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Subject: Re: [boost] [interprocess] atomic_write32 on GCC/x86/x68_64 lacks memory barrier
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-08-08 13:48:39

On Fri, Aug 8, 2014 at 9:21 PM, Ion Gaztañaga <igaztanaga_at_[hidden]> wrote:
> El 08/08/2014 18:15, Andrey Semashev wrote:
>> - Looking at your current implementation, I think some platforms are
>> not directly supported by Boost.Atomic. That is, I don't test for e.g.
>> __IBMCPP__ or availability of Solaris functions in Boost.Atomic, so
>> unless these platforms are supported by other backends (like gcc asm
>> based, for example) these platforms will fall back to the lock-based
>> backend. The list of the supported lock-free platforms can be inferred
>> from the boost/atomic/detail/platform.hpp file (or the list of
>> caps_*.hpp files there). Needless to say, I'm all for improving
>> portability of Boost.Atomic, and I'll appreciate any help in
>> supporting these platforms.
> I think maintainers of those compilers and platforms should help us
> supporting atomic operations. Interprocess can only require
> BOOST_ATOMIC_INT32_LOCK_FREE to be 1 after including
> boost/atomic/capabilities.hpp to test if needed atomics are available.

Correction, the macro value should be 2, not 1.

> It would be nice if Boost.Atomic used
> C++11 atomics in case they were available, that would make Boost libraries
> easier to maintain, but for now Boost.Interprocess can use C++11 if
> available.

Boost.Atomic is planned to have a more extended feature set than
std::atomic provides, so I can't use it.

>> - The underlying storage type of atomic<> is not officially fixed for
>> a given element type and may differ from one platform to another
>> (including from one compiler to another on the same target hardware).
>> For example, some versions of MSVC don't support 8 and 16-bit
>> interlocked intrinsics and Boost.Atomic uses 32-bit atomic ops
>> instead. Not sure how critical this is for Boost.Interprocess.
> Not critical, all atomics are now only 32 bit. Thanks for your quick reply.

I used MSVC as an example. In theory, it is possible that some
platform only provides 64-bit atomics and Boost.Atomic has to emulate
32-bit ops. I don't know if such a platform really exists though.

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