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

[Richard Earnshaw <rearnsha@arm.com>]

On 13/01/15 21:08, Andreas Tobler wrote:
> 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.
> 

I agree with Ramana on this one, I feel that FreeBSD trying to plough a
slightly different furrow is just going to cause major problems for
everyone.

I don't believe a claim that LLVM can't be made to work with the
standard EHABI data structures.  It can do this on Linux, so I can't
conceive of any reason why it cannot also do so on FreeBSD.

R.

> Thanks for the review and the feedback.
> 
> Gruss,
> Andreas
> 
>