From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f173.google.com (mail-pl1-f173.google.com [209.85.214.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 33D1F3FC0 for ; Tue, 24 Aug 2021 02:56:53 +0000 (UTC) Received: by mail-pl1-f173.google.com with SMTP id e1so4421043plt.11 for ; Mon, 23 Aug 2021 19:56:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=29Fb0DQb3lLM1RT0ONfW5Rl3roBtF0uhGZi6Clr4PQ8=; b=GDUCXarovw3ukF3RwAsETeElR5eGZPBu70ayB2wBQ/TkJiUQsrK/LrueDGJCxotLKi ONJSxgXeWrngVIsd79smxVSF174v3ocrKyIRx2FpMXOk9kzCNFVH2niEcM+R+iqNEmI7 ZlvmjevY3GS6Hme6xtZ3+Ptx+FnFMI9Yq/daLTAA4UCGxALn/6hgG5JG2QLb1sp4M1em kl2plY0zoiACgUQbnHH2YRDzcEhkPyZQdD+mbfmlKIV9Vtf7FbLGrEhvgfrzAiF0T/Ex QkqbMid4GkGvIAcMfdK7VJ5jWqv1njIol6NTXXsAqKIUXQJuVN0TAmsN7MSk3SSdFPmG d1Zw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=29Fb0DQb3lLM1RT0ONfW5Rl3roBtF0uhGZi6Clr4PQ8=; b=W3uE3APdnQq7xRZE5JeDQeYptAzTh4v3nGvasi1/McsZwR8jq8wAooMtCE6LP2hMDT ZgrFpC8KECAa+yOwQq9PjJwH+yDXkNU8vLUKSGbcO1qgYAJyqW93KMFiH3GvGIYdwYZw ol44fTPInCV22/+PaPJQjw2wGTWhsvqH0HCVJ4XrUWalvJ57I9h+N1VwzVGKnfULRNcz +ylnspNI8AaArb8SdWUJJetY4sHAqqVoj9TSPgJh27dFvt7qiRE2EkPMSc+50xFQjlQF CDrJZTgxTSM8eJ6oD4RpUplMSZfiIsuokWwLf4OMqitZ9LzjgBEtipV5E6yrZ6kqSqY1 OXmg== X-Gm-Message-State: AOAM530BVtDF/W2g7mpdf85r/9ldY6OXBaqIjUlw3mDR0aMMGqnedwJf iwochVqgh2o3lzCMCVczinanIg== X-Google-Smtp-Source: ABdhPJwXFrQMieCMh9x9c+mtDLu/YP/iQ36j1IknXDrvOvDegeb32ZiTPGsKdkfnQ8HYxv43bwJgqw== X-Received: by 2002:a17:903:310a:b0:133:9bb6:98bd with SMTP id w10-20020a170903310a00b001339bb698bdmr9300777plc.19.1629773812408; Mon, 23 Aug 2021 19:56:52 -0700 (PDT) Received: from google.com ([2620:15c:2ce:200:196f:5297:b0e8:1313]) by smtp.gmail.com with ESMTPSA id y12sm18770264pgl.65.2021.08.23.19.56.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Aug 2021 19:56:51 -0700 (PDT) Date: Mon, 23 Aug 2021 19:56:47 -0700 From: Fangrui Song To: Nathan Chancellor Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, Masahiro Yamada , "H. Peter Anvin" , Nick Desaulniers , linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com, llvm@lists.linux.dev, kernel test robot Subject: Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang Message-ID: <20210824025647.tssnp7qtccbgvdq7@google.com> References: <20210824022640.2170859-1-nathan@kernel.org> <20210824022640.2170859-2-nathan@kernel.org> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20210824022640.2170859-2-nathan@kernel.org> On 2021-08-23, Nathan Chancellor wrote: >clang does not support -falign-jumps and only recently gained support >for -falign-loops. When one of the configuration options that adds these >flags is enabled, clang warns and all cc-{disable-warning,option} that >follow fail because -Werror gets added to test for the presence of this >warning: [I implemented clang -falign-loops :) It doesn't affect LTO, though. LTO ld.lld may use -Wl,-mllvm,-align-loops=32 for now. ] >clang-14: warning: optimization flag '-falign-jumps=0' is not supported >[-Wignored-optimization-argument] grub made a similar mistake:) It thought the availability of -falign-X implies the availability of other -falign-* https://lists.gnu.org/archive/html/grub-devel/2021-08/msg00076.html >To resolve this, add a couple of cc-option calls when building with >clang; gcc has supported these options since 3.2 so there is no point in >testing for their support. -falign-functions was implemented in clang-7, >-falign-loops was implemented in clang-14, and -falign-jumps has not >been implemented yet. > >Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ >Reported-by: kernel test robot >Signed-off-by: Nathan Chancellor >--- > arch/x86/Makefile_32.cpu | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > >diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu >index cd3056759880..e8c65f990afd 100644 >--- a/arch/x86/Makefile_32.cpu >+++ b/arch/x86/Makefile_32.cpu >@@ -10,6 +10,12 @@ else > tune = $(call cc-option,-mcpu=$(1),$(2)) > endif > >+ifdef CONFIG_CC_IS_CLANG >+align := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0) >+else >+align := -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+endif >+ > cflags-$(CONFIG_M486SX) += -march=i486 > cflags-$(CONFIG_M486) += -march=i486 > cflags-$(CONFIG_M586) += -march=i586 >@@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6) += -march=k6 > # They make zero difference whatsosever to performance at this time. > cflags-$(CONFIG_MK7) += -march=athlon > cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8,-march=athlon) >-cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 >-cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+cflags-$(CONFIG_MCRUSOE) += -march=i686 $(align) >+cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) $(align) > cflags-$(CONFIG_MWINCHIPC6) += $(call cc-option,-march=winchip-c6,-march=i586) > cflags-$(CONFIG_MWINCHIP3D) += $(call cc-option,-march=winchip2,-march=i586) >-cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) $(align) > cflags-$(CONFIG_MVIAC3_2) += $(call cc-option,-march=c3-2,-march=i686) > cflags-$(CONFIG_MVIAC7) += -march=i686 > cflags-$(CONFIG_MCORE2) += -march=i686 $(call tune,core2) >-- >2.33.0 https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html says "If n is not specified or is zero, use a machine-dependent default." Unless some other files specify -falign-loops=N and expect 0 to reset to the machine default, -falign-jumps=0 -falign-loops=0 -falign-functions=0 should just be dropped. BTW: I believe GCC 8 (likely when fixing another issue with a large refactor https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84100) introduced a bug that -falign-X=0 was essentially -falign-X=1. GCC 11.0 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96247) fixed the bug.