nir/lower_tex: Add lower_array_layer_round_even option.

Signed-off-by: Georg Lehmann <dadschoorse@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16871>

src/compiler/nir/nir.h

@@ -5100,6 +5100,11 @@ typedef struct nir_lower_tex_options {
     */
    bool lower_invalid_implicit_lod;
 
+   /* If true, round the layer component of the coordinates source to the nearest
+    * integer for all array ops.
+    */
+   bool lower_array_layer_round_even;
+
    /**
     * Payload data to be sent to callback / filter functions.
     */

src/compiler/nir/nir_lower_tex.c

@@ -1061,6 +1061,27 @@ lower_tex_packing(nir_builder *b, nir_tex_instr *tex,
                                   color->parent_instr);
 }
 
+static bool
+lower_array_layer_round_even(nir_builder *b, nir_tex_instr *tex)
+{
+   int coord_index = nir_tex_instr_src_index(tex, nir_tex_src_coord);
+   if (coord_index < 0 || nir_tex_instr_src_type(tex, coord_index) != nir_type_float)
+      return false;
+
+   assert(tex->src[coord_index].src.is_ssa);
+   nir_ssa_def *coord = tex->src[coord_index].src.ssa;
+
+   b->cursor = nir_before_instr(&tex->instr);
+
+   unsigned layer = tex->coord_components - 1;
+   nir_ssa_def *rounded_layer = nir_fround_even(b, nir_channel(b, coord, layer));
+   nir_ssa_def *new_coord = nir_vector_insert_imm(b, coord, rounded_layer, layer);
+
+   nir_instr_rewrite_src_ssa(&tex->instr, &tex->src[coord_index].src, new_coord);
+
+   return true;
+}
+
 static bool
 sampler_index_lt(nir_tex_instr *tex, unsigned max)
 {
@@ -1450,6 +1471,11 @@ nir_lower_tex_block(nir_block *block, nir_builder *b,
          progress = true;
       }
 
+      if (options->lower_array_layer_round_even && tex->is_array &&
+          tex->op != nir_texop_lod) {
+         progress |= lower_array_layer_round_even(b, tex);
+      }
+
       if (tex->op == nir_texop_txd &&
           (options->lower_txd ||
            (options->lower_txd_shadow && tex->is_shadow) ||