Experimental: this is a substantial change to allow the virtual frame...

src/register-allocator-ia32.h

 // Copyright 2008 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 23 matching lines @@
 
 
 // -------------------------------------------------------------------------
 // Results
 //
 // Results encapsulate the compile-time values manipulated by the code
 // generator.  They can represent registers or constants.
 
 class Result BASE_EMBEDDED {
  public:
+  enum Type {
+    INVALID,
+    REGISTER,
+    CONSTANT
+  };
+
   // Construct an invalid result.
   explicit Result(CodeGenerator* cgen) : type_(INVALID), cgen_(cgen) {}
 
   // Construct a register Result.
   Result(Register reg, CodeGenerator* cgen);
 
   // Construct a Result whose value is a compile-time constant.
   Result(Handle<Object> value, CodeGenerator * cgen)
       : type_(CONSTANT),
         cgen_(cgen) {
     data_.handle_ = value.location();
   }
 
   // The copy constructor and assignment operators could each create a new
   // register reference.
   Result(const Result& other) {
     other.CopyTo(this);
   }
 
-  Result& operator=(Result& other) {
+  Result& operator=(const Result& other) {
     if (this != &other) {
       Unuse();
       other.CopyTo(this);
-      other.Unuse();
+      // other.Unuse();
     }
     return *this;
   }
 
   ~Result() { Unuse(); }
 
   void Unuse();
 
+  Type type() const { return type_; }
+
   bool is_valid() const { return type() != INVALID; }
   bool is_register() const { return type() == REGISTER; }
   bool is_constant() const { return type() == CONSTANT; }
 
   Register reg() const {
     ASSERT(type() == REGISTER);
     return data_.reg_;
   }
 
   Handle<Object> handle() const {
     ASSERT(type() == CONSTANT);
     return Handle<Object>(data_.handle_);
   }
 
-  // Change a result to a register result.  If the result is not already
-  // in a register, allocate a register from the code generator, and emit
-  // code to move the value into that register.
+  // Move this result to an arbitrary register.  The register is not
+  // necessarily spilled from the frame or even singly-referenced outside
+  // it.

[William Hesse, 2008/12/22 13:55:21]

How can we move a result into a register that isn't spilled? Are you referring
to the case that the result is already in a register?  Then it isn't being
moved.  Otherwise, aren't we messing up the value already in the register?

[Kevin Millikin (Chromium), 2008/12/22 14:40:00]

Yes, the comment refers to the case where the result is in a register.  In that
case, the current register is as arbitrary as any other.

   void ToRegister();
 
+  // Move this result to a specified register.  The register is spilled from
+  // the frame, and the register is singly-referenced (by this result)
+  // outside the frame.
+  void ToRegister(Register reg);
+
  private:
-  enum Type {
-    INVALID,
-    REGISTER,
-    CONSTANT
-  };
-
   Type type_;
 
   union {
     Register reg_;
     Object** handle_;
   } data_;
 
   CodeGenerator* cgen_;
 
-  Type type() const { return type_; }
-
   void CopyTo(Result* destination) const;
 };
 
 
 // -------------------------------------------------------------------------
 // Register file
 //
 // The register file tracks reference counts for the processor registers.
 // It is used by both the register allocator and the virtual frame.
 
 class RegisterFile BASE_EMBEDDED {
  public:
   RegisterFile() { Reset(); }
 
   void Reset() {
     for (int i = 0; i < kNumRegisters; i++) {
       ref_counts_[i] = 0;
     }
   }
 
-  // Predicates and accessors for the reference counts.  They take a
-  // register code rather than a register because they are frequently used
-  // in a loop over the register codes.
+  // Predicates and accessors for the reference counts.  The versions
+  // that take a register code rather than a register are for
+  // convenience in loops over the register codes.
   bool is_used(int reg_code) const { return ref_counts_[reg_code] > 0; }
+  bool is_used(Register reg) const { return is_used(reg.code()); }
   int count(int reg_code) const { return ref_counts_[reg_code]; }
+  int count(Register reg) const { return count(reg.code()); }
 
   // Record a use of a register by incrementing its reference count.
   void Use(Register reg) {
     ref_counts_[reg.code()]++;
   }
 
   // Record that a register will no longer be used by decrementing its
   // reference count.
   void Unuse(Register reg) {
     ASSERT(is_used(reg.code()));
@@ skipping 14 matching lines @@
 //
 
 class RegisterAllocator BASE_EMBEDDED {
  public:
   explicit RegisterAllocator(CodeGenerator* cgen) : cgen_(cgen) {}
 
   int num_registers() const { return RegisterFile::kNumRegisters; }
 
   // Predicates and accessors for the registers' reference counts.
   bool is_used(int reg_code) const { return registers_.is_used(reg_code); }
-  int count(int reg_code) { return registers_.count(reg_code); }
+  bool is_used(Register reg) const { return registers_.is_used(reg.code()); }
+  int count(int reg_code) const { return registers_.count(reg_code); }
+  int count(Register reg) const { return registers_.count(reg.code()); }
 
   // Explicitly record a reference to a register.
   void Use(Register reg) { registers_.Use(reg); }
 
   // Explicitly record that a register will no longer be used.
   void Unuse(Register reg) { registers_.Unuse(reg); }
 
   // Initialize the register allocator for entry to a JS function.  On
   // entry, esp, ebp, esi, and edi are externally referenced (ie, outside
   // the virtual frame); and the other registers are free.
   void Initialize();
 
   // Allocate a free register and return a register result if possible or
   // fail and return an invalid result.
   Result Allocate();
 
+  // Allocate a specific register if possible, spilling it from the frame if
+  // necessary, or else fail and regutrn an invalid result.

[William Hesse, 2008/12/22 13:55:21]

and return an invalid result.

+  Result Allocate(Register target);
+
   // Allocate a free register without spilling any from the current frame or
   // fail and return an invalid result.
   Result AllocateWithoutSpilling();
 
  private:
   CodeGenerator* cgen_;
   RegisterFile registers_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_REGISTER_ALLOCATOR_IA32_H_