Messages in this thread | | | Date | Mon, 23 Aug 2021 19:56:47 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang |
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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@intel.com> >Signed-off-by: Nathan Chancellor <nathan@kernel.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.
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