Re: [Qemu-devel] [PATCH] kvm: workaround build break on gcc-7.1.1 / fedora26

[Eric Blake <eblake@redhat.com>]

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On 08/03/2017 01:38 PM, Greg Kurz wrote:
> The following code snippet spits a warning with gcc-7.1.1-3.fc26 and -Wall on
> ppc64le AND x86_64:
> 
> int foo(void)
> {
>     char empty_array[] = { };
>     int i, ret = 0;
> 
>     for (i = 0; i < (int) (sizeof(empty_array) / sizeof(empty_array[0])); i++) {
>         ret = empty_array[i];
>     }
> 
>     return ret;
> }

Confirmed.  No idea why the cast makes gcc think i is uninitialized.

> 
> If I drop the (int), the warning goes away... and so does the build break
> of qemu-system-ppc64 on my ppc64le host.
> 
> I don't have 4.7.1 or 4.6 compilers around but I could check 4.8.5
> is okay with that change FWIW.

I tested with gcc 4.4.7 on RHEL 6, but -Wtype-limits there didn't warn
whether or not the (int) was present; it's possible that we still need
to test with 4.6 or 4.7.1 to see whether it makes a difference.

> 
>> 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.

That's also a possibility, if we still hit old compilers that require
the (int).

>>
> 
> If I also convert 'int i' to 'size_t i' then I get the same error as
> in commit 61c7bbd2:
> 
> error: comparison of unsigned expression < 0 is always
>  false [-Werror=type-limits]

Confirmed with newer gcc, whether or not the (int) cast is present.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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