Remove a race between sweepers and the free space skipping code (while iterating new space pointers)

src/store-buffer.cc

Index: src/store-buffer.cc
diff --git a/src/store-buffer.cc b/src/store-buffer.cc
index b0f7d2f80309a67fe35faeb7d39d8d6e0362abed..547359a09d50d8a33c70b73fdb3e3b3085480572 100644
--- a/src/store-buffer.cc
+++ b/src/store-buffer.cc
@@ -332,27 +332,6 @@ void StoreBuffer::GCPrologue() {
 
 
 #ifdef VERIFY_HEAP
-static void DummyScavengePointer(HeapObject** p, HeapObject* o) {
-  // Do nothing.
-}
-
-
-void StoreBuffer::VerifyPointers(PagedSpace* space,
-                                 RegionCallback region_callback) {
-  PageIterator it(space);
-
-  while (it.has_next()) {
-    Page* page = it.next();
-    FindPointersToNewSpaceOnPage(
-        reinterpret_cast<PagedSpace*>(page->owner()),
-        page,
-        region_callback,
-        &DummyScavengePointer,
-        false);
-  }
-}
-
-
 void StoreBuffer::VerifyPointers(LargeObjectSpace* space) {
   LargeObjectIterator it(space);
   for (HeapObject* object = it.Next(); object != NULL; object = it.Next()) {
@@ -378,10 +357,6 @@ void StoreBuffer::VerifyPointers(LargeObjectSpace* space) {
 
 void StoreBuffer::Verify() {
 #ifdef VERIFY_HEAP
-  VerifyPointers(heap_->old_pointer_space(),
-                 &StoreBuffer::FindPointersToNewSpaceInRegion);
-  VerifyPointers(heap_->map_space(),
-                 &StoreBuffer::FindPointersToNewSpaceInMapsRegion);
   VerifyPointers(heap_->lo_space());
 #endif
 }
@@ -482,94 +457,6 @@ void StoreBuffer::FindPointersToNewSpaceInMapsRegion(
 }
 
 
-// This function iterates over all the pointers in a paged space in the heap,
-// looking for pointers into new space.  Within the pages there may be dead
-// objects that have not been overwritten by free spaces or fillers because of
-// concurrent sweeping.  These dead objects may not contain pointers to new
-// space. The garbage areas that have been swept properly (these will normally
-// be the large ones) will be marked with free space and filler map words.  In
-// addition any area that has never been used at all for object allocation must
-// be marked with a free space or filler.  Because the free space and filler
-// maps do not move we can always recognize these even after a compaction.
-// Normal objects like FixedArrays and JSObjects should not contain references
-// to these maps.  Constant pool array objects may contain references to these
-// maps, however, constant pool arrays cannot contain pointers to new space
-// objects, therefore they are skipped.  The special garbage section (see
-// comment in spaces.h) is skipped since it can contain absolutely anything.
-// Any objects that are allocated during iteration may or may not be visited by
-// the iteration, but they will not be partially visited.
-void StoreBuffer::FindPointersToNewSpaceOnPage(
-    PagedSpace* space,
-    Page* page,
-    RegionCallback region_callback,
-    ObjectSlotCallback slot_callback,
-    bool clear_maps) {
-  Address visitable_start = page->area_start();
-  Address end_of_page = page->area_end();
-
-  Address visitable_end = visitable_start;
-
-  Object* free_space_map = heap_->free_space_map();
-  Object* two_pointer_filler_map = heap_->two_pointer_filler_map();
-  Object* constant_pool_array_map = heap_->constant_pool_array_map();
-
-  while (visitable_end < end_of_page) {
-    // The sweeper thread concurrently may write free space maps and size to
-    // this page. We need acquire load here to make sure that we get a
-    // consistent view of maps and their sizes.
-    Object* o = reinterpret_cast<Object*>(
-        Acquire_Load(reinterpret_cast<AtomicWord*>(visitable_end)));
-    // Skip fillers or constant pool arrays (which never contain new-space
-    // pointers but can contain pointers which can be confused for fillers)
-    // but not things that look like fillers in the special garbage section
-    // which can contain anything.
-    if (o == free_space_map ||
-        o == two_pointer_filler_map ||
-        o == constant_pool_array_map ||
-        (visitable_end == space->top() && visitable_end != space->limit())) {
-      if (visitable_start != visitable_end) {
-        // After calling this the special garbage section may have moved.
-        (this->*region_callback)(visitable_start,
-                                 visitable_end,
-                                 slot_callback,
-                                 clear_maps);
-        if (visitable_end >= space->top() && visitable_end < space->limit()) {
-          visitable_end = space->limit();
-          visitable_start = visitable_end;
-          continue;
-        }
-      }
-      if (visitable_end == space->top() && visitable_end != space->limit()) {
-        visitable_start = visitable_end = space->limit();
-      } else {
-        // At this point we are either at the start of a filler, a
-        // constant pool array, or we are at the point where the space->top()
-        // used to be before the visit_pointer_region call above.  Either way we
-        // can skip the object at the current spot:  We don't promise to visit
-        // objects allocated during heap traversal, and if space->top() moved
-        // then it must be because an object was allocated at this point.
-        visitable_start =
-            visitable_end + HeapObject::FromAddress(visitable_end)->Size();
-        visitable_end = visitable_start;
-      }
-    } else {
-      ASSERT(o != free_space_map);
-      ASSERT(o != two_pointer_filler_map);
-      ASSERT(o != constant_pool_array_map);
-      ASSERT(visitable_end < space->top() || visitable_end >= space->limit());
-      visitable_end += kPointerSize;
-    }
-  }
-  ASSERT(visitable_end == end_of_page);
-  if (visitable_start != visitable_end) {
-    (this->*region_callback)(visitable_start,
-                             visitable_end,
-                             slot_callback,
-                             clear_maps);
-  }
-}
-
-
 void StoreBuffer::IteratePointersInStoreBuffer(
     ObjectSlotCallback slot_callback,
     bool clear_maps) {
@@ -656,14 +543,15 @@ void StoreBuffer::IteratePointersToNewSpace(ObjectSlotCallback slot_callback,
         } else {
           Page* page = reinterpret_cast<Page*>(chunk);
           PagedSpace* owner = reinterpret_cast<PagedSpace*>(page->owner());
-          FindPointersToNewSpaceOnPage(
-              owner,
-              page,
-              (owner == heap_->map_space() ?
-                 &StoreBuffer::FindPointersToNewSpaceInMapsRegion :
-                 &StoreBuffer::FindPointersToNewSpaceInRegion),
-              slot_callback,
-              clear_maps);
+          Address start = page->area_start();
+          Address end = page->area_end();
+          if (owner == heap_->map_space()) {
+            FindPointersToNewSpaceInMapsRegion(
+                start, end, slot_callback, clear_maps);
+          } else {
+            FindPointersToNewSpaceInRegion(
+                start, end, slot_callback, clear_maps);
+          }
         }
       }
     }