From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 74285C433DF for ; Thu, 6 Aug 2020 22:00:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5F6A622C9F for ; Thu, 6 Aug 2020 22:00:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1596751248; bh=OmPyAHrc8XMXpip9NvERg00Q2YyQjkyAT/0lynti33s=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=JCCPiAebPYp+j5nynSILnGqOIpFSVWVHVBkzPWllapyPMOmh4n3k/J8GPa1brsBNx BdZZPslAyRIIPZZ5Le+FyCt3LMFjrAi9JRCxE94QxWQc52f1NoQmQ7/QqrWVC7w7wx sOOaCnXA0EOHo4eU/1TWaomnpQqHmIgMcDbzdSLU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726589AbgHFWAs (ORCPT ); Thu, 6 Aug 2020 18:00:48 -0400 Received: from conssluserg-05.nifty.com ([210.131.2.90]:19146 "EHLO conssluserg-05.nifty.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726078AbgHFWAr (ORCPT ); Thu, 6 Aug 2020 18:00:47 -0400 Received: from mail-vs1-f51.google.com (mail-vs1-f51.google.com [209.85.217.51]) (authenticated) by conssluserg-05.nifty.com with ESMTP id 076M0TUn006651; Fri, 7 Aug 2020 07:00:29 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-05.nifty.com 076M0TUn006651 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1596751230; bh=Xfs85PHXdOhRcMjUjbm4/NBLl7dcQvPuB0IgyJvQcNY=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=xItFWfax66V1ZlVuvEA/5RZID3kXTCWeBtcMORvBRgSfWXPwamCAkrwdwA0S8/b/k /154VHpCDvYt7vYSXrKjIz0Y8CPUmE9GkVL2xNRNFxKKJyMO8QRKxjDC/2iNVbLyUg bYHXHjmXKfqxuRglfmAYs6gqFzpxtWrr+LQOl+aeIrraoJ40THoJRZuS/Sz9oh1rPz 7HtXBZh6y7B2IHNbSxim8Giv6ZhkXqc26FALl+KIQAV2fu2N9aGrjU75pLdIEjGLD6 V601vnzM/ZBII0ObJyn9TfYh4Iqc1CWGBbEjTHIRthG73/QOPOnxPvEOKPbM6WaIcy 6E9R3JceQurXA== X-Nifty-SrcIP: [209.85.217.51] Received: by mail-vs1-f51.google.com with SMTP id n25so11186415vsq.6; Thu, 06 Aug 2020 15:00:29 -0700 (PDT) X-Gm-Message-State: AOAM5334jNectuqMkMvnDpY6yL+quG3Pj7Qbptv/VvLyCko/eK76tK1e ArbT0Nb8VUfgBxoIUNUVzassql2sGbFuGBVgCxQ= X-Google-Smtp-Source: ABdhPJzz7ZZOU/6VCnvmI3r7gzwomd0hOWCZWY3UlQMHSS3+9qv2lWLJwLf6DZJtEiuUw3mJA7PzeBefGQiHc/jfmQ8= X-Received: by 2002:a67:d908:: with SMTP id t8mr8112536vsj.215.1596751228524; Thu, 06 Aug 2020 15:00:28 -0700 (PDT) MIME-Version: 1.0 References: <20200702233929.181409-1-danny@kdrag0n.dev> <20200705024318.GA433@Ryzen-9-3900X.localdomain> In-Reply-To: <20200705024318.GA433@Ryzen-9-3900X.localdomain> From: Masahiro Yamada Date: Fri, 7 Aug 2020 06:59:51 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] kbuild: Allow Clang global merging if !MODULES To: Nathan Chancellor , Danny Lin Cc: Michal Marek , Nick Desaulniers , Sami Tolvanen , Kees Cook , Linux Kbuild mailing list , Linux Kernel Mailing List , clang-built-linux Content-Type: text/plain; charset="UTF-8" Sender: linux-kbuild-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org On Sun, Jul 5, 2020 at 11:43 AM Nathan Chancellor wrote: > > Hi Danny, > > On Thu, Jul 02, 2020 at 04:39:29PM -0700, Danny Lin wrote: > > The old reasoning for disabling Clang's global merging optimization is > > that it breaks modpost by coalescing many symbols into _MergedGlobals. > > However, modpost is only used in builds with dynamic modules; > > vmlinux.symvers is still created during standalone builds, but it's > > effectively just an empty dummy file. I am not convinced with this statement. modpost is used even if CONFIG_MODULES=n You can see the following log for allnoconfig. MODPOST vmlinux.symvers Yes, vmlinux.symver is empty. This is because EXPORT_SYMBOL is no-op when CONFIG_MODULES=n. > > > > Enabling the optimization whenever possible allows us to reap the > > benefits of reduced register pressure when many global variables are > > used in the same function. > > Have you run into any place within the kernel that this is the case or > this more of a "could help if this ever happens" type of deal? > > > An x86 defconfig kernel built with this optimization boots fine in qemu, > > and a downstream 4.14 kernel has been used on arm64 for nearly a year > > without any issues caused by this optimization. > > If I am reading LLVM's source correctly, this option only seems relevant > for ARM and AArch64? > > $ rg --no-heading createGlobalMergePass > llvm/lib/CodeGen/GlobalMerge.cpp:679:Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset, > llvm/lib/Target/AArch64/AArch64TargetMachine.cpp:524: addPass(createGlobalMergePass(TM, 4095, OnlyOptimizeForSize, > llvm/lib/Target/ARM/ARMTargetMachine.cpp:456: addPass(createGlobalMergePass(TM, 127, OnlyOptimizeForSize, > llvm/include/llvm/CodeGen/Passes.h:419: Pass *createGlobalMergePass(const TargetMachine *TM, unsigned MaximalOffset, > > Otherwise, I think this is probably okay. According to [1], when the > optimization level is less than -O3, we get a less aggressive version of > this optimization level, which is good for code size: > > https://github.com/llvm/llvm-project/commit/8207641251706ea808df6d2a1ea8f87b8ee04c6d I do not understand how -mglobal-merge works. Do you have test code to generate _MergedGlobals ? Thanks. > However, we do potentially get merging of extern globals if we do not > specify -mglobal-merge (if I am reading the source correctly), which > this commit claims might hurt performance? Not sure if there is any way > to test or verify that? > > https://github.com/llvm/llvm-project/commit/de73404b8c4332190750537eb93ce0d5b6451300 > > > Signed-off-by: Danny Lin > > --- > > Makefile | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/Makefile b/Makefile > > index a60c98519c37..f04c3639cf61 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -772,10 +772,13 @@ ifdef CONFIG_CC_IS_CLANG > > KBUILD_CPPFLAGS += -Qunused-arguments > > KBUILD_CFLAGS += -Wno-format-invalid-specifier > > KBUILD_CFLAGS += -Wno-gnu > > + > > +ifdef CONFIG_MODULES > > # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the > > # source of a reference will be _MergedGlobals and not on of the whitelisted names. > > # See modpost pattern 2 > > KBUILD_CFLAGS += -mno-global-merge > > +endif > > else > > > > # These warnings generated too much noise in a regular build. > > -- > > 2.27.0 > > > > Cheers, > Nathan -- Best Regards Masahiro Yamada