InvokeDynamic

From APIDesign

Revision as of 14:30, 17 August 2014

When I was younger I believed that having invokeDynamic instruction in HotSpot VM can be beneficial. I even argued that the instruction should not be used just for languages like Ruby but rather by the core Java to implement lambdas. Now, after spending time to implement lambdas in my Bck2Brwsr VM and seeing things from the other side I have to admit I was wrong. invokeDynamic is wrong idea (especially for implementation of lambdas).

Benefits

Implementing different languages on top of HotSpot virtual machine is of different complexity. The most problematic thing is to properly and effectively dispatch methods calls. Not every language uses the Java rules. Some support type conversions, implicit arguments. Some can dynamically alter the existing dispatch target or strategies. More about that in an excellent summary Bytecodes meet Combinators.

To address all these different needs the new invokeDynamic bytecode operand does not hardcode the actual invocation, but delegates it to software controllable MethodHandles. What is a method handle? A pointer to method of some signature (for example plus would take two ints and return their sum as an int) and an object - a receiver to call the method on. However this is nothing else than a closure.

The proposed improvements to the HotSpot virtual machine may help the JDK to support different languages, but first and foremost they open the door to effective implementation of Closures.

Drawbacks

The major problem with invokeDynamic is, well, that it is dynamic! Java is statically typed language and all variable, field, method and parameter types are known to JavaC before its emits the bytecode. Yet (as JavaC from JDK8 is emulating lambdas with invokeDynamic) it forgets all the derived type information and generates invokeDynamic - which is supposed to do late binding - e.g. find out the right types at the invocation time.

One of the key ideas that I had in mind when advocating use of MethodHandles for implementation of lambdas was reduction in the size of constant pool - you know, the list of referenced symbols like Ljava/lang/String which generally needs to be repeated in every Java class. If lambdas were simulated by inner classes, the constant pool might get enormous. With invokeDynamic I was hoping for the pool to be reduced to one shared pool for a single source code (with as many lambdas as needed).

However the JDK8 lambdas are generating innerclasses behind the scene and on the fly! So the main benefit is in my opinion gone.

Summary

As a result we have implementation of lambdas that is needlessly forgetting the type information gained during compilation, re-creates it during each startup, is generating bytecode on the fly. It is even surprising it performs acceptably.