Branch Prediction and the Performance of Interpreters – Don’t Trust Folklore

Re:Branch Prediction and the Performance of Interpreters – D... by sklogic on Monday August 10, 2015 @01:43PM (#10036078)

Exactly, you got it right. One indirection less, and no jump table in the cache. And CPUs are very well optimised for this kind of indirect jumps, because of vtables.

There is also one step further - emit the actual jump instructions, making a direct threaded code. It is a bit more complicated and not very portable, but if you want to squeeze all the cycles you can it's still an easy and fast option before resorting to a full-scale compiler.

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Re:Branch Prediction and the Performance of Interpreters – D... by ehaliewicz2 on Monday August 10, 2015 @06:28PM (#10037874)

I think direct threading is usually implemented just by reducing the indirection another level,

e.g.

  goto *lp

rather than

  goto **lp

You could implement an interpreter with generated JMP instructions, which might be called direct-threading, but at least in the forth world, where most jumps are to nested user-defined subroutines, subroutine-threading using JSR instructions is typically used.

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