Re: [Qemu-ppc] [PATCH v1 4/8] target-ppc: add vabsdu[b, h, w] instructions

[Nikunj A Dadhania]

Richard Henderson <address@hidden> writes:

> On 07/28/2016 12:19 PM, Nikunj A Dadhania wrote:
>> +        r->element[i] = abs(a->element[i] - b->element[i]);             \
>> +    }                                                                   \
>> +}
>> +
>> +/* VABSDU - Vector absolute difference unsigned
>> + *   name    - instruction mnemonic suffix (b: byte, h: halfword, w: word)
>> + *   element - element type to access from vector
>> + */
>> +#define VABSDU(type, element)                   \
>> +    VABSDU_DO(absdu##type, element)
>> +VABSDU(b, u8)
>> +VABSDU(h, u16)
>> +VABSDU(w, u32)
>
> From whence are you receiving this abs definition, and how do you expect it 
> to 
> work with an unsigned input?
>
> I can only imagine you're getting abs(3), aka int abs(int), from stdlib.h. 
> Which technically does work post-arithmetic promotion for u8 and u16, but it 
> does not for u32.

Thanks for pointing that, wasn't aware about that.

> I think we'd prefer an explicit (a > b ? a - b : b - a).

That is what ISA also says, i thought of using library function abs().

Regards
Nikunj