linux-kernel - Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

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Message-ID: <20220417155205.GI2731@worktop.programming.kicks-ass.net>
Date:   Sun, 17 Apr 2022 17:52:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christophe de Dinechin <dinechin@...hat.com>, trivial@...nel.org,
        Ben Segall <bsegall@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

On Thu, Apr 14, 2022 at 01:30:50PM -0700, Andrew Morton wrote:
> On Thu, 14 Apr 2022 17:21:01 +0200 Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > +/* The + 1 below places the pointers within the range of their array */
> > >  #define for_class_range(class, _from, _to) \
> > > -	for (class = (_from); class != (_to); class--)
> > > +	for (class = (_from); class + 1 != (_to) + 1; class--)
> > 
> > Urgh, so now we get less readable code, just because GCC is being
> > stupid?
> > 
> > What's wrong with negative array indexes? memory is memory, stuff works.
> 
> What's more, C is C.  Glorified assembly language in which people do odd
> stuff.
> 
> But this is presumably a released gcc version and we need to do
> something.  And presumably, we need to do a backportable something, so
> people can compile older kernels with gcc-12.
> 
> Is it possible to suppress just this warning with a gcc option?  And if
> so, are we confident that this warning will never be useful in other
> places in the kernel?
> 
> If no||no then we'll need to add workarounds such as these?

-Wno-array-bounds

Is the obvious fix-all cure. The thing is, I want to hear if this new
warning has any actual use or is just crack induced madness like many of
the warnings we turn off.