diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -436,7 +436,9 @@
 
     if (auto *C = dyn_cast<Constant>(V)) {
       // Undef values remain unknown.
-      if (!isa<UndefValue>(V))
+      if (isa<UndefValue>(V))
+        LV.markOverdefined();
+      else
         LV.markConstant(C);          // Constants are constant
     }
 
@@ -750,6 +752,10 @@
       continue;
 
     LatticeVal &Res = getValueState(&PN);
+    if (isa<UndefValue>(PN.getIncomingValue(i))) {
+      if (isConstant(Res))
+        continue;
+    }
     Changed |= Res.mergeIn(IV, DL);
     if (Res.isOverdefined())
       break;