AArch64 calling convention in assembly code

Alexander Fedotov <[email protected]> Tue, 31 Jul 2018 21:34:07 +0300

Newsgroups	gmane.comp.lib.newlib,gmane.comp.gdb.bugs.general
Message-ID	<CAN8C2CqGq4OUNXq5=ur87nqwVjYF-D6aAcVKGGB1pQT1S0b6Jg@mail.gmail.com>

Hello dear AArch64 maintainers
Please look into code snippet below from newlib/libgloss/aarch64/rdimon-aem-el3.

Seems to me this code violates AArch64 calling convention and actually
breaks debugging in GDB. GDB tries to unwind call stack and got
endless reentrancy...

FUNCTION (_cpu_init_hook):
    sub    sp, sp, #16
    str    x30, [sp, xzr]
    bl    _init_vectors
    bl    _flat_map
    ldr    x30, [sp, xzr]
    add    sp, sp, #16
    ret


We have couple of calls there (_init_vectors, _flat_map). If you'll
try to step into any subroutine you will found that GDB hangs and
can't step anymore.

Pushing LR on the stack resolves a problem.

So my message is that:
1. Current code in _cpu_init_hook is incorrect
2. GDB should handle this and do not hang

Alex