- Add free list to be used for sweep phase of mark-sweep collector.

vm/freelist.cc

+// Copyright (c) 2011, 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.
+
+#include "vm/freelist.h"
+
+#include "vm/object.h"
+#include "vm/raw_object.h"
+
+namespace dart {
+
+// Allocate a fake class to be used as the class for elements of the free list.
+// These raw classes are only used to identify free list elements in the heap.
+// These classes cannot be allocated in the heap as the elements of the free
+// list are not live objects and their class references would not be updated
+// during a moving collection. In the general case these classes are also used
+// to implement RawObject::Size() to allow other code to safely traverse
+// the heap without any knowledge of the embedded free list elements.
+RawClass* AllocateFakeClass() {
+  RawClass* result =
+      reinterpret_cast<RawClass*>(calloc(1, Class::InstanceSize()));

[antonm, 2011/12/15 10:00:25]

maybe should check the alignment of allocated data?  or allocate with the proper
alignement.

[Ivan Posva, 2011/12/16 23:22:14]

If calloc does not allocate on word boundaries I think there would be bigger
problems.

[Anton Muhin, 2011/12/19 15:31:27]

I am more concerned with the case when objects may (in the future) require
bigger alignment say to distinguish between new and old space objects---the
places that need adjustment might be tricky to spot.

On 2011/12/16 23:22:14, Ivan Posva wrote:

+  result->instance_kind_ = kFreeListElement;
+  return reinterpret_cast<RawClass*>(RawObject::FromAddr(
+      reinterpret_cast<uword>(result)));
+}
+
+
+RawClass* FreeListElement::minimal_element_class_ = NULL;
+RawClass* FreeListElement::element_class_ = NULL;
+
+
+FreeListElement* FreeListElement::AsElement(uword addr, intptr_t size) {
+  ASSERT(size >= kObjectAlignment);
+  ASSERT(Utils::IsAligned(size, kObjectAlignment));
+
+  FreeListElement* result = reinterpret_cast<FreeListElement*>(addr);
+  if (size == kObjectAlignment) {
+    result->class_ = minimal_element_class_;
+  } else {
+    result->class_ = element_class_;
+    *result->SizeAddress() = size;
+  }
+  result->set_next(NULL);
+  ASSERT(result->Size() == size);
+  return result;
+}
+
+
+void FreeListElement::InitOnce() {
+  ASSERT(sizeof(FreeListElement) == kObjectAlignment);
+  ASSERT(minimal_element_class_ == NULL);
+  ASSERT(element_class_ == NULL);
+  minimal_element_class_ = AllocateFakeClass();
+  element_class_ = AllocateFakeClass();
+}
+
+
+FreeList::FreeList() {
+  Reset();
+}
+
+
+FreeList::~FreeList() {
+  // Nothing to release.
+}
+
+
+uword FreeList::TryAllocate(intptr_t size) {
+  int index = IndexForSize(size);
+  if ((index != kNumLists) && (free_lists_[index] != NULL)) {
+    return reinterpret_cast<uword>(DequeueElement(index));
+  }
+
+  if (index < kNumLists) {
+    index++;
+    while (index < kNumLists) {
+      if (free_lists_[index] != NULL) {
+        // Dequeue an element from the list, split and enqueue the remainder in
+        // the appropriate list.
+        FreeListElement* element = DequeueElement(index);
+        SplitElementAfterAndEnqueue(element, size);
+        return reinterpret_cast<uword>(element);
+      }
+      index++;
+    }
+  }
+
+  FreeListElement* previous = NULL;
+  FreeListElement* current = free_lists_[kNumLists];
+  while (current != NULL) {
+    if (current->Size() >= size) {
+      // Found an element large enough to hold the requested size. Dequeue,
+      // split and enqueue the remainder.
+      if (previous == NULL) {
+        free_lists_[kNumLists] = current->next();
+      } else {
+        previous->set_next(current->next());
+      }
+      SplitElementAfterAndEnqueue(current, size);
+      return reinterpret_cast<uword>(current);
+    }
+    previous = current;
+    current = current->next();
+  }
+  return 0;
+}
+
+
+void FreeList::Free(uword addr, intptr_t size) {
+  intptr_t index = IndexForSize(size);
+  FreeListElement* element = FreeListElement::AsElement(addr, size);
+  EnqueueElement(element, index);
+}
+
+
+void FreeList::Reset() {
+  for (int i = 0; i < (kNumLists + 1); i++) {
+    free_lists_[i] = NULL;
+  }
+}
+
+intptr_t FreeList::IndexForSize(intptr_t size) {
+  ASSERT(size >= kObjectAlignment);
+  ASSERT(Utils::IsAligned(size, kObjectAlignment));
+
+  intptr_t index = size / kObjectAlignment;
+  if (index >= kNumLists) {
+    index = kNumLists;
+  }
+  return index;
+}
+
+
+void FreeList::EnqueueElement(FreeListElement* element, intptr_t index) {
+  element->set_next(free_lists_[index]);
+  free_lists_[index] = element;
+}
+
+
+FreeListElement* FreeList::DequeueElement(intptr_t index) {
+  FreeListElement* result = free_lists_[index];
+  free_lists_[index] = result->next();
+  return result;
+}
+
+
+void FreeList::SplitElementAfterAndEnqueue(FreeListElement* element,
+                                           intptr_t size) {
+  intptr_t remainder_size = element->Size() - size;
+  if (remainder_size == 0) return;
+
+  element = FreeListElement::AsElement(reinterpret_cast<uword>(element) + size,
+                                       remainder_size);
+  intptr_t remainder_index = IndexForSize(remainder_size);
+  EnqueueElement(element, remainder_index);
+}
+
+}  // namespace dart