[Mesa-dev] [PATCH] intel/compiler: Properly consider UBO loads that cross 32B boundaries.

[Kenneth Graunke]

On Tuesday, June 12, 2018 1:38:03 PM PDT Rafael Antognolli wrote:
> On Mon, Jun 11, 2018 at 02:01:49PM -0700, Kenneth Graunke wrote:
> > The UBO push analysis pass incorrectly assumed that all values would fit
> > within a 32B chunk, and only recorded a bit for the 32B chunk containing
> > the starting offset.
> > 
> > For example, if a UBO contained the following, tightly packed:
> > 
> >    vec4 a;  // [0, 16)
> >    float b; // [16, 20)
> >    vec4 c;  // [20, 36)
> > 
> > then, c would start at offset 20 / 32 = 0 and end at 36 / 32 = 1,
> > which means that we ought to record two 32B chunks in the bitfield.
> > 
> > Similarly, dvec4s would suffer from the same problem.
> > ---
> >  src/intel/compiler/brw_nir_analyze_ubo_ranges.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/intel/compiler/brw_nir_analyze_ubo_ranges.c b/src/intel/compiler/brw_nir_analyze_ubo_ranges.c
> > index d58fe3dd2e3..6d6ccf73ade 100644
> > --- a/src/intel/compiler/brw_nir_analyze_ubo_ranges.c
> > +++ b/src/intel/compiler/brw_nir_analyze_ubo_ranges.c
> > @@ -141,10 +141,16 @@ analyze_ubos_block(struct ubo_analysis_state *state, nir_block *block)
> >           if (offset >= 64)
> >              continue;
> >  
> > +         /* The value might span multiple 32-byte chunks. */
> > +         const int bytes = nir_intrinsic_dest_components(intrin) *
> > +                           (nir_dest_bit_size(intrin->dest) / 8);
> > +         const int end = DIV_ROUND_UP(offset_const->u32[0] + bytes, 32);
> > +         const int regs = end - offset + 1;
> > +
> 
> But if I understood it correctly, offset is the first 32B chunk within
> the UBO block (it's actually an ubo "chunk offset"). And you calculate
> bytes by taking the number of components times the size of each
> component of the nir_intrinsic_load_ubo instruction (which apparently
> supports multiple components). So yeah, this makes sense to me.

Yeah, that's exactly right.  load_ubo can load up to 4 components.

> Take this review with a grain of salt (assuming what I wrote above is
> correct), but this looks simple enough. So it is
> 
> Reviewed-by: Rafael Antognolli <rafael.antognolli at intel.com>

Thanks!
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