Re: [Lightning] setarg

[Paolo Bonzini]

> core-common.h:
> [...]
> #ifndef JIT_FP
> #define jit_setarg_i(ofs, reg)        jit_movr_i((ofs), (reg))
> #else
> #define jit_setarg_i(ofs, reg)        jit_stxi_i((ofs), JIT_FP, (reg));
> #endif
> [...]
>
> of course, it would still be a bit of work, if you would also
> like to support jit_setarg_c and all the variants. and don't forget
> the floating point stuff! but i should still be possible to implement
> in few hours (including tests).
>
> i'd be glad to learn, why such a thing has never been implemented
> before. what's the reason? what would we lose by doing so?

Sorry for the late answer.

The main reason why it's not implemented is that:

1) in lightning arguments are caller-save (even if in some platform they 
might not be), which limits a lot the usefulness of setarg, including 
your proposed usage;

2) in lightning arguments are not usable as general registers (even if 
on some platforms they might actually be), which also suggests that you 
load the argument in a register if you want portable and efficient code.

The two limitations are related, in that usually arguments are 
caller-save on platforms where they are general-purpose registers, and 
dually they are not general-purpose registers where they are callee-save.

Thanks anyway for your interest in lightning.

Paolo