Simple BlinkGC heap compaction.

third_party/WebKit/Source/platform/heap/SparseHeapBitmap.h

Index: third_party/WebKit/Source/platform/heap/SparseHeapBitmap.h
diff --git a/third_party/WebKit/Source/platform/heap/SparseHeapBitmap.h b/third_party/WebKit/Source/platform/heap/SparseHeapBitmap.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SparseHeapBitmap_h
+#define SparseHeapBitmap_h
+
+#include "platform/heap/BlinkGC.h"
+#include "platform/heap/HeapPage.h"
+#include "wtf/Alignment.h"
+#include "wtf/PtrUtil.h"
+#include <bitset>
+#include <memory>
+
+namespace blink {
+
+// A sparse bitmap of heap addresses where the (very few) addresses that are
+// set are likely to be in small clusters. The abstraction is tailored to
+// support heap compaction, assuming the following:
+//
+//   - Addresses will be bitmap-marked from lower to higher addresses.
+//   - Bitmap lookups are performed for each object that is compacted
+//     and moved to some new location, supplying the (base, size)
+//     pair of the object's heap allocation.
+//   - If the sparse bitmap has any marked addresses in that range, it
+//     returns a sub-bitmap that can be quickly iterated over to check which
+//     addresses within the range are actually set.
+//   - The bitmap is needed to support something that is very rarely done
+//     by the current Blink codebase, which is to have nested collection
+//     part objects. Consequently, it is safe to assume sparseness.
+//
+// Support the above by having a sparse bitmap organized as a binary
+// tree with nodes covering fixed size ranges via a simple bitmap/set.
+// That is, each SparseHeapBitmap node will contain a bitmap/set for
+// some fixed size range, along with pointers to SparseHeapBitmaps
+// for addresses on each side its range.
+//
+// This bitmap tree isn't kept balanced across the Address additions
+// made.
+//
+class PLATFORM_EXPORT SparseHeapBitmap {
+ public:
+  static std::unique_ptr<SparseHeapBitmap> create(Address base) {
+    return WTF::wrapUnique(new SparseHeapBitmap(base));
+  }
+
+  ~SparseHeapBitmap() {}
+
+  // Return the sparse bitmap subtree that at least covers the
+  // [address, address + size) range, or nullptr if none.
+  //
+  // The returned SparseHeapBitmap can be used to quickly lookup what
+  // addresses in that range are set or not; see |isSet()|. Its
+  // |isSet()| behavior outside that range is not defined.
+  SparseHeapBitmap* hasRange(Address, size_t);
+
+  // True iff |address| is set for this SparseHeapBitmap tree.
+  bool isSet(Address);
+
+  // Mark |address| as present/set.
+  void add(Address);
+
+  // The assumed minimum alignment of the pointers being added. Cannot
+  // exceed |log2(allocationGranularity)|; having it be equal to
+  // the platform pointer alignment is what's wanted.
+  static const int s_pointerAlignmentInBits = WTF_ALIGN_OF(void*) == 8 ? 3 : 2;
+  static const size_t s_pointerAlignmentMask =
+      (0x1u << s_pointerAlignmentInBits) - 1;
+
+  // Represent ranges in 0x100 bitset chunks; bit I is set iff Address
+  // |m_base + I * (0x1 << s_pointerAlignmentInBits)| has been added to the
+  // |SparseHeapBitmap|.
+  static const size_t s_bitmapChunkSize = 0x100;
+
+  // A SparseHeapBitmap either contains a single Address or a bitmap
+  // recording the mapping for [m_base, m_base + s_bitmapChunkRange)
+  static const size_t s_bitmapChunkRange = s_bitmapChunkSize
+                                           << s_pointerAlignmentInBits;
+
+  // Return the number of nodes; for debug stats.
+  size_t intervalCount() const;
+
+ private:
+  explicit SparseHeapBitmap(Address base) : m_base(base), m_size(1) {
+    DCHECK(!(reinterpret_cast<uintptr_t>(m_base) & s_pointerAlignmentMask));
+    static_assert(s_pointerAlignmentMask <= allocationMask,
+                  "address shift exceeds heap pointer alignment");
+    // For now, only recognize 8 and 4.
+    static_assert(WTF_ALIGN_OF(void*) == 8 || WTF_ALIGN_OF(void*) == 4,
+                  "unsupported pointer alignment");
+  }
+
+  Address base() const { return m_base; }
+  size_t size() const { return m_size; }
+  Address end() const { return base() + (m_size - 1); }
+
+  Address maxEnd() const { return base() + s_bitmapChunkRange; }
+
+  Address minStart() const {
+    // If this bitmap node represents the sparse [m_base, s_bitmapChunkRange)
+    // range, do not allow it to be "left extended" as that would entail
+    // having to shift down the contents of the std::bitset somehow.
+    //
+    // This shouldn't be a real problem as any clusters of set addresses
+    // will be marked while iterating from lower to higher addresses, hence
+    // "left extension" are unlikely to be common.
+    if (m_bitmap)
+      return base();
+    return (m_base > reinterpret_cast<Address>(s_bitmapChunkRange))
+               ? (base() - s_bitmapChunkRange + 1)
+               : nullptr;
+  }
+
+  Address swapBase(Address address) {
+    DCHECK(!(reinterpret_cast<uintptr_t>(address) & s_pointerAlignmentMask));
+    Address oldBase = m_base;
+    m_base = address;
+    return oldBase;
+  }
+
+  void createBitmap();
+
+  Address m_base;
+  // Either 1 or |s_bitmapChunkRange|.
+  size_t m_size;
+
+  // If non-null, contains a bitmap for addresses within [m_base, m_size)
+  std::unique_ptr<std::bitset<s_bitmapChunkSize>> m_bitmap;
+
+  std::unique_ptr<SparseHeapBitmap> m_left;
+  std::unique_ptr<SparseHeapBitmap> m_right;
+};
+
+}  // namespace blink
+
+#endif  // SparseHeapBitmap_h