[Mesa-dev] [PATCH 07/10] nir: add imprecise flrp optimisation

[Timothy Arceri]

On Sat, 2017-01-07 at 12:35 -0800, Jason Ekstrand wrote:
> On Jan 7, 2017 12:07 PM, "Timothy Arceri" <timothy.arceri at collabora.c
> om> wrote:
> On Sat, 2017-01-07 at 11:14 -0800, Matt Turner wrote:
> > On Sat, Jan 7, 2017 at 3:58 AM, Timothy Arceri
> > <timothy.arceri at collabora.com> wrote:
> > > On BDW:
> > >
> > > total instructions in shared programs: 13061890 -> 13061877 (-
> > > 0.00%)
> > > instructions in affected programs: 2441 -> 2428 (-0.53%)
> > > helped: 13
> > > HURT: 0
> > >
> > > total cycles in shared programs: 256612254 -> 256611784 (-0.00%)
> > > cycles in affected programs: 16418 -> 15948 (-2.86%)
> > > helped: 10
> > > HURT: 2
> > > ---
> > >  src/compiler/nir/nir_opt_algebraic.py | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/src/compiler/nir/nir_opt_algebraic.py
> > > b/src/compiler/nir/nir_opt_algebraic.py
> > > index fa9f3f1..7cee378 100644
> > > --- a/src/compiler/nir/nir_opt_algebraic.py
> > > +++ b/src/compiler/nir/nir_opt_algebraic.py
> > > @@ -111,6 +111,7 @@ optimizations = [
> > >     (('~flrp', a, b, 1.0), b),
> > >     (('~flrp', a, a, b), a),
> > >     (('~flrp', 0.0, a, b), ('fmul', a, b)),
> > > +   (('~flrp', a, 0.0, c), ('ffma', ('fneg', a), c, a),
> '!options-
> > > >lower_ffma'),
> >
> > I don't think we want to be generating ffma directly in
> > nir_opt_algebraic?
> 
> We already have:
> 
> (('~fadd', ('fmul', a, b), c), ('ffma', a, b, c), 'options-
> >fuse_ffma'),
> 
> Is you concern backends that don't want ffma? The !options-
> >lower_ffma
> should avoid  problems there. Or I could switch to using options-
> >fuse_ffma (I assume i965 is using it).
> 
> In i965 we've generally taken the approach that you should do mul+add
> and let the ffma peephole sort it out.

Yeah ok that makes sense. I'll change it.

> 
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