Nouveau - Nov 2020 - [PATCH 000/141] Fix fall-through warnings for Clang

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva
wrote:
> This series aims to fix almost all remaining fall-through warnings in
> order to enable -Wimplicit-fallthrough for Clang.
> 
> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
> add multiple break/goto/return/fallthrough statements instead of just
> letting the code fall through to the next case.
> 
> Notice that in order to enable -Wimplicit-fallthrough for Clang, this
> change[1] is meant to be reverted at some point. So, this patch helps
> to move in that direction.
> 
> Something important to mention is that there is currently a discrepancy
> between GCC and Clang when dealing with switch fall-through to empty case
> statements or to cases that only contain a break/continue/return
> statement[2][3][4].
Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped
find?

IMVHO compiler warnings are supposed to warn about issues that could
be bugs. Falling through to default: break; can hardly be a bug?!

On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:
> On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:
> > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  
> > > This series aims to fix almost all remaining fall-through
warnings in
> > > order to enable -Wimplicit-fallthrough for Clang.
> > > 
> > > In preparation to enable -Wimplicit-fallthrough for Clang,
explicitly
> > > add multiple break/goto/return/fallthrough statements instead of
just
> > > letting the code fall through to the next case.
> > > 
> > > Notice that in order to enable -Wimplicit-fallthrough for Clang,
this
> > > change[1] is meant to be reverted at some point. So, this patch
helps
> > > to move in that direction.
> > > 
> > > Something important to mention is that there is currently a
discrepancy
> > > between GCC and Clang when dealing with switch fall-through to
empty case
> > > statements or to cases that only contain a break/continue/return
> > > statement[2][3][4].  
> > 
> > Are we sure we want to make this change? Was it discussed before?
> > 
> > Are there any bugs Clangs puritanical definition of fallthrough helped
> > find?
> > 
> > IMVHO compiler warnings are supposed to warn about issues that could
> > be bugs. Falling through to default: break; can hardly be a bug?!  
> 
> It's certainly a place where the intent is not always clear. I think
> this makes all the cases unambiguous, and doesn't impact the machine
> code, since the compiler will happily optimize away any behavioral
> redundancy.
If none of the 140 patches here fix a real bug, and there is no change
to machine code then it sounds to me like a W=2 kind of a warning.

I think clang is just being annoying here, but if I'm the only one who
feels this way chances are I'm wrong :)