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

David Gow
2022-03-31T01:59:31Z

On Thu, Mar 31, 2022 at 7:46 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>
> ---

This seems to work fine here under the KUnit tooling, with both x86_64
and arm64. That admittedly doesn't do anything with modules, so
wouldn't reveal any of those issues, but at least the warnings about
--mno-global-merge existing remain gone.

Tested-by: David Gow <davidgow@google.com>

Cheers,
-- David