/clang++ has\n>>> -Wconstant-logical-operand warning enabled by default.\n>>> Their behavior is:\n>>> 1) only warns for rhs of &&/|| (why?)\n>>> 2) don't warn if rhs is bool\n>>> 3) for C++ warn if rhs is constant or folds into constant,\n>>> for C warn if rhs is constant or folds into constant and\n>>> that constant is not 0 or 1\n>>> 4) I think it doesn't warn if rhs comes from a macro\n>>> I think having\n>>> && 1 or && 0 is common even in C++, people don't always write\n>>> && true or && false etc. and don't see why C++ would be different\n>>> in that from C, I think people sometimes write\n>>> if (1\n>>> #ifdef ABC\n>>> && ABC\n>>> #endif\n>>> #ifdef DEF\n>>> && DEF\n>>> #endif\n>>> && 1)\n>>> and similar (or similarly with 0/true/false or ||).\n>>\n>> Hmm, I often write things like this, but only locally/temporarily to disable\n>> a change for testing; I wouldn't mind a warning about it. But we wouldn't\n>> want to warn if it comes from a macro or enumerator, and I suppose it's\n>> unlikely to be an accident, so not warning also seems fine.\n>>\n>>> And the warning\n>>> is only enabled in -Wall, not by default. This won't warn about\n>>> the ff_genericize case, because that is actually data->flags && 1\n>>> and I think that isn't that suspicious.\n>>\n>> I think it would still be good to warn if the original operand has enum type\n>> with flag_enum attribute (and use that attribute on more enums).\n> \n> I can certainly implement that. Or just warn also whenever the constant is\n> 1 with some enum type originally, I don't see why people actually would write\n> enum { A, B }; if (x && B) unless it is a poor man's pre-C11 replacement of bool (so\n> say typedef enum BOOL { FALSE, TRUE } BOOL;\n> and using if (x && TRUE)). Or warn for constant 1 if it has enum type and\n> the enum type contains more enumerators than just 0/1. One can always get\n> rid of the warning by using && ENUM != 0 instead of just && ENUM and make\n> the code even more readable that way.\n\nSounds good.\n\nJason","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=eiCsW0aD;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=eiCsW0aD","sourceware.org; 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Myers\" ,\n Marek Polacek , Patrick Palka ,\n gcc-patches@gcc.gnu.org","References":"\n <8d188ded-0e3a-426d-b3ef-6357cc98440f@redhat.com> ","From":"Jason Merrill ","In-Reply-To":"","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"qvGVGDQjnxw3A16zwxarnwnb5xDlJskVdLnH6uOEnEk_1777572529","X-Mimecast-Originator":"redhat.com","Content-Language":"en-US","Content-Type":"text/plain; charset=UTF-8; format=flowed","Content-Transfer-Encoding":"7bit","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"}}]