Index: llvm/include/llvm/ADT/APInt.h
===================================================================
--- llvm/include/llvm/ADT/APInt.h
+++ llvm/include/llvm/ADT/APInt.h
@@ -2239,12 +2239,17 @@
 /// Splat/Merge neighboring bits to widen/narrow the bitmask represented
 /// by \param A to \param NewBitWidth bits.
 ///
+/// Default (Down = false):
+/// e.g. ScaleBitMask(0b0101, 8) -> 0b00110011
+/// e.g. ScaleBitMask(0b00011011, 4) -> 0b0111
+///
+/// Round down (Down = true):
 /// e.g. ScaleBitMask(0b0101, 8) -> 0b00110011
 /// e.g. ScaleBitMask(0b00011011, 4) -> 0b0111
 /// A.getBitwidth() or NewBitWidth must be a whole multiples of the other.
 ///
 /// TODO: Do we need a mode where all bits must be set when merging down?
-APInt ScaleBitMask(const APInt &A, unsigned NewBitWidth);
+APInt ScaleBitMask(const APInt &A, unsigned NewBitWidth, bool Down = false);
 } // namespace APIntOps
 
 // See friend declaration above. This additional declaration is required in
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2712,7 +2712,15 @@
         SubDemandedElts &= ScaledDemandedElts;
         if (!isSplatValue(Src, SubDemandedElts, SubUndefElts, Depth + 1))
           return false;
-        UndefElts |= APIntOps::ScaleBitMask(SubUndefElts, NumElts);
+
+        // Here we can't do "OR" operation merge for undef bits.
+        // Because some operation only use part value of the source.
+        // Take llvm.fshl.* for example:
+        // t1: v4i32 = Constant:i32<12>, undef:i32, Constant:i32<12>, undef:i32
+        // t2: v2i64 = bitcast t1
+        // t5: v2i64 = fshl t3, t4, t2
+        // We can not convert t2 to {i64 undef, i64 undef}
+        UndefElts |= APIntOps::ScaleBitMask(SubUndefElts, NumElts, true);
       }
       return true;
     }
Index: llvm/lib/Support/APInt.cpp
===================================================================
--- llvm/lib/Support/APInt.cpp
+++ llvm/lib/Support/APInt.cpp
@@ -2968,7 +2968,8 @@
   return A.getBitWidth() - ((A ^ B).countLeadingZeros() + 1);
 }
 
-APInt llvm::APIntOps::ScaleBitMask(const APInt &A, unsigned NewBitWidth) {
+APInt llvm::APIntOps::ScaleBitMask(const APInt &A, unsigned NewBitWidth,
+                                   bool Down) {
   unsigned OldBitWidth = A.getBitWidth();
   assert((((OldBitWidth % NewBitWidth) == 0) ||
           ((NewBitWidth % OldBitWidth) == 0)) &&
@@ -2994,9 +2995,15 @@
   } else {
     // Merge bits - if any old bit is set, then set scale equivalent new bit.
     unsigned Scale = OldBitWidth / NewBitWidth;
-    for (unsigned i = 0; i != NewBitWidth; ++i)
-      if (!A.extractBits(Scale, i * Scale).isZero())
-        NewA.setBit(i);
+    for (unsigned i = 0; i != NewBitWidth; ++i) {
+      if (Down) {
+        if (A.extractBits(Scale, i * Scale).isAllOnes())
+          NewA.setBit(i);
+      } else {
+        if (!A.extractBits(Scale, i * Scale).isZero())
+          NewA.setBit(i);
+      }
+    }
   }
 
   return NewA;