[Mesa-dev] [PATCH 16/18] intel/blorp: Properly handle color compression in blorp_copy

[Jason Ekstrand]

On Nov 4, 2016 2:30 AM, "Pohjolainen, Topi" <topi.pohjolainen at gmail.com>
wrote:
>
> On Fri, Nov 04, 2016 at 11:17:13AM +0200, Pohjolainen, Topi wrote:
> > On Fri, Oct 28, 2016 at 02:17:12AM -0700, Jason Ekstrand wrote:
> > > Previously, blorp copy operations were CCS-unaware so you had to
perform
> > > resolves on the source and destination before performing the copy.
This
> > > commit makes blorp_copy capable of handling CCS-compressed images
without
> > > any resolves.
> > >
> > > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > > ---
> > >  src/intel/blorp/blorp_blit.c | 173
++++++++++++++++++++++++++++++++++++++++++-
> > >  src/intel/blorp/blorp_priv.h |   6 ++
> > >  2 files changed, 177 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/src/intel/blorp/blorp_blit.c
b/src/intel/blorp/blorp_blit.c
> > > index 07bb181..598b7c9 100644
> > > --- a/src/intel/blorp/blorp_blit.c
> > > +++ b/src/intel/blorp/blorp_blit.c
> > > @@ -851,6 +851,66 @@ blorp_nir_manual_blend_bilinear(nir_builder *b,
nir_ssa_def *pos,
> > >                        frac_y);
> > >  }
> > >
> > > +/** Perform a color bit-cast operation
> > > + *
> > > + * For copy operations involving CCS, we may need to use different
formats for
> > > + * the source and destination surfaces.  The two formats must both
be UINT
> > > + * formats and must have the same size but may have different bit
layouts.
> > > + * For instance, we may be copying from R8G8B8A8_UINT to R32_UINT or
R32_UINT
> > > + * to R16G16_UINT.  This function generates code to shuffle bits
around to get
> > > + * us from one to the other.
> > > + */
> > > +static nir_ssa_def *
> > > +bit_cast_color(struct nir_builder *b, nir_ssa_def *color,
> > > +               const struct brw_blorp_blit_prog_key *key)
> > > +{
> > > +   assert(key->texture_data_type == nir_type_uint);
> > > +
> > > +   if (key->dst_bpc > key->src_bpc) {
> > > +      nir_ssa_def *u = nir_ssa_undef(b, 1, 32);
> > > +      nir_ssa_def *dst_chan[2] = { u, u };
> > > +      unsigned shift = 0;
> > > +      unsigned dst_idx = 0;
> > > +      for (unsigned i = 0; i < 4; i++) {
> > > +         nir_ssa_def *shifted = nir_ishl(b, nir_channel(b, color, i),
> > > +                                            nir_imm_int(b, shift));
> > > +         if (shift == 0) {
> > > +            dst_chan[dst_idx] = shifted;
> > > +         } else {
> > > +            dst_chan[dst_idx] = nir_ior(b, dst_chan[dst_idx],
shifted);
> > > +         }
> > > +
> > > +         shift += key->src_bpc;
> > > +         if (shift >= key->dst_bpc) {
> > > +            dst_idx++;
> > > +            shift = 0;
> > > +         }
> > > +      }
> >
> > This iterates unconditionally over 4 source components regardless how
many
> > there are in source, right? Lets assume we have dst == R32_UINT and
> > src == R16G16_UINT, don't we get the following where iterations with i
= 2
> > and i = 3 try to access non-existing z and w components:
> >
> > i = 0:
> >             nir_ssa_def *shifted = nir_ishl(b, nir_channel(b, color, 0),
> >                                             nir_imm_int(b, 0));
> >             dst_chan[0] = shifted;
> >
> > i = 1:
> >             nir_ssa_def *shifted = nir_ishl(b, nir_channel(b, color, 1),
> >                                             nir_imm_int(b, 16));
> >             dst_chan[0] = nir_ior(b, dst_chan[0], shifted);
> >
> > i = 2:
> >             nir_ssa_def *shifted = nir_ishl(b, nir_channel(b, color, 2),
> >                                             nir_imm_int(b, 0));
> >             dst_chan[1] = shifted;
> >
> > i = 3:
> >             nir_ssa_def *shifted = nir_ishl(b, nir_channel(b, color, 3),
> >                                             nir_imm_int(b, 16));
> >             dst_chan[1] = nir_ior(b, dst_chan[1], shifted);
>
> If this is intentional and the idea is to rely on optimization passes to
> remove unnecessary writes to "dst_chan[1]" then a comment would be nice.

It is intentional but instead of relying on any compiler magic, we're just
relying on the fact that the texturing instruction always returns a vec4
and the render target write always takes a vec4.  I could have included the
number of components in the key and used that to not generate pointless
conversion code in a few places but that would have ended up making it more
complicated and we would end up generating more distinct shader programs
for only marginal benefit.
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