[utils] Improve BitVector performance

Avoid most of the {is_inline()} checks by having a {data_begin_} pointer
which either points at the inline storage or at the zone-allocated
memory.
This replaces a dynamic branch by a memory indirection, which is
beneficial for big (non-inline) BitVectors. For small BitVectors we will
have to see what the bots say; the hypothesis is that a memory load is
still faster than a dynamic branch.

Apart from better performance, this change allows for simpler code in
many places, including the iterator implementation.

R=jkummerow@chromium.org

Bug: v8:13063
Change-Id: I1e28279d1a438598e0b8403a6a4078c2cd2a4c48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3776685Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81902}

src/utils/bit-vector.cc

@@ -4,6 +4,8 @@
 
 #include "src/utils/bit-vector.h"
 
+#include <numeric>
+
 #include "src/base/bits.h"
 #include "src/utils/utils.h"
 
@@ -26,13 +28,10 @@ void BitVector::Print() const {
 #endif
 
 int BitVector::Count() const {
-  if (is_inline()) return base::bits::CountPopulation(data_.inline_);
-
-  int count = 0;
-  for (int i = 0; i < data_length_; i++) {
-    count += base::bits::CountPopulation(data_.ptr_[i]);
-  }
-  return count;
+  auto accumulate_popcnt = [](int cnt, uintptr_t word) -> int {
+    return cnt + base::bits::CountPopulation(word);
+  };
+  return std::accumulate(data_begin_, data_end_, 0, accumulate_popcnt);
 }
 
 }  // namespace internal

test/unittests/utils/bit-vector-unittest.cc

@@ -126,5 +126,40 @@ TEST_F(BitVectorTest, Resize) {
   EXPECT_TRUE(!v.Contains(243));
 }
 
+TEST_F(BitVectorTest, BigBitVectorIterator) {
+  // Big BitVector with big and small entries.
+  BitVector v(500, zone());
+  v.Add(27);
+  v.Add(300);
+  v.Add(499);
+  auto iter = v.begin();
+  auto end = v.end();
+  EXPECT_NE(iter, end);
+  EXPECT_EQ(27, *iter);
+  ++iter;
+  EXPECT_NE(iter, end);
+  EXPECT_EQ(300, *iter);
+  ++iter;
+  EXPECT_NE(iter, end);
+  EXPECT_EQ(499, *iter);
+  ++iter;
+  EXPECT_EQ(iter, end);
+
+  // Remove small entries, add another big one.
+  v.Resize(1000, zone());
+  v.Remove(27);
+  v.Remove(300);
+  v.Add(500);
+  iter = v.begin();
+  end = v.end();
+  EXPECT_NE(iter, end);
+  EXPECT_EQ(499, *iter);
+  ++iter;
+  EXPECT_NE(iter, end);
+  EXPECT_EQ(500, *iter);
+  ++iter;
+  EXPECT_EQ(iter, end);
+}
+
 }  // namespace internal
 }  // namespace v8