Re: [Lightning] jit_calli requires jit_prolog

[Paulo César Pereira de Andrade]

2014-08-18 10:59 GMT-03:00 Paul Cercueil <address@hidden>:
> Hi,

  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

  This code should never crash, but you are probably having trouble
with save/reload in elsewhere.
  I suggest making sure to test with lightning built with --enable-assertions,
so that it would tell when/where there is a fault.

  lightning 2.x has a simple register allocator, and requires entering/leaving
the jit as a function call, so that it knows and can handle "bounds" of live
registers.

> 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.

  For mips this should work if using only to JIT_Rn, and not using too much
complex constructs with too long live ranges for JIT_Rn (or it could allocate
a callee save as a temporary). It probably is crashing because some JIT_Vn
is being changed, and without a prolog/epilog it cannot save (in prolog)
and reload (in epilog) the callee save register.

> 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.

  I suggest you make some benchmarking to make sure it really is
better to not use a function call sequence. If no callee save registers
are changed, it would be basically a sub/dec of the sp register.
But would be better if you could construct the jit to avoid jumps
for every small chunk; the major point of jit is avoiding branches
and jumps, usually at the cost of some code duplication.

> 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?

  There is a lightning 1.2 compatible mips port at
https://github.com/pcpa/lightning that probably will work for the need
you describe; main difference should be right to left argument
pushing (to match lightning 1.2 api).

> Thanks,
>
> -Paul

Paulo