[Interpreter] Reduce temporary register usage in generated bytecode.

src/interpreter/bytecode-generator.h

 // Copyright 2015 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_INTERPRETER_BYTECODE_GENERATOR_H_
 #define V8_INTERPRETER_BYTECODE_GENERATOR_H_
 
 #include "src/ast.h"
 #include "src/interpreter/bytecode-array-builder.h"
 #include "src/interpreter/bytecodes.h"
@@ skipping 12 matching lines @@
 #define DECLARE_VISIT(type) void Visit##type(type* node) override;
   AST_NODE_LIST(DECLARE_VISIT)
 #undef DECLARE_VISIT
 
   // Visiting function for declarations list is overridden.
   void VisitDeclarations(ZoneList<Declaration*>* declarations) override;
 
  private:
   class ControlScope;
   class ControlScopeForIteration;
+  class ExpressionResultScope;
+  class AccumulatorResultScope;
+  class RegisterResultScope;
 
   DEFINE_AST_VISITOR_SUBCLASS_MEMBERS();
 
   void VisitArithmeticExpression(BinaryOperation* binop);
   void VisitPropertyLoad(Register obj, Property* expr);
   void VisitVariableLoad(Variable* variable, FeedbackVectorSlot slot);
   void VisitVariableAssignment(Variable* variable, FeedbackVectorSlot slot);
 
   // Dispatched from VisitUnaryOperation.
   void VisitVoid(UnaryOperation* expr);
   void VisitTypeOf(UnaryOperation* expr);
   void VisitNot(UnaryOperation* expr);
 
+  void VisitForAccumulatorValue(Expression* expression);

[rmcilroy, 2015/10/08 16:54:26]

Could we have some comments on when these are expected to be used.

[oth, 2015/10/09 12:50:48]

Done.

+  void VisitForRegisterValue(Expression* expression,

[rmcilroy, 2015/10/08 16:54:26]

Could VisitForRegisterValue just return the Register (with MUST_USE_RESULT)
rather than having a separate GetResultRegister? Also, would it make sense to
have the register stored in the RegisterResultScope instead of in the
BytecodeGenerator?

[oth, 2015/10/09 12:50:48]

Done.

[rmcilroy, 2015/10/09 13:02:56]

What did you think of storing the register in the RegisterResultScope  instead
of in the BytecodeGenerator? Doesn't seem to be done. It would allow the removal
of GetResultRegister and ResultRegisterIsEmpty functions.

+                             TemporaryRegisterScope* temporary_register_scope);
+  MUST_USE_RESULT Register GetResultRegister();
+  void DropResultRegister();

[rmcilroy, 2015/10/08 16:54:26]

Doesn't look like this is necessary as a seperate function since it's only used
by GetResultRegister

[oth, 2015/10/09 12:50:48]

Done. Right there was an intermediate step that also used this, but that
disappeared before the CL.

+  void SetResultRegister(Register reg);
+
   inline BytecodeArrayBuilder* builder() { return &builder_; }
   inline Scope* scope() const { return scope_; }
   inline void set_scope(Scope* scope) { scope_ = scope; }
   inline ControlScope* control_scope() const { return control_scope_; }
   inline void set_control_scope(ControlScope* scope) { control_scope_ = scope; }
   inline CompilationInfo* info() const { return info_; }
   inline void set_info(CompilationInfo* info) { info_ = info; }
   ZoneVector<Handle<Object>>* globals() { return &globals_; }
+  inline void set_result_scope(ExpressionResultScope* result_scope) {
+    result_scope_ = result_scope;
+  }
+  inline ExpressionResultScope* result_scope() const {
+    return result_scope_;
+  }
 
   LanguageMode language_mode() const;
   Strength language_mode_strength() const;
   int feedback_index(FeedbackVectorSlot slot) const;
   Register current_context() const;
 
   BytecodeArrayBuilder builder_;
   CompilationInfo* info_;
   Scope* scope_;
   ZoneVector<Handle<Object>> globals_;
   ControlScope* control_scope_;
 
   // TODO(rmcilroy): Encapsulate this in an environment object.
   Register current_context_;
+  ExpressionResultScope* result_scope_;
+  Maybe<Register> result_register_;
 };
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_INTERPRETER_BYTECODE_GENERATOR_H_