Re: AArch64 calling convention in assembly code

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 01/08/18 14:28, Alexander Fedotov wrote:
>>> Pushing LR on the stack resolves a problem
> FP of course, not LR.
> So the correct code must be like this:
> 
> _cpu_init_hook:
>     stp        x29, x30, [sp, #-16]!
>     mov        x29, sp
>     bl        _init_vectors
>     bl        _flat_map
>     ldp        x29, x30, [sp], #16
>     ret
> 
> But still my point is that GDB should catch such an error and do not hang.
> 
> Alex
> 
> On Tue, Jul 31, 2018 at 9:34 PM, Alexander Fedotov <alfedotov@gmail.com> wrote:
>> 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.

X30 is LR.

All we really need is a CFI unwind record that GDB can understand.

R.

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