nir: add an optimization to remove useless phi nodes

This removes phi nodes whose sources all point to the same thing.

Shader-db results:

total NIR instructions in shared programs: 2045293 -> 2041209 (-0.20%)
NIR instructions in affected programs:     126564 -> 122480 (-3.23%)
helped:                                615
HURT:                                  0

total FS instructions in shared programs: 4321840 -> 4320392 (-0.03%)
FS instructions in affected programs:     24622 -> 23174 (-5.88%)
helped:                                138
HURT:                                  0

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Tested-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/Makefile.sources

@@ -45,6 +45,7 @@ NIR_FILES = \
 	nir/nir_opt_dce.c \
 	nir/nir_opt_global_to_local.c \
 	nir/nir_opt_peephole_select.c \
+	nir/nir_opt_remove_phis.c \
 	nir/nir_print.c \
 	nir/nir_remove_dead_variables.c \
 	nir/nir_search.c \

src/glsl/nir/nir.h

@@ -1561,6 +1561,8 @@ bool nir_opt_dce(nir_shader *shader);
 bool nir_opt_peephole_select(nir_shader *shader);
 bool nir_opt_peephole_ffma(nir_shader *shader);
 
+bool nir_opt_remove_phis(nir_shader *shader);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif

src/glsl/nir/nir_opt_remove_phis.c

@@ -0,0 +1,109 @@
+/*
+ * Copyright © 2015 Connor Abbott
+ *
+ * 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:
+ *    Connor Abbott (cwabbott0@gmail.com)
+ *
+ */
+
+#include "nir.h"
+
+/*
+ * This is a pass for removing phi nodes that look like:
+ * a = phi(b, b, b, ...)
+ *
+ * Note that we can't ignore undef sources here, or else we may create a
+ * situation where the definition of b isn't dominated by its uses. We're
+ * allowed to do this since the definition of b must dominate all of the
+ * phi node's predecessors, which means it must dominate the phi node as well
+ * as all of the phi node's uses. In essence, the phi node acts as a copy
+ * instruction. b can't be another phi node in the same block, since the only
+ * time when phi nodes can source other phi nodes defined in the same block is
+ * at the loop header, and in that case one of the sources of the phi has to
+ * be from before the loop and that source can't be b.
+ */
+
+static bool
+remove_phis_block(nir_block *block, void *state)
+{
+   bool *progress = state;
+
+   void *mem_ctx = ralloc_parent(block);
+
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+      nir_ssa_def *def = NULL;
+      bool srcs_same = true;
+
+      nir_foreach_phi_src(phi, src) {
+         assert(src->src.is_ssa);
+         
+         if (def == NULL) {
+            def  = src->src.ssa;
+         } else {
+            if (src->src.ssa != def) {
+               srcs_same = false;
+               break;
+            }
+         }
+      }
+
+      if (!srcs_same)
+         continue;
+
+      assert(phi->dest.is_ssa);
+      nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def),
+                               mem_ctx);
+      nir_instr_remove(instr);
+
+      *progress = true;
+   }
+
+   return true;
+}
+
+static bool
+remove_phis_impl(nir_function_impl *impl)
+{
+   bool progress = false;
+
+   nir_foreach_block(impl, remove_phis_block, &progress);
+
+   return progress;
+}
+
+bool
+nir_opt_remove_phis(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_overload(shader, overload)
+      if (overload->impl)
+         progress = remove_phis_impl(overload->impl) || progress;
+
+   return progress;
+}
+