On Fri, 2016-09-16 at 16:52 +0200, Erik Faye-Lund wrote:
> On Thu, Sep 15, 2016 at 9:03 AM, Timothy Arceri
> <timothy.arc...@collabora.com> wrote:
> >
> > This will be used by the loop unroll and lcssa passes.
> >
> > V2:
> > - Check instruction count is not too large for unrolling
> > - Add helper for complex loop unrolling
> > ---
> > src/compiler/nir/nir.h | 31 +++++++++++++++++++++++++++++++
> > 1 file changed, 31 insertions(+)
> >
> > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> > index 49e8cd8..3a2a13a 100644
> > --- a/src/compiler/nir/nir.h
> > +++ b/src/compiler/nir/nir.h
> > @@ -2590,6 +2590,37 @@ bool nir_normalize_cubemap_coords(nir_shader
> > *shader);
> >
> > void nir_live_ssa_defs_impl(nir_function_impl *impl);
> >
> > +static inline bool
> > +is_loop_small_enough_to_unroll(nir_shader *shader, nir_loop_info
> > *li)
> > +{
> > + unsigned max_iter = shader->options->max_unroll_iterations;
> > +
> > + if (li->trip_count > max_iter)
> > + return false;
> > +
> > + if (li->force_unroll)
> > + return true;
> > +
> > + bool loop_not_too_large =
> > + li->num_instructions * li->trip_count <= max_iter * 25;
>
>
> "max_iter * 25" seems like a pretty arbirary limit at first glance.
> How was it found? Perhaps a comment explaining a bit could be added?
Well it is :P I just tried to match it somewhat to the GLSL IR pass. I
don't think there was even a great explanation for the value that was
chosen.
>
> >
> > +static inline bool
> > +is_complex_loop(nir_shader *shader, nir_loop_info *li)
> > +{
> > + unsigned num_lt = list_length(&li->loop_terminator_list);
> > + return is_loop_small_enough_to_unroll(shader, li) && num_lt ==
> > 2;
>
> Perhaps you could add a comment to explain the "num_lt == 2"-part?
Sure. Basically if we don't know the trip count of all the exits (not a
simple loop) we can only possibly unroll loops with two exit points.
Anything more would be extra code for a scenario that is unlikely to
come up very often.
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