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Date:   Sun, 12 Sep 2021 00:33:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     kernel test robot <lkp@intel.com>,
        Yafang Shao <laoar.shao@gmail.com>, kbuild-all@lists.01.org,
        linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org
Subject: Re: [peterz-queue:sched/core 13/19] kernel/sched/fair.c:892:34:
 warning: variable 'stats' set but not used
Message-ID: <20210911223313.GC4323@worktop.programming.kicks-ass.net>
References: <202109110214.oh62aoIq-lkp@intel.com>
 <20210911122049.GW4323@worktop.programming.kicks-ass.net>
 <20210911213041.GD1583@gate.crashing.org>
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On Sat, Sep 11, 2021 at 04:30:41PM -0500, Segher Boessenkool wrote:
> Hi!
> 
> On Sat, Sep 11, 2021 at 02:20:49PM +0200, Peter Zijlstra wrote:
> > On Sat, Sep 11, 2021 at 02:21:26AM +0800, kernel test robot wrote:
> > > compiler: m68k-linux-gcc (GCC) 11.2.0
> > 
> > >    kernel/sched/fair.c: In function 'update_curr':
> > >    kernel/sched/fair.c:860:42: warning: unused variable 'stats' [-Wunused-variable]
> > >      860 |                 struct sched_statistics *stats = __schedstats_from_se(curr);
> > >          |                                          ^~~~~
> > 
> > OK, compiler guys, this code reads like:
> > 
> > #define schedstats_enabled()	(0)
> > #define __schedstat_set(x, y)	do { } while (0)
> > 
> > 
> > 	if (schedstats_enabled()) {
> > 		struct sched_statistics *stats = __schedstats_from_se(curr);
> > 
> > 		__schedstat_set(stats->exec_max,
> > 				max(delta_exec, stats->exec_max));
> > 	}
> > 
> > So yes, we initialize a variable that then isn't used, but the whole
> > bloody thing is inside if (0) which will not ever get ran *anyway*.
> > 
> > This is a crap warning if ever I saw one...
> 
> Yes, we really should warn "do not use a preprocessor macro if what you
> want is a function"?  The variable really *is* unused, with this macro.

Why would I want to write a bunch of one-off functions and preprocessor
guard them? That's going to be a mess.

> If we would remove dead code before warning about unused variables
> there would be many *more* false positives, fwiw.

Not if you also remove any variables declared in dead code and all
variables only used in the dead code.