Re: [Lightning] Support for an "automatic area" on stack

[Paolo Bonzini]

>   /* Prolog of a 3-arg function, with 8 octets reserved on the stack.  */
>   jit_prolog_with_stack_area (3, 8);
>   
We can have instead jit_allocai or something like that, which could 
either do a JIT_SP subtraction or patch the save instruction on the 
SPARC.  I don't remember the details for leaf functions on the sparc.
> Since the exact location of this "automatic area" may vary from one
> architecture to another, we can't just let the user assume that this
> area is located right above %sp (and we should discourage referring to
> `JIT_SP').  Thus we may need additional load and store instructions:
This I'd rather avoid.  The i386 has an undocumented JIT_FP macro, we 
could provide something similar for other architectures.  My preference 
would be something like

   jit_ldxi_p (regd, JIT_FP, offset)

but I could live with

   jit_ldxi_p (regd, JIT_FP, jit_auto (offset))

or

   jit_ldxi_p (regd, JIT_FP, JIT_AUTO_BASE + offset)

(Note that I don't like neither jit_auto nor JIT_AUTO_BASE, but this is 
just a proof of concept).  JIT_AUTO_BASE could access the global state 
in _jit, so it it is exactly as powerful as jit_auto.

Paolo