Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -420,40 +420,37 @@
   }
   case Instruction::SExt: {
     // Compute the bits in the result that are not present in the input.
-    unsigned SrcBitWidth =I->getOperand(0)->getType()->getScalarSizeInBits();
+    unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();
 
-    APInt InputDemandedBits = DemandedMask &
-                              APInt::getLowBitsSet(BitWidth, SrcBitWidth);
+    // If the extend bits are not demanded, convert this into a zero extension.
+    if (DemandedMask.getActiveBits() <= SrcBitWidth) {
+      // Convert to ZExt cast.
+      CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy, I->getName());
+      return InsertNewInstWith(NewCast, *I);
+    }
 
-    APInt NewBits(APInt::getBitsSetFrom(BitWidth, SrcBitWidth));
-    // If any of the sign extended bits are demanded, we know that the sign
-    // bit is demanded.
-    if ((NewBits & DemandedMask) != 0)
-      InputDemandedBits.setBit(SrcBitWidth-1);
+    APInt InputDemandedBits = DemandedMask.trunc(SrcBitWidth);
 
-    InputDemandedBits = InputDemandedBits.trunc(SrcBitWidth);
-    KnownZero = KnownZero.trunc(SrcBitWidth);
-    KnownOne = KnownOne.trunc(SrcBitWidth);
-    if (SimplifyDemandedBits(I, 0, InputDemandedBits, KnownZero, KnownOne,
-                             Depth + 1))
-      return I;
-    InputDemandedBits = InputDemandedBits.zext(BitWidth);
-    KnownZero = KnownZero.zext(BitWidth);
-    KnownOne = KnownOne.zext(BitWidth);
-    assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
+    // Sign extended bits are demanded, so the sign bit is demanded.
+    InputDemandedBits.setSignBit();
 
-    // If the sign bit of the input is known set or clear, then we know the
-    // top bits of the result.
+    APInt InputKnownZero(SrcBitWidth, 0), InputKnownOne(SrcBitWidth, 0);
+    if (SimplifyDemandedBits(I, 0, InputDemandedBits, InputKnownZero,
+                             InputKnownOne, Depth + 1))
+      return I;
 
-    // If the input sign bit is known zero, or if the NewBits are not demanded
-    // convert this into a zero extension.
-    if (KnownZero[SrcBitWidth-1] || (NewBits & ~DemandedMask) == NewBits) {
-      // Convert to ZExt cast
+    // If the input sign bit is known zero, convert this into a zero extension.
+    if (InputKnownZero.isSignBitSet()) {
+      // Convert to ZExt cast.
       CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy, I->getName());
       return InsertNewInstWith(NewCast, *I);
-    } else if (KnownOne[SrcBitWidth-1]) {    // Input sign bit known set
-      KnownOne |= NewBits;
     }
+
+    // If the sign bit of the input is known set or clear, then we know the
+    // top bits of the result.
+    KnownZero = InputKnownZero.sext(BitWidth);
+    KnownOne = InputKnownOne.sext(BitWidth);
+    assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
     break;
   }
   case Instruction::Add: