Re: [boost] [lockfree] arm / iOS builds against Boost.Atomic

Subject: Re: [boost] [lockfree] arm / iOS builds against Boost.Atomic
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2013-05-16 11:29:23

Hi Rich,

Rich E wrote:
> I noticed today when moving my work from OS X / clang / libc++ to iOS /
> clang / libc++ that Boost.Lockfree must link against libboost_atomic.a to
> compile.  This shouldn't be necessary, since there is std::atomic on iOS as
> well.

I wrote the Boost.Atomic support for ARM.

As you may have noticed, Boost.Atomic was a long time in gestation and 
has been
somewhat overtaken by events, i.e. there is vendor-supplied <atomic> 
support on
many systems, making Boost.Atomic useful primarily for people who 
cannot use
latest versions of compilers.  Although I'm still using this code 
myself on
iOS and ARM Linux, I probably won't be for much longer.

Having said that, I suggest always checking to see what code is 
actually being
generated i.e. is it inline atomic instructions (ldrex & strex on ARM) 
or some kind
of library call, even with vendor-supplied <atomic> implementations.  
Just look at
the generated assembler for a trivial program.  The fact that you're 
seeing a
requirement to link with Boost.Atomic suggests that it is not using my
inline-assembler version... explained by this post from Tim:

> not sure if arm supports a double-width compare-and-swap or ll/sc

IIRC, ARMv7and later does - but my code doesn't support it.  It could 
probably be
added quite easily.  I didn't have a platform to test that on when I 
wrote it.

Regards,  Phil.