[vm] Compute per-HeapPage usage during sweeping.

runtime/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/heap.h"
 #include "vm/lockers.h"
 #include "vm/pages.h"
 #include "vm/safepoint.h"
 #include "vm/thread_pool.h"
 #include "vm/timeline.h"
 
 namespace dart {
 
 bool GCSweeper::SweepPage(HeapPage* page, FreeList* freelist, bool locked) {
   if (page->is_image_page()) {
     // Don't clear mark bits.
     return true;
   }
 
   // Keep track whether this page is still in use.
-  bool in_use = false;
+  intptr_t used_in_bytes = 0;
 
   bool is_executable = (page->type() == HeapPage::kExecutable);
   uword start = page->object_start();
   uword end = page->object_end();
   uword current = start;
 
   while (current < end) {
     intptr_t obj_size;
     RawObject* raw_obj = RawObject::FromAddr(current);
     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 = true;
+      used_in_bytes += obj_size;
     } else {
       uword free_end = current + raw_obj->Size();
       while (free_end < end) {
         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();
@@ skipping 12 matching lines @@
           freelist->FreeLocked(current, obj_size);
         } else {
           freelist->Free(current, obj_size);
         }
       }
     }
     current += obj_size;
   }
   ASSERT(current == end);
 
-  return in_use;
+  page->set_used_in_bytes(used_in_bytes);
+  return used_in_bytes != 0;  // In use.
 }
 
 intptr_t GCSweeper::SweepLargePage(HeapPage* page) {
   intptr_t words_to_end = 0;
   RawObject* raw_obj = RawObject::FromAddr(page->object_start());
   if (raw_obj->IsMarked()) {
     raw_obj->ClearMarkBit();
     words_to_end = (raw_obj->Size() >> kWordSizeLog2);
   }
 #ifdef DEBUG
@@ skipping 87 matching lines @@
                                 HeapPage* first,
                                 HeapPage* last,
                                 FreeList* freelist) {
   SweeperTask* task = new SweeperTask(isolate, isolate->heap()->old_space(),
                                       first, last, freelist);
   ThreadPool* pool = Dart::thread_pool();
   pool->Run(task);
 }
 
 }  // namespace dart