Give a length field to stack bitmaps.

runtime/vm/bitmap.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_BITMAP_H_
 #define VM_BITMAP_H_
 
 #include "vm/allocation.h"
+#include "vm/isolate.h"
+#include "vm/zone.h"
 
 namespace dart {
 
 // Forward declarations.
 class RawStackmap;
 class Stackmap;
 
 
-// BitmapBuilder is used to build a bitmap. The implementation is optimized for
-// a dense set of small bit maps without an upper bound (e.g: a pointer map
-// description of a stack).
+// BitmapBuilder is used to build a bitmap. The implementation is optimized
+// for a dense set of small bit maps without a fixed upper bound (e.g: a
+// pointer map description of a stack).
 class BitmapBuilder : public ZoneAllocated {
  public:
-  BitmapBuilder() : size_in_bytes_(kInitialSizeInBytes),
-                    bit_list_(bit_list_data_) {
-    memset(bit_list_data_, 0, kInitialSizeInBytes);
+  BitmapBuilder()
+      : length_(0),
+        size_in_bytes_(kInitialSizeInBytes),
+        bit_list_(Isolate::Current()->current_zone()->Alloc<uint8_t>(
+            kInitialSizeInBytes)) {
+    memset(bit_list_, 0, kInitialSizeInBytes);
   }
 
-  intptr_t SizeInBits() const { return (size_in_bytes_ * kBitsPerByte); }
-  intptr_t SizeInBytes() const { return size_in_bytes_; }
+  intptr_t Length() const { return length_; }
+  void SetLength(intptr_t length);
 
-  // Get/Set individual bits in the bitmap, set expands the underlying bitmap
-  // if needed.
+  // Get/Set individual bits in the bitmap, setting bits beyond the bitmap's
+  // length increases the length and expands the underlying bitmap if
+  // needed.
   bool Get(intptr_t bit_offset) const;
   void Set(intptr_t bit_offset, bool value);
 
   // Return the bit offset of the highest bit set.
   intptr_t Maximum() const;
 
   // Return the bit offset of the lowest bit set.
   intptr_t Minimum() const;
 
   // Sets min..max (inclusive) to value.
   void SetRange(intptr_t min, intptr_t max, bool value);
 
  private:
   static const intptr_t kInitialSizeInBytes = 16;
   static const intptr_t kIncrementSizeInBytes = 16;
 
   bool InRange(intptr_t offset) const {
-    return (offset >= 0) && (offset < SizeInBits());
+    return (offset >= 0) && (offset < length_);
   }
 
+  // Get/Set a bit that is known to be covered by the backing store.
   bool GetBit(intptr_t bit_offset) const;
   void SetBit(intptr_t bit_offset, bool value);
 
+  intptr_t length_;
   intptr_t size_in_bytes_;

[Vyacheslav Egorov (Google), 2012/08/22 11:49:40]

I would rename size_in_bytes_ and bit_list_ in such a way that connection
between them would become apparent. 

I would also add a comment here that reading bits outside of backing store is
allowed and they are assumed to be false.

[Kevin Millikin (Google), 2012/08/22 11:56:27]

OK.  (bit_list_, size_in_bytes_) => (data_, data_size_in_bytes_).

-  uint8_t bit_list_data_[kInitialSizeInBytes];
   uint8_t* bit_list_;
 
   DISALLOW_COPY_AND_ASSIGN(BitmapBuilder);
 };
 
 }  // namespace dart
 
 #endif  // VM_BITMAP_H_