linux-kernel - Re: [PATCH] kbuild: add most of Clang-specific flags unconditionally

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Message-ID: <20190509111534.GA32696@archlinux-i9>
Date:   Thu, 9 May 2019 04:15:34 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        clang-built-linux@...glegroups.com,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: add most of Clang-specific flags unconditionally

On Thu, May 09, 2019 at 03:44:55PM +0900, Masahiro Yamada wrote:
> We do not support old Clang versions. Upgrade your clang version
> if any of these flags is unsupported.
> 
> Let's add flags within ifdef CONFIG_CC_IS_CLANG unconditionally,
> except -fcatch-undefined-behavior.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
> 
>  Makefile                   | 10 +++++-----
>  scripts/Makefile.extrawarn | 12 ++++++------
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index bd7ae11947cb..c71ffb6f55b5 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -731,15 +731,15 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
>  KBUILD_CFLAGS += $(stackp-flags-y)
>  
>  ifdef CONFIG_CC_IS_CLANG
> -KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
> -KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
> -KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> +KBUILD_CPPFLAGS += -Qunused-arguments
> +KBUILD_CFLAGS += -Wno-format-invalid-specifier
> +KBUILD_CFLAGS += -Wno-gnu
>  # Quiet clang warning: comparison of unsigned expression < 0 is always false
> -KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
> +KBUILD_CFLAGS += -Wno-tautological-compare
>  # 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 += $(call cc-option, -mno-global-merge,)
> +KBUILD_CFLAGS += -mno-global-merge
>  KBUILD_CFLAGS += $(call cc-option, -fcatch-undefined-behavior)

I think we should just remove this, I'm fairly confident the kernel
can't be reliably compiled with anything earlier than Clang 4 (Pixel 2
was shipped with it but had some hacks, this commit is from Clang 3.5):

https://github.com/llvm/llvm-project/commit/cb3f812b6b9fab8f3b41414f24e90222170417b4

Otherwise:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

>  else
>  
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 768306add591..523c4cafe2dc 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -66,11 +66,11 @@ KBUILD_CFLAGS += $(warning)
>  else
>  
>  ifdef CONFIG_CC_IS_CLANG
> -KBUILD_CFLAGS += $(call cc-disable-warning, initializer-overrides)
> -KBUILD_CFLAGS += $(call cc-disable-warning, unused-value)
> -KBUILD_CFLAGS += $(call cc-disable-warning, format)
> -KBUILD_CFLAGS += $(call cc-disable-warning, sign-compare)
> -KBUILD_CFLAGS += $(call cc-disable-warning, format-zero-length)
> -KBUILD_CFLAGS += $(call cc-disable-warning, uninitialized)
> +KBUILD_CFLAGS += -Wno-initializer-overrides
> +KBUILD_CFLAGS += -Wno-unused-value
> +KBUILD_CFLAGS += -Wno-format
> +KBUILD_CFLAGS += -Wno-sign-compare
> +KBUILD_CFLAGS += -Wno-format-zero-length
> +KBUILD_CFLAGS += -Wno-uninitialized
>  endif
>  endif
> -- 
> 2.17.1
>