[utils] Fix BitVector::Count with an inline backing store

The condition to detect an inline backing store was wrong and we would
try to access the heap-allocated store even for inline stores.

Drive-by: Use kBitsPerSystemPointer and the new
kBitsPerSystemPointerLog2 constants.

Change-Id: I19d0245ae82642a788c967534ab2a84464d56a67
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1890093
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64667}

src/common/globals.h

@@ -283,6 +283,8 @@ constexpr int kMaxRegularHeapObjectSize = (1 << (kPageSizeBits - 1));
 constexpr int kBitsPerByte = 8;
 constexpr int kBitsPerByteLog2 = 3;
 constexpr int kBitsPerSystemPointer = kSystemPointerSize * kBitsPerByte;
+constexpr int kBitsPerSystemPointerLog2 =
+    kSystemPointerSizeLog2 + kBitsPerByteLog2;
 constexpr int kBitsPerInt = kIntSize * kBitsPerByte;
 
 // IEEE 754 single precision floating point number bit layout.

src/utils/bit-vector.cc

@@ -41,15 +41,13 @@ void BitVector::Iterator::Advance() {
 }
 
 int BitVector::Count() const {
-  if (data_length_ == 0) {
-    return base::bits::CountPopulation(data_.inline_);
-  } else {
+  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;
-  }
 }
 
 }  // namespace internal

src/utils/bit-vector.h

@@ -66,8 +66,8 @@ class V8_EXPORT_PRIVATE BitVector : public ZoneObject {
   };
 
   static const int kDataLengthForInline = 1;
-  static const int kDataBits = kSystemPointerSize * 8;
-  static const int kDataBitShift = kSystemPointerSize == 8 ? 6 : 5;
+  static const int kDataBits = kBitsPerSystemPointer;
+  static const int kDataBitShift = kBitsPerSystemPointerLog2;
   static const uintptr_t kOne = 1;  // This saves some static_casts.
 
   BitVector() : length_(0), data_length_(kDataLengthForInline), data_(0) {}