nir/loop_analyze: Fix phi-of-identical-alu detection

One issue was that the original version didn't check that swizzles
matched when comparing ALU instructions so it could end up matching
very different instructions.  Using the nir_instrs_equal function from
nir_instr_set.c which we use for CSE should be much more reliable.
Another was that the loop assumes it will only run two iterations which
may not be true.  If there's something which guarantees that this case
only happens for phis after ifs, it wasn't documented.

Fixes: 9e6b39e1d521 "nir: detect more induction variables"
Reviewed-by: Timothy Arceri <[email protected]>
(cherry picked from commit 9f7ffe41dd185487479ea8846df1f5cdbf1b83a6)

1 files changed, 29 insertions, 26 deletions

diff --git a/src/compiler/nir/nir_loop_analyze.c b/src/compiler/nir/nir_loop_analyze.c
index 0ae9533e007..20905be25f9 100644
--- a/src/compiler/nir/nir_loop_analyze.c
+++ b/src/compiler/nir/nir_loop_analyze.c
@@ -274,6 +274,31 @@ compute_invariance_information(loop_info_state *state)
    }
 }
 
+/* If all of the instruction sources point to identical ALU instructions (as
+ * per nir_instrs_equal), return one of the ALU instructions.  Otherwise,
+ * return NULL.
+ */
+static nir_alu_instr *
+phi_instr_as_alu(nir_phi_instr *phi)
+{
+   nir_alu_instr *first = NULL;
+   nir_foreach_phi_src(src, phi) {
+      assert(src->src.is_ssa);
+      if (src->src.ssa->parent_instr->type != nir_instr_type_alu)
+         return NULL;
+
+      nir_alu_instr *alu = nir_instr_as_alu(src->src.ssa->parent_instr);
+      if (first == NULL) {
+         first = alu;
+      } else {
+         if (!nir_instrs_equal(&first->instr, &alu->instr))
+            return NULL;
+      }
+   }
+
+   return first;
+}
+
 static bool
 compute_induction_information(loop_info_state *state)
 {
@@ -313,33 +338,11 @@ compute_induction_information(loop_info_state *state)
          if (is_var_phi(src_var)) {
             nir_phi_instr *src_phi =
                nir_instr_as_phi(src_var->def->parent_instr);
-
-            nir_op alu_op = nir_num_opcodes; /* avoid uninitialized warning */
-            nir_ssa_def *alu_srcs[2] = {0};
-            nir_foreach_phi_src(src2, src_phi) {
-               nir_loop_variable *src_var2 =
-                  get_loop_var(src2->src.ssa, state);
-
-               if (!src_var2->in_if_branch || !is_var_alu(src_var2))
+            nir_alu_instr *src_phi_alu = phi_instr_as_alu(src_phi);
+            if (src_phi_alu) {
+               src_var = get_loop_var(&src_phi_alu->dest.dest.ssa, state);
+               if (!src_var->in_if_branch)
                   break;
-
-               nir_alu_instr *alu =
-                  nir_instr_as_alu(src_var2->def->parent_instr);
-               if (nir_op_infos[alu->op].num_inputs != 2)
-                  break;
-
-               if (alu->src[0].src.ssa == alu_srcs[0] &&
-                   alu->src[1].src.ssa == alu_srcs[1] &&
-                   alu->op == alu_op) {
-                  /* Both branches perform the same calculation so we can use
-                   * one of them to find the induction variable.
-                   */
-                  src_var = src_var2;
-               } else {
-                  alu_srcs[0] = alu->src[0].src.ssa;
-                  alu_srcs[1] = alu->src[1].src.ssa;
-                  alu_op = alu->op;
-               }
             }
          }