javace 2012

invokedynamic crash course

- Douglas Campos / @qmx

whoami

@qmx

disclaimer

hardcore stuff

lingua franca

languages?

Java?

						Car car = new Car();
					

Ruby?

						car = Car.new
					

JavaScript?

						var x = new Car();
					

different?

back to

java

JVM

stack-based

Virtual Machine

1+2 = ?

(+ 1 2)

ZOMG, lisp???

le stack

le stack - bytecode

iadd?

integer add

^^^^------

statically typed

Java

Virtual Machine

let's go back to

Ruby

isn't this cool?

						2.days.ago
					

what is

2

in

Ruby?

2

						> 2.class
 => Fixnum
> 2.methods[1..10]
 => [:even?, :magnitude, :&, :[], :div, :<<, :modulo, :times, :==, :/]
					

object!!!

let's do this in java

						public class Fixnum {
  public Fixnum +(Fixnum f) { 
    return new Fixnum(
      this.intValue()+f.intValue()
    );
  }
}
					

oops

syntax error

fixing the java class

						public class Fixnum {
  public Fixnum op_add(Fixnum f) { 
    return new Fixnum(
      this.intValue()+f.intValue()
    );
  }
}
				

ugly, I know :(

Java

Y U NO OPERATOR OVERLOADING

Java (language) is

limited

(and that's ok)

there and back again

similar?

						car.honk();
					

similar?

						car.honk();
					

similar?

						car.honk();
					

on the jvm

it's all about

invocation

bytecodes!!!

						invokevirtual #3, #8
					

bytecode is hard

let's go shopping!

this java code

						Car car = new Car();
car.honk();
					

jitescript

						// new instance
  newobj(p(Car.class));
  dup();

					

jitescript

						// calls the default constructor
  invokespecial(p(Car.class),
    "<init>", sig(void.class));
  // calls the "honk" method
  invokevirtual(p(Car.class), 
    "honk", sig(void.class));

					

WTF?

summary

requires

ALL THE TYPES

during compilation

What about some

JavaScript?

JavaScript

						var car = new Car();
car.honk();
					

let's do this in Java

OHWAIT

from ECMA 262 specification, Topic 4.3.3

An object is a collection of properties and has a single prototype object.

let's follow the spec

						public class JSObject {
  // yuck!
  private Map<String, Map<String, Object>> properties = ...;
  private Object prototype;
}
					

where is honk()?

The Car Object

						JSObject obj = new JSObject();
obj.getProperties().put("honk",
 new HashMap<String, Object>(){{
  put("call", SomeClass.class);
}});

					

invoking honk()

						// cast to Class!
Class honk = (Class) obj.getProperties().get("honk").get("call");
honk.getDeclaredMethod("call").invoke(obj);
					

forced casts

reflection

slow

JVM

what if we could

help the JVM?

what if we could

avoid slow reflection?

what if we could

teach the JVM the **fast** path?

what if we could

						// put a car object on the stack
invokedynamic("dyn:call:honk", sig(Object.class, Object.class));
					

YES, WE CAN

(almost)

avoiding reflection

int (*foo)(int)

MethodHandles

aka method pointers

MethodHandles

						MethodHandle honk = MethodHandles
  .lookup()
  .findVirtual(Car.class, "honk", 
MethodType.methodType(void.class));
honk.invoke(new Car());
					

reflection is slow because of

security checks

at each call

MethodHandles do all

security checks

during creation

and they are immutable too :)

what about the types?

polymorphic signatures

polymorphic signatures

						handle.invoke(1,2.0,3L,"lol");
					

teaching the JVM

who calls who?

main calls car.honk()

the place where the caller calls the callee is called a

Call Site

(inception?)

a Call Site

						public static void main(...) {
  new Car().honk();
}
					

to help the JVM, we need to

create the CallSites

back to the invokedynamic instruction

						// put a car object on the stack
invokedynamic("dyn:call:honk", sig(Object.class, Object.class), ?, ?);
					

we need a

bootstrap method

bootstrap method (BSM)

						public class MyBootstrapper {
public static CallSite(caller, name, methodType) {
if (name.equals("dyn:call:honk")) {
  // lookup a Method Handle
 }
}
}
					

back to the JS code, invokedynamic version

JS

• invokedynamic("dyn:call:honk", signature, bootstrap, bootstrap_signature)
• bootstrap method
• MethodHandles that maps straight to the desired object
• CallSite

Inline Caching

monomorphic CallSite

						Object[] objects = {1L, 2L, 3L};
for(Object o : objects {
  System.out.println(o);
}
					

polymorphic CallSite

						Object[] objects = {1L, 2.0, 3f};
for(Object o : objects {
  System.out.println(o);
}
					

megamorphic CallSite

						Runnable[] objects = {.....};
for(Runnable o : objects {
	o.run();
}
					

that's all, folks!

?

thanks!