[llvm-dev] ScalarEvolution "add nsw" question

[Johannes Doerfert via llvm-dev]

Replace context with flow in the last mail... sry.

On 04/12, Johannes Doerfert via llvm-dev wrote:
> Hi Bjarke,
> 
> Thanks for your response. I agree that Sanjoy's email already answered
> my question. To be honest, I am not 100% sure about the loop code and
> the consequences but that doesn't matter to much at the moment. What
> matters is that I have made progress on explaining the mismatch between
> the ScalarEvolution results and what I would have expected, or better,
> liked. At the moment I think that the context in-sensitivity of
> ScalarEvolution is what is preventing me from the results I would like
> to see. Therefor, I have to look at the IR during Polly's context
> sensitive analysis/modeling step. As our "context" includes information
> about possibly "non-returning function calls", infinite loops, etc. I
> think that should allow to get precise but also sound results.
> 
> Thank you both again,
>   Johannes
> 
> On 04/12, 'Bjarke Roune' via Polly Development wrote:
> > Hi Johannes,
> > 
> > Sanjoy has given you great information already.
> > 
> > On Sun, Apr 10, 2016 at 5:19 PM, Sanjoy Das <sanjoy at playingwithpointers.com>
> > wrote:
> > 
> > > Johannes Doerfert wrote:
> > > > Is there any plan to use e.g., post-dominance information to
> > > > propagate wrapping flags?
> > >
> > > None that I'm aware of.
> > >
> > > > If x +nsw y post-dominates the entry block
> > > > [and is used in some side-effect instruction] the SCEV could be marked
> > > > as non-wrapping, couldn't it?
> > >
> > > Yes, but we have to be careful to account for exceptions, infinitely
> > > looping functions, exit(0) etc.  I.e. in
> > >
> > > void foo() {
> > >   use(x + y);
> > >   exit(0);
> > >   use(x +nsw y);
> > > }
> > >
> > > the nsw in the later x+y cannot be transferred to the former x+y
> > > (assume use is a side effecting use).
> > >
> > > The code I wrote to handle this could be made more powerful, e.g. by
> > introducing an analysis pass for strong post-dominance (as opposed to the
> > usual kind of post-dominance). There was a brief amount of discussion on
> > this a while back:
> > 
> >   https://groups.google.com/forum/#!topic/llvm-dev/CBJY51__wOE
> > 
> > I don't know if there's been any developments on that since then.
> > 
> > Bjarke
> > 
> > -- 
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> 
> -- 
> 
> Johannes Doerfert
> Researcher / PhD Student
> 
> Compiler Design Lab (Prof. Hack)
> Saarland University, Computer Science
> Building E1.3, Room 4.31
> 
> Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
> Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert



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-- 

Johannes Doerfert
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
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