[Mesa-dev] [PATCH 4/4] nir: Make nir_opt_remove_phis see through moves.

[Kenneth Graunke]

I found a shader in Tales of Maj'Eyal that contains:

        if ssa_21 {
                block block_1:
                /* preds: block_0 */
                ...instructions that prevent the select peephole...
                vec1 32 ssa_23 = imov ssa_4
                vec1 32 ssa_24 = imov ssa_4.y
                vec1 32 ssa_25 = imov ssa_4.z
                /* succs: block_3 */
        } else {
                block block_2:
                /* preds: block_0 */
                vec1 32 ssa_26 = imov ssa_4
                vec1 32 ssa_27 = imov ssa_4.y
                vec1 32 ssa_28 = imov ssa_4.z
                /* succs: block_3 */
        }
        block block_3:
        /* preds: block_1 block_2 */
        vec1 32 ssa_29 = phi block_1: ssa_23, block_2: ssa_26
        vec1 32 ssa_30 = phi block_1: ssa_24, block_2: ssa_27
        vec1 32 ssa_31 = phi block_1: ssa_25, block_2: ssa_28

Here, copy propagation will bail because phis cannot perform swizzles,
and CSE won't do anything because there is no dominance relationship
between the imovs.  By making nir_opt_remove_phis handle identical moves,
we can eliminate the phis and rewrite everything to use ssa_4 directly,
so all the moves become dead and get eliminated.

I don't think we need to check "exact" - just the alu sources.
Presumably phi sources should match in their exactness.

On Broadwell:

total instructions in shared programs: 11639872 -> 11638535 (-0.01%)
instructions in affected programs: 134222 -> 132885 (-1.00%)
helped: 338
HURT: 0

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/compiler/nir/nir_opt_remove_phis.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/compiler/nir/nir_opt_remove_phis.c b/src/compiler/nir/nir_opt_remove_phis.c
index f1e7bb0..ebcc856 100644
--- a/src/compiler/nir/nir_opt_remove_phis.c
+++ b/src/compiler/nir/nir_opt_remove_phis.c
@@ -27,6 +27,26 @@
 
 #include "nir.h"
 
+static nir_alu_instr *
+get_parent_mov(nir_ssa_def *ssa)
+{
+   if (ssa->parent_instr->type != nir_instr_type_alu)
+      return false;
+
+   nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
+   return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
+}
+
+static bool
+matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
+{
+   if (!mov1)
+      return false;
+
+   nir_alu_instr *mov2 = get_parent_mov(ssa);
+   return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
+}
+
 /*
  * This is a pass for removing phi nodes that look like:
  * a = phi(b, b, b, ...)
@@ -54,6 +74,7 @@ remove_phis_block(nir_block *block)
       nir_phi_instr *phi = nir_instr_as_phi(instr);
 
       nir_ssa_def *def = NULL;
+      nir_alu_instr *mov = NULL;
       bool srcs_same = true;
 
       nir_foreach_phi_src(src, phi) {
@@ -75,8 +96,9 @@ remove_phis_block(nir_block *block)
          
          if (def == NULL) {
             def  = src->src.ssa;
+            mov = get_parent_mov(def);
          } else {
-            if (src->src.ssa != def) {
+            if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
                srcs_same = false;
                break;
             }
-- 
2.9.0