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

[Nick Desaulniers]

On Thu, Aug 15, 2019 at 1:45 PM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> On Thu, Aug 15, 2019 at 11:20:29AM -0700, 'Nathan Huckleberry' via Clang Built Linux 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@xxxxxxxxxx>
> > ---
> >  Makefile                   | 4 ++++
> >  scripts/Makefile.extrawarn | 3 +++
> >  2 files changed, 7 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index 1b23f95db176..93b9744e66a2 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -846,7 +846,11 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> >  KBUILD_CFLAGS += -Wdeclaration-after-statement
> >
> >  # Warn about unmarked fall-throughs in switch statement.
> > +# If the compiler is clang, this warning is only enabled if W=1 in
> > +# Makefile.extrawarn
> > +ifndef CONFIG_CC_IS_CLANG
> >  KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
> > +endif
> >
> >  # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
> >  KBUILD_CFLAGS += -Wvla
> > diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> > index a74ce2e3c33e..e12359d69bb7 100644
> > --- a/scripts/Makefile.extrawarn
> > +++ b/scripts/Makefile.extrawarn
> > @@ -30,6 +30,9 @@ warning-1 += $(call cc-option, -Wunused-but-set-variable)
> >  warning-1 += $(call cc-option, -Wunused-const-variable)
> >  warning-1 += $(call cc-option, -Wpacked-not-aligned)
> >  warning-1 += $(call cc-option, -Wstringop-truncation)
> > +ifdef CONFIG_CC_IS_CLANG
> > +KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
>
> Shouldn't this be warning-1?

+1

>
> > +endif
> >  # The following turn off the warnings enabled by -Wextra
> >  warning-1 += -Wno-missing-field-initializers
> >  warning-1 += -Wno-sign-compare
> > --
> > 2.23.0.rc1.153.gdeed80330f-goog
> >
>
> I am still not a huge fan of the CONFIG_CC_IS_CLANG ifdefs but I don't
> really see a much cleaner way to get around this. Some that come to
> mind:
>
> * Leave Makefile alone and add
>
> KBUILD_CFLAGS += -Wno-implicit-fallthrough
>
> in the CONFIG_CC_IS_CLANG section of scripts/Makefile.extrawarn

Yeah, I think this might be cleaner.  -Wimplicit-fallthrough
-Wno-implicit-fallthrough will be passed to Clang, but "last one
wins."  A smaller patch makes it more likely to be revertable without
potentially having to resolve any conflicts.  Would you mind sending a
V2 with that change? You can include Nathan's Suggested-by tag.
-- 
Thanks,
~Nick Desaulniers