diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 4739d1b881a..b9a48eaf571 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -714,9 +714,16 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
-      case Intrinsic::ctpop:
       case Intrinsic::ctlz:
       case Intrinsic::cttz: {
+        unsigned LowBits = Log2_32(BitWidth)+1;
+        // If this call is undefined for 0, the result will be less than 2^n.
+        if (II->getArgOperand(1) == ConstantInt::getTrue(II->getContext()))
+          LowBits -= 1;
+        KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
+        break;
+      }
+      case Intrinsic::ctpop: {
         unsigned LowBits = Log2_32(BitWidth)+1;
         KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
         break;
diff --git a/test/Transforms/InstCombine/intrinsics.ll b/test/Transforms/InstCombine/intrinsics.ll
index e31bd7dfee0..7ae6d5a969a 100644
--- a/test/Transforms/InstCombine/intrinsics.ll
+++ b/test/Transforms/InstCombine/intrinsics.ll
@@ -181,10 +181,10 @@ entry:
 
 define void @cmp.simplify(i32 %a, i32 %b, i1* %c) {
 entry:
-  %lz = tail call i32 @llvm.ctlz.i32(i32 %a, i1 true) nounwind readnone
+  %lz = tail call i32 @llvm.ctlz.i32(i32 %a, i1 false) nounwind readnone
   %lz.cmp = icmp eq i32 %lz, 32
   store volatile i1 %lz.cmp, i1* %c
-  %tz = tail call i32 @llvm.cttz.i32(i32 %a, i1 true) nounwind readnone
+  %tz = tail call i32 @llvm.cttz.i32(i32 %a, i1 false) nounwind readnone
   %tz.cmp = icmp ne i32 %tz, 32
   store volatile i1 %tz.cmp, i1* %c
   %pop = tail call i32 @llvm.ctpop.i32(i32 %b) nounwind readnone
@@ -203,7 +203,7 @@ entry:
 
 
 define i32 @cttz_simplify1(i32 %x) nounwind readnone ssp {
-  %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %x, i1 true)    ; <i32> [#uses=1]
+  %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %x, i1 false)    ; <i32> [#uses=1]
   %shr3 = lshr i32 %tmp1, 5                       ; <i32> [#uses=1]
   ret i32 %shr3
   
diff --git a/test/Transforms/InstCombine/sub-xor.ll b/test/Transforms/InstCombine/sub-xor.ll
index 41e01fbdae8..bfa9f408c6f 100644
--- a/test/Transforms/InstCombine/sub-xor.ll
+++ b/test/Transforms/InstCombine/sub-xor.ll
@@ -10,3 +10,16 @@ define i32 @test1(i32 %x) nounwind {
 ; CHECK-NEXT: xor i32 %and, 63
 ; CHECK-NEXT: ret
 }
+
+declare i32 @llvm.ctlz.i32(i32, i1) nounwind readnone
+
+define i32 @test2(i32 %x) nounwind {
+  %count = tail call i32 @llvm.ctlz.i32(i32 %x, i1 true) nounwind readnone
+  %sub = sub i32 31, %count
+  ret i32 %sub
+
+; CHECK: @test2
+; CHECK-NEXT: ctlz
+; CHECK-NEXT: xor i32 %count, 31
+; CHECK-NEXT: ret
+}