[ADT] Add BitVector::find_prev

[zturner]

Summary

This almost completes the matrix of all possible find operations.

*EXISTING*
----------
find_first
find_first_unset
find_next
find_next_unset
find_last
find_last_unset

*NEW*
----
find_prev

*STILL MISSING*
---------------
find_prev_unset

[chandlerc]

llvm/include/llvm/ADT/BitVector.h

220 ↗

(On Diff #97888)

Unrelated change?

232 ↗

(On Diff #97888)

Feel free to just commit typo fixes etc.

256 ↗

(On Diff #97888)

I'd really like to move the doxygen comments away from repeating the name. Maybe in a follow-up? Definitely not in this patch.

257–258 ↗

(On Diff #97888)

I find all the wording really confusing here.

"the next bit set *preceding* the Next bit" huh?

I think the argument would make much more sense named Pos or Index or something. Then you can talk about finding the *previous* bit set to match the name find_prev?

llvm/unittests/ADT/BitVectorTest.cpp

236–258 ↗

(On Diff #97888)

Feel free to land everything but find_prev right away with a separate patch?

[zturner]

Removed all unrelated changes and committed them separately.

[zturner]

Re-word the comment and parameter name to be more clear.

[chandlerc]

LGTM, nice!

llvm/trunk/include/llvm/ADT/BitVector.h

int find_next_unset(unsigned Prev) const {
 
     // Check subsequent words.
     for (unsigned i = WordPos + 1; i < NumBitWords(size()); ++i)
       if (Bits[i] != ~0UL)
         return next_unset_in_word(i, Bits[i]);
     return -1;
   }
 
+  /// 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.
+  int find_prev(unsigned PriorTo) {
+    if (PriorTo == 0)
+      return -1;
+
+    --PriorTo;
+
+    unsigned WordPos = PriorTo / BITWORD_SIZE;
+    unsigned BitPos = PriorTo % 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;
   }
 
   /// resize - Grow or shrink the bitvector.
   void resize(unsigned N, bool t = false) {
     if (N > getBitCapacity()) {

llvm/trunk/include/llvm/ADT/SmallBitVector.h

if (isSmall()) {
 
       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.
+  int find_prev(unsigned PriorTo) const {
+    if (isSmall()) {
+      if (PriorTo == 0)
+        return -1;
+
+      --PriorTo;
+      uintptr_t Bits = getSmallBits();
+      Bits &= maskTrailingOnes<uintptr_t>(PriorTo + 1);
+      if (Bits == 0)
+        return -1;
+
+      return NumBaseBits - countLeadingZeros(Bits) - 1;
+    }
+    return getPointer()->find_prev(PriorTo);
+  }
+
   /// Clear all bits.
   void clear() {
     if (!isSmall())
       delete getPointer();
     switchToSmall(0, 0);
   }
 
   /// Grow or shrink the bitvector.

llvm/trunk/unittests/ADT/BitVectorTest.cpp

TYPED_TEST(BitVectorTest, FindOperations) {
   A.set(75);
 
   EXPECT_EQ(75, A.find_last());
   EXPECT_EQ(12, A.find_first());
   EXPECT_EQ(13, A.find_next(12));
   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));
   EXPECT_EQ(14, A.find_next_unset(12));
   EXPECT_EQ(14, A.find_next_unset(13));
   EXPECT_EQ(16, A.find_next_unset(15));
   EXPECT_EQ(76, A.find_next_unset(74));
   EXPECT_EQ(76, A.find_next_unset(75));
TYPED_TEST(BitVectorTest, FindOperations) {
   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));
 }