Re: [PATCH][ARM] FreeBSD ARM support, EABI, v3

[Andreas Tobler <andreast-list@fgznet.ch>]

On 13.01.15 11:25, Ramana Radhakrishnan wrote:
> On Thu, Jan 8, 2015 at 8:51 PM, Andreas Tobler <andreast-list@fgznet.ch> wrote:
>> On 08.01.15 17:27, Richard Earnshaw wrote:
>>>
>>> On 29/12/14 18:44, Andreas Tobler wrote:
>>>>
>>>> All,
>>>>
>>>> here is the third attempt to support ARM with FreeBSD.
>>>>
>>>> In the meantime we found another issue in the unwinder where I had to
>>>> adapt some stuff.
>>>>
>>>> The unwind_phase2_forced function in libgcc calls a stop_fn function.
>>>> This stop_fn is in FreeBSD's libthr implementation and is called
>>>> thread_unwind_stop. This thread_unwind_stop is a generic function used
>>>> on all FreeBSD archs.
>>>>
>>>> The issue is now that this thread_unwind_stop expects a double int for
>>>> the exception_class, like on every other arch. For ARM EABI this
>>>> exception_class is an array of char which is passed in one register as
>>>> pointer vs. two registers for a double int.
>>>>
>>>> To solve this issue we defined the exception_class as double integer for
>>>> FreeBSD.
>
> My apologies for the slow response, some other work and then holidays
> intervened.

Np, the only issue which made me hurry was the stage 4 entering this week.

>From my understanding of the ABI document the implementation is
> currently as mandated by the ABI. Also this isn't a part of the ABI
> that's available for the platform (here FreeBSD to manipulate and
> change as per it's wishes). ARM EHABI is special for software, making
> FreeBSD more "special" for ARM appears to be counter intuitive from my
> point of view. A number of exception unwinding libraries. for e.g.
> libobjc , libstdc++ all use this implementation of exception_class.
> Therefore this creates a divergence for the FreeBSD port which is
> different from everything else. I expect that a number of language run
> time support libraries that supported the ARM EHABI would be using
> such an implementation, therefore you need to fix every single
> implementation of this in every unwinder that supports the ARM EHABI
> which I expect to have been ported to in a number of libraries
> already. (I already see this in libobjc and libstdc++ in the GCC tree)

Grr ;) I didn't want to hear this answer, but I expected it somehow.
My proposal was the least effort for me.
The other way round is going to be very hard. Maybe impossible.

It is not only FreeBSD which is affected but also llvm and friends. They 
use for exception_class uint64_t.

I have to take a picture how the effort is and if it would be possible 
to do such a change in FreeBSD and more important in llvm etc.

> I would rather fix the thread_unwind_stop implementation in libthr for
> ARM EHABI rather than make this change.

It wouldn't be a 'fix' but more a wrapper I think.

>>>> This adaptation reduced the failure count in libstdc++ by about 40 fails.
>>>>
>>>> I build and test this port on a regular basis and I post the results to
>>>> the usual place.
>
> Thanks for doing this. I'm really glad that FreeBSD is finally moving to EABI.

Thanks for the review and the feedback.

Gruss,
Andreas