invokedynamic: Is It What We Really Need?

John Rose wrote interesting post regarding dynamic invocation in the VM. Here are my thoughts as implementor of Groovy language.

DECLAIMER: It is not official opinion of Groovy Development Team or G2One Inc. but only my personal opinion.

First of all I think John and JSR 292 team finally start to do very good job. After long period of uncertainty we have a lot of communication last months about what’s going on and ideas under discussion. It is really helpful.

Finally we have very clear explanation of method handles idea, which sounds very promising. To make long story short it is about avoiding expensive calls to Reflection API, which is one of the biggest problems each dynamic runtime meet. And now we have more or less clear picture (or suggestion) for invokedynamic.

Recent significant progress in JRuby and Groovy runtime performance proved clearly that dynamic languages has a lot of space for improvements in terms of performance. It is also more clear now what kind of support is required from JVM for dynamic language runtime.

In fact, the main challenge for implementor of dynamic language sounds pretty trivial: you have call to the method and in general can assume nothing about type of receiver and types of arguments. So execution of a call consist <i>logically</i> of two important steps:

• method selection
• method invocation

IMPORTANT NOTE: Current Groovy MOP doesn’t separate these steps, which also leads to some difficulties. Of course, implicitly (logically) both steps are presented. For Groovy MOP 2.0 we consider possibility to separate them explicitly.

Method invocation in Groovy is expensive. We have to use Reflection API, which does a lot of checks, requires allocating of array for result, boxing/unboxing of arguments and return value and several nested calls before we come to execution of real method. As I said before there is some hope that methods handle will help here. What is especially beautiful in method handles is the fact that they don’t require new bytecode but can be handled directly by JVM.

Method selection in Groovy is even more expensive. Following diagram gives very simplified overview of what’s going on. Let me repeat this: it is simplified overview and real process even more complicated (categories, ExpandoMetaClass etc.)

Fortunately there is solution for the method selection problem: if we store result of previous method selection there is very good chance that next time our choice will be the same and check of this fact is much cheaper compare to real selection process. This technique is implemented in Groovy 1.6 and proved to be very efficient (of course some memory overhead is involved)

Here is pseudo-code of API used by Groovy 1.6

public abstract class CallSite {
    // index of call site
    protected final int index;

    // name of method
    public final String name;

    // array of all call sites in compiled class
    protected final CallSiteArray array;

   // calls to this methods appear in bytecode
   public Object call (Object receiver, Object [] args) {
      retutn acceptCall(receiver,args).invoke(receiver,args);
   }

   // check if receiver, arguments, categories in use, etc. fits to memorized choice
   // if so, we can continue to use same call site
   // if not, new call site created to replace current one
   protected CallSite acceptCall (Object receiver, Object [] args) {
      if (checkCall(receiver,args))
         return this;
      else
         return createCallSite(this,receiver,args);
   }  

   // check if receiver, arguments, categories in use, etc. fits to memorized choice
   // depends on type of call site
   protected abstract boolean checkCall (Object receiver, Object [] args);

   // real invocation
   // depends on type of call site
   protected abstract Object invoke (Object receiver, Object [] args);

   // creates new call site, which will replace existing one
   protected final CallSite createCallSite(CallSite base, Object receiver, Object [] args) {
      ....
   }
}

In bytecode receiver.method (arg1, arg2) becomes

callSiteArray[callSiteIndex].call (receiver, new Object[] {arg1, arg2} );

callSiteArray is synthetic local variable filled in the beginning of each method from static soft referenced field. Initially array is filled by dummy call sites, which needed to keep name of target method and index of call site, because we don’t want to send two additional parameters each time and we also don’t want to do null check for call site on each invocation.

So what happen when we visit call site first time is creation of real call site and invocation. On second visit we check if it still to be valid and either recreate or use it.

Truely speaking, API above is not real API. There are special paths reflecting the fact that in Groovy there is several different types of method call but it is out of scope of this note and I hope the idea is clear. Another thing, which makes API much more complex is special paths for binary operations and in the future for calls with small number of parameters, which allow to avoid creating of argument arrays.

So how all that is related to invokedynamic? Very simply. Bootstrap method will call our createCallSite. Target handle will be CallSite.call doing check and either invoking another method handle (really selected method) or setting new target. And voila!

The only thing, which really worries me is do we need special bytecode or Java API (of course specially recognized and optimized by JVM) is enough.

