deps: backport 7a88ff3 from V8 upstream

This backport does not include the changes to `src/heap/scavenger.cc`
as it does not exist in the V8 included in the v4.x stream.

Original commit message:
  Filter out stale left-trimmed handles for scavenges

  The missing part from
    https://codereview.chromium.org/2078403002/

  R=jochen@chromium.org
  BUG=chromium:621869
  LOG=N

  Review-Url: https://codereview.chromium.org/2077353004
  Cr-Commit-Position: refs/heads/master@{#37184}

PR-URL: #10668
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>

Author: MylesBorins

deps/v8/src/heap/heap-inl.h

@@ -393,12 +393,35 @@ bool Heap::AllowedToBeMigrated(HeapObject* obj, AllocationSpace dst) {
   return false;
 }
 
-
 void Heap::CopyBlock(Address dst, Address src, int byte_size) {
   CopyWords(reinterpret_cast<Object**>(dst), reinterpret_cast<Object**>(src),
             static_cast<size_t>(byte_size / kPointerSize));
 }
 
+bool Heap::PurgeLeftTrimmedObject(Object** object) {
+  HeapObject* current = reinterpret_cast<HeapObject*>(*object);
+  const MapWord map_word = current->map_word();
+  if (current->IsFiller() && !map_word.IsForwardingAddress()) {
+#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() == one_pointer_filler_map()) {
+        next += kPointerSize;
+      } else if (current->map() == two_pointer_filler_map()) {
+        next += 2 * kPointerSize;
+      } else {
+        next += current->Size();
+      }
+      current = reinterpret_cast<HeapObject*>(next);
+    }
+    DCHECK(current->IsFixedArrayBase());
+#endif  // DEBUG
+    *object = nullptr;
+    return true;
+  }
+  return false;
+}
 
 void Heap::MoveBlock(Address dst, Address src, int byte_size) {
   DCHECK(IsAligned(byte_size, kPointerSize));

deps/v8/src/heap/heap.h

@@ -590,6 +590,12 @@ class Heap {
   // jslimit_/real_jslimit_ variable in the StackGuard.
   void SetStackLimits();
 
+  // 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).
+  inline bool PurgeLeftTrimmedObject(Object** object);
+
   // Notifies the heap that is ok to start marking or other activities that
   // should not happen during deserialization.
   void NotifyDeserializationComplete();

deps/v8/src/heap/mark-compact.cc

@@ -1650,31 +1650,7 @@ class RootMarkingVisitor : public ObjectVisitor {
 
     HeapObject* object = ShortCircuitConsString(p);
 
-    // We cannot avoid stale handles to left-trimmed objects, but can only make
-    // sure all handles still needed are updated. Filter out any stale pointers
-    // and clear the slot to allow post processing of handles (needed because
-    // the sweeper might actually free the underlying page).
-    if (object->IsFiller()) {
-#ifdef DEBUG
-      // We need to find a FixedArrayBase map after walking the fillers.
-      Heap* heap = collector_->heap();
-      HeapObject* current = object;
-      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;
-      return;
-    }
+    if (collector_->heap()->PurgeLeftTrimmedObject(p)) return;
 
     MarkBit mark_bit = Marking::MarkBitFrom(object);
     if (Marking::IsBlackOrGrey(mark_bit)) return;

deps/v8/src/objects-inl.h

@@ -1351,7 +1351,7 @@ Map* MapWord::ToMap() {
 }
 
 
-bool MapWord::IsForwardingAddress() {
+bool MapWord::IsForwardingAddress() const {
   return HAS_SMI_TAG(reinterpret_cast<Object*>(value_));
 }
 

deps/v8/src/objects.h

@@ -1382,7 +1382,7 @@ class MapWord BASE_EMBEDDED {
   // True if this map word is a forwarding address for a scavenge
   // collection.  Only valid during a scavenge collection (specifically,
   // when all map words are heap object pointers, i.e. not during a full GC).
-  inline bool IsForwardingAddress();
+  inline bool IsForwardingAddress() const;
 
   // Create a map word from a forwarding address.
   static inline MapWord FromForwardingAddress(HeapObject* object);

deps/v8/test/mjsunit/regress/regress-621869.js

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+// Flags: --expose-gc
+
+var o0 = [];
+var o1 = [];
+var cnt = 0;
+var only_scavenge = true;
+o1.__defineGetter__(0, function() {
+  if (cnt++ > 2) return;
+  o0.shift();
+  gc(only_scavenge);
+  o0.push((64));
+  o0.concat(o1);
+});
+o1[0];