Re: [Lightning] [BUG] [AMD64 ABI] %r12 not preserved and restored in trampoline function prolog/epilog

[Paulo César Pereira de Andrade]

Em qui, 12 de set de 2019 às 03:39, Marc Nieper-Wißkirchen
<address@hidden> escreveu:
>
> Hi Paulo,

  Hi Marc,

> when creating a function prolog and epilogue for a trampoline function
> as this one
>
> prolog
> frame (...)
> ...
> epilog
>
> GNU lightning saves all callee-save registers in the prolog and
> restores them in the epilog. On the Unix port of x86_64, however, it
> fails to save and restore %r12, which is a callee-saved register.
>
> (This error has led to subtle bugs when I compiled my code with "-O2"
> and gcc began to use %r12 and expected it to be preserved over a call
> into a GNU lightning trampoline.)
>
> The error maybe related to the fact that on GNU lightning, %r12 is
> mapped to JIT_R3, which appears as a caller-save register to the user.
> See also this discussion here:
> https://lists.gnu.org/archive/html/lightning/2017-09/msg00006.html

  I believe quite some time ago I had made a commit for this issue
in another computer, that I no longer have, and never pushed it.
  For sure it will be added in the next release of GNU Lightning that
I am already working on. It just is taking more time than expected
due to issues with powerpc 32 bit abi, that previously I had tested
only on AIX, and when testing on Linux, it uses another ABI...

> I'd suggest to apply the patch of this discussion so that %r12 is
> treated like %13, %r14, %15 and will be automatically saved in the
> prolog and restored in the epilog. The side effect will be that JIT_R3
> goes away and JIT_V4 appears.

 The problem should go away as well if just changing:
{ rc(gpr) | rc(rg8) | 12,          "%r12" },
to
{ rc(sav)|rc(gpr) | rc(rg8) | 12,          "%r12" },

  Then, there would be a JIT_R3 that actually is callee save, and could
be checked with jit_callee_save_p. It does not really matter much,
but probably better to make it indeed JIT_V4, to make it more explicit
it is callee_save.
  The issue is hard to find because it explicitly save/restore _R12,
in prolog/epilog if used in a jit function. But logic checking for
jit_class_sav will not work correctly when computing live registers,
%r12 will be treated as a scratch register.

> If you don't want this, the bug has to be fixed differently by
> patching the _prolog and _epilog procedures.
>
> In any case, a fix of this bug is urgent because it is a critical one
> (as non-ABI-conforming code is produced).
>
> Thanks,
>
> Marc

Thanks,
Paulo