[LLVMdev] parallel loop metadata question

[Hal Finkel]

----- Original Message -----
> From: "Tobias Grosser" <tobias at grosser.es>
> To: "Pekka Jääskeläinen" <pekka.jaaskelainen at tut.fi>, "Jonathan Humphreys" <j-humphreys at ti.com>, llvmdev at cs.uiuc.edu
> Sent: Monday, May 5, 2014 3:36:07 AM
> Subject: Re: [LLVMdev] parallel loop metadata question
> 
> On 05/05/2014 10:14, Pekka Jääskeläinen wrote:
> > On 05/02/2014 07:22 PM, Humphreys, Jonathan wrote:
> >> Thanks for the link.  I understand your concern of caution with
> >> metadata.
> >> I cannot, though, imagine how the dependence relation
> >> (independence)
> >> of two
> >> memory references can be affected by a third memory reference.  If
> >> two
> >> references are independent across loop iterations, then they are
> >> independent, and any other load or store cannot change that.
> >>  Right?
> >
> > Yes, it makes sense. I'm mostly concerned about accesses to stack,
> > but even those at this point should remain independent. Otherwise
> > even
> > the current semantics might produce broken code with parallel stack
> > accesses.
> >
> > However, as this is such a major semantics change to the original
> > one, I'd
> > like to hear more opinions on it. I suggest you create a
> > (documentation)
> > patch where the new semantics is articulated and request comments
> > for it at
> > the LLVM-commits list.
> 
> I agree with both. I think the extension is very reasonable and I
> also
> do not see a reason why this interpretation should cause troubles.
> However, to get it right it would be good to get this throughly
> reviewed.

I agree, I think this sounds reasonable. You'll certainly need to be careful, however, that the associated instruction has not been hoisted/sunk out of the associated loops. Even if the load is one that can be speculated, that does not mean that there was not a control dependence on the independence information itself.

 -Hal

> 
> Tobias
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory