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

[Nikita Popov via llvm-commits]

================
@@ -1096,6 +1096,53 @@ Value *InstCombinerImpl::foldUsingDistributiveLaws(BinaryOperator &I) {
   return SimplifySelectsFeedingBinaryOp(I, LHS, RHS);
 }
 
+std::optional<std::pair<Value *, Value *>>
+InstCombinerImpl::matchSymmetricPhiNodesPair(PHINode *LHS, PHINode *RHS) {
+
+  if (LHS->getParent() != RHS->getParent())
+    return std::nullopt;
+
+  if (LHS->getNumIncomingValues() < 2)
+    return std::nullopt;
+
+  BasicBlock *B0 = LHS->getIncomingBlock(0);
+  Value *N1 = LHS->getIncomingValueForBlock(B0);
+  Value *N2 = RHS->getIncomingValueForBlock(B0);
+
+  for (unsigned I = 1, E = LHS->getNumIncomingValues(); I != E; ++I) {
+    BasicBlock *B1 = LHS->getIncomingBlock(I);
+    Value *N3 = LHS->getIncomingValueForBlock(B1);
+    Value *N4 = RHS->getIncomingValueForBlock(B1);
+    if ((N1 == N3 && N2 == N4) || (N1 == N4 && N2 == N3))
+      continue;
----------------
nikic wrote:

```suggestion
    Value *L1 = LHS->getIncomingValueForBlock(B1);
    Value *R1 = RHS->getIncomingValueForBlock(B1);
    if ((L0 == L1 && R0 == R1) || (L0 == R1 && R0 == L1))
      continue;
```

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