Re: [PATCH v2] target/i386: Remove compilation errors when -Werror=maybe-uninitialized

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Dec 22, 2022 at 11:07:31AM +0100, Eric Auger wrote:
> Hi Philippe,
> 
> On 12/22/22 10:01, Philippe Mathieu-Daudé wrote:
> > On 22/12/22 09:18, Paolo Bonzini wrote:
> >> On 12/21/22 17:36, Eric Auger wrote:
> >>> To avoid compilation errors when -Werror=maybe-uninitialized is used,
> >>> replace 'case 3' by 'default'.
> >>>
> >>> Otherwise we get:
> >>>
> >>> ../target/i386/ops_sse.h: In function ‘helper_vpermdq_ymm’:
> >>> ../target/i386/ops_sse.h:2495:13: error: ‘r3’ may be used
> >>> uninitialized in this function [-Werror=maybe-uninitialized]
> >>>     2495 |     d->Q(3) = r3;
> >>>          |     ~~~~~~~~^~~~
> >>> ../target/i386/ops_sse.h:2494:13: error: ‘r2’ may be used
> >>> uninitialized in this function [-Werror=maybe-uninitialized]
> >>>     2494 |     d->Q(2) = r2;
> >>>          |     ~~~~~~~~^~~~
> >>> ../target/i386/ops_sse.h:2493:13: error: ‘r1’ may be used
> >>> uninitialized in this function [-Werror=maybe-uninitialized]
> >>>     2493 |     d->Q(1) = r1;
> >>>          |     ~~~~~~~~^~~~
> >>> ../target/i386/ops_sse.h:2492:13: error: ‘r0’ may be used
> >>> uninitialized in this function [-Werror=maybe-uninitialized]
> >>>     2492 |     d->Q(0) = r0;
> >>>          |     ~~~~~~~~^~~~
> >
> > With what compiler? Is that a supported one?
> https://lore.kernel.org/qemu-devel/3aab489e-9d90-c1ad-0b6b-b2b5d80db723@redhat.com/
> >
> >>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>> Suggested-by: Stefan Weil <sw@weilnetz.de>
> >>> Fixes: 790684776861 ("target/i386: reimplement 0x0f 0x3a, add AVX")
> >>> ---
> >>>   target/i386/ops_sse.h | 4 ++--
> >>>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/target/i386/ops_sse.h b/target/i386/ops_sse.h
> >>> index 3cbc36a59d..c442c8c10c 100644
> >>> --- a/target/i386/ops_sse.h
> >>> +++ b/target/i386/ops_sse.h
> >>> @@ -2466,7 +2466,7 @@ void helper_vpermdq_ymm(Reg *d, Reg *v, Reg
> >>> *s, uint32_t order)
> >>>           r0 = s->Q(0);
> >>>           r1 = s->Q(1);
> >>>           break;
> >>> -    case 3:
> >>> +    default:
> >>>           r0 = s->Q(2);
> >>>           r1 = s->Q(3);
> >>>           break;
> >>> @@ -2484,7 +2484,7 @@ void helper_vpermdq_ymm(Reg *d, Reg *v, Reg
> >>> *s, uint32_t order)
> >>>           r2 = s->Q(0);
> >>>           r3 = s->Q(1);
> >>>           break;
> >>> -    case 3:
> >>> +    default:
> >>>           r2 = s->Q(2);
> >>>           r3 = s->Q(3);
> >>>           break;
> >>
> >> Queued, but this compiler sucks. :)
> >
> > Can't we simply add a dumb 'default' case? So when reviewing we don't
> > have to evaluate 'default' means 3 here.
> >
> > -- >8 --
> > --- a/target/i386/ops_sse.h
> > +++ b/target/i386/ops_sse.h
> > @@ -2470,6 +2470,8 @@ void helper_vpermdq_ymm(Reg *d, Reg *v, Reg *s,
> > uint32_t order)
> >          r0 = s->Q(2);
> >          r1 = s->Q(3);
> >          break;
> > +    default:
> > +        qemu_build_not_reached();
> >      }
> >      switch ((order >> 4) & 3) {
> >      case 0:
> > @@ -2488,6 +2490,8 @@ void helper_vpermdq_ymm(Reg *d, Reg *v, Reg *s,
> > uint32_t order)
> >          r2 = s->Q(2);
> >          r3 = s->Q(3);
> >          break;
> > +    default:
> > +        qemu_build_not_reached();
> >      }
> I guess this won't fix the fact r0, r1, r2, r3 are not initialized, will it?

This ultimately expands to assert() and the compiler should see that it
terminates the control flow at this point, so shouldn't have a reason
to warn.


With regards,
Daniel
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