Internalize strings in-place

src/heap/scavenger.cc

Index: src/heap/scavenger.cc
diff --git a/src/heap/scavenger.cc b/src/heap/scavenger.cc
index aea41dd552ae06151a6c21ddc3e23cbed346f72c..1173bb60c6027dd59a25dd5efc13be71abf08eda 100644
--- a/src/heap/scavenger.cc
+++ b/src/heap/scavenger.cc
@@ -30,6 +30,7 @@ class ScavengingVisitor : public StaticVisitorBase {
     table_.Register(kVisitSeqOneByteString, &EvacuateSeqOneByteString);
     table_.Register(kVisitSeqTwoByteString, &EvacuateSeqTwoByteString);
     table_.Register(kVisitShortcutCandidate, &EvacuateShortcutCandidate);
+    table_.Register(kVisitThinString, &EvacuateThinString);
     table_.Register(kVisitByteArray, &EvacuateByteArray);
     table_.Register(kVisitFixedArray, &EvacuateFixedArray);
     table_.Register(kVisitFixedDoubleArray, &EvacuateFixedDoubleArray);
@@ -89,6 +90,12 @@ class ScavengingVisitor : public StaticVisitorBase {
     return &table_;
   }
 
+  static void EvacuateThinStringNoShortcut(Map* map, HeapObject** slot,
+                                           HeapObject* object) {
+    EvacuateObject<POINTER_OBJECT, kWordAligned>(map, slot, object,
+                                                 ThinString::kSize);
+  }
+
  private:
   enum ObjectContents { DATA_OBJECT, POINTER_OBJECT };
 
@@ -335,6 +342,22 @@ class ScavengingVisitor : public StaticVisitorBase {
                                                  object_size);
   }
 
+  static inline void EvacuateThinString(Map* map, HeapObject** slot,
+                                        HeapObject* object) {
+    if (marks_handling == IGNORE_MARKS) {
+      HeapObject* actual = ThinString::cast(object)->actual();
+      *slot = actual;
+      // ThinStrings always refer to internalized strings, which are
+      // always in old space.
+      DCHECK(!map->GetHeap()->InNewSpace(actual));
+      object->set_map_word(MapWord::FromForwardingAddress(actual));
+      return;
+    }
+
+    EvacuateObject<POINTER_OBJECT, kWordAligned>(map, slot, object,
+                                                 ThinString::kSize);
+  }
+
   template <ObjectContents object_contents>
   class ObjectEvacuationStrategy {
    public:
@@ -419,6 +442,10 @@ void Scavenger::SelectScavengingVisitorsTable() {
           StaticVisitorBase::kVisitShortcutCandidate,
           scavenging_visitors_table_.GetVisitorById(
               StaticVisitorBase::kVisitConsString));
+      scavenging_visitors_table_.Register(

[Hannes Payer (out of office), 2016/12/21 09:53:14]

Instead of using a special visitor when compacting, we could just record the
slot when updating the slot with the actual string (above).

[Jakob Kummerow, 2017/01/04 12:45:06]

I don't understand what you mean here, can you please clarify?
(I realize that the patch as-is doesn't make a lot of sense, as in particular
the ConsString corner case being addressed here doesn't apply to ThinStrings,
but I don't understand what alternative precisely you have in mind.)

[Jakob Kummerow, 2017/01/09 17:42:31]

As discussed offline, this appears to be much harder than it sounds, so since
the Scavenger will likely get replaced by the MinorMC anyway, we're not going to
do it for now.

+          StaticVisitorBase::kVisitThinString,
+          &ScavengingVisitor<TRANSFER_MARKS, LOGGING_AND_PROFILING_DISABLED>::
+              EvacuateThinStringNoShortcut);
     }
   }
 }