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Re: [Qemu-devel] [PATCH] kvm: workaround build break on gcc-7.1.1 / fedo


From: Greg Kurz
Subject: Re: [Qemu-devel] [PATCH] kvm: workaround build break on gcc-7.1.1 / fedora26
Date: Thu, 3 Aug 2017 16:31:34 +0200

On Thu, 3 Aug 2017 10:34:45 -0300
Philippe Mathieu-Daudé <address@hidden> wrote:

> Hi Greg,
> 
> On 08/02/2017 11:47 AM, Greg Kurz wrote:
> > Building QEMU on fedora26 with the latest gcc package fails:
> > 
> >    CC      ppc64-softmmu/target/ppc/kvm.o
> > In file included from include/sysemu/hw_accel.h:16:0,
> >                   from target/ppc/kvm.c:31:
> > target/ppc/kvm.c: In function ‘kvmppc_booke_watchdog_enable’:
> > include/sysemu/kvm.h:449:35: error: ‘args_tmp[i]’ may be used uninitialized
> >   in this function [-Werror=maybe-uninitialized]
> >               cap.args[i] = args_tmp[i];                               \
> >                                     ^
> > target/ppc/kvm.c: In function ‘kvmppc_set_papr’:
> > include/sysemu/kvm.h:449:35: error: ‘args_tmp[i]’ may be used uninitialized
> >   in this function [-Werror=maybe-uninitialized]
> > cc1: all warnings being treated as errors
> > 
> > $ rpm -q gcc
> > gcc-7.1.1-3.fc26.ppc64le
> > 
> > Testing the size of args_tmp seems to be enough to prevent the warning
> > to pop up.  
> 
> This sizeof() use looks unnatural to me. I wonder why not use size_t, 
> since this is about sizeof()/ARRAY_SIZE(). The problem seems to come 
> from the commit this cast was introduced (61c7bbd236):
> 
> target-ppc/kvm.c:1302:21: error: comparison of unsigned expression < 0 
> is always false [-Werror=type-limits]
> 

Switching from int to size_t causes this error ^^ to show up again.

> So I'd rather suggest this code, which looks more natural to read to me:
> 
>      if (ARRAY_SIZE(args_tmp)) {

Yeah, it's maybe more in phase with the context but anyway, the truly
unnatural thing is to be forced to do:

    if (foo) {
        for (i = 0; i < foo; ...


>          for (i = 0; i < ARRAY_SIZE(args_tmp) && ...
> 
> I Cc'ed Eric :)
> 
> Regards,
> 
> Phil.
> 
> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> >   include/sysemu/kvm.h |   12 ++++++++----
> >   1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> > index 91fc07ee9afe..41fc1005a9ea 100644
> > --- a/include/sysemu/kvm.h
> > +++ b/include/sysemu/kvm.h
> > @@ -429,9 +429,11 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
> > extension);
> >           };                                                           \
> >           uint64_t args_tmp[] = { __VA_ARGS__ };                       \
> >           int i;                                                       \
> > -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
> > +        if (sizeof(args_tmp)) {                                      \
> > +            for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&             \
> >                        i < ARRAY_SIZE(cap.args); i++) {                \
> > -            cap.args[i] = args_tmp[i];                               \
> > +                cap.args[i] = args_tmp[i];                           \
> > +            }                                                        \
> >           }                                                            \
> >           kvm_vm_ioctl(s, KVM_ENABLE_CAP, &cap);                       \
> >       })
> > @@ -444,9 +446,11 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
> > extension);
> >           };                                                           \
> >           uint64_t args_tmp[] = { __VA_ARGS__ };                       \
> >           int i;                                                       \
> > -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
> > +        if (sizeof(args_tmp)) {                                      \
> > +            for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&             \
> >                        i < ARRAY_SIZE(cap.args); i++) {                \
> > -            cap.args[i] = args_tmp[i];                               \
> > +                cap.args[i] = args_tmp[i];                           \
> > +            }                                                        \
> >           }                                                            \
> >           kvm_vcpu_ioctl(cpu, KVM_ENABLE_CAP, &cap);                   \
> >       })
> > 
> >   

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