[heap] Iterate handles with special left-trim visitor

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 1a768fbc920ec6116bda538e741d205ce41744ae..bf9e0b403e39a10b1064a726b799d3b3d53634c7 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4755,6 +4755,49 @@ void Heap::IterateSmiRoots(ObjectVisitor* v) {
   v->Synchronize(VisitorSynchronization::kSmiRootList);
 }
 
+// We cannot avoid stale handles to left-trimmed objects, but can only make
+// sure all handles still needed are updated. Filter out a stale pointer
+// and clear the slot to allow post processing of handles (needed because
+// the sweeper might actually free the underlying page).
+class FixStaleLeftTrimmedHandlesVisitor : public ObjectVisitor {
+ public:
+  explicit FixStaleLeftTrimmedHandlesVisitor(Heap* heap) : heap_(heap) {
+    USE(heap_);
+  }
+
+  void VisitPointer(Object** p) override { FixHandle(p); }
+
+  void VisitPointers(Object** start, Object** end) override {
+    for (Object** p = start; p < end; p++) FixHandle(p);
+  }
+
+ private:
+  inline void FixHandle(Object** p) {
+    HeapObject* current = reinterpret_cast<HeapObject*>(*p);
+    if (!current->IsHeapObject()) return;
+    const MapWord map_word = current->map_word();
+    if (!map_word.IsForwardingAddress() && current->IsFiller()) {
+#ifdef DEBUG
+      // We need to find a FixedArrayBase map after walking the fillers.
+      while (current->IsFiller()) {
+        Address next = reinterpret_cast<Address>(current);
+        if (current->map() == heap_->one_pointer_filler_map()) {
+          next += kPointerSize;
+        } else if (current->map() == heap_->two_pointer_filler_map()) {
+          next += 2 * kPointerSize;
+        } else {
+          next += current->Size();
+        }
+        current = reinterpret_cast<HeapObject*>(next);
+      }
+      DCHECK(current->IsFixedArrayBase());
+#endif  // DEBUG
+      *p = nullptr;
+    }
+  }
+
+  Heap* heap_;
+};
 
 void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->VisitPointers(&roots_[0], &roots_[kStrongRootListLength]);
@@ -4777,6 +4820,8 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->Synchronize(VisitorSynchronization::kCompilationCache);
 
   // Iterate over local handles in handle scopes.
+  FixStaleLeftTrimmedHandlesVisitor left_trim_visitor(this);
+  isolate_->handle_scope_implementer()->Iterate(&left_trim_visitor);
   isolate_->handle_scope_implementer()->Iterate(v);
   isolate_->IterateDeferredHandles(v);
   v->Synchronize(VisitorSynchronization::kHandleScope);