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
@@ -113,10 +113,17 @@
     return KnownBits(Zero.trunc(BitWidth), One.trunc(BitWidth));
   }
 
-  /// Zero extends the underlying known Zero and One bits. This is equivalent
-  /// to zero extending the value we're tracking.
-  KnownBits zext(unsigned BitWidth) {
-    return KnownBits(Zero.zext(BitWidth), One.zext(BitWidth));
+  /// Extends the underlying known Zero and One bits.
+  /// By setting ExtendedBitsAreKnownZero=true this will be equivalent to
+  /// zero extending the value we're tracking.
+  /// With ExtendedBitsAreKnownZero=false the extended bits are set to unknown.
+  KnownBits zext(unsigned BitWidth, bool ExtendedBitsAreKnownZero) {
+    unsigned OldBitWidth = getBitWidth();
+    Zero.zext(BitWidth);
+    One.zext(BitWidth);
+    if (ExtendedBitsAreKnownZero)
+      Zero.setBitsFrom(OldBitWidth);
+    return *this;
   }
 
   /// Sign extends the underlying known Zero and One bits. This is equivalent
@@ -125,10 +132,17 @@
     return KnownBits(Zero.sext(BitWidth), One.sext(BitWidth));
   }
 
-  /// Zero extends or truncates the underlying known Zero and One bits. This is
-  /// equivalent to zero extending or truncating the value we're tracking.
-  KnownBits zextOrTrunc(unsigned BitWidth) {
-    return KnownBits(Zero.zextOrTrunc(BitWidth), One.zextOrTrunc(BitWidth));
+  /// Extends or truncates the underlying known Zero and One bits. When
+  /// extending the extended bits can either be set as known zero (if
+  /// ExtendedBitsAreKnownZero=true) or as unknown (if
+  /// ExtendedBitsAreKnownZero=false).
+  KnownBits zextOrTrunc(unsigned BitWidth, bool ExtendedBitsAreKnownZero) {
+    unsigned OldBitWidth = getBitWidth();
+    Zero.zextOrTrunc(BitWidth);
+    One.zextOrTrunc(BitWidth);
+    if (ExtendedBitsAreKnownZero && BitWidth > OldBitWidth)
+      Zero.setBitsFrom(OldBitWidth);
+    return *this;
   }
 
   /// Returns the minimum number of trailing zero bits.
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1128,12 +1128,9 @@
       Q.DL.getTypeSizeInBits(ScalarTy);
 
     assert(SrcBitWidth && "SrcBitWidth can't be zero");
-    Known = Known.zextOrTrunc(SrcBitWidth);
+    Known = Known.zextOrTrunc(SrcBitWidth, false);
     computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
-    Known = Known.zextOrTrunc(BitWidth);
-    // Any top bits are known to be zero.
-    if (BitWidth > SrcBitWidth)
-      Known.Zero.setBitsFrom(SrcBitWidth);
+    Known = Known.zextOrTrunc(BitWidth, true /* ExtendedBitsAreKnownZero */);
     break;
   }
   case Instruction::BitCast: {
diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -400,7 +400,7 @@
 
   if (BitWidth > LOI->Known.getBitWidth()) {
     LOI->NumSignBits = 1;
-    LOI->Known = LOI->Known.zextOrTrunc(BitWidth);
+    LOI->Known = LOI->Known.zext(BitWidth, false /* => any extend */);
   }
 
