Fix bug in SmallBitVector::find_next_unset

[bmoody]

Summary

find_next_unset was returning size() instead of -1 in small-mode, when no unset bits are found.

[bmoody]

Ping! Could anyone take a look at this?

[dblaikie]

Sounds good - thanks!

[dantrushin]

llvm/include/llvm/ADT/SmallBitVector.h

296

Wouldn't it be simpler to leave ++Prev from line 290 in place and then just use Prev ?

[bmoody]

Great! Please commit this on my behalf, as I don't have commit access. Thanks.

llvm/include/llvm/ADT/SmallBitVector.h

296

IMO modifying parameter values can make code more confusing at a glance, so I try to avoid it. But the main reason I did this was to make the code as close to the find_next implementation above as possible.

llvm/include/llvm/ADT/SmallBitVector.h

int find_next(unsigned Prev) const {
     }
     return getPointer()->find_next(Prev);
   }
 
   /// Returns the index of the next unset bit following the "Prev" bit.
   /// Returns -1 if the next unset bit is not found.
   int find_next_unset(unsigned Prev) const {
     if (isSmall()) {
-      ++Prev;
       uintptr_t Bits = getSmallBits();
       // Mask in previous bits.
-      uintptr_t Mask = (uintptr_t(1) << Prev) - 1;
-      Bits |= Mask;
+      Bits |= (uintptr_t(1) << (Prev + 1)) - 1;
+      // Mask in unused bits.
+      Bits |= ~uintptr_t(0) << getSmallSize();
 
       if (Bits == ~uintptr_t(0) || Prev + 1 >= getSmallSize())
         return -1;
       return countTrailingOnes(Bits);
     }
     return getPointer()->find_next_unset(Prev);
   }
 
   /// find_prev - Returns the index of the first set bit that precedes the
   /// the bit at \p PriorTo.  Returns -1 if all previous bits are unset.

llvm/unittests/ADT/BitVectorTest.cpp

TYPED_TEST(BitVectorTest, SimpleFindOpsSingleWord) {
   TypeParam A;
   ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
   EXPECT_EQ(-1, A.find_first());
   EXPECT_EQ(-1, A.find_last());
   EXPECT_EQ(-1, A.find_first_unset());
   EXPECT_EQ(-1, A.find_last_unset());
 
   A.resize(20);
+  ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
+  EXPECT_EQ(-1, A.find_first());
+  EXPECT_EQ(-1, A.find_last());
+  EXPECT_EQ(-1, A.find_next(5));
+  EXPECT_EQ(-1, A.find_next(19));
+  EXPECT_EQ(-1, A.find_prev(5));
+  EXPECT_EQ(-1, A.find_prev(20));
+  EXPECT_EQ(0, A.find_first_unset());
+  EXPECT_EQ(19, A.find_last_unset());
+  EXPECT_EQ(6, A.find_next_unset(5));
+  EXPECT_EQ(-1, A.find_next_unset(19));
+
   A.set(3);
   A.set(4);
   A.set(16);
   ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
   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));
+
+  A.set();
+  ASSERT_TRUE(SmallBitVectorIsSmallMode(A));
+  EXPECT_EQ(0, A.find_first());
+  EXPECT_EQ(19, A.find_last());
+  EXPECT_EQ(6, A.find_next(5));
+  EXPECT_EQ(-1, A.find_next(19));
+  EXPECT_EQ(4, A.find_prev(5));
+  EXPECT_EQ(19, A.find_prev(20));
+  EXPECT_EQ(-1, A.find_first_unset());
+  EXPECT_EQ(-1, A.find_last_unset());
+  EXPECT_EQ(-1, A.find_next_unset(5));
+  EXPECT_EQ(-1, A.find_next_unset(19));
 }
 
 TEST(BitVectorTest, FindInRangeMultiWord) {
   BitVector Vec;
 
   Vec.resize(200);
   Vec.set(3, 7);
   Vec.set(24, 35);