[Lightning] jit_calli requires jit_prolog

[Paul Cercueil]

Hi,

I am currently developing a simple MIPS-to-everything dynamic recompiler 
based on Lightning. The recompiler generates blocks of machine code that 
are meant to be chained: each block jumps to the following one. This is 
what I want to do (MIPS code):

# lightrec_emit_end_of_block:emitter.c:37
        0x77d97060      move    a0,s6
        0x77d97064      lui     t9,0x77d7
        0x77d97068      ori     t9,t9,0x2198
        0x77d9706c      jalr    t9
        0x77d97070      nop
        0x77d97074      jr      v0
        0x77d97078      nop

Basically, jump to a C function that will return the address of the next 
block of code (optionally compile it beforehand), then jump to the 
address returned.

The problem is that Lightning will segfault when generating this code, 
as soon as jit_calli() is executed. Apparently, jit_prolog() and 
jit_epilog() are required for jit_calli() to work.

However, if I add jit_prolog() and jit_epilog(), Lightning will generate 
some code to save/restore registers, while I don't want that. This is 
especially bad since the epilog will never be reached (as the code jumps 
to another block), so the stack pointer will decrease at each prolog but 
never increase.

What I would need, are either variants of jit_prolog() and jit_epilog() 
which don't save/restore the registers on the stack, or alternatively to 
have jit_calli() not require jit_prolog().

Do you have suggestions on how to fix this issue?

Thanks,

-Paul