Here are the steps to use this functionality:
1) Arrange mappable registers like this:
char <type>val_map[] = { non-volatiles, volatiles }:
2) define PRESERVED_<uc_type>_REGS as the count of non-volatiles
3) denote a function call in core.jit as
CALL_FUNCTION(jit_info, the_function);
NB: the only effect of this statement is that the extcall flag of that opcode is set to -1. It does not call the function or even try to setup function arguments. Function call is done by ...
4) define that macro on top of core.jit
jit/i386/core.jit is using this functionality now.
Have fun, leo