Incorporate constness into inferred interfaces

src/interface.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 18 matching lines @@
 #define V8_INTERFACE_H_
 
 #include "zone-inl.h"  // For operator new.
 
 namespace v8 {
 namespace internal {
 
 
 // This class implements the following abstract grammar of interfaces
 // (i.e. module types):
-//   interface ::= UNDETERMINED | VALUE | MODULE(exports)
+//   interface ::= UNDETERMINED | VALUE | CONST | MODULE(exports)
 //   exports ::= {name : interface, ...}
-// A frozen module type is one that is fully determined. Unification does not
-// allow adding additional exports to frozen interfaces.
-// Otherwise, unifying modules merges their exports.
+// A frozen type is one that is fully determined. Unification does not
+// allow to turn non-const values into const, or adding additional exports to
+// frozen interfaces. Otherwise, unifying modules merges their exports.
 // Undetermined types are unification variables that can be unified freely.
+// There is a natural subsort lattice that reflects the increase of knowledge:
+//
+//            undetermined
+//           //     |    \\                                                    .
+//       value  (frozen)  module
+//      //   \\  /    \  //
+//  const   fr.value  fr.module
+//      \\    /
+//     fr.const
+//
+// where the bold lines are the only transitions allowed.
 
 class Interface : public ZoneObject {
  public:
   // ---------------------------------------------------------------------------
   // Factory methods.
 
+  static Interface* NewUnknown(Zone* zone) {
+    return new(zone) Interface(NONE);
+  }
+
   static Interface* NewValue() {
     static Interface value_interface(VALUE + FROZEN);  // Cached.
     return &value_interface;
   }
 
-  static Interface* NewUnknown(Zone* zone) {
-    return new(zone) Interface(NONE);
+  static Interface* NewConst() {
+    static Interface value_interface(VALUE + CONST + FROZEN);  // Cached.
+    return &value_interface;
   }
 
   static Interface* NewModule(Zone* zone) {
     return new(zone) Interface(MODULE);
   }
 
   // ---------------------------------------------------------------------------
   // Mutators.
 
   // Add a name to the list of exports. If it already exists, unify with
   // interface, otherwise insert unless this is closed.
   void Add(Handle<String> name, Interface* interface, Zone* zone, bool* ok) {
     DoAdd(name.location(), name->Hash(), interface, zone, ok);
   }
 
   // Unify with another interface. If successful, both interface objects will
   // represent the same type, and changes to one are reflected in the other.
   void Unify(Interface* that, Zone* zone, bool* ok);
 
   // Determine this interface to be a value interface.
   void MakeValue(bool* ok) {
     *ok = !IsModule();
     if (*ok) Chase()->flags_ |= VALUE;
   }
 
+  // Determine this interface to be an immutable interface.
+  void MakeConst(bool* ok) {
+    *ok = !IsModule() && (IsConst() || !IsFrozen());
+    if (*ok) Chase()->flags_ |= VALUE + CONST;
+  }
+
   // Determine this interface to be a module interface.
   void MakeModule(bool* ok) {
     *ok = !IsValue();
     if (*ok) Chase()->flags_ |= MODULE;
   }
 
   // Set associated instance object.
   void MakeSingleton(Handle<JSModule> instance, bool* ok) {
     *ok = IsModule() && Chase()->instance_.is_null();
     if (*ok) Chase()->instance_ = instance;
   }
 
   // Do not allow any further refinements, directly or through unification.
   void Freeze(bool* ok) {
     *ok = IsValue() || IsModule();
     if (*ok) Chase()->flags_ |= FROZEN;
   }
 
   // ---------------------------------------------------------------------------
   // Accessors.
 
   // Check whether this is still a fully undetermined type.
   bool IsUnknown() { return Chase()->flags_ == NONE; }
 
   // Check whether this is a value type.
   bool IsValue() { return Chase()->flags_ & VALUE; }
 
+  // Check whether this is a constant type.
+  bool IsConst() { return Chase()->flags_ & CONST; }
+
   // Check whether this is a module type.
   bool IsModule() { return Chase()->flags_ & MODULE; }
 
   // Check whether this is closed (i.e. fully determined).
   bool IsFrozen() { return Chase()->flags_ & FROZEN; }
 
   Handle<JSModule> Instance() { return Chase()->instance_; }
 
   // Look up an exported name. Returns NULL if not (yet) defined.
   Interface* Lookup(Handle<String> name, Zone* zone);
@@ skipping 34 matching lines @@
 #ifdef DEBUG
   void Print(int n = 0);  // n = indentation; n < 0 => don't print recursively
 #endif
 
   // ---------------------------------------------------------------------------
   // Implementation.
  private:
   enum Flags {    // All flags are monotonic
     NONE = 0,
     VALUE = 1,    // This type describes a value
-    MODULE = 2,   // This type describes a module
-    FROZEN = 4    // This type is fully determined
+    CONST = 2,    // This type describes a constant
+    MODULE = 4,   // This type describes a module
+    FROZEN = 8    // This type is fully determined
   };
 
   int flags_;
   Interface* forward_;     // Unification link
   ZoneHashMap* exports_;   // Module exports and their types (allocated lazily)
   Handle<JSModule> instance_;
 
   explicit Interface(int flags)
     : flags_(flags),
       forward_(NULL),
@@ skipping 12 matching lines @@
   }
 
   void DoAdd(void* name, uint32_t hash, Interface* interface, Zone* zone,
              bool* ok);
   void DoUnify(Interface* that, bool* ok, Zone* zone);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_INTERFACE_H_