Only swap undefined for the global object if necessary in the prologue

src/compiler.h

 // Copyright 2012 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.
 
 #ifndef V8_COMPILER_H_
 #define V8_COMPILER_H_
 
 #include "src/allocation.h"
 #include "src/ast.h"
 #include "src/bailout-reason.h"
@@ skipping 130 matching lines @@
   // TODO(titzer): inline and delete accessors of ParseInfo
   // -----------------------------------------------------------
   Handle<Script> script() const;
   bool is_eval() const;
   bool is_native() const;
   bool is_module() const;
   LanguageMode language_mode() const;
   Handle<JSFunction> closure() const;
   FunctionLiteral* function() const;
   Scope* scope() const;
+  bool MayUseThis() const;
   Handle<Context> context() const;
   Handle<SharedFunctionInfo> shared_info() const;
   bool has_shared_info() const;
   // -----------------------------------------------------------
 
   Isolate* isolate() const {
     return isolate_;
   }
   Zone* zone() { return zone_; }
   bool is_osr() const { return !osr_ast_id_.IsNone(); }
@@ skipping 513 matching lines @@
   Zone zone_;
   size_t info_zone_start_allocation_size_;
   base::ElapsedTimer timer_;
 
   DISALLOW_COPY_AND_ASSIGN(CompilationPhase);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_COMPILER_H_