1 files changed, 29 insertions, 2 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 8fbaf68..ff063f9 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -5169,6 +5169,7 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
   // - or: pick -1
   // - select's condition: if the true value is constant, choose it by making
   //                       the condition true.
+  // - phi: pick the common constant across operands
   // - default: pick 0
   //
   // Note that this transform is intentionally done here rather than
@@ -5179,9 +5180,32 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
   // TODO: This could use getBinopAbsorber() / getBinopIdentity() to avoid
   //       duplicating logic for binops at least.
   auto getUndefReplacement = [&](Type *Ty) {
-    Value *BestValue = nullptr;
+    auto pickCommonConstantFromPHI = [](PHINode &PN) -> Value * {
+      // phi(freeze(undef), C, C). Choose C for freeze so the PHI can be
+      // removed.
+      Constant *BestValue = nullptr;
+      for (Value *V : PN.incoming_values()) {
+        if (match(V, m_Freeze(m_Undef())))
+          continue;
+
+        Constant *C = dyn_cast<Constant>(V);
+        if (!C)
+          return nullptr;
+
+        if (!isGuaranteedNotToBeUndefOrPoison(C))
+          return nullptr;
+
+        if (BestValue && BestValue != C)
+          return nullptr;
+
+        BestValue = C;
+      }
+      return BestValue;
+    };
+
     Value *NullValue = Constant::getNullValue(Ty);
-    for (const auto *U : I.users()) {
+    Value *BestValue = nullptr;
+    for (auto *U : I.users()) {
       Value *V = NullValue;
       if (match(U, m_Or(m_Value(), m_Value())))
         V = ConstantInt::getAllOnesValue(Ty);
@@ -5190,6 +5214,9 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
       else if (match(U, m_c_Select(m_Specific(&I), m_Value(V)))) {
         if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
           V = NullValue;
+      } else if (auto *PHI = dyn_cast<PHINode>(U)) {
+        if (Value *MaybeV = pickCommonConstantFromPHI(*PHI))
+          V = MaybeV;
       }
 
       if (!BestValue)