Re: [Qemu-devel] [PATCH] util: Fix MIN_NON_ZERO

[Paolo Bonzini]

On 12/07/2016 17:54, Eric Blake wrote:
> On 07/12/2016 12:48 AM, Fam Zheng wrote:
>> MIN_NON_ZERO(0, 1) is evaluated to 0. Rewrite the macro to fix it.
> 
> Huh?
> 
> Old expansion, in various stages:
> 
> (((0) != 0 && (0) < (1)) ? (0) : (1))
> ((0 && 1) ? 0 : 1)
> (0 ? 0 : 1)
> 1
> 
> Maybe you meant MIN_NON_ZERO(1, 0), which evaluates to:
> 
> (((1) != 0 && (1) < (0)) ? (1) : (0))
> ((1 && 0) ? 1 : 0)
> (0 ? 1 : 0)
> 0
> 
> in which case, you are correct that there is a bug.

Commit message fixed, patch queued.

Paolo

>>
>> Reported-by: Miroslav Rezanina <address@hidden>
>> Signed-off-by: Fam Zheng <address@hidden>
>> ---
>>  include/qemu/osdep.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>> index e63da28..e4c6ae6 100644
>> --- a/include/qemu/osdep.h
>> +++ b/include/qemu/osdep.h
>> @@ -151,7 +151,8 @@ extern int daemon(int, int);
>>  /* Minimum function that returns zero only iff both values are zero.
>>   * Intended for use with unsigned values only. */
>>  #ifndef MIN_NON_ZERO
>> -#define MIN_NON_ZERO(a, b) (((a) != 0 && (a) < (b)) ? (a) : (b))
>> +#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \
>> +                                ((b) == 0 ? (a) : (MIN(a, b))))
> 
> Another way might be:
> 
> (!(a) == !(b) ? MIN(a, b) : (a) + (b))
> 
> but you have to put more mental thought into that.  I'm just fine with
> your version.
> 
> With the commit message fixed,
> Reviewed-by: Eric Blake <address@hidden>
>