From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:55636 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728683AbgBKNBj (ORCPT ); Tue, 11 Feb 2020 08:01:39 -0500 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 01BCx83Y125930 for ; Tue, 11 Feb 2020 08:01:38 -0500 Received: from e06smtp05.uk.ibm.com (e06smtp05.uk.ibm.com [195.75.94.101]) by mx0a-001b2d01.pphosted.com with ESMTP id 2y1tpcs4ma-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 11 Feb 2020 08:01:38 -0500 Received: from localhost by e06smtp05.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Feb 2020 13:01:36 -0000 Date: Tue, 11 Feb 2020 14:01:29 +0100 From: Vasily Gorbik Subject: Re: -Wtautological-constant-compare in arch/s390/include/asm/page.h References: <20200208125714.GA9164@ubuntu-x2-xlarge-x86> <1f54ae4c-8748-496b-0833-80749d8d4f6c@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1f54ae4c-8748-496b-0833-80749d8d4f6c@de.ibm.com> Message-Id: Sender: linux-s390-owner@vger.kernel.org List-ID: To: Nathan Chancellor Cc: Christian Borntraeger , Heiko Carstens , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com On Mon, Feb 10, 2020 at 08:55:46AM +0100, Christian Borntraeger wrote: > > > On 08.02.20 13:57, Nathan Chancellor wrote: > > Hi all, > > > > We noticed that you all added support for building s390 with clang, > > which is great! I have noticed a few warnings for which I will send > > patches but this one has me stumped. > > > > In file included from ../lib/crypto/sha256.c:16: > > In file included from ../include/linux/module.h:13: > > In file included from ../include/linux/stat.h:19: > > In file included from ../include/linux/time.h:6: > > In file included from ../include/linux/seqlock.h:36: > > In file included from ../include/linux/spinlock.h:51: > > In file included from ../include/linux/preempt.h:78: > > In file included from ../arch/s390/include/asm/preempt.h:6: > > In file included from ../include/linux/thread_info.h:38: > > In file included from ../arch/s390/include/asm/thread_info.h:26: > > ../arch/s390/include/asm/page.h:45:6: warning: converting the result of '<<' to a boolean always evaluates to false [-Wtautological-constant-compare] > > if (PAGE_DEFAULT_KEY) > > ^ > > ../arch/s390/include/asm/page.h:23:44: note: expanded from macro 'PAGE_DEFAULT_KEY' > > #define PAGE_DEFAULT_KEY (PAGE_DEFAULT_ACC << 4) > > ^ > > 1 warning generated. This warning only shows up for the decompressor code and purgatory which have separate set of build flags not derived from top level KBUILD_CFLAGS. For the rest of the code this warning is suppressed by: Makefile: 740 ifdef CONFIG_CC_IS_CLANG ... 744 # Quiet clang warning: comparison of unsigned expression < 0 is always false 745 KBUILD_CFLAGS += -Wno-tautological-compare At the same time both decompressor and purgatory Makefiles include CLANG_FLAGS into their CFLAGS. And this -Wno-tautological-compare is clang specific. So I believe this option belongs to CLANG_FLAGS rather than being included into KBUILD_CFLAGS under ifdef CONFIG_CC_IS_CLANG. But this raises question about other clang specific options inside that ifdef CONFIG_CC_IS_CLANG. Should they all be made part of CLANG_FLAGS? > > > > PAGE_DEFAULT_PAGE is always 0, meaning this function never does what it > > is supposed to. Is this intentional? It seems that commit 0b642ede4796 > > ("[PATCH] s390: default storage key") added this and it mentions that it > > can be overwritten at build time but I do not see any infrastructure for > > doing that. Any clarification that you can give so we can solve this > > warning would be much appreciated! > > Yes, it is a debugging tool that we use from time to time. The user would then > change PAGE_DEFAULT_ACC in the header file when needed. It was not worth a config > option as normal users should not use it. >