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Re: forcing the compiler to reload from memory with c++0x

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From	Anthony Williams <anthony.ajw@gmail.com>
Newsgroups	comp.programming.threads
Subject	Re: forcing the compiler to reload from memory with c++0x
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Date	Fri, 28 Jan 2011 08:01:05 +0000
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frege <gottlobfrege@gmail.com> writes:

> Now once you start talking C++0x, I don't *think* anything changes
> (particularly since the standard tries to follow current practice, not
> make up something new), but I can't actually quote chapter and verse
> as to why.  In particular, even though we code in it every day, it is
> hard to say exactly what memory is, and thus what volatile does and
> doesn't do, what the atomic ops do, etc - in a precise, technical-
> standard kind of way.  For me at least.

In C++0x, if there are non-atomic accesses to an object from multiple
threads, and at least one of them is a write then you have a "data race"
and undefined behaviour. Volatile still has nothing to do with
threading.

If you don't think any ordering guarantees are required then use atomic
operations with memory_order_relaxed. However, be aware that writes with
relaxed operations can be reordered just like non-atomic writes, so you
might well need a fence just to prevent the compiler reordering.

If you are only using your fence to prevent compiler reordering then you
can use atomic_signal_fence rather than atomic_thread_fence. The former
is primarily a compiler directive, and will likely not issue any CPU
instructions.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976

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Re: forcing the compiler to reload from memory with c++0x frege <gottlobfrege@gmail.com> - 2011-01-27 21:56 -0800
  Re: forcing the compiler to reload from memory with c++0x Dmitriy Vyukov <dvyukov@gmail.com> - 2011-01-28 00:54 -0800
  Re: forcing the compiler to reload from memory with c++0x Andy Venikov <swojchelowek@gmail.com> - 2011-01-28 15:28 -0500
    Re: forcing the compiler to reload from memory with c++0x frege <gottlobfrege@gmail.com> - 2011-01-28 19:58 -0800
  Re: forcing the compiler to reload from memory with c++0x Anthony Williams <anthony.ajw@gmail.com> - 2011-01-28 08:01 +0000

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