Index: llvm/include/llvm/ADT/BitVector.h
===================================================================
--- llvm/include/llvm/ADT/BitVector.h
+++ llvm/include/llvm/ADT/BitVector.h
@@ -217,7 +217,7 @@
     unsigned BitPos = Prev % BITWORD_SIZE;
     BitWord Copy = Bits[WordPos];
     // Mask off previous bits.
-    Copy &= ~0UL << BitPos;
+    Copy &= maskTrailingZeros<BitWord>(BitPos);
 
     if (Copy != 0)
       return WordPos * BITWORD_SIZE + countTrailingZeros(Copy);
@@ -229,7 +229,7 @@
     return -1;
   }
 
-  /// find_next_unset - Returns the index of the next usnet bit following the
+  /// find_next_unset - Returns the index of the next unset bit following the
   /// "Prev" bit.  Returns -1 if all remaining bits are set.
   int find_next_unset(unsigned Prev) const {
     ++Prev;
@@ -253,6 +253,33 @@
     return -1;
   }
 
+  /// find_prev - Returns the index of the next set bit *preceding* the
+  /// \p Next bit.  Returns -1 if all previous bits are unset.
+  int find_prev(unsigned Next) {
+    if (Next == 0)
+      return -1;
+
+    --Next;
+
+    unsigned WordPos = Next / BITWORD_SIZE;
+    unsigned BitPos = Next % BITWORD_SIZE;
+    BitWord Copy = Bits[WordPos];
+    // Mask off next bits.
+    Copy &= maskTrailingOnes<BitWord>(BitPos + 1);
+
+    if (Copy != 0)
+      return (WordPos + 1) * BITWORD_SIZE - countLeadingZeros(Copy) - 1;
+
+    // Check previous words.
+    for (unsigned i = 1; i <= WordPos; ++i) {
+      unsigned Index = WordPos - i;
+      if (Bits[Index] == 0)
+        continue;
+      return (Index + 1) * BITWORD_SIZE - countLeadingZeros(Bits[Index]) - 1;
+    }
+    return -1;
+  }
+
   /// clear - Clear all bits.
   void clear() {
     Size = 0;
Index: llvm/include/llvm/ADT/SmallBitVector.h
===================================================================
--- llvm/include/llvm/ADT/SmallBitVector.h
+++ llvm/include/llvm/ADT/SmallBitVector.h
@@ -278,6 +278,24 @@
     return getPointer()->find_next_unset(Prev);
   }
 
+  /// find_prev - Returns the index of the next set bit *preceding* the
+  /// \p Next bit.  Returns -1 if all previous bits are unset.
+  int find_prev(unsigned Next) const {
+    if (isSmall()) {
+      if (Next == 0)
+        return -1;
+
+      --Next;
+      uintptr_t Bits = getSmallBits();
+      Bits &= maskTrailingOnes<uintptr_t>(Next + 1);
+      if (Bits == 0)
+        return -1;
+
+      return NumBaseBits - countLeadingZeros(Bits) - 1;
+    }
+    return getPointer()->find_prev(Next);
+  }
+
   /// Clear all bits.
   void clear() {
     if (!isSmall())
Index: llvm/include/llvm/Support/MathExtras.h
===================================================================
--- llvm/include/llvm/Support/MathExtras.h
+++ llvm/include/llvm/Support/MathExtras.h
@@ -214,6 +214,18 @@
   return ~maskTrailingOnes<T>(CHAR_BIT * sizeof(T) - N);
 }
 
+/// \brief Create a bitmask with the N right-most bits set to 0, and all other
+/// bits set to 1.  Only unsigned types are allowed.
+template <typename T> T maskTrailingZeros(unsigned N) {
+  return maskLeadingOnes<T>(CHAR_BIT * sizeof(T) - N);
+}
+
+/// \brief Create a bitmask with the N left-most bits set to 0, and all other
+/// bits set to 1.  Only unsigned types are allowed.
+template <typename T> T maskLeadingZeros(unsigned N) {
+  return maskTrailingOnes<T>(CHAR_BIT * sizeof(T) - N);
+}
+
 /// \brief Get the index of the last set bit starting from the least
 ///   significant bit.
 ///
Index: llvm/unittests/ADT/BitVectorTest.cpp
===================================================================
--- llvm/unittests/ADT/BitVectorTest.cpp
+++ llvm/unittests/ADT/BitVectorTest.cpp
@@ -204,6 +204,11 @@
   EXPECT_EQ(75, A.find_next(13));
   EXPECT_EQ(-1, A.find_next(75));
 
+  EXPECT_EQ(-1, A.find_prev(12));
+  EXPECT_EQ(12, A.find_prev(13));
+  EXPECT_EQ(13, A.find_prev(75));
+  EXPECT_EQ(75, A.find_prev(90));
+
   EXPECT_EQ(0, A.find_first_unset());
   EXPECT_EQ(99, A.find_last_unset());
   EXPECT_EQ(14, A.find_next_unset(11));
@@ -227,6 +232,30 @@
   EXPECT_EQ(-1, A.find_last());
   EXPECT_EQ(0, A.find_first_unset());
   EXPECT_EQ(99, A.find_last_unset());
+
+  // Also test with a vector that is small enough to fit in 1 word.
+  A.resize(20);
+  A.set(3);
+  A.set(4);
+  A.set(16);
+  EXPECT_EQ(16, A.find_last());
+  EXPECT_EQ(3, A.find_first());
+  EXPECT_EQ(3, A.find_next(1));
+  EXPECT_EQ(4, A.find_next(3));
+  EXPECT_EQ(16, A.find_next(4));
+  EXPECT_EQ(-1, A.find_next(16));
+
+  EXPECT_EQ(-1, A.find_prev(3));
+  EXPECT_EQ(3, A.find_prev(4));
+  EXPECT_EQ(4, A.find_prev(16));
+  EXPECT_EQ(16, A.find_prev(18));
+
+  EXPECT_EQ(0, A.find_first_unset());
+  EXPECT_EQ(19, A.find_last_unset());
+  EXPECT_EQ(5, A.find_next_unset(3));
+  EXPECT_EQ(5, A.find_next_unset(4));
+  EXPECT_EQ(13, A.find_next_unset(12));
+  EXPECT_EQ(17, A.find_next_unset(15));
 }
 
 TYPED_TEST(BitVectorTest, CompoundAssignment) {