[boost] Help needed for shared_ptr issue on iPad2 (dual core ARM)

Subject: [boost] Help needed for shared_ptr issue on iPad2 (dual core ARM)
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-03-28 08:03:44

Hi everyone,

Ticket #5372:

https://svn.boost.org/trac/boost/ticket/5372

says that shared_ptr's ARM spinlock implementation (which uses the swp 
instruction) doesn't work properly on iPad2 (which has a dual core ARM 
processor). The sample program in the ticket compares it to a loop using 
__sync_fetch_and_add, which means that the __sync intrinsics are implemented 
by the compiler the submitter is using. These didn't work on gcc for ARM 
when we tested them, but may have been added meanwhile. (I can see some code 
samples that test for 4.4, but the official docs state that ARM intrinsics 
are only supported on Linux before 4.6, which was released yesterday.)

So, we have two questions; first, why does the swp-based spinlock fail, and 
second, how can we detect support for __sync intrinsics and use them.

Anybody with ARM knowledge and iPad2 development access?