Re: [PATCH] S/390: Disable vectorization for loops with few iterations

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

On Thu, Mar 2, 2017 at 4:39 PM, Robin Dapp <rdapp@linux.vnet.ibm.com> wrote:
> Hi,
>
> the following patch defines the PARAM_MIN_VECT_LOOP_BOUND parameter in
> the s390 backend.  It helps with the vectorization epilogue problem
> described here [1].
> I see an overall performance increase of > 1% in SPECfp2006, yet some
> cases like cactusADM regress.  This seems to be caused by the vectorizer
> creating an epilogue guard for one more iteration than before, which, in
> turn, causes e.g. predcom to run on the epilogue that it used to ignore
> before ("Loop iterates only 1 time, nothing to do.").  Subsequent,
> minor, effects cause an eventual slowdown.
>
> Until the reason for the bad epilogue code is understood, this patch
> mitigates the problem.  When investigating the issue, I stumbled across
> an attempt to vectorize the epilogue itself as well as combine it with
> the vectorized loop in addition to vector masking [2].  A similar
> approach might also help here.  My original observation of high register
> pressure within the epilogue still stands.  In this specific case, it
> would most likely suffice to save all registers once, run the epilogue
> and restore the registers.  I'm pretty sure this would be faster than
> the "spill fest" that's currently happening.

Note it also honors PARAM_MIN_VECT_LOOP_BOUND if there's no epilouge.

Richard.

> Regards
>  Robin
>
> [1] https://gcc.gnu.org/ml/gcc/2017-01/msg00234.html
> [2] https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01562.html
>
> --
>
> gcc/ChangeLog:
>
> 2017-03-02  Robin Dapp  <rdapp@linux.vnet.ibm.com>
>
>         * config/s390/s390.c (s390_option_override_internal): Set
>         PARAM_MIN_VECT_LOOP_BOUND