linux-kernel - Re: [PATCH 3/3] kbuild: add -Wno-unused-but-set-variable compiler flag unconditionally

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Message-ID: <CAK7LNARuyo89pK9Fmd4s6CG8yhjpmzmiOVrN1fGmWkSyHDbfgA@mail.gmail.com>
Date:   Tue, 2 Oct 2018 07:23:27 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] kbuild: add -Wno-unused-but-set-variable compiler
 flag unconditionally

2018年10月2日(火) 4:58 Nick Desaulniers <ndesaulniers@...gle.com>:
>
> On Mon, Oct 1, 2018 at 12:02 PM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > Hi Nick,
> >
> > 2018年10月2日(火) 2:18 Nick Desaulniers <ndesaulniers@...gle.com>:
> > >
> > > On Mon, Oct 1, 2018 at 2:45 AM Masahiro Yamada
> > > <yamada.masahiro@...ionext.com> wrote:
> > > >
> > > > We have raised the compiler requirement from time to time.
> > > > With commit cafa0010cd51 ("Raise the minimum required gcc version
> > > > to 4.6"), the minimum for GCC is 4.6 now.
> > > >
> > > > This flag was added by GCC 4.6, and it is recognized by Clang and
> > > > ICC as well.
> > >
> > > This doesn't seem to be the case for Clang:
> > > https://godbolt.org/z/qesF5o
> > >
> > > Nacked-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >
> > Hmm, I tested this patch with pre-built Clang 6.0.1 / 7.0.0
> > downloaded from http://releases.llvm.org/download.html
> >
> > Was this option dropped by clang 8 ?
>
> From the Godbolt link above, it seems all versions of Clang do not
> recognize this flag.  It doesn't look like the kernel sets
> -Wno-unknown-warning-option to prevent this warning.  Can you please
> triple check that compiling with Clang and this patch causes no
> warnings?  I suspect something might be amiss then if this patch
> doesn't produce warnings with Clang, since on the smaller Godbolt
> example it does.

I got it.

Clang does NOT recognize -Wno-unused-but-set-variable.

But, the code I changed is not parsed for clang.
That is why I did not see any problem with this patch.

See the following code.

ifeq ($(cc-name),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_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
# Quiet clang warning: comparison of unsigned expression < 0 is always false
KBUILD_CFLAGS += $(call cc-disable-warning, 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 += $(call cc-option, -fcatch-undefined-behavior)
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
endif

-- 
Best Regards
Masahiro Yamada