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Date:   Wed, 16 Dec 2020 10:58:55 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: New objtool warning..

On Wed, Dec 16, 2020 at 10:46:31AM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 16 December 2020 04:49
> > 
> > On Tue, Dec 15, 2020 at 08:22:23PM -0800, Linus Torvalds wrote:
> > > I only see this on my laptop, but that's probably because my desktop
> > > is built using clang. So it's a gcc code generation interaction, I
> > > suspect..
> > >
> > > Anyway, the new warning is
> > >
> > >     drivers/gpu/drm/drm_edid.o: warning: objtool: do_cvt_mode() falls
> > > through to next function drm_mode_detailed.isra.0()
> > >
> > > and googling around a bit I see that 0day ended up reporting it on the
> > > linux-next lists, and blames commit 991fcb77f490 ("drm/edid: Fix
> > > uninitialized variable in drm_cvt_modes()").
> > >
> > > That presumably then makes gcc generate that odd code.
> > >
> > > That "unreachable()" is because the compiler isn't smart enough to see
> > > that yes, there really are case statements for every single possible
> > > case. Oh well. Maybe the code should just make one of the possible
> > > cases also be the "default:" case, and that might fix it.
> > >
> > > But maybe this is worth looking into for objtool too?
> > >
> > > Anyway, I see it with gcc-10.2.1 as per current F32. Holler if you
> > > can't reproduce it, I can send the object file around.
> > 
> > I can't recreate with my compiler, but I think I've seen one like this
> > before.  I suspect s/unreachable()/BUG()/ would fix it?
> 
> Then a smart(er) compiler will report that the BUG() is unreachable.

It shouldn't, BUG() already has unreachable().

-- 
Josh

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