[LLVMdev] BasicAA unable to analyze recursive PHI nodes

Tobias Edler von Koch
Thu, 11 Jun 2015 15:02:37 -0000

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.

>
> >  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).

Thanks for your comments!

Tobias

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