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

[Marc Nieper-Wißkirchen]

Hi Paulo,

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'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.

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