nir: Add a nir_lower_load_const_to_scalar() pass.

This is useful to increase the CSE opportunities for a scalar backend.  It
avoids regressions when dropping vc4's custom CSE implementation.

v2: Cleanups by Matt (decl in the for loop, and unreachable()).
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/Makefile.sources

@@ -33,6 +33,7 @@ NIR_FILES = \
 	nir/nir_lower_alu_to_scalar.c \
 	nir/nir_lower_atomics.c \
 	nir/nir_lower_global_vars_to_local.c \
+	nir/nir_lower_load_const_to_scalar.c \
 	nir/nir_lower_locals_to_regs.c \
 	nir/nir_lower_idiv.c \
 	nir/nir_lower_io.c \

src/glsl/nir/nir.h

@@ -1654,6 +1654,7 @@ void nir_remove_dead_variables(nir_shader *shader);
 
 void nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alu_to_scalar(nir_shader *shader);
+void nir_lower_load_const_to_scalar(nir_shader *shader);
 
 void nir_lower_phis_to_scalar(nir_shader *shader);
 

src/glsl/nir/nir_lower_load_const_to_scalar.c

@@ -0,0 +1,103 @@
+/*
+ * Copyright © 2015 Broadcom
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "util/macros.h"
+#include "nir.h"
+#include "nir_builder.h"
+
+/** @file nir_lower_load_const_to_scalar.c
+ *
+ * Replaces vector nir_load_const instructions with a series of loads and a
+ * vec[234] to reconstruct the original vector (on the assumption that
+ * nir_lower_alu_to_scalar() will then be used to split it up).
+ *
+ * This gives NIR a chance to CSE more operations on a scalar shader, when the
+ * same value was used in different vector contant loads.
+ */
+
+static void
+lower_load_const_instr_scalar(nir_load_const_instr *lower)
+{
+   if (lower->def.num_components == 1)
+      return;
+
+   nir_builder b;
+   nir_builder_init(&b, nir_cf_node_get_function(&lower->instr.block->cf_node));
+   nir_builder_insert_before_instr(&b, &lower->instr);
+
+   /* Emit the individual loads. */
+   nir_ssa_def *loads[4];
+   for (unsigned i = 0; i < lower->def.num_components; i++) {
+      nir_load_const_instr *load_comp = nir_load_const_instr_create(b.shader, 1);
+      load_comp->value.u[0] = lower->value.u[i];
+      nir_builder_instr_insert(&b, &load_comp->instr);
+      loads[i] = &load_comp->def;
+   }
+
+   /* Batch things back together into a vector. */
+   nir_ssa_def *vec;
+   switch (lower->def.num_components) {
+   case 2:
+      vec = nir_vec2(&b, loads[0], loads[1]);
+      break;
+   case 3:
+      vec = nir_vec3(&b, loads[0], loads[1], loads[2]);
+      break;
+   case 4:
+      vec = nir_vec4(&b, loads[0], loads[1], loads[2], loads[3]);
+      break;
+   default:
+      unreachable("Unknown load_const component count.");
+   }
+
+   /* Replace the old load with a reference to our reconstructed vector. */
+   nir_ssa_def_rewrite_uses(&lower->def, nir_src_for_ssa(vec),
+                            ralloc_parent(b.impl));
+   nir_instr_remove(&lower->instr);
+}
+
+static bool
+lower_load_const_to_scalar_block(nir_block *block, void *data)
+{
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type == nir_instr_type_load_const)
+         lower_load_const_instr_scalar(nir_instr_as_load_const(instr));
+   }
+
+   return true;
+}
+
+static void
+nir_lower_load_const_to_scalar_impl(nir_function_impl *impl)
+{
+   nir_foreach_block(impl, lower_load_const_to_scalar_block, NULL);
+}
+
+void
+nir_lower_load_const_to_scalar(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl)
+         nir_lower_load_const_to_scalar_impl(overload->impl);
+   }
+}