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

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Date:   Wed, 30 Mar 2022 21:57:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-um@...ts.infradead.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev
Subject: Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

On Wed, Mar 30, 2022 at 04:45:27PM -0700, Nathan Chancellor 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@...nel.org>

Yeah, this looks right -- the history of this option seems to show it
is no longer needed.

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook