arm access to stack slot out of allocated area

[Olivier Hainque]

Hi Wilco,

> On 8 Feb 2019, at 15:49, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> 
> Hi Olivier,
> 
>> Below is a description of a very annoying bug we are witnessing
>> on ARM.
> ...
>> compiled with -Og -mapcs
> 
> Do you know -mapcs has been deprecated for more than 4 years now?
> Is there a reason you are still using it? It was deprecated since -mapcs
> is both extremely inefficient and buggy. I would strongly suggest to stop
> using this option completely in all your software.

Sorry, I had -mapcs-frame in mind. The investigation I described was
conducted with the default settings of the VxWorks port (no particular option)
and thought it might be of greater potential interest if exposed with a bare
eabi compiler. The conditions of interest are guarded with

      IS_NESTED (arm_current_func_type ())
        && ((TARGET_APCS_FRAME && frame_pointer_needed && TARGET_ARM)

Turns out that the arm-eabi code with -mapcs-frame is very slightly
different than the vxworks one, which I missed on the first reading.

Let me double check again ...

> It's useful to create a bug report with the details, however it might not be
> worth fixing if the issue cannot be reproduced without -mapcs. -mapcs
> was never meant to support a static chain, especially not spills of the
> static chain.

Understood. I'll start by examining the differences
between the vxworks and the eabi+aapcs-frame configurations
more closely, and I'll report back.

Thanks for your feedback!