Index: llvm/trunk/include/llvm/ADT/APInt.h
===================================================================
--- llvm/trunk/include/llvm/ADT/APInt.h
+++ llvm/trunk/include/llvm/ADT/APInt.h
@@ -212,6 +212,9 @@
   /// out-of-line slow case for intersects.
   bool intersectsSlowCase(const APInt &RHS) const LLVM_READONLY;
 
+  /// out-of-line slow case for isSubsetOf.
+  bool isSubsetOfSlowCase(const APInt &RHS) const LLVM_READONLY;
+
   /// out-of-line slow case for setBits.
   void setBitsSlowCase(unsigned loBit, unsigned hiBit);
 
@@ -1219,6 +1222,14 @@
     return intersectsSlowCase(RHS);
   }
 
+  /// This operation checks that all bits set in this APInt are also set in RHS.
+  bool isSubsetOf(const APInt &RHS) const {
+    assert(BitWidth == RHS.BitWidth && "Bit widths must be the same");
+    if (isSingleWord())
+      return (VAL & ~RHS.VAL) == 0;
+    return isSubsetOfSlowCase(RHS);
+  }
+
   /// @}
   /// \name Resizing Operators
   /// @{
Index: llvm/trunk/lib/Support/APInt.cpp
===================================================================
--- llvm/trunk/lib/Support/APInt.cpp
+++ llvm/trunk/lib/Support/APInt.cpp
@@ -730,6 +730,14 @@
   return false;
 }
 
+bool APInt::isSubsetOfSlowCase(const APInt &RHS) const {
+  for (unsigned i = 0, e = getNumWords(); i != e; ++i)
+    if ((pVal[i] & ~RHS.pVal[i]) != 0)
+      return false;
+
+  return true;
+}
+
 APInt APInt::byteSwap() const {
   assert(BitWidth >= 16 && BitWidth % 16 == 0 && "Cannot byteswap!");
   if (BitWidth == 16)
Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -38,7 +38,7 @@
 
   // If there are no bits set that aren't demanded, nothing to do.
   Demanded = Demanded.zextOrTrunc(C->getBitWidth());
-  if ((~Demanded & *C) == 0)
+  if (C->isSubsetOf(Demanded))
     return false;
 
   // This instruction is producing bits that are not demanded. Shrink the RHS.
Index: llvm/trunk/unittests/ADT/APIntTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/APIntTest.cpp
+++ llvm/trunk/unittests/ADT/APIntTest.cpp
@@ -2057,4 +2057,33 @@
   EXPECT_EQ(0, neg_one.shl(128));
 }
 
+TEST(APIntTest, isSubsetOf) {
+  APInt i32_1(32, 1);
+  APInt i32_2(32, 2);
+  APInt i32_3(32, 3);
+  EXPECT_FALSE(i32_3.isSubsetOf(i32_1));
+  EXPECT_TRUE(i32_1.isSubsetOf(i32_3));
+  EXPECT_FALSE(i32_2.isSubsetOf(i32_1));
+  EXPECT_FALSE(i32_1.isSubsetOf(i32_2));
+  EXPECT_TRUE(i32_3.isSubsetOf(i32_3));
+
+  APInt i128_1(128, 1);
+  APInt i128_2(128, 2);
+  APInt i128_3(128, 3);
+  EXPECT_FALSE(i128_3.isSubsetOf(i128_1));
+  EXPECT_TRUE(i128_1.isSubsetOf(i128_3));
+  EXPECT_FALSE(i128_2.isSubsetOf(i128_1));
+  EXPECT_FALSE(i128_1.isSubsetOf(i128_2));
+  EXPECT_TRUE(i128_3.isSubsetOf(i128_3));
+
+  i128_1 <<= 64;
+  i128_2 <<= 64;
+  i128_3 <<= 64;
+  EXPECT_FALSE(i128_3.isSubsetOf(i128_1));
+  EXPECT_TRUE(i128_1.isSubsetOf(i128_3));
+  EXPECT_FALSE(i128_2.isSubsetOf(i128_1));
+  EXPECT_FALSE(i128_1.isSubsetOf(i128_2));
+  EXPECT_TRUE(i128_3.isSubsetOf(i128_3));
+}
+
 } // end anonymous namespace