nir: add new linking opt nir_link_constant_varyings()

This pass moves constant outputs to the consuming shader stage
where possible.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir.h

@@ -2813,6 +2813,7 @@ bool nir_remove_unused_io_vars(nir_shader *shader, struct exec_list *var_list,
 void nir_compact_varyings(nir_shader *producer, nir_shader *consumer,
                           bool default_to_smooth_interp);
 void nir_link_xfb_varyings(nir_shader *producer, nir_shader *consumer);
+bool nir_link_constant_varyings(nir_shader *producer, nir_shader *consumer);
 
 typedef enum {
    /* If set, this forces all non-flat fragment shader inputs to be

src/compiler/nir/nir_linking_helpers.c

@@ -22,6 +22,7 @@
  */
 
 #include "nir.h"
+#include "nir_builder.h"
 #include "util/set.h"
 #include "util/hash_table.h"
 
@@ -556,3 +557,112 @@ nir_link_xfb_varyings(nir_shader *producer, nir_shader *consumer)
       }
    }
 }
+
+static bool
+try_replace_constant_input(nir_shader *shader,
+                           nir_intrinsic_instr *store_intr)
+{
+   nir_variable *out_var =
+      nir_deref_instr_get_variable(nir_src_as_deref(store_intr->src[0]));
+
+   if (out_var->data.mode != nir_var_shader_out)
+      return false;
+
+   /* Skip types that require more complex handling.
+    * TODO: add support for these types.
+    */
+   if (glsl_type_is_array(out_var->type) ||
+       glsl_type_is_dual_slot(out_var->type) ||
+       glsl_type_is_matrix(out_var->type) ||
+       glsl_type_is_struct(out_var->type))
+      return false;
+
+   /* Limit this pass to scalars for now to keep things simple. Most varyings
+    * should have been lowered to scalars at this point anyway.
+    */
+   if (store_intr->num_components != 1)
+      return false;
+
+   if (out_var->data.location < VARYING_SLOT_VAR0 ||
+       out_var->data.location - VARYING_SLOT_VAR0 >= MAX_VARYING)
+      return false;
+
+   nir_function_impl *impl = nir_shader_get_entrypoint(shader);
+
+   nir_builder b;
+   nir_builder_init(&b, impl);
+
+   bool progress = false;
+   nir_foreach_block(block, impl) {
+      nir_foreach_instr(instr, block) {
+         if (instr->type != nir_instr_type_intrinsic)
+            continue;
+
+         nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+         if (intr->intrinsic != nir_intrinsic_load_deref)
+            continue;
+
+         nir_variable *in_var =
+            nir_deref_instr_get_variable(nir_src_as_deref(intr->src[0]));
+
+         if (in_var->data.mode != nir_var_shader_in)
+            continue;
+
+         if (in_var->data.location != out_var->data.location ||
+             in_var->data.location_frac != out_var->data.location_frac)
+            continue;
+
+         b.cursor = nir_before_instr(instr);
+
+         nir_load_const_instr *out_const =
+            nir_instr_as_load_const(store_intr->src[1].ssa->parent_instr);
+
+         /* Add new const to replace the input */
+         nir_ssa_def *nconst = nir_build_imm(&b, store_intr->num_components,
+                                             intr->dest.ssa.bit_size,
+                                             out_const->value);
+
+         nir_ssa_def_rewrite_uses(&intr->dest.ssa, nir_src_for_ssa(nconst));
+
+         progress = true;
+      }
+   }
+
+   return progress;
+}
+
+bool
+nir_link_constant_varyings(nir_shader *producer, nir_shader *consumer)
+{
+   /* TODO: Add support for more shader stage combinations */
+   if (consumer->info.stage != MESA_SHADER_FRAGMENT ||
+       (producer->info.stage != MESA_SHADER_VERTEX &&
+        producer->info.stage != MESA_SHADER_TESS_EVAL))
+      return false;
+
+   bool progress = false;
+
+   nir_function_impl *impl = nir_shader_get_entrypoint(producer);
+
+   /* If we find a store in the last block of the producer we can be sure this
+    * is the only possible value for this output.
+    */
+   nir_block *last_block = nir_impl_last_block(impl);
+   nir_foreach_instr_reverse(instr, last_block) {
+      if (instr->type != nir_instr_type_intrinsic)
+         continue;
+
+      nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+
+      if (intr->intrinsic != nir_intrinsic_store_deref)
+         continue;
+
+      if (intr->src[1].ssa->parent_instr->type != nir_instr_type_load_const) {
+         continue;
+      }
+
+      progress |= try_replace_constant_input(consumer, intr);
+   }
+
+   return progress;
+}