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Date: Wed, 30 Mar 2011 14:39:00 -0000
From: Michael Matz <matz@suse.de>
To: Aldy Hernandez <aldyh@redhat.com>
Cc: Richard Guenther <richard.guenther@gmail.com>,	gcc-patches <gcc-patches@gcc.gnu.org>,	Jakub Jelinek <jakub@redhat.com>
Subject: Re: [cxx-mem-model] bitfield tests
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Hi,

On Wed, 30 Mar 2011, Aldy Hernandez wrote:

> 
> > The memory model is not implementable on strict-alignment targets
> > that do not have a byte store operation.  But we previously said that ;)
> 
> Yes.  I think we should issue an error when we have such a target and the user
> tries -fmemory-model=c++0x.  However, how many strict-alignment targets are
> not byte addressable nowadays?

Consider cache aliasing, where the unit of coherence (absent using atomic 
instructions) is for instance 64 bytes.  I'm not sure how the mem-model 
could be implemented without generally falling back to atomics.

Or CPU internal write buffers that could (again if there are just normal 
writes, not atomics) reorder or merge write requests.  I think also that 
would destroy guarantees that the cxx-mem-model tries to provide.


Ciao,
Michael.