Re: [PATCH v3] osdep: Make MIN/MAX evaluate arguments only once

[Eric Blake <eblake@redhat.com>]

On 6/3/20 11:32 AM, Richard Henderson wrote:

>>> I'd prefer we generate a compile-time error than a runtime trap (or nothing,
>>> depending on compiler flags controlling __builtin_unreachable).
>>
>> What we have DOES produce a compile-time error.  If either expression to
>> MIN_CONST() is not actually const, the fact that __builtin_unreachable()
>> returns void causes a compilation failure because a value is expected.
> 
> Ah!  Well, that's good and certainly sufficient for my needs.
> 
> I do now wonder if it wouldn't be clearer to use "(void)0"
> instead of __builtin_unreachable, and add a note to the comment just above.

Yes, I just tested; using "((void)0)" in place of 
"__builtin_unreachable()" has the same effect (no change to valid use, 
and still a compiler error on misuse). gcc:

/home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’:
/home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as 
it ought to be
  1225 |             i = MIN_CONST(i, n);
       |               ^

clang:

/home/eblake/qemu/qemu-img.c:1225:15: error: assigning to 'int' from 
incompatible type 'void'
             i = MIN_CONST(i, n);
               ^ ~~~~~~~~~~~~~~~


Of course, a comment explaining the intent can't hurt either.  I'll wait 
to see if this gathers any other comments before spinning a v4 with that 
change.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org