Re: [boost] [atomic] atomic<int>::is_lock_free() returns false on iPad 4 (armv7s)

Subject: Re: [boost] [atomic] atomic<int>::is_lock_free() returns false on iPad 4 (armv7s)
From: Tim Blechmann (tim_at_[hidden])
Date: 2013-09-06 03:20:16

>>   boost:atomic<int>::is_lock_free() returns false on iPad 4 (armv7s) but
>> returns true on iPad 3 (armv7). Is this expected or did I build boost
>> incorrectly?
> 
> Most of Boost.Atomic code is header-only, so the problem is not related to 
> building Boost.
> 
> I suppose, noone added detection for the armv7s target because noone knows how 
> to detect it at compile time. If you could provide a compiler predefined macro 
> that can be used for this purpose, we could add the support.

my experience with arm is rather limited (ccing phil, who wrote the
arm-part), just two points:

* if armv7s is similar to armv7, it may be safe to adapt
atomic/detail/platform.hpp with __ARM_ARCH_7S__

* the boost.atomic part for armv7 currently does not support
double-width compare_exchange, although the hardware provides these
instructions.

btw, it may be worth to note that on iOS devices std::atomic should be
available ...

tim