diff --git a/llvm/include/llvm/ADT/BitVector.h b/llvm/include/llvm/ADT/BitVector.h
--- a/llvm/include/llvm/ADT/BitVector.h
+++ b/llvm/include/llvm/ADT/BitVector.h
@@ -203,9 +203,10 @@
     return !any();
   }
 
-  /// find_first_in - Returns the index of the first set bit in the range
-  /// [Begin, End).  Returns -1 if all bits in the range are unset.
-  int find_first_in(unsigned Begin, unsigned End) const {
+  /// find_first_in - Returns the index of the first set / unset bit,
+  /// depending on \p Set, in the range [Begin, End).
+  /// Returns -1 if all bits in the range are unset / set.
+  int find_first_in(unsigned Begin, unsigned End, bool Set = true) const {
     assert(Begin <= End && End <= Size);
     if (Begin == End)
       return -1;
@@ -214,8 +215,14 @@
     unsigned LastWord = (End - 1) / BITWORD_SIZE;
 
     // Check subsequent words.
+    // The code below is based on search for the first _set_ bit. If
+    // we're searching for the first _unset_, we just take the
+    // complement of each word before we use it and apply
+    // the same method.
     for (unsigned i = FirstWord; i <= LastWord; ++i) {
       BitWord Copy = Bits[i];
+      if (!Set)
+        Copy = ~Copy;
 
       if (i == FirstWord) {
         unsigned FirstBit = Begin % BITWORD_SIZE;
@@ -266,32 +273,7 @@
   /// find_first_unset_in - Returns the index of the first unset bit in the
   /// range [Begin, End).  Returns -1 if all bits in the range are set.
   int find_first_unset_in(unsigned Begin, unsigned End) const {
-    assert(Begin <= End && End <= Size);
-    if (Begin == End)
-      return -1;
-
-    unsigned FirstWord = Begin / BITWORD_SIZE;
-    unsigned LastWord = (End - 1) / BITWORD_SIZE;
-
-    // Check subsequent words.
-    for (unsigned i = FirstWord; i <= LastWord; ++i) {
-      BitWord Copy = Bits[i];
-
-      if (i == FirstWord) {
-        unsigned FirstBit = Begin % BITWORD_SIZE;
-        Copy |= maskTrailingOnes<BitWord>(FirstBit);
-      }
-
-      if (i == LastWord) {
-        unsigned LastBit = (End - 1) % BITWORD_SIZE;
-        Copy |= maskTrailingZeros<BitWord>(LastBit + 1);
-      }
-      if (Copy != ~BitWord(0)) {
-        unsigned Result = i * BITWORD_SIZE + countTrailingOnes(Copy);
-        return Result < size() ? Result : -1;
-      }
-    }
-    return -1;
+    return find_first_in(Begin, End, /* Set = */ false);
   }
 
   /// find_last_unset_in - Returns the index of the last unset bit in the