Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2443,8 +2443,43 @@
   return nullptr;
 }
 
-static Instruction *foldSelectToPhi(SelectInst &Sel, const DominatorTree &DT,
-                                    InstCombiner::BuilderTy &Builder) {
+static bool isAvailableAt(Value *V, Instruction *P, const DominatorTree &DT) {
+  if (auto *I = dyn_cast<Instruction>(V))
+    return DT.dominates(I, P);
+  return true;
+}
+
+static Value *propagatePhiInputs(SelectInst &Sel, const DominatorTree &DT,
+                                 InstCombiner::BuilderTy &Builder,
+                                 BasicBlock *TruePred, BasicBlock *FalsePred) {
+  Value *TrueInput = Sel.getTrueValue();
+  Value *FalseInput = Sel.getFalseValue();
+  auto GetPhiInput = [&](Value *V, BasicBlock *BB) {
+    if (auto *PN = dyn_cast<PHINode>(V))
+      if (PN->getParent() == Sel.getParent()) {
+        Value *PhiInput = PN->getIncomingValueForBlock(BB);
+        if (isAvailableAt(PhiInput, &Sel, DT))
+          return PhiInput;
+      }
+    return V;
+  };
+
+  Value *NewTrueInput = GetPhiInput(TrueInput, TruePred);
+  Value *NewFalseInput = GetPhiInput(FalseInput, FalsePred);
+
+  if (NewTrueInput == TrueInput && NewFalseInput == FalseInput)
+    return nullptr;
+
+  Builder.SetInsertPoint(&Sel);
+  auto *NewSelect =
+      Builder.CreateSelect(Sel.getCondition(), NewTrueInput, NewFalseInput);
+  NewSelect->takeName(&Sel);
+  return NewSelect;
+}
+
+static Value *foldSelectUsingPhiDominance(SelectInst &Sel,
+                                          const DominatorTree &DT,
+                                          InstCombiner::BuilderTy &Builder) {
   // Find the block's immediate dominator that ends with a conditional branch
   // that matches select's condition (maybe inverted).
   BasicBlock *BB = Sel.getParent();
@@ -2472,28 +2507,42 @@
   BasicBlockEdge TrueEdge(IDom, TrueSucc);
   BasicBlockEdge FalseEdge(IDom, FalseSucc);
   DenseMap<BasicBlock *, Value *> Inputs;
+  SmallVector<BasicBlock *, 2> TrueIncomingBlocks, FalseIncomingBlocks;
+  bool CanReplaceWithPhi = true;
   for (auto *Pred : predecessors(BB)) {
     // Check implication.
     BasicBlockEdge Incoming(Pred, BB);
-    if (DT.dominates(TrueEdge, Incoming))
+    if (DT.dominates(TrueEdge, Incoming)) {
       Inputs[Pred] = IfTrue;
-    else if (DT.dominates(FalseEdge, Incoming))
+      TrueIncomingBlocks.push_back(Pred);
+    } else if (DT.dominates(FalseEdge, Incoming)) {
       Inputs[Pred] = IfFalse;
-    else
+      FalseIncomingBlocks.push_back(Pred);
+    } else
       return nullptr;
     // Check availability.
-    if (auto *Insn = dyn_cast<Instruction>(Inputs[Pred]))
-      if (!DT.dominates(Insn, Pred->getTerminator()))
-        return nullptr;
+    if (!isAvailableAt(Inputs[Pred], Pred->getTerminator(), DT))
+      CanReplaceWithPhi = false;
   }
 
-  Builder.SetInsertPoint(&*BB->begin());
-  auto *PN = Builder.CreatePHI(Sel.getType(), Inputs.size());
-  for (auto *Pred : predecessors(BB)) {
-    PN->addIncoming(Inputs[Pred], Pred);
+  // Try to replace this select with a phi node.
+  if (CanReplaceWithPhi) {
+    Builder.SetInsertPoint(&*BB->begin());
+    auto *PN = Builder.CreatePHI(Sel.getType(), Inputs.size());
+    for (auto *Pred : predecessors(BB)) {
+      PN->addIncoming(Inputs[Pred], Pred);
+    }
+    PN->takeName(&Sel);
+    return PN;
   }
-  PN->takeName(&Sel);
-  return PN;
+
+  // If this select has a Phi argument from the same block, take its input.
+  if (TrueIncomingBlocks.size() == 1 && FalseIncomingBlocks.size() == 1)
+    if (auto *V = propagatePhiInputs(Sel, DT, Builder, TrueIncomingBlocks[0],
+                                     FalseIncomingBlocks[0]))
+      return V;
+
+  return nullptr;
 }
 
 Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
@@ -2957,8 +3006,8 @@
   if (Instruction *Copysign = foldSelectToCopysign(SI, Builder))
     return Copysign;
 
-  if (Instruction *PN = foldSelectToPhi(SI, DT, Builder))
-    return replaceInstUsesWith(SI, PN);
+  if (Value *V = foldSelectUsingPhiDominance(SI, DT, Builder))
+    return replaceInstUsesWith(SI, V);
 
   return nullptr;
 }
Index: llvm/test/Transforms/InstCombine/select.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select.ll
+++ llvm/test/Transforms/InstCombine/select.ll
@@ -1755,7 +1755,6 @@
   ret i32 %s
 }
 
-; TODO: Replace with phi[x, z].
 define i32 @select_phi_same_condition(i1 %cond, i32 %x, i32 %y, i32 %z) {
 ; CHECK-LABEL: @select_phi_same_condition(
 ; CHECK-NEXT:  entry:
@@ -1765,8 +1764,7 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 0, [[IF_TRUE]] ], [ [[Z:%.*]], [[IF_FALSE]] ]
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND]], i32 [[X:%.*]], i32 [[PHI]]
+; CHECK-NEXT:    [[S:%.*]] = phi i32 [ [[Z:%.*]], [[IF_FALSE]] ], [ [[X:%.*]], [[IF_TRUE]] ]
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
 entry: