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

[Zack Rusin]

> 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.

> And if you want more conditional ops, in theory we also have
> predication, albeit support for that depends on the driver
> (PIPE_SHADER_CAP_MAX_PREDS).

No, that's a completely different thing. 

z