diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index e25dc0242878..339e3c37ee25 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -6528,11 +6528,12 @@ static bool getTargetConstantBitsFromNode(SDValue Op, unsigned EltSizeInBits,
 
 namespace llvm {
 namespace X86 {
-bool isConstantSplat(SDValue Op, APInt &SplatVal) {
+bool isConstantSplat(SDValue Op, APInt &SplatVal, bool AllowPartialUndefs) {
   APInt UndefElts;
   SmallVector<APInt, 16> EltBits;
   if (getTargetConstantBitsFromNode(Op, Op.getScalarValueSizeInBits(),
-                                    UndefElts, EltBits, true, false)) {
+                                    UndefElts, EltBits, true,
+                                    AllowPartialUndefs)) {
     int SplatIndex = -1;
     for (int i = 0, e = EltBits.size(); i != e; ++i) {
       if (UndefElts[i])
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 0da22acc99de..825ddedad176 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -722,7 +722,10 @@ namespace llvm {
 
     /// If Op is a constant whose elements are all the same constant or
     /// undefined, return true and return the constant value in \p SplatVal.
-    bool isConstantSplat(SDValue Op, APInt &SplatVal);
+    /// If we have undef bits that don't cover an entire element, we treat these
+    /// as zero if AllowPartialUndefs is set, else we fail and return false.
+    bool isConstantSplat(SDValue Op, APInt &SplatVal,
+                         bool AllowPartialUndefs = true);
   } // end namespace X86
 
   //===--------------------------------------------------------------------===//