[LLVMdev] BasicAA unable to analyze recursive PHI nodes

[Hal Finkel]

----- Original Message -----
> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: "Tobias Edler von Koch" <tobias at codeaurora.org>
> Cc: reviews+D10368+public+78f7503e373eac35 at reviews.llvm.org, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>,
> "Hal Finkel" <hfinkel at anl.gov>
> Sent: Saturday, June 20, 2015 11:55:55 AM
> Subject: Re: [LLVMdev] BasicAA unable to analyze recursive PHI nodes
> 
> > Not a huge change here although the numbers tend to be more on the
> > side
> > of a slowdown. This is interesting - you'd think that better alias
> > analysis shouldn't have that effect. Any idea what could be causing
> > this?
> 
> 
> LLVM's alias analysis is good but not amazing.
> If you improve it significantly (as you appear to have :P), you give
> the compiler more freedom to do things.
> What it chooses to do with that freedom  may not always be good :)
> 
> Basically, with any significant alias analysis change comes a lot of
> tuning of heuristics/etc, or looking through regressions, to
> determine
> what's going on now.  *Usually* it's "something used to not be able
> to
> do a ton of <whatever>, and now it can.  The cost model for
> <whatever>
> needs to be worked on so it doesn't go too crazy".

I agree. In the mean time, let's get this committed turned off by default. Please update your patch so that the new behavior is controlled by some cl::opt that is off by default. This will give us a better way to do more-extensive benchmarking and help isolate the problems.

 -Hal

> 

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