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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 28 Aug 2019 16:28:30 -0700
Message-ID: <CAKwvOd=r5Y8hQQBeKZ6zAokPdyeT2AVKFsdviTvwV5AyDQQHrw@mail.gmail.com>
Subject: Re: [PATCH 2/2] kbuild: allow Clang to find unused static inline
 functions for W=1 build
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Masahiro Yamada <yamada.masahiro@socionext.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Sven Schnelle <svens@stackframe.org>,
        Xiaozhou Liu <liuxiaozhou@bytedance.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Aug 28, 2019 at 11:20 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Wed, Aug 28, 2019 at 02:54:25PM +0900, Masahiro Yamada wrote:
> > GCC and Clang have different policy for -Wunused-function; GCC does not
> > warn unused static inline functions at all whereas Clang does if they
> > are defined in source files instead of included headers although it has
> > been suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> > warning for unused static inline functions").
> >
> > We often miss to delete unused functions where 'static inline' is used
> > in *.c files since there is no tool to detect them. Unused code remains
> > until somebody notices. For example, commit 075ddd75680f ("regulator:
> > core: remove unused rdev_get_supply()").
> >
> > Let's remove __maybe_unused from the inline macro to allow Clang to
> > start finding unused static inline functions. For now, we do this only
> > for W=1 build since it is not a good idea to sprinkle warnings for the
> > normal build.
> >
> > My initial attempt was to add -Wno-unused-function for no W=1 build
> > (https://lore.kernel.org/patchwork/patch/1120594/)
> >
> > Nathan Chancellor pointed out that would weaken Clang's checks since
> > we would no longer get -Wunused-function without W=1. It is true GCC
> > would detect unused static non-inline functions, but it would weaken
> > Clang as a standalone compiler at least.

Got it. No problem.

> >
> > Here is a counter implementation. The current problem is, W=... only
> > controls compiler flags, which are globally effective. There is no way
> > to narrow the scope to only 'static inline' functions.
> >
> > This commit defines KBUILD_EXTRA_WARN[123] corresponding to W=[123].
> > When KBUILD_EXTRA_WARN1 is defined, __maybe_unused is omitted from
> > the 'inline' macro.
> >
> > This makes the code a bit uglier, so personally I do not want to carry
> > this forever. If we can manage to fix most of the warnings, we can
> > drop this entirely, then enable -Wunused-function all the time.

How many warnings?

> >
> > If you contribute to code clean-up, please run "make CC=clang W=1"
> > and check -Wunused-function warnings. You will find lots of unused
> > functions.
> >
> > Some of them are false-positives because the call-sites are disabled
> > by #ifdef. I do not like to abuse the inline keyword for suppressing
> > unused-function warnings because it is intended to be a hint for the
> > compiler optimization. I prefer #ifdef around the definition, or
> > __maybe_unused if #ifdef would make the code too ugly.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>
> I can still see warnings from static unused functions and with W=1, I
> see plenty more. I agree that this is uglier because of the
> __inline_maybe_unused but I think this is better for regular developers.
> I will try to work on these unused-function warnings!

How many are we talking here?

>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>

This is getting kind of messy.  I was more ok when the goal seemed to
be simplifying the definition of `inline`, but this is worse IMO.

-- 
Thanks,
~Nick Desaulniers