linux-kernel - Re: [PATCH 2/2] kbuild: allow Clang to find unused static inline functions for W=1 build

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Date:   Wed, 28 Aug 2019 17:05:51 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Sven Schnelle <svens@...ckframe.org>,
        Xiaozhou Liu <liuxiaozhou@...edance.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kbuild: allow Clang to find unused static inline
 functions for W=1 build

On Wed, Aug 28, 2019 at 04:28:30PM -0700, Nick Desaulniers wrote:
> On Wed, Aug 28, 2019 at 11:20 AM Nathan Chancellor
> <natechancellor@...il.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?

In an x86 defconfig build (one of the smallest builds we do), I see an
additional 35 warnings that crop up:

https://gist.github.com/003ba86ba60b4ac7e8109089d6cb1a5a

> > >
> > > 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@...ionext.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@...il.com>
> > Tested-by: Nathan Chancellor <natechancellor@...il.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.

I guess if you want, we can just go back to v1 and have all unused
function warnings hidden by default with clang. Fixing these warnings
will take a significant amount of time given there will probably be a
few hundred so I don't think having this warning hidden behind W=1 for
that long is a good thing.

Cheers,
Nathan