Re: [PATCH v2] kbuild: Require W=1 for -Wimplicit-fallthrough with clang

[Nathan Chancellor <natechancellor@gmail.com>]

On Mon, Aug 19, 2019 at 01:43:08AM +0900, Masahiro Yamada wrote:
> 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

I like this more than anything suggested so far. I think a comment
should be added regarding why this is only enabled for GCC right now but
that is pretty easy to revert once we have figured out the right course
of action.

Cheers,
Nathan