linux-kernel - Re: drivers/pinctrl/intel/pinctrl-intel.c:1663:14: warning: variable 'dummy' set but not used

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Date:   Fri, 30 Apr 2021 15:54:14 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     kernel test robot <lkp@...el.com>, Arnd Bergmann <arnd@...db.de>,
        kbuild-all@...ts.01.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: drivers/pinctrl/intel/pinctrl-intel.c:1663:14: warning: variable
 'dummy' set but not used

On Wed, Apr 28, 2021 at 1:47 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Wed, Apr 28, 2021 at 11:43:33AM +0300, Andy Shevchenko wrote:
> > +Cc: Arnd and Nick
> >
> > (Seems like a clang bug)
> >
> > On Wed, Apr 28, 2021 at 08:10:34AM +0800, kernel test robot wrote:
> > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > > head:   e359bce39d9085ab24eaa0bb0778bb5f6894144a
> > > commit: 10c857f063fc63902160755a6e23fa594290c6d3 pinctrl: cherryview: Switch to use intel_pinctrl_get_soc_data()
> > > date:   8 months ago
> > > config: x86_64-randconfig-a005-20210426 (attached as .config)
> > > compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 87fc97169e7901dfe56ca0da0d92da0c02d2ef48)
> > > reproduce (this is a W=1 build):
> > >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > >         chmod +x ~/bin/make.cross
> > >         # install x86_64 cross compiling tool for clang build
> > >         # apt-get install binutils-x86-64-linux-gnu
> > >         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=10c857f063fc63902160755a6e23fa594290c6d3
> > >         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> > >         git fetch --no-tags linus master
> > >         git checkout 10c857f063fc63902160755a6e23fa594290c6d3
> > >         # save the attached .config to linux build tree
> > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 ARCH=x86_64
> > >
> > > If you fix the issue, kindly add following tag as appropriate
> > > Reported-by: kernel test robot <lkp@...el.com>
> > >
> > > All warnings (new ones prefixed by >>):
> > >
> > > >> drivers/pinctrl/intel/pinctrl-intel.c:1663:14: warning: variable 'dummy' set but not used [-Wunused-but-set-variable]
> > >            const char *dummy;
> > >                        ^
> > >    1 warning generated.
> >
> > Seems to me a bug in clang. The variable is used to check if the body of the
> > loop should be executed or not.
>
> The only way of make this variable being unused is to avoid the loop being
> executed at all. This may be the case iff (mind to f:s) the supplied size
> parameter is 0, but how on earth clang can prove it (esp. taking into
> consideration that it's not true in real life)?!

Yep, the patch that enabled support for this warning in clang was
reported after multiple reports from projects; I'm working on helping
the author revise it based on cases it still flags from the kernel.
-- 
Thanks,
~Nick Desaulniers