linux-kernel - Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang

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Date:   Mon, 23 Aug 2021 19:56:47 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Masahiro Yamada <masahiroy@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com, llvm@...ts.linux.dev,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for
 clang

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 <lkp@...el.com>
>Signed-off-by: Nathan Chancellor <nathan@...nel.org>
>---
> 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.