microsoft/compiler: Back-propagate interpolator modes from FS

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41067>

src/gallium/drivers/d3d12/d3d12_compiler.cpp

@@ -1177,6 +1177,9 @@ select_shader_variant(struct d3d12_selection_context *sel_ctx, d3d12_shader_sele
 
    /* Remove not-read outputs and re-sort */
    if (next) {
+      if (next->stage == MESA_SHADER_FRAGMENT)
+         dxil_nir_propagate_interp_to_outputs(new_nir_variant, next->initial);
+
       NIR_PASS(_, new_nir_variant, dxil_nir_kill_unused_outputs, key.next_varying_inputs,
                  next->initial->info.patch_inputs_read, key.next_varying_frac_inputs);
       dxil_reassign_driver_locations(new_nir_variant, nir_var_shader_out, key.next_varying_inputs,

src/microsoft/compiler/dxil_nir.c

@@ -2876,3 +2876,33 @@ dxil_nir_kill_unused_outputs(nir_shader *shader, uint64_t next_stage_read_mask,
    progress |= nir_remove_dead_variables(shader, nir_var_shader_out, &options);
    return progress;
 }
+
+void
+dxil_nir_propagate_interp_to_outputs(nir_shader *prev_stage_nir,
+                                     const nir_shader *fs_nir)
+{
+   /* Work around a bug in some D3D12 drivers where the interpolation mode on
+    * outputs of the previous shader stage must match the interpolation mode on
+    * the corresponding fragment shader inputs. Back-propagate interpolation
+    * qualifiers (interpolation, centroid, sample) from FS inputs to the
+    * previous stage's outputs.
+    */
+   assert(fs_nir->info.stage == MESA_SHADER_FRAGMENT);
+
+   nir_foreach_variable_with_modes(fs_input, fs_nir, nir_var_shader_in) {
+      unsigned loc = fs_input->data.location;
+
+      /* Skip system values - only propagate for user varyings */
+      if (loc < VARYING_SLOT_VAR0)
+         continue;
+
+      nir_foreach_variable_with_modes(prev_output, prev_stage_nir,
+                                      nir_var_shader_out) {
+         if (prev_output->data.location == loc) {
+            prev_output->data.interpolation = fs_input->data.interpolation;
+            prev_output->data.centroid = fs_input->data.centroid;
+            prev_output->data.sample = fs_input->data.sample;
+         }
+      }
+   }
+}

src/microsoft/compiler/dxil_nir.h

@@ -89,6 +89,8 @@ bool dxil_nir_kill_undefined_varyings(nir_shader *shader, uint64_t prev_stage_wr
                                       uint32_t prev_stage_patch_written_mask, const BITSET_WORD *prev_stage_frac_output_mask);
 bool dxil_nir_kill_unused_outputs(nir_shader *shader, uint64_t next_stage_read_mask,
                                   uint32_t next_stage_patch_read_mask, const BITSET_WORD *next_stage_frac_input_mask);
+void dxil_nir_propagate_interp_to_outputs(nir_shader *prev_stage_nir,
+                                          const nir_shader *fs_nir);
 
 #ifdef __cplusplus
 }

src/microsoft/spirv_to_dxil/dxil_spirv_nir.c

@@ -804,6 +804,8 @@ dxil_spirv_nir_link(nir_shader *nir, nir_shader *prev_stage_nir,
             NIR_PASS(_, nir, dxil_spirv_compute_pntc);
             metadata->requires_runtime_data = true;
          }
+
+         dxil_nir_propagate_interp_to_outputs(prev_stage_nir, nir);
       }
 
       NIR_PASS(_, nir, dxil_nir_kill_undefined_varyings, prev_stage_nir->info.outputs_written, prev_stage_nir->info.patch_outputs_written, NULL);