Hi David,
On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> The things built with USER_CFLAGS don't seem to recognise it as a
> compiler option, and print a warning:
> clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
>
> Fixes: 744814d2fa ("um: Allow builds with Clang")
> Signed-off-by: David Gow <davidgow@xxxxxxxxxx>
> ---
>
> This warning shows up after merging:
> https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@xxxxxxxxxxxx/
>
> I'm not 100% sure why this is necessary, but it does seem to work. All
> the attempts to get rid of -mno-global-merge entirely have been met with
> skepticism, but I'm guessing that it's not a problem for just the UML
> "user" files, as they shouldn't(?) interact too much with modules.
Thank you for the patch! I think it is correct, as this flag only works
for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
which are obviously never called with UML. I am not sure why we do not
see warning during regular kernel builds, maybe something about how UML
objects are compiled exposes this?
Regardless, I would definitely like to clean up this instance of the
warning because I would like to make this warning a hard error so that
we do not get cryptic cc-option failures:
https://github.com/ClangBuiltLinux/linux/issues/1587
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
One small comment below.
> arch/um/Makefile | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index f2fe63bfd819..320b09cd513c 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> -D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
> -idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
>
> +ifdef CONFIG_CC_IS_CLANG
Is this ifdef needed?
> +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> +endif
> +
> #This will adjust *FLAGS accordingly to the platform.
> include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
>
> --
> 2.35.1.616.g0bdcbb4464-goog
>
