A few years ago :), I implemented the mixing of bytecode and native
code in JoCaml. The bytecode interpreter included in native code
runtimes was modified so that bytecode functions were wrapped inside
native code functions, using a special tag. This tag was checked
before executing a closure, to either call a wrapper for real native
code functions, or to find the bytecode pointer in the closure for
bytecode functions. In native code, calling directly the wrapped
function started a new interpreter to execute its bytecode. 

For exceptions, a flag is used to know whether the first exception
handler is executing native code or bytecode. Depending on this flag,
the exception is raised in bytecode or native style.

I did not measure the cost of this, but I noticed a slowdown. I don't
know if there is a better solution. One of my (unfortunately long list
of) projects is to improve asmdynlink to produce JIT native code for
bytecode ... maybe next year !

- Fabrice

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