From: Alexander Fedotov <alfedotov@gmail.com>
To: "Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>
Cc: newlib@sourceware.org, gdb@sourceware.org
Subject: Re: AArch64 calling convention in assembly code
Date: Wed, 01 Aug 2018 15:56:00 -0000 [thread overview]
Message-ID: <CAN8C2CpFLhxZ5Veca=y2=wbeOtxgnm16LPpCdN7ZKCKh6WEVSw@mail.gmail.com> (raw)
In-Reply-To: <cc3376cb-0735-bff6-7417-77eb3f08675a@arm.com>
Yes, but X29 is not saved in original version. I can understand this
with 'frame-less functions" approach. So this code has a right for a
life.
I suspect that such a problem could happen if user will pass
"-fomit-frame-pointer" option to GCC as well.
>>All we really need is a CFI unwind record that GDB can understand.
For plain assembly code ?
Alex
On Wed, Aug 1, 2018 at 6:48 PM, Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com> wrote:
> 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
>>
>>
>>
>
--
Best regards,
AF
next prev parent reply other threads:[~2018-08-01 15:56 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAN8C2CqGq4OUNXq5=ur87nqwVjYF-D6aAcVKGGB1pQT1S0b6Jg@mail.gmail.com>
2018-08-01 13:28 ` Alexander Fedotov
2018-08-01 15:49 ` Richard Earnshaw (lists)
2018-08-01 15:56 ` Alexander Fedotov [this message]
2018-08-01 16:19 ` Richard Earnshaw (lists)
2018-08-01 18:03 ` Jeff Johnston
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