[boost] [shared_ptr] m68k and ia64 implementations by Ben Hutchings, RFC

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-04-07 06:13:07

Ben Hutchings has kindly contributed the attached sp_counted_base 
implementations for g++ on IA64 and 68020+. The IA64 version of decrement 
should be changed to:

Ben Hutchings wrote:
>>> It isn't a memory barrier (unless you add .acq or .rel to it) but I
>>> don't believe a barrier is needed for the simple increment and
>>> decrement operations.  It is always atomic, like lock xadd.
>>
>> For increment, no. For decrement, yes. It needs to be a release when
>> the new value is nonzero,
>
> Oh yes, of course it does.
>
>> and an acquire when the new value is zero.
>
> Hmm, tricky.  There would be need to be another instruction afterward
> that creates the acquire barrier.  Maybe this would do:
>
>    __asm__ ("     fetchaddl.rel %0=[%4],-1 \n"
>             "     cmp.eq        %2,%3=1,%0 \n"
>             "(%2) ldl.acq       %0=[%4]    " :
>      "=r"(rv), "=m"(*pw), "=c", "=c" :
>      "r"(pw) :
>      );
>
> Don't ask me whether that's the most efficient way to do this!

In your (Boost developers) opinion,

1. Do we need a platform-specific version for 68020+ at all? (It is my 
understanding that threads aren't very popular there ;-) but I may be wrong)

2. Should we try to get an IA64 version in 1.33 or should this be postponed 
for 1.34?