[LLVMdev] BasicAA unable to analyze recursive PHI nodes

[Hal Finkel]

----- Original Message -----
> From: "Tobias Edler von Koch" <tobias at codeaurora.org>
> To: "Daniel Berlin" <dberlin at dberlin.org>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Thursday, June 11, 2015 10:02:37 AM
> Subject: Re: [LLVMdev] BasicAA unable to analyze recursive PHI nodes
> 
> Hi Daniel,
> 
> On Wed, 10 Jun 2015 17:11:02 -0700 Daniel Berlin
> <dberlin at dberlin.org>
> wrote:
> 
> > 
> > I didn't see numbers one way or the other. It probably has some
> > overhead, if it's the right solution, i'm sure it can be ade
> > faster.
> 
> As far as I remember, scev-aa is off because of limitations in the
> current pass manager that would mean it would have to be recalculated
> far too often.

That's correct. It also may need some caching, or other performance-related work, to be sufficiently efficient to be on by default.

> 
> > 
> > >  I think this is a common enough case for it to be
> > > handled by BasicAA.
> > >
> > 
> > Can you provide some evidence?
> > 
> > IE hard numbers of how often it occurs on some set of benchmarks or
> > something, and what the compile time cost is?
> > 
> > It's really hard to say whether this is the right tradeoff without
> > any data.
> 
> I agree. We're seeing about 3% improvement on a number of internal
> benchmarks, but I'll give this a try on SPEC etc. and share the
> results
> here (including compile time).

Sounds good; please do.

 -Hal

> 
> Thanks for your comments!
> 
> Tobias
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory