Previously languages like Ruby suffered on the JVM as the set of JVM instructions available were designed around the Java programming language. This meant that for invoking Ruby methods and other things like accessing instance variables could not take advantage of many of the JVM's features such as method inlining.
Since HotSpot is unaware of Ruby semantics, invoking Ruby methods involved a bunch of custom logic implemented in the form of bytecodes which were completely opaque to HotSpot and thus could not be optimized away.
InvokeDynamic allows JRuby to teach the JVM how to invoke Ruby methods in a way that plays friendly with the HotSpot inliner. Here's a contrived example of HotSpot inlining a Ruby method and optimizing it down to a single assembly instruction:
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If you're confused about what InvokeDynamic is...
Previously languages like Ruby suffered on the JVM as the set of JVM instructions available were designed around the Java programming language. This meant that for invoking Ruby methods and other things like accessing instance variables could not take advantage of many of the JVM's features such as method inlining.
Since HotSpot is unaware of Ruby semantics, invoking Ruby methods involved a bunch of custom logic implemented in the form of bytecodes which were completely opaque to HotSpot and thus could not be optimized away.
InvokeDynamic allows JRuby to teach the JVM how to invoke Ruby methods in a way that plays friendly with the HotSpot inliner. Here's a contrived example of HotSpot inlining a Ruby method and optimizing it down to a single assembly instruction:
https://gist.github.com/3728777