d3d12: Lower load_sample_pos to load_sample_pos_at_id

D3D doesn't have an intrinsic for loading the current sample's
position, only for loading a specific sample's intrinsic. Fortunately,
we can also just get the current sample. So do that.

Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14624>

src/gallium/drivers/d3d12/d3d12_compiler.cpp

@@ -1160,6 +1160,7 @@ d3d12_create_shader(struct d3d12_context *ctx,
                                             next ? next->current->nir->info.inputs_read : 0);
    } else {
       NIR_PASS_V(nir, nir_lower_fragcoord_wtrans);
+      NIR_PASS_V(nir, d3d12_lower_sample_pos);
       dxil_sort_ps_outputs(nir);
    }
 

src/gallium/drivers/d3d12/d3d12_nir_passes.c

@@ -857,3 +857,24 @@ d3d12_lower_triangle_strip(nir_shader *shader)
    nir_metadata_preserve(impl, nir_metadata_none);
    NIR_PASS_V(shader, nir_lower_var_copies);
 }
+
+static bool
+is_sample_pos(const nir_instr *instr, const void *_data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+   nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+   return intr->intrinsic == nir_intrinsic_load_sample_pos;
+}
+
+static nir_ssa_def *
+lower_sample_pos(nir_builder *b, nir_instr *instr, void *_data)
+{
+   return nir_load_sample_pos_from_id(b, 32, nir_load_sample_id(b));
+}
+
+bool
+d3d12_lower_sample_pos(nir_shader *s)
+{
+   return nir_shader_lower_instructions(s, is_sample_pos, lower_sample_pos, NULL);
+}

src/gallium/drivers/d3d12/d3d12_nir_passes.h

@@ -98,6 +98,9 @@ d3d12_lower_triangle_strip(nir_shader *shader);
 bool
 d3d12_lower_image_casts(nir_shader *s, struct d3d12_image_format_conversion_info *info);
 
+bool
+d3d12_lower_sample_pos(nir_shader *s);
+
 #ifdef __cplusplus
 }
 #endif