[Mesa-dev] [PATCH 05/16] i965: Fix bitcast operations with negate

[Iago Toral]

On Fri, 2014-12-12 at 09:36 -0800, Matt Turner wrote:
> On Thu, Dec 11, 2014 at 2:34 PM, Eduardo Lima Mitev <elima at igalia.com> wrote:
> > From: Iago Toral Quiroga <itoral at igalia.com>
> >
> > For code such as this in a vertex or fragment shader:
> >
> > uniform float in0;
> > flat out float out0;
> > ...
> > out0 = ceil(in0)
> >
> > We generate this IR:
> >
> > (assign  (x) (var_ref packed:out0)
> >    (expression int bitcast_f2i
> >        (expression float ceil (var_ref in0) ) ) )
> >
> > In i965, this would eventually become:
> >
> > rndd(8)         g14<1>.xF       -g3<0,4,1>.xF
> > mov(8)          g6<1>.xD        -g14<4,4,1>.xD
> > mov(8)          g116<1>D        g6<4,4,1>D
> >
> > which does not produce the expected result.
> >
> > The problem has to do with the second instruction, that changes the
> > type to D at the same time it does the negate. For this to work we
> > need to negate first with the original type, then change the type
> > to the bitcast destination so we produce code like this instead:
> >
> > rndd(8)         g14<1>.xF       -g3<0,4,1>.xF
> > mov(8)          g6<1>.xF        -g14<4,4,1>.xF
> > mov(8)          g116<1>D        g6<4,4,1>D
> 
> I found and fixed a problem (in commit 0ae9ca12a8) last year where we
> would put move the source modifiers out of the bitcast_*(...).
> 
> In that commit, I just emit a MOV to do the source modifier as a
> separate step, and then let copy propagation handle getting rid of it
> when possible. It looks like I didn't notice that ceil also does the
> in place op[0].negate = !op[0].negate.
> 
> Instead of special casing the bit cast functions, we should just emit
> the extra MOV in ceil and let copy propagation clean it up when
> possible.

Thanks Matt, I'll do this and send a new version.

Iago