[GlobalISel] Fix the infinite loop issue in commute_int_constant_to_rhs

- When both operands are constant, the matcher runs into an infinite
  loop as the commutation should be applied only when LHS is a constant
  and RHS is not.

Reviewers: arsenm

Reviewed By: arsenm

Pull Request: #87426

Author: darkbuck

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -6276,14 +6276,15 @@ bool CombinerHelper::matchShiftsTooBig(MachineInstr &MI) {
 bool CombinerHelper::matchCommuteConstantToRHS(MachineInstr &MI) {
   Register LHS = MI.getOperand(1).getReg();
   Register RHS = MI.getOperand(2).getReg();
-  auto *LHSDef = MRI.getVRegDef(LHS);
-  if (getIConstantVRegVal(LHS, MRI).has_value())
-    return true;
-
-  // LHS may be a G_CONSTANT_FOLD_BARRIER. If so we commute
-  // as long as we don't already have a constant on the RHS.
-  if (LHSDef->getOpcode() != TargetOpcode::G_CONSTANT_FOLD_BARRIER)
-    return false;
+  if (!getIConstantVRegVal(LHS, MRI)) {
+    // Skip commuting if LHS is not a constant. But, LHS may be a
+    // G_CONSTANT_FOLD_BARRIER. If so we commute as long as we don't already
+    // have a constant on the RHS.
+    if (MRI.getVRegDef(LHS)->getOpcode() !=
+        TargetOpcode::G_CONSTANT_FOLD_BARRIER)
+      return false;
+  }
+  // Commute as long as RHS is not a constant or G_CONSTANT_FOLD_BARRIER.
   return MRI.getVRegDef(RHS)->getOpcode() !=
              TargetOpcode::G_CONSTANT_FOLD_BARRIER &&
          !getIConstantVRegVal(RHS, MRI);

llvm/test/CodeGen/AArch64/GlobalISel/combine-commute-int-const-lhs.mir

@@ -0,0 +1,28 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 4
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-prelegalizer-combiner %s -o - \
+# RUN:     --aarch64prelegalizercombiner-disable-rule=constant_fold_binop | FileCheck %s
+
+# `constant_fold_binop` is disabled to trigger the infinite loop in `commute_int_constant_to_rhs`.
+
+---
+name:            add
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $s0
+
+    ; CHECK-LABEL: name: add
+    ; CHECK: liveins: $s0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %c0:_(s32) = G_CONSTANT i32 1
+    ; CHECK-NEXT: %c1:_(s32) = G_CONSTANT i32 2
+    ; CHECK-NEXT: %add:_(s32) = G_ADD %c0, %c1
+    ; CHECK-NEXT: $s0 = COPY %add(s32)
+    ; CHECK-NEXT: RET_ReallyLR
+    %c0:_(s32) = G_CONSTANT i32 1
+    %c1:_(s32) = G_CONSTANT i32 2
+    %add:_(s32) = G_ADD %c0, %c1
+    $s0 = COPY %add
+    RET_ReallyLR
+
+...