Re: [boost] Help needed for shared_ptr issue on iPad2 (dualcore ARM)

Subject: Re: [boost] Help needed for shared_ptr issue on iPad2 (dualcore ARM)
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-03-28 15:11:02

Phil Endecott wrote:

> It's in Helge's repository -
> http://git.chaoticmind.net/cgi-bin/cgit.cgi/boost.atomic/tree/boost/atomic/detail/gcc-armv6+.hpp

Thanks. Your barriers look fine. I see that you also switch to ARM mode 
automatically if in __thumb__. I wonder whether there's some easier way to 
do that, for example by tagging the appropriate functions as ARM by some 
magic GCC __attribute__.

> > How do you detect the ARMv7 architecture to enable ldrex/strex use?
>
> The compiler defines a macro.  Unfortunately it appears not to have a 
> simple "v6" or "v7" but instead something more detailed, and I'm not at 
> all convinced that this detects all of the variants:
>
> // This list of ARM architecture versions comes from Apple's arm/arch.h 
> header.
> // I don't know how complete it is.
> #elif defined(__GNUC__) && (defined(__ARM_ARCH_6__)  || 
> defined(__ARM_ARCH_6J__) \
>                           || defined(__ARM_ARCH_6Z__) || 
> defined(__ARM_ARCH_6ZK__) \
>                           || defined(__ARM_ARCH_6K__) || 
> defined(__ARM_ARCH_7A__))

Thanks for that, too. It helps to have a starting point for googling. It 
looks like v6 is:

__ARM_ARCH_6__
__ARM_ARCH_6J__
__ARM_ARCH_6K__
__ARM_ARCH_6Z__
__ARM_ARCH_6ZK__
__ARM_ARCH_6T2__

and v7 is:

__ARM_ARCH_7__
__ARM_ARCH_7A__
__ARM_ARCH_7R__
__ARM_ARCH_7M__

and maybe

__ARM_ARCH_7EM__

but it looks like the only one seen in practice is indeed __ARM_ARCH_7A__. 
They don't make that easy.

> > There should be __GCC_HAVE_COMPARE_AND_SWAP_4, but I'm not sure if it's 
> > actually defined by the whatever compiler people use for iOS 
> > development.
>
> I'm using Apple's version of gcc 4.2, and it is NOT defined there - but 
> the intrinsic DOES exist:
>
> #ifdef __GCC_HAVE_COMPARE_AND_SWAP_4
> #warning "__GCC_HAVE_COMPARE_AND_SWAP_4 defined"
> #else
> #warning "__GCC_HAVE_COMPARE_AND_SWAP_4 NOT defined"
> int a, b, c;
> __sync_bool_compare_and_swap(&a,b,c);
> #endif
>
> That outputs the "NOT defined" warning, but compiles and links OK.

Yeah, thought so. The instrinsics _probably_ work in ARMv6 and ARMv7 mode, 
but we can't be sure. I assume you tried that in ARMv6 mode?

> > I'm not quite sure what exactly DSB and ISB can be used for - maybe one 
> > of those could be enough.
>
> I think they are used when you've changed the page tables, and similar 
> things, and aren't appropriate here.  What might be appropriate are some 
> of the options to the DMB instruction e.g. "DMB #ST"; I think they might 
> be new though.

I think that it's better to stay away from these.