Re: [boost] Help with a shared_ptr problem on Android (ARM)

Subject: Re: [boost] Help with a shared_ptr problem on Android (ARM)
From: Peter Dimov (lists_at_[hidden])
Date: 2014-04-17 10:11:34

Andrey Semashev wrote:
> On Thu, Apr 17, 2014 at 4:53 PM, Peter Dimov <lists_at_[hidden]> wrote:
> > https://svn.boost.org/trac/boost/ticket/9912
> >
> > reports that spinlock_gcc_arm.hpp segfaults on a Nexus 7.
> >
> > Unfortunately, I know next to nothing about ARM or Android, so I wonder 
> > if someone can help with that.
>
> I'm probably even less experienced with ARM than you are, but there is an 
> interesting comment in boost/atomic/detail/gcc-armv6plus.hpp that explains 
> some things about different ARM ISA versions.
>
> From Wikipedia it seems that Nexus 7 has an ARMv7 CPU, but the compiler is 
> probably reducing the target CPU to ARMv6 or even lower (judging by the 
> fact that __thumb__ is not defined). In this version mcr instruction is 
> used for a memory barrier. As I understand it, it's basically an 
> instruction for a co-processor (15 in this case). I suspect that that 
> particular SoC could have this co-processor number reserved. Of course, 
> all this is just a speculation on my part, so take it with a grain of 
> salt. It would be useful to see the backtrace and disassembly to verify my 
> theory.

I suspect that the target CPU is not even set to ARMv6, in which case no 
memory barrier instructions are inserted.