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

Alive2 proof: https://alive2.llvm.org/ce/z/2P8gq-
This patch closes #73905

Author: sun-jacobi

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -1539,6 +1539,9 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     if (Instruction *I = foldCommutativeIntrinsicOverSelects(*II))
       return I;
 
+    if (Instruction *I = foldCommutativeIntrinsicOverPhis(*II))
+      return I;
+
     if (CallInst *NewCall = canonicalizeConstantArg0ToArg1(CI))
       return NewCall;
   }
@@ -4237,3 +4240,22 @@ InstCombinerImpl::foldCommutativeIntrinsicOverSelects(IntrinsicInst &II) {
 
   return nullptr;
 }
+
+Instruction *
+InstCombinerImpl::foldCommutativeIntrinsicOverPhis(IntrinsicInst &II) {
+  assert(II.isCommutative() && "Instruction should be commutative");
+
+  PHINode *LHS = dyn_cast<PHINode>(II.getOperand(0));
+  PHINode *RHS = dyn_cast<PHINode>(II.getOperand(1));
+
+  if (!LHS || !RHS)
+    return nullptr;
+
+  if (auto P = matchSymmetricPhiNodesPair(LHS, RHS)) {
+    replaceOperand(II, 0, P->first);
+    replaceOperand(II, 1, P->second);
+    return &II;
+  }
+
+  return nullptr;
+}

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -278,6 +278,16 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
                                               IntrinsicInst &Tramp);
   Instruction *foldCommutativeIntrinsicOverSelects(IntrinsicInst &II);
 
+  // Match a pair of Phi Nodes like
+  // phi [a, BB0], [b, BB1] & phi [b, BB0], [a, BB1]
+  // Return the matched two operands.
+  std::optional<std::pair<Value *, Value *>>
+  matchSymmetricPhiNodesPair(PHINode *LHS, PHINode *RHS);
+
+  // Tries to fold (op phi(a, b) phi(b, a)) -> (op a, b)
+  // while op is a commutative intrinsic call.
+  Instruction *foldCommutativeIntrinsicOverPhis(IntrinsicInst &II);
+
   Value *simplifyMaskedLoad(IntrinsicInst &II);
   Instruction *simplifyMaskedStore(IntrinsicInst &II);
   Instruction *simplifyMaskedGather(IntrinsicInst &II);
@@ -492,6 +502,11 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   /// X % (C0 * C1)
   Value *SimplifyAddWithRemainder(BinaryOperator &I);
 
+  // Tries to fold (Binop phi(a, b) phi(b, a)) -> (Binop a, b)
+  // while Binop is commutative.
+  Value *SimplifyPhiCommutativeBinaryOp(BinaryOperator &I, Value *LHS,
+                                        Value *RHS);
+
   // Binary Op helper for select operations where the expression can be
   // efficiently reorganized.
   Value *SimplifySelectsFeedingBinaryOp(BinaryOperator &I, Value *LHS,

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1096,6 +1096,54 @@ 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;
+
+  if (!equal(LHS->blocks(), RHS->blocks()))
+    return std::nullopt;
+
+  Value *L0 = LHS->getIncomingValue(0);
+  Value *R0 = RHS->getIncomingValue(0);
+
+  for (unsigned I = 1, E = LHS->getNumIncomingValues(); I != E; ++I) {
+    Value *L1 = LHS->getIncomingValue(I);
+    Value *R1 = RHS->getIncomingValue(I);
+
+    if ((L0 == L1 && R0 == R1) || (L0 == R1 && R0 == L1))
+      continue;
+
+    return std::nullopt;
+  }
+
+  return std::optional(std::pair(L0, R0));
+}
+
+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;
+
+  if (auto P = matchSymmetricPhiNodesPair(LHS, RHS)) {
+    Value *BI = Builder.CreateBinOp(I.getOpcode(), P->first, P->second);
+    if (auto *BO = dyn_cast<BinaryOperator>(BI))
+      BO->copyIRFlags(&I);
+    return BI;
+  }
+
+  return nullptr;
+}
+
 Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
                                                         Value *LHS,
                                                         Value *RHS) {
@@ -1529,6 +1577,11 @@ Instruction *InstCombinerImpl::foldBinopWithPhiOperands(BinaryOperator &BO) {
       BO.getParent() != Phi1->getParent())
     return nullptr;
 
+  if (BO.isCommutative()) {
+    if (Value *V = SimplifyPhiCommutativeBinaryOp(BO, Phi0, Phi1))
+      return replaceInstUsesWith(BO, V);
+  }
+
   // Fold if there is at least one specific constant value in phi0 or phi1's
   // incoming values that comes from the same block and this specific constant
   // value can be used to do optimization for specific binary operator.