[LLVMdev] Proposal for a new LLVM concurrency memory model

David Greene dag at cray.com
Tue Apr 27 09:02:54 PDT 2010

On Monday 26 April 2010 16:09:48 Jeffrey Yasskin wrote:

> > Vector atomics are extremely useful on architectures that support them.
> I'm curious about the architectures/instructions you're thinking of.
> Something like 'lock; movdqa'?

Don't think X86.  Think traditional vector machines like the Cray X1/X2.
Atomic vector adds and logicals are common operations.

> > I'm not sure we need atomicity across vector elements, so decomposing
> > shouldn't be a problem, but I will have to think about it a bit.
> That's interesting. Naïvely, it seems to violate the whole point of
> atomics, since it means their side-effects don't appear atomic to
> other threads. We could relax that if it's useful, of course. How do
> you imagine specifying the program's constraints on vector/word
> tearing?

I wrote up a response to Renato's question.  In the end I don't think
the tearing idea is worth the effort.  The compiler should just not
generate vector atomics if the target doesn't support them.


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