The real beauty of API is that you can use it (backported versions) for old JVMs. Even now a lot of people stay with 1.4 and I am not sure how many already switched to 1.6. For us, as language developers, very important to support old platforms. For sure, to support 1.5. And I would prefer not to have two versions of compiler for platform supporting invokedynamic and for one, which doesn’t.

The only really good side of invokedynamic compare to fixed API is ability to make calls with absolutely any signature. I don’t know how important is that for other languages, for Groovy I think it will be enough to have rich enough fixed API for small number of arguments and Reflection-like general API.

So if I was member of EG, which I am not, I would prefer not to introduce new bytecode.

Dixi

Joint compilation of Groovy and Java – III

Believe you or not but it happened. Now you can compile Groovy and Java together.

Read Jochen Theodorou article on implementation details. And hey… it is based on my patch

This is the house what Jack built

Today I realized what we (or may be just I) live in very strange world. Let me describe why.

I work now on problem of making GORM domain classes accessible in GWT client-side code. It is an interesting task by itself and I plan  to dedicate  several separated posts to it but in general it is very simple. I want to define my server data objects in Groovy and be able to request them in from client-side code without coding too much.

The  solution is almost trivial. GWT provides great idea of generators which can create Java code to be integrated in to compilation of client-side JavaScript, so all what I need is to implement generator, which read groovy classes and generate all additional Java code I need. It is not too compilcated especially with Groovy in your hands.

What I realized debugging my code is how many different compilers, generators and even instrumenters involved in run of my one single test.

1. Javac. I use it to compile Java part of my generator and the test to be run by JUnit.
2. Groovyc. Most interesting part of my generator is written on Groovy
3. GWT compiler from Java to JavaScript. It calls my generator and compile my test and generated Java code. If you curious, it contains built-in parser from Eclipse compiler
4. BTW, compilation of unit tests in GWT involves generation of client-side part of the test. It’s a long story, which I will probably need to blog separately later. For now we just notice one more generation step.
5. My generator use Groovy compiler to parse Groovy sources of domain classes. Main task for the generator is  to create client-side twins for domain classes and make them serializable.
6. Of course, serialization may not happen for free. Fortunately GWT provide default generator, which takes care for generation such code for java source, generated by me.
7. Again, just for record we should notice that my generator not only written on Groovy but also depends on Groovy runtime, which does some code instrumentation in magical GroovyClassLoader.
8. There is one more thing, which I am afraid to speak about. GORM uses Hibernate and Spring. How many code generation and instrumentation used inside? God knows.
9. I know only that even GORM iteself uses some to access Hibernate and Spring.

That’t it. But something is wrong in this world, isn’t it?

Loopy with Groovy

There’s been a lot of people asking for a Java loop syntax in groovy. A lot if not almost all usages however are cleaner without it, once you know some idiomatic Groovy.

Marc Palmer’s post blog post demonstrates it almost obviously.

Joint compilation of Groovy and Java-II : The Quest for Solution

In my previous post I promised to describe how to compile Groovy and Java together without writing complicated ant scripts or limiting yourself in design decisions.

Let us analyze the problem and probably it will lead us to a solution:

