Better pack fields in Variable

src/ast/variables.h

 // Copyright 2011 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_AST_VARIABLES_H_
 #define V8_AST_VARIABLES_H_
 
 #include "src/ast/ast-value-factory.h"
 #include "src/zone.h"
 
 namespace v8 {
 namespace internal {
 
 // The AST refers to variables via VariableProxies - placeholders for the actual
 // variables. Variables themselves are never directly referred to from the AST,
 // they are maintained by scopes, and referred to from VariableProxies and Slots
 // after binding and variable allocation.
 class Variable final : public ZoneObject {
  public:
-  enum Kind { NORMAL, FUNCTION, THIS, ARGUMENTS };
+  enum Kind : uint8_t {
+    NORMAL,
+    FUNCTION,
+    THIS,
+    ARGUMENTS,
+    kLastKind = ARGUMENTS
+  };
 
   Variable(Scope* scope, const AstRawString* name, VariableMode mode, Kind kind,
            InitializationFlag initialization_flag,
            MaybeAssignedFlag maybe_assigned_flag = kNotAssigned);
 
   // Printing support
   static const char* Mode2String(VariableMode mode);
 
   // The source code for an eval() call may refer to a variable that is
   // in an outer scope about which we don't know anything (it may not
@@ skipping 67 matching lines @@
   void AllocateTo(VariableLocation location, int index) {
     location_ = location;
     index_ = index;
   }
 
   static int CompareIndex(Variable* const* v, Variable* const* w);
 
  private:
   Scope* scope_;
   const AstRawString* name_;
-  VariableMode mode_;
-  Kind kind_;
-  VariableLocation location_;
-  int index_;
-  int initializer_position_;
-
   // If this field is set, this variable references the stored locally bound
   // variable, but it might be shadowed by variable bindings introduced by
   // sloppy 'eval' calls between the reference scope (inclusive) and the
   // binding scope (exclusive).
   Variable* local_if_not_shadowed_;
+  int index_;
+  int initializer_position_;
+
+  STATIC_ASSERT(kLastVariableMode < (1 << 3));
+  VariableMode mode_ : 3;
+  STATIC_ASSERT(kLastKind < (1 << 2));
+  Kind kind_ : 2;
+  STATIC_ASSERT(static_cast<uint8_t>(VariableLocation::kLastVariableLocation) <
+                (1 << 3));
+  VariableLocation location_ : 3;
 
   // Usage info.
-  bool force_context_allocation_;  // set by variable resolver
-  bool is_used_;
-  InitializationFlag initialization_flag_;
-  MaybeAssignedFlag maybe_assigned_;
+  bool force_context_allocation_ : 1;  // set by variable resolver
+  bool is_used_ : 1;
+  InitializationFlag initialization_flag_ : 2;
+  MaybeAssignedFlag maybe_assigned_ : 2;
 };
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_VARIABLES_H_