linux-kernel - Re: [PATCH] kbuild: Do not enable -Wimplicit-fallthrough for clang for now

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Date:   Thu, 29 Aug 2019 01:20:19 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Huckleberry <nhuck@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] kbuild: Do not enable -Wimplicit-fallthrough for clang
 for now

On Tue, Aug 27, 2019 at 9:42 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> This functionally reverts commit bfd77145f35c ("Makefile: Convert
> -Wimplicit-fallthrough=3 to just -Wimplicit-fallthrough for clang").
>
> clang enabled support for -Wimplicit-fallthrough in C in r369414 [1],
> which causes a lot of warnings when building the kernel for two reasons:
>
> 1. Clang does not support the /* fall through */ comments. There seems
>    to be a general consensus in the LLVM community that this is not
>    something they want to support. Joe Perches wrote a script to convert
>    all of the comments to a "fallthrough" keyword that will be added to
>    compiler_attributes.h [2] [3], which catches the vast majority of the
>    comments. There doesn't appear to be any consensus in the kernel
>    community when to do this conversion.
>
> 2. Clang and GCC disagree about falling through to final case statements
>    with no content or cases that simply break:
>
>    https://godbolt.org/z/c8csDu
>
>    This difference contributes at least 50 warnings in an allyesconfig
>    build for x86, not considering other architectures. This difference
>    will need to be discussed to see which compiler is right [4] [5].
>
> [1]: https://github.com/llvm/llvm-project/commit/1e0affb6e564b7361b0aadb38805f26deff4ecfc
> [2]: https://lore.kernel.org/lkml/61ddbb86d5e68a15e24ccb06d9b399bbf5ce2da7.camel@perches.com/
> [3]: https://lore.kernel.org/lkml/1d2830aadbe9d8151728a7df5b88528fc72a0095.1564549413.git.joe@perches.com/
> [4]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432
> [5]: https://github.com/ClangBuiltLinux/linux/issues/636
>
> Given these two problems need discussion and coordination, do not enable
> -Wimplicit-fallthrough with clang right now. Add a comment to explain
> what is going on as well. This commit should be reverted once these two
> issues are fully flushed out and resolved.
>
> Suggested-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---

Applied to linux-kbuild. Thanks.

(If other clang folks give tags, I will add them later.)



>  Makefile | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index f125625efd60..6007a56bdbee 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -751,6 +751,11 @@ else
>  # These warnings generated too much noise in a regular build.
>  # Use make W=1 to enable them (see scripts/Makefile.extrawarn)
>  KBUILD_CFLAGS += -Wno-unused-but-set-variable
> +
> +# Warn about unmarked fall-throughs in switch statement.
> +# Disabled for clang while comment to attribute conversion happens and
> +# https://github.com/ClangBuiltLinux/linux/issues/636 is discussed.
> +KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
>  endif
>
>  KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
> @@ -845,9 +850,6 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>  # warn about C99 declaration after statement
>  KBUILD_CFLAGS += -Wdeclaration-after-statement
>
> -# Warn about unmarked fall-throughs in switch statement.
> -KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
> -
>  # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
>  KBUILD_CFLAGS += -Wvla
>
> --
> 2.23.0
>


-- 
Best Regards
Masahiro Yamada