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

[Roland Scheidegger]

Well if the condition is just "any bit set" then it doesn't matter if
the input is a float or int or whatever (of course, for floats, that
definition is different than != zero, as it doesn't hold for negative zero).
That would be the same as for instance the bitwise instructions which
also don't have a "U" or "I" prefix.

Roland

Am 04.04.2013 18:49, schrieb Marek Olšák:
> FWIW, I think UCMP is a misleading name. Whatever the name will be, it
> should be prefixed with "I" or "U", because it's not a floating-point
> opcode. How about UCND? :D
> 
> Marek
> 
> 
> On Thu, Apr 4, 2013 at 6:23 PM, Jose Fonseca <jfonseca at vmware.com
> <mailto:jfonseca at vmware.com>> wrote:
> 
>     There might be some value in renaming UCMP to be MOVC though.  I
>     think everybody here can agree that UCMP, though semantically
>     correct, is misleading.
> 
>     Jose
> 
>     ----- Original Message -----
>     > Hah, yea, I'm sorry, that's a good point. So movc is a bitcast to
>     unsigned
>     > followed by ucmp. Alright, I'm withdrawing the patch.
>     >
>     > z
>     >
>     > ----- Original Message -----
>     > >
>     > >
>     > > ----- 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
>     > >
>     > >
>     >
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> 
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