[Mesa-dev] [PATCH 2/6] nir/lower_tex: Add a way to lower TXS(non-0-LOD) instructions

[Jason Ekstrand]

On Mon, Jun 17, 2019 at 12:42 PM Boris Brezillon <
boris.brezillon at collabora.com> wrote:

> On Mon, 17 Jun 2019 10:53:47 -0500
> Jason Ekstrand <jason at jlekstrand.net> wrote:
>
> > On Mon, Jun 17, 2019 at 5:49 AM Boris Brezillon <
> > boris.brezillon at collabora.com> wrote:
> >
> > > The V3D driver has an open-coded solution for this, and we need the
> > > same thing for Panfrost, so let's add a generic way to lower TXS(LOD)
> > > into max(TXS(0) >> LOD, 1).
> > >
> > > Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> > > ---
> > >  src/compiler/nir/nir.h           |  6 ++++
> > >  src/compiler/nir/nir_lower_tex.c | 49 ++++++++++++++++++++++++++++++++
> > >  2 files changed, 55 insertions(+)
> > >
> > > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> > > index 4270df565111..8972b4af7480 100644
> > > --- a/src/compiler/nir/nir.h
> > > +++ b/src/compiler/nir/nir.h
> > > @@ -3426,6 +3426,12 @@ typedef struct nir_lower_tex_options {
> > >      */
> > >     bool lower_txd_clamp_if_sampler_index_not_lt_16;
> > >
> > > +   /**
> > > +    * If true, lower nir_texop_txs with a non-0-lod into nir_texop_txs
> > > with
> > > +    * 0-lod followed by a nir_ishr.
> > > +    */
> > > +   bool lower_txs_lod;
> > > +
> > >     /**
> > >      * If true, apply a .bagr swizzle on tg4 results to handle
> Broadcom's
> > >      * mixed-up tg4 locations.
> > > diff --git a/src/compiler/nir/nir_lower_tex.c
> > > b/src/compiler/nir/nir_lower_tex.c
> > > index 53719017a87f..c29581d9b048 100644
> > > --- a/src/compiler/nir/nir_lower_tex.c
> > > +++ b/src/compiler/nir/nir_lower_tex.c
> > > @@ -978,6 +978,50 @@ lower_tg4_offsets(nir_builder *b, nir_tex_instr
> *tex)
> > >     return true;
> > >  }
> > >
> > > +static bool
> > > +nir_lower_txs_lod(nir_builder *b, nir_tex_instr *tex)
> > > +{
> > > +   int lod_idx = nir_tex_instr_src_index(tex, nir_tex_src_lod);
> > > +   if (lod_idx < 0 ||
> > > +       (nir_src_is_const(tex->src[lod_idx].src) &&
> > > +        !nir_src_as_int(tex->src[lod_idx].src)))
> > >
> >
> > Please use == 0 instead of ! here.  We're checking an integer, not a
> > boolean.
>
> Sure, I'll change that.
>
> >
> >
> > > +      return false;
> > > +
> > > +   nir_ssa_def *lod = nir_ssa_for_src(b, tex->src[lod_idx].src, 1);
> > >
> >
> > In theory, this can emit instructions.  We should set the cursor before
> > calling it.
>
> Oops, I'll move the cursor initialization before this line.
>
> >
> >
> > > +   unsigned dest_size = nir_tex_instr_dest_size(tex);
> > > +   nir_ssa_def *shift, *min, *result;
> > > +
> > > +   b->cursor = nir_after_instr(&tex->instr);
> > > +
> > > +   switch (dest_size) {
> > > +   case 3:
> > > +      shift = nir_vec3(b, lod, lod, tex->is_array ? nir_imm_int(b, 0)
> :
> > > lod);
> > > +      min = nir_imm_ivec3(b, 1, 1, tex->is_array ? 0 : 1);
> > > +      break;
> > > +   case 2:
> > > +      shift = nir_vec2(b, lod, tex->is_array ? nir_imm_int(b, 0) :
> lod);
> > > +      min = nir_imm_ivec2(b, 1, tex->is_array ? 0 : 1);
> > > +      break;
> > > +   case 1:
> > > +      shift = lod;
> > > +      min = nir_imm_int(b, 1);
> > > +      break;
> > > +   default:
> > > +      unreachable("Invalid nir_tex_instr_dest_size()\n");
> > > +   }
> > > +
> > > +   /* TXS(LOD) = max(TXS(0) >> LOD, 1) */
> > > +   result = nir_imax(b, nir_ishr(b, &tex->dest.ssa, shift), min);
> > >
> >
> > I think it will actually emit less code and be slightly simpler if you do
> >
> > nir_ssa_def *minified =
> >    nir_imax(b, nir_ushr(b, &tex->dest.ssa, lod), nir_imm_int(b, 1));
>
> Are you sure for the ushr()? lod is supposed to be an int, so I'd
> expect to have an ishr() here (note that I don't even handle the
> lod < 0 case as I'm not sure what's supposed to be done in that case).
>

I don't think it's going to matter in practice.  Neither the lod nor the
size are ever validly going to be negative.  GLSL might require that we
clamp the LOD to a valid range before using it in the shift.


> > if (tex->is_array) {
> >    nir_ssa_def *comp[4];
> >    for (unsigned i = 0; i < dest_size - 1; i++)
> >       comp[i] = nir_component(b, minified, i);
> >    comp[dest_size - 1] = nir_component(b, &tex->dest.ssa, dest_size - 1);
> >    minified = nir_vec(b, comp, dest_size);
> > }
> >
> > That way, it generates one vec() operation instead of two.  Note that you
> > don't need to explcitly expand lod out to three components as nir_builder
> > will do that for you.
>
> Oh, good to know.
>
> >
> >
> > > +   nir_ssa_def_rewrite_uses_after(&tex->dest.ssa,
> nir_src_for_ssa(result),
> > > +                                  result->parent_instr);
> > > +
> > > +   /* Replace the non-0-LOD in the initial TXS operation by a 0-LOD.
> */
> > > +   b->cursor = nir_before_instr(&tex->instr);
> > > +   nir_instr_rewrite_src(&tex->instr, &tex->src[lod_idx].src,
> > > +                         nir_src_for_ssa(nir_imm_int(b, 0)));
> > >
> >
> > Personally, I'd put this right after we grab the LOD from the instruction
> > but this is fine too.
>
> Okay, I'll move it there.
>
> Thanks for the review.
>
> Boris
>
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