Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

Sedat Dilek
2022-03-31T07:13:17Z

On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> This flag is specific to clang, where it is only used by the 32-bit and
> 64-bit ARM backends. In certain situations, the presence of this flag
> will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> out -mno-global-merge from USER_CFLAGS").
>
> Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> building kernel with Clang") that added this flag back in 2014, there
> have been quite a few changes to the GlobalMerge pass in LLVM. Building
> several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> (minimum) and 15 (current main version) with this flag removed (i.e.,
> with the default of '-mglobal-merge') reveals no modpost warnings, so it
> is likely that the issue noted in the comment is no longer relevant due
> to changes in LLVM or modpost, meaning this flag can be removed.
>
> If any new warnings show up that are a result of the removal of this
> flag, it can be added back under arch/arm{,64}/Makefile to avoid
> warnings on other architectures.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> Makefile | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index daeb5c88b50b..f2723d9bfca4 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> KBUILD_CPPFLAGS += -Qunused-arguments
> # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> KBUILD_CFLAGS += -Wno-gnu
> -# 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
> else
>
> # gcc inanely warns about local variables called 'main'
> --
> 2.35.1
>

I have tested this several times and was able to boot into bar metal -
no problems with building and/or booting my kernel-modules.

Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>

Just as a side-note:
As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?

# The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
KBUILD_CFLAGS += -Wno-gnu

- Sedat -