[PATCH] D19950: Use frequency info to guide Loop Invariant Code Motion.

[Xinliang David Li via llvm-commits]

On Tue, May 10, 2016 at 11:01 AM, Dehao Chen <danielcdh at gmail.com> wrote:

> danielcdh added a comment.
>
> In http://reviews.llvm.org/D19950#425287, @hfinkel wrote:
>
> > In http://reviews.llvm.org/D19950#425286, @hfinkel wrote:
> >
> > > In http://reviews.llvm.org/D19950#425285, @davidxl wrote:
> > >
> > > > Static prediction has been conservative in estimating loop trip
> count -- it produces something like 30ish iterations. If the a very hot
> loop has a big if-then-else (or switch), it is very likely to mark many
> bbs' to be colder than the loop header.   Turning on this for static
> prediction really depends on the false rate. It seems to be this can get
> wrong pretty easily for very hot loops (which is also the most important
> thing to optimize for).
> > >
> > >
> > > This is a good point. There's no universal conservative choice
> (assuming a small trip count is conservative in some cases, and assuming a
> large trip count is conservative in other cases).
> >
> >
> > Would it be better (and practical) if there were some way for the BFI
> client to specify which kind of 'conservative' is desired?
> >
> > Also, why are we doing this instead of sinking later (in CGP or
> similar)? LICM can expose optimization opportunities, plus represents a
> code pattern the user might input manually. Sinking later seems more robust.
>
>
> I looked at CGP pass, looks like it's handling the sinking case-by-case
> (e.g. there is separate routine to handle sinking of load, gep, etc. I'm
> afraid this would miss opportunities. Additionally, the file-level comment
> of CGP pass says "This works around limitations in it's
> basic-block-at-a-time approach. It should eventually be removed."
>


Perhaps you can do profile driven sinking CGP separately to handle manually
hoisted code situation mentioned by Hal.

David


>
> I'm not quite clear why it helps to move code out of loop early and later
> sink it inside. Could you give an example or some more context?
>
> Thanks,
> Dehao
>
>
> http://reviews.llvm.org/D19950
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160510/28d4416f/attachment.html>