After moving unreachable weak global handles only process harmony collections

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 2060 matching lines @@
   EmptyMarkingDeque();
   while (marking_deque_.overflowed()) {
     RefillMarkingDeque();
     EmptyMarkingDeque();
   }
 }
 
 
 // Mark all objects reachable (transitively) from objects on the marking
 // stack including references only considered in the atomic marking pause.
-void MarkCompactCollector::ProcessEphemeralMarking(ObjectVisitor* visitor) {
+void MarkCompactCollector::ProcessEphemeralMarking(
+    ObjectVisitor* visitor, bool only_process_harmony_weak_collections) {
   bool work_to_do = true;
   DCHECK(marking_deque_.IsEmpty());
   while (work_to_do) {
-    isolate()->global_handles()->IterateObjectGroups(
-        visitor, &IsUnmarkedHeapObjectWithHeap);
-    MarkImplicitRefGroups();
+    if (!only_process_harmony_weak_collections) {
+      isolate()->global_handles()->IterateObjectGroups(
+          visitor, &IsUnmarkedHeapObjectWithHeap);
+      MarkImplicitRefGroups();
+    }
     ProcessWeakCollections();
     work_to_do = !marking_deque_.IsEmpty();
     ProcessMarkingDeque();
   }
 }
 
 
 void MarkCompactCollector::ProcessTopOptimizedFrame(ObjectVisitor* visitor) {
   for (StackFrameIterator it(isolate(), isolate()->thread_local_top());
        !it.done(); it.Advance()) {
@@ skipping 91 matching lines @@
   }
 
   RootMarkingVisitor root_visitor(heap());
   MarkRoots(&root_visitor);
 
   ProcessTopOptimizedFrame(&root_visitor);
 
   // The objects reachable from the roots are marked, yet unreachable
   // objects are unmarked.  Mark objects reachable due to host
   // application specific logic or through Harmony weak maps.
-  ProcessEphemeralMarking(&root_visitor);
+  ProcessEphemeralMarking(&root_visitor, false);
 
   // The objects reachable from the roots, weak maps or object groups
-  // are marked, yet unreachable objects are unmarked.  Mark objects
+  // are marked. Objects pointed to only by weak global handles cannot be
+  // immediately reclaimed. Instead, we have to mark them as pending and mark
+  // objects reachable from them.
   // reachable only from weak global handles.

[Michael Starzinger, 2014/12/02 10:22:42]

nit: This dangling part of the sentence looks like a left-over, can we drop it?

[jochen (gone - plz use gerrit), 2014/12/02 10:37:18]

done

   //
   // First we identify nonlive weak handles and mark them as pending
   // destruction.
   heap()->isolate()->global_handles()->IdentifyWeakHandles(
       &IsUnmarkedHeapObject);
-  // Then we mark the objects and process the transitive closure.
+  // Then we mark the objects.
   heap()->isolate()->global_handles()->IterateWeakRoots(&root_visitor);
-  while (marking_deque_.overflowed()) {
-    RefillMarkingDeque();
-    EmptyMarkingDeque();
-  }
 
-  // Repeat host application specific and Harmony weak maps marking to
-  // mark unmarked objects reachable from the weak roots.
-  ProcessEphemeralMarking(&root_visitor);
+  // Repeat Harmony weak maps marking to mark unmarked objects reachable from
+  // the weak roots we just marked as pending destruction.

[Michael Starzinger, 2014/12/02 10:22:42]

This requires more explanation as to why we can skip other embedder-specific
marking here, e.g. all objects groups have been fully processed and no weakly
reachable node can discover new object groups.

[jochen (gone - plz use gerrit), 2014/12/02 10:37:18]

done

+  ProcessEphemeralMarking(&root_visitor, true);
 
   AfterMarking();
 
   if (FLAG_print_cumulative_gc_stat) {
     heap_->tracer()->AddMarkingTime(base::OS::TimeCurrentMillis() - start_time);
   }
 }
 
 
 void MarkCompactCollector::AfterMarking() {
@@ skipping 2177 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal