Re: [PATCH] (ARM Cortex-M) FPU and PSP aware exception frame unwinder

[James-Adam Renquinha Henri <arenquinha@cimeq.qc.ca>]

> On 04/07/2016 05:07 PM, James-Adam Renquinha Henri wrote:
>> I submitted it as a bug to the GNU ARM Embedded initially, see here for
>> details: https://bugs.launchpad.net/gcc-arm-embedded/+bug/1566054
>>
>> Basically, this patch allow gdb to unwind properly an extended stack
>> frame, that is an exception frame with FPU state stacked. Additionally,
>> because all Cortex-M variants have 2 stack pointers, the Main Stack
>> Pointer (MSP) and the Process Stack Pointer (PSP), the code in the patch
>> also check which stack was used prior to the exception. That way,
>> backtraces work beautifully.
>>
>> In my original submission, I mentioned a known issue that I didn't try
>> to fix *yet*, because that would involve a lot more work, and the impact
>> is relatively minor: for a given outer frame, some FPU registers may not
>> be reported correctly. I hope you don't mind too much. I consider the
>> current patch still useful, because at least backtraces work, and it's
>> an annoyance not to be able to get them.
>
> I have feeling people will mind. Ideally it should keep the old behavior
> intact if possible. So if you can fallback to the old code, it should be
> ok.

Sorry I don't get it. The old code didn't work in the cases I'm 
providing a fix for, so falling back to the old behavior means just 
giving wrong results? *scratches head*

As I said, getting the behavior 100% correct would require much more 
work, and I felt that it was better to provide an almost correct 
solution so others would benefit quickly of this fix. It might be more 
honest to report a warning to the user that s0-s16 and fpscr could be 
incorrect upon detection of an extended frame. Mind that the old 
situation was "I can't even backtrace past the (CPU) exception if I 
happen to use the FPU", so IMHO it's less harmful to give inaccurate FPU 
information.

Of course I or someone else can work to get it 100% right and we can 
throw all that altogether if it's better that way.


James-Adam Renquinha Henri, Ing. jr
Ingénieur d'application
CIMEQ INC.