diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index dc5d34f3b1f3..7e06d3d1aecc 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -896,10 +896,9 @@ public:
   /// countLeadingZeros - This function is an APInt version of the
   /// countLeadingZeros_{32,64} functions in MathExtras.h. It counts the number
   /// of zeros from the most significant bit to the first one bit.
-  /// @returns getNumWords() * APINT_BITS_PER_WORD if the value is zero.
+  /// @returns BitWidth if the value is zero.
   /// @returns the number of zeros from the most significant bit to the first
   /// one bits.
-  /// @brief Count the number of leading one bits.
   uint32_t countLeadingZeros() const;
 
   /// countLeadingOnes - This function counts the number of contiguous 1 bits
@@ -911,8 +910,8 @@ public:
 
   /// countTrailingZeros - This function is an APInt version of the 
   /// countTrailingZoers_{32,64} functions in MathExtras.h. It counts 
-  /// the number of zeros from the least significant bit to the first one bit.
-  /// @returns getNumWords() * APINT_BITS_PER_WORD if the value is zero.
+  /// the number of zeros from the least significant bit to the first set bit.
+  /// @returns BitWidth if the value is zero.
   /// @returns the number of zeros from the least significant bit to the first
   /// one bit.
   /// @brief Count the number of trailing zero bits.
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 7af0101ece81..3c83e8a749d9 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -745,7 +745,7 @@ uint32_t APInt::countLeadingZeros() const {
   uint32_t remainder = BitWidth % APINT_BITS_PER_WORD;
   if (remainder)
     Count -= APINT_BITS_PER_WORD - remainder;
-  return Count;
+  return std::min(Count, BitWidth);
 }
 
 static uint32_t countLeadingOnes_64(uint64_t V, uint32_t skip) {
diff --git a/llvm/test/Transforms/ConstProp/2007-11-23-cttz.ll b/llvm/test/Transforms/ConstProp/2007-11-23-cttz.ll
new file mode 100644
index 000000000000..995ce2febf87
--- /dev/null
+++ b/llvm/test/Transforms/ConstProp/2007-11-23-cttz.ll
@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | opt -constprop | llvm-dis | grep {ret i13 13}
+; PR1816
+declare i13 @llvm.cttz.i13(i13)
+
+define i13 @test() {
+	%X = call i13 @llvm.cttz.i13(i13 0)
+	ret i13 %X
+}