AArch64 calling convention in assembly code
Richard Earnshaw (lists)
Richard.Earnshaw@arm.com
Wed Aug 1 15:56:00 GMT 2018
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
>
>
>
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