nir: Introduce a nir_opt_move_comparisons() pass.

This tries to move comparisons (a common source of boolean values)
closer to their first use.  For GPUs which use condition codes,
this can eliminate a lot of temporary booleans and comparisons
which reload the condition code register based on a boolean.

V2: (Timothy Arceri)
 - fix move comparision for phis so we dont end up with:

    vec1 32 ssa_227 = phi block_34: ssa_1, block_38: ssa_240
    vec1 32 ssa_235 = feq ssa_227, ssa_1
    vec1 32 ssa_230 = phi block_34: ssa_221, block_38: ssa_235

 - add nir_op_i2b/nir_op_f2b to the list of comparisons.

V3: (Timothy Arceri)
 - tidy up suggested by Jason.
 - add inot/fnot to move comparison list

V4: (Jason Ekstrand)
 - clean up move_comparison_source
 - get rid of the tuple
 - rework phi handling

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]> [v1]
Reviewed-by: Jason Ekstrand <[email protected]>

3 files changed, 188 insertions, 0 deletions

diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
index 7479afae047..063cf50be5b 100644
--- a/src/compiler/Makefile.sources
+++ b/src/compiler/Makefile.sources
@@ -246,6 +246,7 @@ NIR_FILES = \
 	nir/nir_opt_global_to_local.c \
 	nir/nir_opt_if.c \
 	nir/nir_opt_loop_unroll.c \
+	nir/nir_opt_move_comparisons.c \
 	nir/nir_opt_peephole_select.c \
 	nir/nir_opt_remove_phis.c \
 	nir/nir_opt_trivial_continues.c \
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index 66cb36526bf..8bbc41d326e 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -2590,6 +2590,8 @@ bool nir_opt_if(nir_shader *shader);
 
 bool nir_opt_loop_unroll(nir_shader *shader, nir_variable_mode indirect_mask);
 
+bool nir_opt_move_comparisons(nir_shader *shader);
+
 bool nir_opt_peephole_select(nir_shader *shader, unsigned limit);
 
 bool nir_opt_remove_phis(nir_shader *shader);
diff --git a/src/compiler/nir/nir_opt_move_comparisons.c b/src/compiler/nir/nir_opt_move_comparisons.c
new file mode 100644
index 00000000000..617c2ca9989
--- /dev/null
+++ b/src/compiler/nir/nir_opt_move_comparisons.c
@@ -0,0 +1,185 @@
+/*
+ * Copyright © 2016 Intel Corporation
+ *
+ * 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_move_comparisons.c
+ *
+ * This pass moves ALU comparison operations just before their first use.
+ *
+ * It only moves instructions within a single basic block; cross-block
+ * movement is left to global code motion.
+ *
+ * Many GPUs generate condition codes for comparisons, and use predication
+ * for conditional selects and control flow.  In a sequence such as:
+ *
+ *     vec1 32 ssa_1 = flt a b
+ *     <some other operations>
+ *     vec1 32 ssa_2 = bcsel ssa_1 c d
+ *
+ * the backend would likely do the comparison, producing condition codes,
+ * then save those to a boolean value.  The intervening operations might
+ * trash the condition codes.  Then, in order to do the bcsel, it would
+ * need to re-populate the condition code register based on the boolean.
+ *
+ * By moving the comparison just before the bcsel, the condition codes could
+ * be used directly.  This eliminates the need to reload them from the boolean
+ * (generally eliminating an instruction).  It may also eliminate the need to
+ * create a boolean value altogether (unless it's used elsewhere), which could
+ * lower register pressure.
+ */
+
+static bool
+is_comparison(nir_op op)
+{
+   switch (op) {
+   case nir_op_flt:
+   case nir_op_fge:
+   case nir_op_feq:
+   case nir_op_fne:
+   case nir_op_ilt:
+   case nir_op_ult:
+   case nir_op_ige:
+   case nir_op_uge:
+   case nir_op_ieq:
+   case nir_op_ine:
+   case nir_op_i2b:
+   case nir_op_f2b:
+   case nir_op_inot:
+   case nir_op_fnot:
+      return true;
+   default:
+      return false;
+   }
+}
+
+static bool
+move_comparison_source(nir_src *src, nir_block *block, nir_instr *before)
+{
+   if (!src->is_ssa)
+      return false;
+
+   nir_instr *src_instr = src->ssa->parent_instr;
+
+   if (src_instr->block == block &&
+       src_instr->type == nir_instr_type_alu &&
+       is_comparison(nir_instr_as_alu(src_instr)->op)) {
+
+      exec_node_remove(&src_instr->node);
+
+      if (before)
+         exec_node_insert_node_before(&before->node, &src_instr->node);
+      else
+         exec_list_push_tail(&block->instr_list, &src_instr->node);
+
+      return true;
+   }
+
+   return false;
+}
+
+static bool
+move_comparison_source_cb(nir_src *src, void *data)
+{
+   bool *progress = data;
+
+   nir_instr *instr = src->parent_instr;
+   if (move_comparison_source(src, instr->block, instr))
+      *progress = true;
+
+   return true; /* nir_foreach_src should keep going */
+}
+
+static bool
+move_comparisons(nir_block *block)
+{
+   bool progress = false;
+
+   /* We use a simple approach: walk instructions backwards.
+    *
+    * If the instruction's source is a comparison from the same block,
+    * simply move it here.  This may break SSA if it's used earlier in
+    * the block as well.  However, as we walk backwards, we'll find the
+    * earlier use and move it again, further up.  It eventually ends up
+    * dominating all uses again, restoring SSA form.
+    *
+    * Before walking instructions, we consider the if-condition at the
+    * end of the block, if one exists.  It's effectively a use at the
+    * bottom of the block.
+    */
+   nir_if *iff = nir_block_get_following_if(block);
+   if (iff) {
+      progress |= move_comparison_source(&iff->condition, block, NULL);
+   }
+
+   nir_foreach_instr_reverse(instr, block) {
+      /* The sources of phi instructions happen after the predecessor block
+       * but before this block.  (Yes, that's between blocks).  This means
+       * that we don't need to move them in order for them to be correct.
+       * We could move them to encourage comparisons that are used in a phi to
+       * the end of the block, doing so correctly would make the pass
+       * substantially more complicated and wouldn't gain us anything since
+       * the phi can't use a flag value anyway.
+       */
+      if (instr->type == nir_instr_type_phi) {
+         /* We're going backwards so everything else is a phi too */
+         break;
+      } else if (instr->type == nir_instr_type_alu) {
+         /* Walk ALU instruction sources backwards so that bcsel's boolean
+          * condition is processed last.
+          */
+         nir_alu_instr *alu = nir_instr_as_alu(instr);
+         for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
+            progress |= move_comparison_source(&alu->src[i].src,
+                                               block, instr);
+         }
+      } else {
+         nir_foreach_src(instr, move_comparison_source_cb, &progress);
+      }
+   }
+
+   return progress;
+}
+
+bool
+nir_opt_move_comparisons(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(func, shader) {
+      if (!func->impl)
+         continue;
+
+      nir_foreach_block(block, func->impl) {
+         if (move_comparisons(block)) {
+            nir_metadata_preserve(func->impl, nir_metadata_block_index |
+                                              nir_metadata_dominance |
+                                              nir_metadata_live_ssa_defs);
+            progress = true;
+         }
+      }
+   }
+
+   return progress;
+}