Re: [PULL 25/31] osdep: Make MIN/MAX evaluate arguments only once

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 6/24/20 2:13 PM, Eric Blake wrote:
> On 6/24/20 5:50 AM, Paolo Bonzini wrote:
>> From: Eric Blake <eblake@redhat.com>
>>
>> I'm not aware of any immediate bugs in qemu where a second runtime
>> evalution of the arguments to MIN() or MAX() causes a problem, but
> 
> evaluation
> 
>> Update the MIN/MAX macros to only evaluate their argument once at
>> runtime;
> 
>> Use of MIN when MIN_CONST is needed:
>>
>> In file included from /home/eblake/qemu/qemu-img.c:25:
>> /home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group
>> within expression allowed only inside a function
>>    249 |     ({                                                  \
>>        |     ^
>> /home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’
> 
> UTF-8 mojibake in the commit message ;(

Oh, this is not a git-publish bug:
https://github.com/bonzini/qemu/commit/6f9ff58baae

> 
> 
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> Message-Id: <20200604215236.2798244-1-eblake@redhat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
> 
>> +#define MIN_CONST(a, b)                                         \
>> +    __builtin_choose_expr(                                      \
>> +        __builtin_constant_p(a) && __builtin_constant_p(b),     \
>> +        (a) < (b) ? (a) : (b),                                  \
>> +        ((void)0))
> 
> This one is correct...
> 
>> +#undef MAX
>> +#define MAX(a, b)                                       \
>> +    ({                                                  \
>> +        typeof(1 ? (a) : (b)) _a = (a), _b = (b);       \
>> +        _a > _b ? _a : _b;                              \
>> +    })
>> +#define MAX_CONST(a, b)                                         \
>> +    __builtin_choose_expr(                                      \
>> +        __builtin_constant_p(a) && __builtin_constant_p(b),     \
>> +        (a) > (b) ? (a) : (b),                                  \
>> +        __builtin_unreachable())
> 
> ...but this one should also use ((void)0), to match the commit message.
> 
>> +
>> +/* Minimum function that returns zero only if both values are zero.
>>    * Intended for use with unsigned values only. */
> 
> And checkpatch complains about the formatting here.
> 
> Ah well.  I had all these things fixed in my tree, but failed to post a
> v5.  Not worth holding up this pull request in isolation, but if there
> are any other build issues, I'll post a v5 of this patch, otherwise a
> followup.
>