Add ImmutableHeap class consisting of parts

src/vm/immutable_heap.h

+// Copyright (c) 2015, the Fletch 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.md file.
+
+#ifndef SRC_VM_IMMUTABLE_HEAP_H_
+#define SRC_VM_IMMUTABLE_HEAP_H_
+
+#include "src/shared/globals.h"
+#include "src/vm/heap.h"
+
+namespace fletch {
+
+class ImmutableHeap {
+ public:
+  ImmutableHeap();
+  ~ImmutableHeap();
+
+  // Will return a [Heap] which will have an allocation budget which is
+  // `known_live_memory / number_of_hw_threads_`. This is an approximation of a
+  // 2x growth strategy.
+  //
+  // TODO(kustermann): instead of `number_of_hw_threads_` we could make this
+  // better by keeping track of the current number of used scheduler threads.
+  Heap* AcquirePart();
+
+  // Will return `true` if the caller should trigger an immutable GC.
+  //
+  // It is assumed that this function is only called on allocation failures.
+  bool ReleasePart(Heap* part);
+
+  // Merges all parts which have been acquired and subsequently released into
+  // the accumulated immutable heap.
+  //
+  // This function assumes that there are no parts outstanding.
+  void MergeParts();
+
+  // This method can only be called if
+  //   * all acquired parts were released again
+  //   * all cached parts were merged via [MergeParts]
+  void IterateProgramPointers(PointerVisitor* visitor);
+
+  // This method can only be called if
+  //   * all acquired parts were released again
+  //   * all cached parts were merged via [MergeParts]
+  Heap* heap() {
+    ASSERT(outstanding_parts_ == 0 && unmerged_parts_ == NULL);
+    return &heap_;
+  }
+
+ private:
+  // TODO(kustermann): Instead of having a linked list which requires heap
+  // allocations we should make a simple version of `std::vector` and use it
+  // here.
+  // [The number of parts is almost always fixed (i.e. the number of threads)]
+  class HeapPart {
+   public:
+    HeapPart(HeapPart* next_part) : heap_part(NULL), next(next_part) {}
+
+    Heap* heap_part;

[ricow1, 2015/08/05 13:18:18]

why is this called heap_part and not just heap (judging purely by the name I
would assume this is of type HeapPart)

[kustermann, 2015/08/05 13:20:53]

Well, because it is only part of the [ImmutableHeap]. Several parts can be
merged together. The "real merged heap" is on line 70.

+    HeapPart* next;
+  };
+
+  bool HasUnmergedParts() { return unmerged_parts_ != NULL; }
+  void AddUnmergedPart(Heap* heap);
+  Heap* RemoveUnmergedPart();
+
+  int number_of_hw_threads_;
+
+  Mutex* heap_mutex_;
+  Heap heap_;
+  int outstanding_parts_;
+  HeapPart* unmerged_parts_;
+  int ticks_;
+};
+
+}  // namespace fletch
+
+
+#endif  // SRC_VM_IMMUTABLE_HEAP_H_