Interpreters are very hard to optimise automatically. A huge case statement for the whole language in effect. C compilers don't do that well, still often written in assembler.
But in this case they're talking about writing it in Java, and they have control over the Java compiler. In other words they have plenty of opportunities to ensure it optimises well:
- Focus on ensuring the overall pattern optimises well.
- Or recognising the special case of the JVM interpreter loop
- Or adding a pragma of some sort to trigger special optimisations.
Making it fast when compiled with an arbitrary compiler is another matter. But then even a lot of interpreters written in C often resorts to compiler specific hackery.
Recognising special cases is fragile. And the special optimisation may as well be "replace with this hand written code"... Compilers, well parts of them, are not really an example of the typical code you are trying to optimise.
