nir: add pass to move load_const

Run this pass late (after opt loop) to move load_const instructions back
into the basic blocks which use the result, in cases where a load_const
is only consumed in a single block.

This helps reduce register usage in cases where the backend driver
cannot lower the load_const to a uniform.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/Makefile.sources

@@ -254,6 +254,7 @@ NIR_FILES = \
 	nir/nir_lower_wpos_center.c \
 	nir/nir_lower_wpos_ytransform.c \
 	nir/nir_metadata.c \
+	nir/nir_move_load_const.c \
 	nir/nir_move_vec_src_uses_to_dest.c \
 	nir/nir_normalize_cubemap_coords.c \
 	nir/nir_opt_conditional_discard.c \

src/compiler/nir/meson.build

@@ -145,6 +145,7 @@ files_libnir = files(
   'nir_lower_wpos_ytransform.c',
   'nir_lower_bit_size.c',
   'nir_metadata.c',
+  'nir_move_load_const.c',
   'nir_move_vec_src_uses_to_dest.c',
   'nir_normalize_cubemap_coords.c',
   'nir_opt_conditional_discard.c',

src/compiler/nir/nir.h

@@ -2632,6 +2632,7 @@ bool nir_remove_dead_variables(nir_shader *shader, nir_variable_mode modes);
 bool nir_lower_constant_initializers(nir_shader *shader,
                                      nir_variable_mode modes);
 
+bool nir_move_load_const(nir_shader *shader);
 bool nir_move_vec_src_uses_to_dest(nir_shader *shader);
 bool nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alpha_test(nir_shader *shader, enum compare_func func,

src/compiler/nir/nir_move_load_const.c

@@ -0,0 +1,141 @@
+/*
+ * Copyright © 2018 Red Hat
+ *
+ * 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.
+ *
+ * Authors:
+ *    Rob Clark (robdclark@gmail.com>
+ *
+ */
+
+#include "nir.h"
+
+
+/*
+ * A simple pass that moves load_const's into consuming block if
+ * they are only consumed in a single block, to try to counter-
+ * act nir's tendency to move all load_const to the top of the
+ * first block.
+ */
+
+/* iterate a ssa def's use's and try to find a more optimal block to
+ * move it to, using the dominance tree.  In short, if all of the uses
+ * are contained in a single block, the load will be moved there,
+ * otherwise it will be move to the least common ancestor block of all
+ * the uses
+ */
+static nir_block *
+get_preferred_block(nir_ssa_def *def)
+{
+   nir_block *lca = NULL;
+
+   /* hmm, probably ignore if-uses: */
+   if (!list_empty(&def->if_uses))
+      return NULL;
+
+   nir_foreach_use(use, def) {
+      nir_instr *instr = use->parent_instr;
+      nir_block *use_block = instr->block;
+
+      /*
+       * Kind of an ugly special-case, but phi instructions
+       * need to appear first in the block, so by definition
+       * we can't move a load_immed into a block where it is
+       * consumed by a phi instruction.  We could conceivably
+       * move it into a dominator block.
+       */
+      if (instr->type == nir_instr_type_phi) {
+         nir_phi_instr *phi = nir_instr_as_phi(instr);
+         nir_block *phi_lca = NULL;
+         nir_foreach_phi_src(src, phi)
+            phi_lca = nir_dominance_lca(phi_lca, src->pred);
+         use_block = phi_lca;
+      }
+
+      lca = nir_dominance_lca(lca, use_block);
+   }
+
+   return lca;
+}
+
+/* insert before first non-phi instruction: */
+static void
+insert_after_phi(nir_instr *instr, nir_block *block)
+{
+   nir_foreach_instr(instr2, block) {
+      if (instr2->type == nir_instr_type_phi)
+         continue;
+
+      exec_node_insert_node_before(&instr2->node,
+                                   &instr->node);
+
+      return;
+   }
+
+   /* if haven't inserted it, push to tail (ie. empty block or possibly
+    * a block only containing phi's?)
+    */
+   exec_list_push_tail(&block->instr_list, &instr->node);
+}
+
+bool
+nir_move_load_const(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(function, shader) {
+      if (!function->impl)
+         continue;
+
+      nir_foreach_block(block, function->impl) {
+         nir_metadata_require(function->impl,
+                              nir_metadata_block_index | nir_metadata_dominance);
+
+         nir_foreach_instr_safe(instr, block) {
+            if (instr->type != nir_instr_type_load_const)
+               continue;
+
+            nir_load_const_instr *load =
+                  nir_instr_as_load_const(instr);
+            nir_block *use_block =
+                  get_preferred_block(&load->def);
+
+            if (!use_block)
+               continue;
+
+            if (use_block == load->instr.block)
+               continue;
+
+            exec_node_remove(&load->instr.node);
+
+            insert_after_phi(&load->instr, use_block);
+
+            load->instr.block = use_block;
+
+            progress = true;
+         }
+      }
+
+      nir_metadata_preserve(function->impl,
+                            nir_metadata_block_index | nir_metadata_dominance);
+   }
+
+   return progress;
+}