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

[Jose Fonseca]

----- Original Message -----
> > > Erm, unsigned < 0 doesn't make sense.
> > 
> > Ah indeed!
> > 
> > > 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);
> > >       }
> > > 
> > 
> > But odd enough, the implementations I happend to look at seemed to do "foo
> > >=
> > 0":
> 
> Yea, like I mentioned it's pretty broken. Sometimes it's implemented as UCMP,
> sometimes it's implemented as MOVC.
> It seems to be used only as MOVC.
> It feels silly writing this, but we should probably make UCMP act like UCMP
> and add MOVC and use it when we need a MOVC.

Zack, I believe Christoph has a point when he says that UCMP is semantically the same as MOVC.

Because for unsigned integers, "foo > 0" is the same as "foo != 0", therefore having UCMP defined as

  dst = src0 > 0 ? src1 : src2

or a MOVC as

  dst = src0 != 0 ? src1 : src2

is pretty much the same.

Jose