- Implement the old sweeper.

vm/gc_sweeper.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/gc_sweeper.h"
+
+#include "vm/freelist.h"
+#include "vm/globals.h"
+#include "vm/pages.h"
+#include "vm/object.h"

[siva, 2011/12/14 16:20:13]

list vm/object.h before vm/pages.h

[Ivan Posva, 2011/12/15 22:29:19]

object.h was only needed for testing. Removed.

+
+namespace dart {
+
+intptr_t GCSweeper::SweepPage(HeapPage* page, FreeList* freelist) {
+  // Keep track of the discovered live object sizes to be able to finish
+  // sweeping early. Reset the per page in_use count for the next marking phase.
+  intptr_t in_use_swept = 0;
+  intptr_t in_use = page->used();

[siva, 2011/12/14 16:20:13]

ASERT(in_use != 0); ?

[Ivan Posva, 2011/12/15 22:29:19]

That would be an incorrect ASSERT for totally empty pages. Once we do not even
attempt to sweep entirely empty pages this assert would make sense.

+  page->set_used(0);
+
+  uword current = page->first_object_start();
+  uword top = page->top();
+
+  while (current < top) {
+    if (in_use_swept == in_use) {
+      // No more marked objects will be found on this page.
+      page->set_top(current);
+      break;
+    }
+    RawObject* raw_obj = RawObject::FromAddr(current);
+    intptr_t obj_size;
+    if (raw_obj->IsMarked()) {
+      // Found marked object. Clear the mark bit and update swept bytes.
+      raw_obj->ClearMarkBit();
+      obj_size = raw_obj->Size();
+      in_use_swept += obj_size;
+    } else {
+      uword free_end = current + raw_obj->Size();

[cshapiro, 2011/12/14 18:48:11]

How does this interact with FreeList elements?

FreeList objects will not be marked so, if I am following this correctly, if you
sweep a page with a FreeList element between two ordinary objects, you may reach
this case.

Either way, a one-liner comment describing the free storage policy would be
great.

If that is the case, it looks like a FreeList element could be coalesced an the
FreeList would be corrupted.

[Ivan Posva, 2011/12/15 22:29:19]

FreeListElements are coalesced and added to the correct free list. Free lists
are entirely emptied before the sweep cycle.

+      while (free_end < top) {
+        RawObject* next_obj = RawObject::FromAddr(free_end);
+        if (next_obj->IsMarked()) {
+          // Reached the end of the free block.
+          break;
+        }
+        // Expand the free block by the size of this object.
+        free_end += next_obj->Size();
+      }
+      obj_size = free_end - current;
+      if ((current + obj_size) == top) {
+        page->set_top(current);
+        break;
+      } else {
+        freelist->Free(current, obj_size);
+      }
+    }
+    current += obj_size;
+  }
+
+  return in_use_swept;
+}
+
+
+intptr_t GCSweeper::SweepLargePage(HeapPage* page) {
+  UNIMPLEMENTED();
+  return 0;
+}
+
+}  // namespace dart