1. To compile Java code, which refers some Groovy code, we have to compile Groovy first because the only way for javac to know about Groovy is .class file
2. We can’t compile Groovy code if it has to know about Java one because again the only way for groovyc to know about Java code is .class file form.
3. But do we really need as much? Obviously, not. We don’t really need compiled Groovy classes during Java compilation. All we need to know are “exported interfaces” of the classes. What are their names, methods, fields, implemented interfaces, superclass etc. We don’t care about real implementation at all. Let me repeat it is once more. We don’t need it at all.
4. So if we are able to supply javac with .class files with the same “exported interfaces” as our Groovy classes we are done. After that we are able to compile Java code and provide compiled code to groovyc. And our circular dependencies will be resolved.
5. It is cool, isn’t it? All we need to do is to write simple compiler which will take Groovy code and create .class file with the same “exported interface” as our Groovy code but ignoring all implementation logic, possible errors etc.
6. But how does it help us with “ant script problem”. Unfortunately, it doesn’t because we need to call this special compiler.
7. Wait a minute. Why do we need additional compiler? We can build it in groovyc itself. It will be one task to call for joint compilation of Groovy and Java code, which will do three passes internally. First pass will generate dummy .class files, usually such things called stubs. The second one will call regular Java compiler, for example javac or eclipse compiler if we like. And the final step will do usual Groovy compilation. And it is exactly what we need, isn’t it? Yes, it is exactly what we were looking for.
8. So the only problem we have is to patch groovyc. We know how to parse and even compile Groovy sources. We know how to call external compiler. We have great ASM library, which will help us to generate stubs. We just need to put it all together. Not too complicated. Right?
9. OK, there is one more problem, which still worry me. I know that ASM is great library but I have no too much experience with it and it may take long time to code and especially debug this code. Sounds like a problem, right? Really not at all! We don’t need compiled classes. What we need is to provide javac with information about “exported interfaces” of our Groovy code. It can be .class files or … Java source. Let us just generate Java sources with exactly the same “exported interfaces” as our Groovy code. It is really easy task if you have parser for Groovy, which we obviously have, and what can be easier than such generator? “Hell, World!” probably.
10. We have last thing to improve. Currently we plan to parse Groovy code twice – once for stubs generation and once more during Groovy compilation. It is not dramatically bad but unpleasant. Fortunately, it is easy to resolve. What we need is to incorporate our patch in the middle of Groovy compilation process. Parse Groovy, generate stubs, compile java and stubs and continue Groovy compilation. What is even more fortunately is that Groovy compiler is modular enough to allow it.

What’s it.  We found solution for all our problems and our quest completed. There is interesting story about different technical details and difficulties, which I met implementing this approach, but I will describe it in the later posts.

And wait a second. The patch gonna be incorporated in to Groovy trunk in nearest days.

Joint compilation of Groovy and Java – I

The first thing I’ve learned about Groovy was that you have to compile it separately from Java. OK, it is understandable because the languages are different.

But the next news was even worse. There is no way to compile it correctly if there are dependencies between Groovy and Java code. Again, of course we are software engineers, everything is possible(c) and there is a such possibility but…

Let me give you a sample. Imagine we have

• Java interface JInterface
• Groovy class GClass implementing JInterface
• Java class JClass, which has to create instance of GClass (probably not thinking too much is it Groovy or Java class) and use it as instance of JInterface

I believe  everybody will agree that such design may be very logical in some situations. But how can we compile this code? Well, we do it in three steps of ant script

• javac for JInterface
• groovyc for GClass
• javac for JClass

And here we are. Our problem resolved. May be the solution is not so elegant (carefully written ant script instead of IDE compilation) but it works. And it is what Groovy coders do every day.

But does the solution above really work? No, unfortunately it doesn’t!

Imagine that by some reasons JInterface has to know about GClass. We can argue long hours is it good design or bad one, so let just think it has to. Now we have circular dependencies between Groovy and Java code. To compile Java we need to compile Groovy first but to compile Groovy we need to compile Groovy. Ooops! We have dead lock.

Obviously, it is not new problem. Have a look on issue http://jira.codehaus.org/browse/GROOVY-1735 The problem is nobody knows how to solve it.

Again, we can refactor our design. Probably introduce some additional class. May be add additional step to our carefully written ant script and finally being able to compile our simple code. But does it make us more productive? I am not sure.

Fortunately I have some good news. There is a solution for joint compilation of Groovy and Java. Some nice trick was invented by guys from JetBrains And I can say even more – the’ve submitted patch, which probably will be accepted before Groovy 1.1 Beta 2 is out.

But I will describe this story tomorrow. Stay tuned

A groovy use case

Luc Dewavrin recently discovered Spring’s ability to declare beans that are script-based. He describes why this is great and helpful to to speed-up development. Sample declarations and scripts are also provided.

The article ends with interesting opinion “…, I like both Groovy and JRuby but now tend to prefer Groovy because it leverages my knowledge of Java API. With Ruby i spend too much time finding the reference web site how to achieve things that i do almost naturally in Java. OTH, Ruby has its core API use closures. Closures are tightly integrated and tend to favor loose coupling.”

Read full story…

Using Spring Factories with Groovy

Spring is an open-source framework created to address the complexity of Java enterprise application development. One of Spring’s goals is to help developers write simple, testable and loosely coupled systems while reducing the amount of scaffolding code required. In this respect, Groovy has a common goal. So, for simple systems, Groovy alone may be sufficient for your needs. However, as your system grows in size and complexity, and especially in hybrid Java/Groovy environments, you might find Spring’s facilities provide great value to your Groovy system development.

Read more in Groovy documentation…