Re: split stack on darwin

[Ian Lance Taylor <iant at google dot com>]

IainS <developer@sandoe-acoustics.co.uk> writes:

> On 28 Sep 2010, at 17:29, Ian Lance Taylor wrote:
>
>> Jack Howarth <howarth@bromo.med.uc.edu> writes:
>>
>>>  Does the new split stack feature in gcc trunk absolutely require
>>> additional
>>> libc support to function or can it be made to work on other targets
>>> like
>>> darwin with just changes to libgcc? Thanks in advance for any
>>> clarifications.
>>
>> The additional support that it needs is a thread local place to store
>> the stack bound which can be accessed efficiently.  On GNU/Linux I'm
>> using a word in the thread block--I've actually stolen a word which
>> was
>> reserved for transactional memory but which RTH says he doesn't need.
>> This can be accessed as an offset from %fs or %gs.  To make this
>> functionality work on Darwin, you need something similar.  I don't
>> know
>> how thread local storage works on Darwin, so I don't know if you can
>> do
>> something similar.
>
> We have emulated TLS (Richard's new code) .

If I understand correctly how that works, that would be a heavy
performance penalty for something like -fsplit-stack.


>> I list some other options at http://gcc.gnu.org/wiki/SplitStacks under
>> "Possible strategies.
>
>
> I sincerely doubt something this involved can be accommodated in the
> remaining time of stage 1
> (unless we magic some additional Darwin devs from somewhere ;-))

If a good strategy can be chosen, the actual changes are probably fairly
small.  But of course it's up to you.

Ian