Index: llvm/test/tools/llvm-reduce/reduce-operands-repeated-phi-input.ll
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-reduce/reduce-operands-repeated-phi-input.ll
@@ -0,0 +1,37 @@
+; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=operands-zero --test FileCheck --test-arg --check-prefixes=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+; RUN: FileCheck --check-prefixes=CHECK %s < %t
+
+; Make sure if we're replacing the value in a phi, it's replaced for
+; all repeats of the same incoming block.
+
+; CHECK-INTERESTINGNESS: switch
+; CHECK-INTERESTINGNESS: phi
+; CHECK-INTERESTINGNESS-SAME: [ %arg1, %bb1 ]
+; CHECK-INTERESTINGNESS: phi
+; CHECK-INTERESTINGNESS-SAME: [ %arg3, %bb1 ]
+; CHECK-INTERESTINGNESS: store volatile i32 %
+; CHECK-INTERESTINGNESS: store volatile float %
+
+; CHECK: %phi.i32 = phi i32 [ 0, %entry ], [ 0, %entry ], [ %arg1, %bb1 ]
+; CHECK: %phi.f32 = phi float [ 0.000000e+00, %entry ], [ 0.000000e+00, %entry ], [ %arg3, %bb1 ]
+define void @foo(i32 %arg0, i32 %arg1, float %arg2, float %arg3) {
+entry:
+  switch i32 %arg0, label %ret [
+    i32 3, label %bb1
+    i32 4, label %bb2
+    i32 12, label %bb2
+  ]
+
+bb1:
+  br label %bb2
+
+bb2:
+  %phi.i32 = phi i32 [ %arg0, %entry ], [ %arg0, %entry ], [ %arg1, %bb1 ]
+  %phi.f32 = phi float [ %arg2, %entry ], [ %arg2, %entry ], [ %arg3, %bb1 ]
+  store volatile i32 %phi.i32, ptr undef
+  store volatile float %phi.f32, ptr undef
+  br label %ret
+
+ret:
+  ret void
+}
Index: llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp
===================================================================
--- llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp
+++ llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp
@@ -22,6 +22,17 @@
                           function_ref<Value *(Use &)> ReduceValue) {
   for (auto &F : Program.functions()) {
     for (auto &I : instructions(&F)) {
+      if (PHINode *Phi = dyn_cast<PHINode>(&I)) {
+        for (auto &Op : Phi->incoming_values()) {
+          if (!O.shouldKeep()) {
+            if (Value *Reduced = ReduceValue(Op))
+              Phi->setIncomingValueForBlock(Phi->getIncomingBlock(Op), Reduced);
+          }
+        }
+
+        continue;
+      }
+
       for (auto &Op : I.operands()) {
         if (!O.shouldKeep()) {
           if (Value *Reduced = ReduceValue(Op))