vc4: Optimize out silly SUBs of 0.

Drops instructions on vs-temp-array-mat4-index-col-row-wr.shader_test,
which I was looking at because it's failing to register allocate.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/gallium/drivers/vc4/vc4_opt_algebraic.c b/src/gallium/drivers/vc4/vc4_opt_algebraic.c
index b5cde0511d0..f88a5c17d2a 100644
--- a/src/gallium/drivers/vc4/vc4_opt_algebraic.c
+++ b/src/gallium/drivers/vc4/vc4_opt_algebraic.c
@@ -117,6 +117,17 @@ qir_opt_algebraic(struct vc4_compile *c)
                         }
                         break;
 
+                case QOP_FSUB:
+                case QOP_SUB:
+                        if (is_zero(c, defs, inst->src[1])) {
+                                dump_from(c, inst);
+                                inst->op = QOP_MOV;
+                                inst->src[1] = c->undef;
+                                progress = true;
+                                dump_to(c, inst);
+                        }
+                        break;
+
                 default:
                         break;
                 }