panfrost: add pass to lower noperspective varyings to a constant

This isn't hooked up yet, but should be a significant performance
improvement when available.

Signed-off-by: Benjamin Lee <benjamin.lee@collabora.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32127>

src/panfrost/util/pan_ir.h

@@ -392,6 +392,8 @@ bool pan_nir_lower_image_ms(nir_shader *shader);
 bool pan_nir_lower_frag_coord_zw(nir_shader *shader);
 bool pan_nir_lower_noperspective_vs(nir_shader *shader);
 bool pan_nir_lower_noperspective_fs(nir_shader *shader);
+bool pan_nir_lower_static_noperspective(nir_shader *shader,
+                                        uint32_t noperspective_varyings);
 
 bool pan_lower_helper_invocation(nir_shader *shader);
 bool pan_lower_sample_pos(nir_shader *shader);

src/panfrost/util/pan_lower_noperspective.c

@@ -290,3 +290,31 @@ pan_nir_lower_noperspective_fs(nir_shader *shader)
 
    return true;
 }
+
+static bool
+lower_static_noperspective(nir_builder *b, nir_intrinsic_instr *intrin,
+                           void *data)
+{
+   uint32_t *noperspective_varyings = data;
+
+   if (intrin->intrinsic != nir_intrinsic_load_noperspective_varyings_pan)
+      return false;
+
+   b->cursor = nir_after_instr(&intrin->instr);
+   nir_def *val = nir_imm_int(b, *noperspective_varyings);
+   nir_def_replace(&intrin->def, val);
+
+   return true;
+}
+
+/**
+ * Lower loads from the noperspective_varyings_pan sysval to a constant.
+ */
+bool
+pan_nir_lower_static_noperspective(nir_shader *shader,
+                                   uint32_t noperspective_varyings)
+{
+   return nir_shader_intrinsics_pass(shader, lower_static_noperspective,
+                                     nir_metadata_control_flow,
+                                     (void *)&noperspective_varyings);
+}