Wait for sweeper threads only if we have to.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index 300d19e0d6dbd6cb78bfd32e47e19b643a0356dd..3c9b34a4c58a29995523c9269927b7846bb95728 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -936,6 +936,7 @@ PagedSpace::PagedSpace(Heap* heap,
     : Space(heap, id, executable),
       free_list_(this),
       was_swept_conservatively_(false),
+      was_swept_concurrently_(false),
       unswept_free_bytes_(0),
       end_of_unswept_pages_(NULL) {
   if (id == CODE_SPACE) {
@@ -2546,8 +2547,8 @@ void PagedSpace::PrepareForMarkCompact() {
 
 
 intptr_t PagedSpace::SizeOfObjects() {
-  ASSERT(heap()->mark_compact_collector()->IsConcurrentSweepingInProgress() ||
-         (unswept_free_bytes_ == 0));
+  ASSERT(heap()->mark_compact_collector()->
+      IsConcurrentSweepingInProgress(this) || (unswept_free_bytes_ == 0));
   return Size() - unswept_free_bytes_ - (limit() - top());
 }
 
@@ -2582,7 +2583,7 @@ HeapObject* PagedSpace::WaitForSweeperThreadsAndRetryAllocation(
   MarkCompactCollector* collector = heap()->mark_compact_collector();
 
   // If sweeper threads are still running, wait for them.
-  if (collector->IsConcurrentSweepingInProgress()) {
+  if (collector->IsConcurrentSweepingInProgress(this)) {
     collector->WaitUntilSweepingCompleted();
 
     // After waiting for the sweeper threads, there may be new free-list
@@ -2598,7 +2599,7 @@ HeapObject* PagedSpace::SlowAllocateRaw(int size_in_bytes) {
 
   // If sweeper threads are active, try to re-fill the free-lists.
   MarkCompactCollector* collector = heap()->mark_compact_collector();
-  if (collector->IsConcurrentSweepingInProgress()) {
+  if (collector->IsConcurrentSweepingInProgress(this)) {
     collector->RefillFreeList(this);
 
     // Retry the free list allocation.