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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 25 Aug 2021 15:27:29 -0700
Message-ID: <CAKwvOdk8nj6Yu=+k+cDx_rKh=ug6+VyYRzzFGDiZjrQQ8q7eCw@mail.gmail.com>
Subject: Re: [PATCH 2/2] kbuild: Add -Werror=ignored-optimization-argument to CLANG_FLAGS
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Masahiro Yamada <masahiroy@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com,
        llvm@lists.linux.dev
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On Mon, Aug 23, 2021 at 7:27 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Similar to commit 589834b3a009 ("kbuild: Add
> -Werror=unknown-warning-option to CLANG_FLAGS").
>
> Clang ignores certain GCC flags that it has not implemented, only
> emitting a warning:
>
> $ echo | clang -fsyntax-only -falign-jumps -x c -
> clang-14: warning: optimization flag '-falign-jumps' is not supported
> [-Wignored-optimization-argument]
>
> When one of these flags gets added to KBUILD_CFLAGS unconditionally, all
> subsequent cc-{disable-warning,option} calls fail because -Werror was
> added to these invocations to turn the above warning and the equivalent
> -W flag warning into errors.
>
> To catch the presence of these flags earlier, turn
> -Wignored-optimization-argument into an error so that the flags can
> either be implemented or ignored via cc-option and there are no more
> weird errors.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Good idea.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  scripts/Makefile.clang | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 4cce8fd0779c..2fe38a9fdc11 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -29,7 +29,11 @@ CLANG_FLAGS  += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
>  else
>  CLANG_FLAGS    += -fintegrated-as
>  endif
> +# By default, clang only warns on unknown warning or optimization flags
> +# Make it behave more like gcc by erroring when these flags are encountered
> +# so they can be implemented or wrapped in cc-option.
>  CLANG_FLAGS    += -Werror=unknown-warning-option
> +CLANG_FLAGS    += -Werror=ignored-optimization-argument
>  KBUILD_CFLAGS  += $(CLANG_FLAGS)
>  KBUILD_AFLAGS  += $(CLANG_FLAGS)
>  export CLANG_FLAGS
> --
> 2.33.0
>


-- 
Thanks,
~Nick Desaulniers