Do deferred migration of maps after deoptimizing once.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 1423c726593c6e88dcfbeccadb1fcf604b977d17..5d71766a96d5c0d50cade72096ec77276c374730 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -2823,7 +2823,9 @@ class HCheckMaps V8_FINAL : public HTemplateInstruction<2> {
  private:
   void Add(Handle<Map> map, Zone* zone) {
     map_set_.Add(Unique<Map>(map), zone);
-    if (!has_migration_target_ && map->is_migration_target()) {
+    if (!has_migration_target_ &&
+        map->is_migration_target() &&
+        map->has_migrated()) {

[Toon Verwaest, 2013/11/20 10:45:54]

The map cannot both be a migration target and have migrated. Currently this
disables the feature entirely. I think what you want is limit the scope in which
"is_migration_target" is set to true; eg only if the map was ever used in a
migration.

       has_migration_target_ = true;
       SetGVNFlag(kChangesNewSpacePromotion);
     }