   return LOI;
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
@@ -2800,15 +2800,12 @@
     EVT InVT = Op.getOperand(0).getValueType();
     APInt InDemandedElts = DemandedElts.zextOrSelf(InVT.getVectorNumElements());
     Known = computeKnownBits(Op.getOperand(0), InDemandedElts, Depth + 1);
-    Known = Known.zext(BitWidth);
-    Known.Zero.setBitsFrom(InVT.getScalarSizeInBits());
+    Known = Known.zext(BitWidth, true /* ExtendedBitsAreKnownZero */);
     break;
   }
   case ISD::ZERO_EXTEND: {
-    EVT InVT = Op.getOperand(0).getValueType();
     Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
-    Known = Known.zext(BitWidth);
-    Known.Zero.setBitsFrom(InVT.getScalarSizeInBits());
+    Known = Known.zext(BitWidth, true /* ExtendedBitsAreKnownZero */);
     break;
   }
   case ISD::SIGN_EXTEND_VECTOR_INREG: {
@@ -2829,7 +2826,7 @@
   }
   case ISD::ANY_EXTEND: {
     Known = computeKnownBits(Op.getOperand(0), Depth+1);
-    Known = Known.zext(BitWidth);
+    Known = Known.zext(BitWidth, false /* ExtendedBitsAreKnownZero */);
     break;
   }
   case ISD::TRUNCATE: {
@@ -3026,7 +3023,7 @@
       Known = computeKnownBits(InVec, Depth + 1);
     }
     if (BitWidth > EltBitWidth)
-      Known = Known.zext(BitWidth);
+      Known = Known.zext(BitWidth, false /* => any extend */);
     break;
   }
   case ISD::INSERT_VECTOR_ELT: {
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
@@ -1163,8 +1163,8 @@
     if (SimplifyDemandedBits(Src, InDemandedBits, Known, TLO, Depth + 1))
       return true;
     assert(!Known.hasConflict() && "Bits known to be one AND zero?");
-    Known = Known.zext(BitWidth);
-    Known.Zero.setBitsFrom(InBits);
+    assert(Known.getBitWidth() == InBits && "Src width has changed?");
+    Known = Known.zext(BitWidth, true /* ExtendedBitsAreKnownZero */);
     break;
   }
   case ISD::SIGN_EXTEND: {
@@ -1217,7 +1217,7 @@
     if (SimplifyDemandedBits(Src, InDemandedBits, Known, TLO, Depth + 1))
       return true;
     assert(!Known.hasConflict() && "Bits known to be one AND zero?");
-    Known = Known.zext(BitWidth);
+    Known = Known.zext(BitWidth, false /* => any extend */);
     break;
   }
   case ISD::TRUNCATE: {
@@ -1312,7 +1312,7 @@
 
     Known = Known2;
     if (BitWidth > EltBitWidth)
-      Known = Known.zext(BitWidth);
+      Known = Known.zext(BitWidth, false /* => any extend */);
     break;
   }
   case ISD::BITCAST: {
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -13706,8 +13706,7 @@
     if (Op.getOpcode() == ARMISD::VGETLANEs)
       Known = Known.sext(DstSz);
     else {
-      Known = Known.zext(DstSz);
-      Known.Zero.setBitsFrom(SrcSz);
+      Known = Known.zext(DstSz, true /* extended bits are known zero */);
     }
     assert(DstSz == Known.getBitWidth());
     break;
diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -6057,12 +6057,10 @@
     case Intrinsic::s390_vuplhw:
     case Intrinsic::s390_vuplf: {
       SDValue SrcOp = Op.getOperand(1);
-      unsigned SrcBitWidth = SrcOp.getScalarValueSizeInBits();
       APInt SrcDemE = getDemandedSrcElements(Op, DemandedElts, 0);
       Known = DAG.computeKnownBits(SrcOp, SrcDemE, Depth + 1);
       if (IsLogical) {
-        Known = Known.zext(BitWidth);
-        Known.Zero.setBitsFrom(SrcBitWidth);
+        Known = Known.zext(BitWidth, true);
       } else
         Known = Known.sext(BitWidth);
       break;
@@ -6091,7 +6089,7 @@
   // Known has the width of the source operand(s). Adjust if needed to match
   // the passed bitwidth.
   if (Known.getBitWidth() != BitWidth)
-    Known = Known.zextOrTrunc(BitWidth);
+    Known = Known.zextOrTrunc(BitWidth, false);
 }
 
 static unsigned computeNumSignBitsBinOp(SDValue Op, const APInt &DemandedElts,
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -30443,7 +30443,7 @@
     APInt DemandedElt = APInt::getOneBitSet(SrcVT.getVectorNumElements(),
                                             Op.getConstantOperandVal(1));
     Known = DAG.computeKnownBits(Src, DemandedElt, Depth + 1);
-    Known = Known.zextOrTrunc(BitWidth);
+    Known = Known.zextOrTrunc(BitWidth, false);
     Known.Zero.setBitsFrom(SrcVT.getScalarSizeInBits());
     break;
   }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -365,10 +365,9 @@
     KnownBits InputKnown(SrcBitWidth);
     if (SimplifyDemandedBits(I, 0, InputDemandedMask, InputKnown, Depth + 1))
       return I;
-    Known = InputKnown.zextOrTrunc(BitWidth);
-    // Any top bits are known to be zero.
-    if (BitWidth > SrcBitWidth)
-      Known.Zero.setBitsFrom(SrcBitWidth);
+    assert(InputKnown.getBitWidth() == SrcBitWidth && "Src width changed?");
+    Known = InputKnown.zextOrTrunc(BitWidth,
+                                   true /* ExtendedBitsAreKnownZero */);
     assert(!Known.hasConflict() && "Bits known to be one AND zero?");
     break;
   }