[Mesa-dev] [PATCH 1/6] glsl: Optimize pow(x, 2) into x * x.

[Paul Berry]

On 10 March 2014 17:23, Ian Romanick <idr at freedesktop.org> wrote:

> I had a pretty similar patch on the top of my pow-optimization branch.
> I also expand x**3 and x**4.  I had hoped that would enable some cases
> to expand then merge to MADs.  It should also be faster on older GENs
> where POW perf sucks.  I didn't send it out because I wanted to add a
> similar optimization in the back end that would turn x*x*x*x back into
> x**4 on GPUs where the POW would be faster.
>

Be careful with that one, though: pow is undefined when x < 0, so x*x*x*x
=> pow(x, 4) isn't a valid conversion in general--it only works if you know
the operation started off as a pow in the first place.  (Note: you can do
x*x*x*x => pow(abs(x), 4), but that doesn't generalize to odd powers).


>
> I also didn't have anything in shader-db that benefitted from x**2 or
> x**3.  It seems like there were a couple that would be modified by a
> x**5 flattening, but I think that would universally be slower....
>
> On 03/10/2014 03:54 PM, Matt Turner wrote:
> > Cuts two instructions out of SynMark's Gl32VSInstancing benchmark.
> > ---
> >  src/glsl/opt_algebraic.cpp | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
> > index 5c49a78..8494bd9 100644
> > --- a/src/glsl/opt_algebraic.cpp
> > +++ b/src/glsl/opt_algebraic.cpp
> > @@ -528,6 +528,14 @@
> ir_algebraic_visitor::handle_expression(ir_expression *ir)
> >        if (is_vec_two(op_const[0]))
> >           return expr(ir_unop_exp2, ir->operands[1]);
> >
> > +      if (is_vec_two(op_const[1])) {
> > +         ir_variable *x = new(ir) ir_variable(ir->operands[1]->type,
> "x",
> > +                                              ir_var_temporary);
> > +         base_ir->insert_before(x);
> > +         base_ir->insert_before(assign(x, ir->operands[0]));
> > +         return mul(x, x);
> > +      }
> > +
> >        break;
> >
> >     case ir_unop_rcp:
> >
>
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