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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Mon, 19 Aug 2019 01:43:08 +0900
Message-ID: <CAK7LNATsA0foyeaE2W3xe=_Hkf9S=q0eD5WHqwPXkMw8udkDPg@mail.gmail.com>
Subject: Re: [PATCH v2] kbuild: Require W=1 for -Wimplicit-fallthrough with clang
To:     Nathan Huckleberry <nhuck@google.com>
Cc:     Michal Marek <michal.lkml@markovi.net>,
        Joe Perches <joe@perches.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <natechancellor@gmail.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi.

On Fri, Aug 16, 2019 at 7:59 AM Nathan Huckleberry <nhuck@google.com> wrote:
>
> Clang is updating to support -Wimplicit-fallthrough on C
> https://reviews.llvm.org/D64838. Since clang does not
> support the comment version of fallthrough annotations
> this update causes an additional 50k warnings. Most
> of these warnings (>49k) are duplicates from header files.
>
> This patch is intended to be reverted after the warnings
> have been cleaned up.
>
> Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> Changes v1->v2
> * Move code to preexisting ifdef
>  scripts/Makefile.extrawarn | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index a74ce2e3c33e..95973a1ee999 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -70,5 +70,6 @@ KBUILD_CFLAGS += -Wno-initializer-overrides
>  KBUILD_CFLAGS += -Wno-format
>  KBUILD_CFLAGS += -Wno-sign-compare
>  KBUILD_CFLAGS += -Wno-format-zero-length
> +KBUILD_CFLAGS += $(call cc-option,-Wno-implicit-fallthrough)
>  endif
>  endif
> --
> 2.23.0.rc1.153.gdeed80330f-goog
>


Perhaps, is the following even cleaner?


diff --git a/Makefile b/Makefile
index 1b23f95db176..cebc6bf5372e 100644
--- a/Makefile
+++ b/Makefile
@@ -751,6 +751,9 @@ 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.
+KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
 endif

 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
@@ -845,9 +848,6 @@ 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


-- 
Best Regards
Masahiro Yamada