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Subject: Re: [Qemu-devel] [PATCH] kvm: workaround build break on gcc-7.1.1 /
 fedora26
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To: Eric Blake <eblake@redhat.com>, Greg Kurz <groug@kaod.org>, qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>

On 08/03/2017 10:55 AM, Eric Blake wrote:
> On 08/03/2017 08:34 AM, Philippe Mathieu-Daudé 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
> 
>>
>> 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]
>>
>> So I'd rather suggest this code, which looks more natural to read to me:
>>
>>      if (ARRAY_SIZE(args_tmp)) {
>>          for (i = 0; i < ARRAY_SIZE(args_tmp) && ...

Oops I forgot to suggest size_t i.

> 
> For that matter, the existing code is doing:
> 
> int i;
> i < (int)ARRAY_SIZE(args_tmp)
> 
> but wouldn't that be better as:
> 
> size_t i;
> i < ARRAY_SIZE(args_temp)
> 
> I guess we have both the old compilers (per commit 61c7bbd2) and the new
> to worry about; although I was unable to reproduce it on Fedora 26 on
> x86_64 (is this an architecture-dependent compiler bug?)
> 

Ok so let's stop losing time about compiler incoherent warnings, using 
-Wno-type-limits for GCC < 5...
So we can keep a sane/understandable codebase, using size_t and no (int) 
cast.

Phil.