Fast allocation of block contexts.

src/full-codegen.cc

Index: src/full-codegen.cc
diff --git a/src/full-codegen.cc b/src/full-codegen.cc
index 8aa763405aba574ef234f31999e8ae694f295382..96e0e8b200a55991cb2179a092f6e9286ce331e6 100644
--- a/src/full-codegen.cc
+++ b/src/full-codegen.cc
@@ -818,9 +818,19 @@ void FullCodeGenerator::VisitBlock(Block* stmt) {
   if (stmt->block_scope() != NULL) {
     { Comment cmnt(masm_, "[ Extend block context");
       scope_ = stmt->block_scope();
-      __ Push(scope_->GetSerializedScopeInfo());
+      Handle<SerializedScopeInfo> scope_info = scope_->GetSerializedScopeInfo();
+      int heap_slots =
+          scope_info->NumberOfContextSlots() - Context::MIN_CONTEXT_SLOTS;
+      __ Push(scope_info);
       PushFunctionArgumentForContextAllocation(result_register());
-      __ CallRuntime(Runtime::kPushBlockContext, 2);
+      if (heap_slots <= FastNewBlockContextStub::kMaximumSlots) {
+        FastNewBlockContextStub stub(heap_slots);
+        __ CallStub(&stub);
+      } else {
+        __ CallRuntime(Runtime::kPushBlockContext, 2);
+      }
+
+      // Replace the context stored in the frame.
       StoreToFrameField(StandardFrameConstants::kContextOffset,
                         context_register());
     }