diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -218,6 +218,13 @@
                      One.extractBits(NumBits, BitPosition));
   }
 
+  /// Concatenate the bits from \p Lo onto the bottom of *this.  This is
+  /// equivalent to:
+  ///   (this->zext(NewWidth) << Lo.getBitWidth()) | Lo.zext(NewWidth)
+  KnownBits concat(const KnownBits &Lo) const {
+    return KnownBits(Zero.concat(Lo.Zero), One.concat(Lo.One));
+  }
+
   /// Return KnownBits based on this, but updated given that the underlying
   /// value is known to be greater than or equal to Val.
   KnownBits makeGE(const APInt &Val) const;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3293,13 +3293,11 @@
     assert((Op.getResNo() == 0 || Op.getResNo() == 1) && "Unknown result");
 
     // Collect lo/hi source values and concatenate.
-    // TODO: Would a KnownBits::concatBits helper be useful?
     unsigned LoBits = Op.getOperand(0).getScalarValueSizeInBits();
     unsigned HiBits = Op.getOperand(1).getScalarValueSizeInBits();
     Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
     Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
-    Known = Known.anyext(LoBits + HiBits);
-    Known.insertBits(Known2, LoBits);
+    Known = Known2.concat(Known);
 
     // Collect shift amount.
     Known2 = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -2235,11 +2235,7 @@
     if (SimplifyDemandedBits(Op.getOperand(1), MaskHi, KnownHi, TLO, Depth + 1))
       return true;
 
-    Known.Zero = KnownLo.Zero.zext(BitWidth) |
-                 KnownHi.Zero.zext(BitWidth).shl(HalfBitWidth);
-
-    Known.One = KnownLo.One.zext(BitWidth) |
-                KnownHi.One.zext(BitWidth).shl(HalfBitWidth);
+    Known = KnownHi.concat(KnownLo);
     break;
   }
   case ISD::ZERO_EXTEND:
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -508,4 +508,29 @@
   });
 }
 
+TEST(KnownBitsTest, ConcatBits) {
+  unsigned Bits = 4;
+  for (unsigned LoBits = 1; LoBits < Bits; ++LoBits) {
+    unsigned HiBits = Bits - LoBits;
+    ForeachKnownBits(LoBits, [&](const KnownBits &KnownLo) {
+      ForeachKnownBits(HiBits, [&](const KnownBits &KnownHi) {
+        KnownBits KnownAll = KnownHi.concat(KnownLo);
+
+        EXPECT_EQ(KnownLo.countMinPopulation() + KnownHi.countMinPopulation(),
+                  KnownAll.countMinPopulation());
+        EXPECT_EQ(KnownLo.countMaxPopulation() + KnownHi.countMaxPopulation(),
+                  KnownAll.countMaxPopulation());
+
+        KnownBits ExtractLo = KnownAll.extractBits(LoBits, 0);
+        KnownBits ExtractHi = KnownAll.extractBits(HiBits, LoBits);
+
+        EXPECT_EQ(KnownLo.One.getZExtValue(), ExtractLo.One.getZExtValue());
+        EXPECT_EQ(KnownHi.One.getZExtValue(), ExtractHi.One.getZExtValue());
+        EXPECT_EQ(KnownLo.Zero.getZExtValue(), ExtractLo.Zero.getZExtValue());
+        EXPECT_EQ(KnownHi.Zero.getZExtValue(), ExtractHi.Zero.getZExtValue());
+      });
+    });
+  }
+}
+
 } // end anonymous namespace