nir: Add a nir_opt_undef() to handle csels with undef.

We may find a cause to do more undef optimization in the future, but for
now this fixes up things after if flattening.  vc4 was handling this
internally most of the time, but a GLB2.7 shader that did a conditional
discard and assign gl_FragColor in the else was still emitting some extra
code.

total instructions in shared programs: 100809 -> 100795 (-0.01%)
instructions in affected programs:     37 -> 23 (-37.84%)

v2: Use nir_instr_rewrite_src() to update def/use on src[0] (by Thomas
    Helland).
v3: Make sure to flag metadata dirties, and copy the swizzle and abs/neg
    over to src[0], too (by anholt).

Reviewed-by: Thomas Helland <thomashelland90@gmail.com> (v2)
Tested-by: Thomas Helland <thomashelland90@gmail.com> (v2)

src/gallium/drivers/vc4/vc4_program.c

@@ -1627,6 +1627,7 @@ vc4_optimize_nir(struct nir_shader *s)
                 progress = nir_opt_peephole_select(s) || progress;
                 progress = nir_opt_algebraic(s) || progress;
                 progress = nir_opt_constant_folding(s) || progress;
+                progress = nir_opt_undef(s) || progress;
         } while (progress);
 }
 

src/glsl/Makefile.sources

@@ -56,6 +56,7 @@ NIR_FILES = \
 	nir/nir_opt_peephole_ffma.c \
 	nir/nir_opt_peephole_select.c \
 	nir/nir_opt_remove_phis.c \
+	nir/nir_opt_undef.c \
 	nir/nir_print.c \
 	nir/nir_remove_dead_variables.c \
 	nir/nir_search.c \

src/glsl/nir/nir.h

@@ -1704,6 +1704,8 @@ bool nir_opt_peephole_ffma(nir_shader *shader);
 
 bool nir_opt_remove_phis(nir_shader *shader);
 
+bool nir_opt_undef(nir_shader *shader);
+
 void nir_sweep(nir_shader *shader);
 
 #ifdef __cplusplus

src/glsl/nir/nir_opt_undef.c

@@ -0,0 +1,104 @@
+/*
+ * 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 "nir.h"
+
+/** @file nir_opt_undef.c
+ *
+ * Handles optimization of operations involving ssa_undef.  For now, we just
+ * make sure that csels between undef and some other value just give the other
+ * value (on the assumption that the condition's going to be choosing the
+ * defined value).  This reduces work after if flattening when each side of
+ * the if is defining a variable.
+ *
+ * Some day, we may find some use for making other operations consuming an
+ * undef arg output undef, but I don't know of any cases currently.
+ */
+
+static bool
+opt_undef_alu(nir_alu_instr *instr)
+{
+   if (instr->op != nir_op_bcsel && instr->op != nir_op_fcsel)
+      return false;
+
+   assert(instr->dest.dest.is_ssa);
+
+   for (int i = 1; i <= 2; i++) {
+      if (!instr->src[i].src.is_ssa)
+         continue;
+
+      nir_instr *parent = instr->src[i].src.ssa->parent_instr;
+      if (parent->type != nir_instr_type_ssa_undef)
+         continue;
+
+      /* We can't just use nir_alu_src_copy, because we need the def/use
+       * updated.
+       */
+      nir_instr_rewrite_src(&instr->instr, &instr->src[0].src,
+                            instr->src[i == 1 ? 2 : 1].src);
+      nir_alu_src_copy(&instr->src[0], &instr->src[i == 1 ? 2 : 1],
+                       ralloc_parent(instr));
+
+      nir_src empty_src;
+      memset(&empty_src, 0, sizeof(empty_src));
+      nir_instr_rewrite_src(&instr->instr, &instr->src[1].src, empty_src);
+      nir_instr_rewrite_src(&instr->instr, &instr->src[2].src, empty_src);
+      instr->op = nir_op_imov;
+
+      return true;
+   }
+
+   return false;
+}
+
+static bool
+opt_undef_block(nir_block *block, void *data)
+{
+   bool *progress = data;
+
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type == nir_instr_type_alu)
+         if (opt_undef_alu(nir_instr_as_alu(instr)))
+             (*progress) = true;
+   }
+
+   return true;
+}
+
+bool
+nir_opt_undef(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl) {
+         nir_foreach_block(overload->impl, opt_undef_block, &progress);
+         if (progress)
+            nir_metadata_preserve(overload->impl,
+                                  nir_metadata_block_index |
+                                  nir_metadata_dominance);
+      }
+   }
+
+   return progress;
+}