Interface
=========

Required reading: Efficient Implementation of Java Interfaces:
Invokeinterface Considered Harmless


* Background
  In the 90's last century, with the popularity of Java, IBM 
  starts a new project to study
  Java compilation and state-of-the-art virtual machine 
  technologies, this project was called "Jalapeno" that time.
  
  Later, this project is open-source and the compiler is renamed
  to "Jikes", which can be found at: 
    http://jikes.sourceforge.net/
  
  The virtual machine technology project is renamed to
  "JikeRVM" and is available at:
    http://jikesrvm.org/
  
* what's this paper about?
  efficient implementation of Java interface

* What's the problem with interface?
  sample code:
  interface I{
    void foo();
  }
  
  class A implements I {
    void bar () {}
    void foo () {}
  }
  
  void f (I a){
    a.foo ();      // how to call foo, if we pass it an object of class "A"?
  }
  
  this problems involves search "foo" in object a, however,
  for any class implements "I", the offset of "foo" maybe different,
  so this searching is inefficient
  [draw a figure of method searching here]
  
* Interface table (section 4.1)
  the first idea is to cook an interface object here
  [draw a figure of interface object here]
  
  thus, by attaching an interface table (itable), interface
  method dispatching is as fast as virtual function dispatching
  
  one must allocate one itable for every class "A" that implements some
  interface "I"
    so, the bad news is that the interface function table occupy extra space
  
* Selector indexed table
  [draw a figure of selector indexed table]
  must handle multiple interfaces efficiently
  again, use the graph coloring algorithm for implmementing multiple inheritance
    however, the global graph coloring method does not apply to Java well
      why?
    
* Interface method table (IMT)
  key idea: attach a hash table in TIB, which maps every interface
  method to its correct offset
  just like the hash table for multiple inheritance