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Date: Wed, 16 Dec 2020 10:46:31 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Josh Poimboeuf' <jpoimboe@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org> CC: Peter Zijlstra <peterz@...radead.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: RE: New objtool warning.. 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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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