[Mesa-dev] [PATCH] tgsi: Add a conditional move inststruction

[Christoph Bumiller]

On 04.04.2013 17:01, Jose Fonseca wrote:
>
> ----- Original Message -----
>>> On 04.04.2013 03:45, Zack Rusin wrote:
>>>> It's part of SM4 (http://goo.gl/4IpeK). It's also fairly
>>>> painful to emulate without branching. Most hardware
>>>> supports it natively and even llvm has a 'select' opcode
>>>> which can handle it without too much hassle.
>>>>
>>>> diff --git a/src/gallium/docs/source/tgsi.rst
>>>> b/src/gallium/docs/source/tgsi.rst
>>>> index 28308cb..6c5a02b 100644
>>>> --- a/src/gallium/docs/source/tgsi.rst
>>>> +++ b/src/gallium/docs/source/tgsi.rst
>>>> @@ -72,6 +72,17 @@ used.
>>>>  
>>>>    dst.w = src.w
>>>>  
>>>> +.. opcode:: MOVC - Conditional move
>>>> +
>>>> +.. math::
>>>> +
>>>> +  dst.x = src0.x ? src1.x : src2.x
>>>> +
>>>> +  dst.y = src0.y ? src1.y : src2.y
>>>> +
>>>> +  dst.z = src0.z ? src1.z : src2.z
>>>> +
>>>> +  dst.w = src0.w ? src1.w : src2.w
>>>>  
>>> I think we already have that:
>>>
>>> .. opcode:: UCMP - Integer Conditional Move
>>>
>>> .. math::
>>>
>>>   dst.x = src0.x ? src1.x : src2.x
>>>
>>>   dst.y = src0.y ? src1.y : src2.y
>>>
>>>   dst.z = src0.z ? src1.z : src2.z
>>>
>>>   dst.w = src0.w ? src1.w : src2.w
>>>
>>>
>>> No difference apart from the source ordering (the "integer" just implies
>>> that any non-zero value counts as true, i.e. also inf, nan and -0).
>> That's really broken. UCMP needs to be a an unsigned version of the CMP
>> instruction which does
>> dst.chan = (src0.chan < 0) ? src1.chan : src2.chan
>> not a whole new instruction. It's what everyone implements anyway. So if
>> st_glsl_to_tgsi needs
>> a conditional move we need to add the above patch and change it to use it.
> Yes, it doesn't seem that any of the TGSI_OPCODE_UCMP implementation does that the spec says it supposedly does -- it seems everybody implements it as an unsigned version of CMP. That is, it seems UCMP's description needs to be fixed.

Erm, unsigned < 0 doesn't make sense.

Definitely what the description says:
static void
micro_ucmp(union tgsi_exec_channel *dst,
           const union tgsi_exec_channel *src0,
           const union tgsi_exec_channel *src1,
           const union tgsi_exec_channel *src2)
{
   dst->u[0] = src0->u[0] ? src1->u[0] : src2->u[0];
   dst->u[1] = src0->u[1] ? src1->u[1] : src2->u[1];
   dst->u[2] = src0->u[2] ? src1->u[2] : src2->u[2];
   dst->u[3] = src0->u[3] ? src1->u[3] : src2->u[3];
}

or

   case TGSI_OPCODE_UCMP:
   case TGSI_OPCODE_CMP:
      FOR_EACH_DST_ENABLED_CHANNEL(0, c, tgsi) {
         src0 = fetchSrc(0, c);
         src1 = fetchSrc(1, c);
         src2 = fetchSrc(2, c);
         if (src1 == src2)
            mkMov(dst0[c], src1);
         else
            mkCmp(OP_SLCT, (srcTy == TYPE_F32) ? CC_LT(less than 0) :
CC_NE(not equal 0),
                  srcTy, dst0[c], src1, src2, src0);
      }


> Jose
>