diff --git a/include/llvm/ADT/APInt.h b/include/llvm/ADT/APInt.h
index fe9d2388f34..8fa1f950bc2 100644
--- a/include/llvm/ADT/APInt.h
+++ b/include/llvm/ADT/APInt.h
@@ -913,8 +913,9 @@ public:
   /// one bits.
   uint32_t countLeadingZeros() const;
 
-  /// countLeadingOnes - This function counts the number of contiguous 1 bits
-  /// in the high order bits. The count stops when the first 0 bit is reached.
+  /// countLeadingOnes - This function is an APInt version of the
+  /// countLeadingOnes_{32,64} functions in MathExtras.h. It counts the number
+  /// of ones from the most significant bit to the first zero bit.
   /// @returns 0 if the high order bit is not set
   /// @returns the number of 1 bits from the most significant to the least
   /// @brief Count the number of leading one bits.
@@ -929,6 +930,15 @@ public:
   /// @brief Count the number of trailing zero bits.
   uint32_t countTrailingZeros() const;
 
+  /// countTrailingOnes - This function is an APInt version of the 
+  /// countTrailingOnes_{32,64} functions in MathExtras.h. It counts 
+  /// the number of ones from the least significant bit to the first zero bit.
+  /// @returns BitWidth if the value is all ones.
+  /// @returns the number of ones from the least significant bit to the first
+  /// zero bit.
+  /// @brief Count the number of trailing one bits.
+  uint32_t countTrailingOnes() const;
+
   /// countPopulation - This function is an APInt version of the
   /// countPopulation_{32,64} functions in MathExtras.h. It counts the number
   /// of 1 bits in the APInt value. 
diff --git a/lib/Support/APInt.cpp b/lib/Support/APInt.cpp
index 88f32810d67..c8482e9c719 100644
--- a/lib/Support/APInt.cpp
+++ b/lib/Support/APInt.cpp
@@ -813,6 +813,18 @@ uint32_t APInt::countTrailingZeros() const {
   return std::min(Count, BitWidth);
 }
 
+uint32_t APInt::countTrailingOnes() const {
+  if (isSingleWord())
+    return std::min(uint32_t(CountTrailingOnes_64(VAL)), BitWidth);
+  uint32_t Count = 0;
+  uint32_t i = 0;
+  for (; i < getNumWords() && pVal[i] == -1; ++i)
+    Count += APINT_BITS_PER_WORD;
+  if (i < getNumWords())
+    Count += CountTrailingOnes_64(pVal[i]);
+  return std::min(Count, BitWidth);
+}
+
 uint32_t APInt::countPopulation() const {
   if (isSingleWord())
     return CountPopulation_64(VAL);