[heap] Fix Heap::EnsureFillerObjectAtTop for non-contiguous new space

src/heap/heap.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/heap/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/ast/scopeinfo.h"
 #include "src/base/bits.h"
@@ skipping 925 matching lines @@
     incremental_marking()->AdvanceIncrementalMarking(
         deadline,
         IncrementalMarking::StepActions(IncrementalMarking::GC_VIA_STACK_GUARD,
                                         IncrementalMarking::FORCE_MARKING,
                                         IncrementalMarking::FORCE_COMPLETION));
   }
 }
 
 
 void Heap::EnsureFillerObjectAtTop() {
-  // There may be an allocation memento behind every object in new space.
-  // If we evacuate a not full new space or if we are on the last page of
-  // the new space, then there may be uninitialized memory behind the top
-  // pointer of the new space page. We store a filler object there to
-  // identify the unused space.
-  Address from_top = new_space_.top();
-  // Check that from_top is inside its page (i.e., not at the end).
-  Address space_end = new_space_.ToSpaceEnd();
-  if (from_top < space_end) {
-    Page* page = Page::FromAddress(from_top);
-    if (page->Contains(from_top)) {
-      int remaining_in_page = static_cast<int>(page->area_end() - from_top);
-      CreateFillerObjectAt(from_top, remaining_in_page,
-                           ClearRecordedSlots::kNo);
-    }
+  // There may be an allocation memento behind objects in new space. Upon
+  // evacuation of a non-full new space (or if we are on the last page) there
+  // may be uninitialized memory behind top. We fill the remainder of the page
+  // with a filler.
+  Address to_top = new_space_.top();
+  NewSpacePage* page = NewSpacePage::FromAddress(to_top - kPointerSize);
+  if (page->Contains(to_top)) {
+    int remaining_in_page = static_cast<int>(page->area_end() - to_top);
+    CreateFillerObjectAt(to_top, remaining_in_page, ClearRecordedSlots::kNo);
   }
 }
 
 
 bool Heap::CollectGarbage(GarbageCollector collector, const char* gc_reason,
                           const char* collector_reason,
                           const v8::GCCallbackFlags gc_callback_flags) {
   // The VM is in the GC state until exiting this function.
   VMState<GC> state(isolate_);
 
@@ skipping 5466 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8