[llvm] [InstCombine] fold (Binop phi(a, b) phi(b, a)) -> (Binop a, b) while Binop is commutative. (PR #75765)

[Thorsten Schütt via llvm-commits]

================
@@ -1096,6 +1096,50 @@ Value *InstCombinerImpl::foldUsingDistributiveLaws(BinaryOperator &I) {
   return SimplifySelectsFeedingBinaryOp(I, LHS, RHS);
 }
 
+bool InstCombinerImpl::matchSymmetricPhiNodesPair(PHINode *LHS, PHINode *RHS) {
+
+  if (LHS->getNumIncomingValues() != 2 || RHS->getNumIncomingValues() != 2)
+    return false;
+
+  BasicBlock *B0 = LHS->getIncomingBlock(0);
+  BasicBlock *B1 = LHS->getIncomingBlock(1);
+
+  bool RHSContainB0 = RHS->getBasicBlockIndex(B0) != -1;
+  bool RHSContainB1 = RHS->getBasicBlockIndex(B1) != -1;
+
+  if (!RHSContainB0 || !RHSContainB1)
+    return false;
+
+  Value *N1 = LHS->getIncomingValueForBlock(B0);
+  Value *N2 = LHS->getIncomingValueForBlock(B1);
+  Value *N3 = RHS->getIncomingValueForBlock(B0);
+  Value *N4 = RHS->getIncomingValueForBlock(B1);
+
+  return N1 == N4 && N2 == N3;
+}
+
+Value *InstCombinerImpl::SimplifyPhiCommutativeBinaryOp(BinaryOperator &I,
+                                                        Value *Op0,
+                                                        Value *Op1) {
+  assert(I.isCommutative() && "Instruction should be commutative");
+
+  PHINode *LHS = dyn_cast<PHINode>(Op0);
+  PHINode *RHS = dyn_cast<PHINode>(Op1);
+
+  if (!LHS || !RHS)
+    return nullptr;
+
----------------
tschuett wrote:

Do you need to restrict to 2 incoming values per phi here? matchSymmetricPhiNodesPair could return true for phis with 10^10 incoming values.

https://github.com/llvm/llvm-project/pull/75765