Re: [PATCH 24/33]New parameter bound on number of selected candidates

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Apr 18, 2017 at 12:50 PM, Bin Cheng <Bin.Cheng@arm.com> wrote:
> Hi,
> IVOPTs still have difficulty for outer loop (especially for large loop nest), and tend to select too many candidates.
> It's generally bad because of unavoidable register spilling.  In this case, we probably want to compute iv_uses with
> small number of bivs.  Though this results in more computation inside of loop, it could improve spilling.
> This patch adds new parameter bound on number of selected candidates, it simply gives up if too many candidates are
> selected.  So far it works loop by loop, I am not sure if we want to by pass whole loop nest once this bound is hit.
> Is it OK?

Hmm, I don't like such kind of caps.  We should simply add less
candidates in such cases?  Like cap this
in find_optimal_iv_set instead, always using the original set of
candidates just not trying harder?

IIRC much of the problem is because the inner loop has been processed
already and thus there may appear
to be a very large number of "original" IVs already?  I may misremeber though.

I think this patch doesn't really belong in the series?

Richard.

>
> Thanks,
> bin
> 2017-04-11  Bin Cheng  <bin.cheng@arm.com>
>
>         * doc/invoke.texi (iv-max-selected-candidates): New.
>         * params.def (PARAM_IV_MAX_SELECTED_CANDIDATES): New.
>         * tree-ssa-loop-ivopts.c (MAX_SELECTED_CANDIDATES): New.
>         (tree_ssa_iv_optimize_loop): Skip if too many cands are selected.