Re: [PATCH][1/n] Fix PR81303

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Fri, Jul 21, 2017 at 8:12 AM, Richard Biener <rguenther@suse.de> wrote:
>
> The following is sth I noticed when looking at a way to fix PR81303.
> We happily compute a runtime cost model threshold that executes the
> vectorized variant even though no vector iteration takes place due
> to the number of prologue/epilogue iterations.  The following fixes
> that -- note that if we do not know the prologue/epilogue counts
> statically they are estimated at vf/2 which means there's still the
> chance the vector iteration won't execute.  To fix that we'd have to
> estimate those as vf-1 instead, sth we might consider doing anyway
> given that we regularly completely peel the epilogues vf-1 times
> in that case.  Maybe as followup.
Hi,
Do we consider disabling epilogue peeling if # of iters is unknown at
compilation time?  When loop is versioned, could we share epilogue
with versioned loop?  Epilogue was shared before?

Thanks,
bin
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk.
>
> Richard.
>
> 2016-07-21  Richard Biener  <rguenther@suse.de>
>
>         PR tree-optimization/81303
>         * tree-vect-loop.c (vect_estimate_min_profitable_iters): Take
>         into account prologue and epilogue iterations when raising
>         min_profitable_iters to sth at least covering one vector iteration.
>
> Index: gcc/tree-vect-loop.c
> ===================================================================
> --- gcc/tree-vect-loop.c        (revision 250384)
> +++ gcc/tree-vect-loop.c        (working copy)
> @@ -3702,8 +3702,9 @@ vect_estimate_min_profitable_iters (loop
>                "  Calculated minimum iters for profitability: %d\n",
>                min_profitable_iters);
>
> -  min_profitable_iters =
> -       min_profitable_iters < vf ? vf : min_profitable_iters;
> +  /* We want the vectorized loop to execute at least once.  */
> +  if (min_profitable_iters < (vf + peel_iters_prologue + peel_iters_epilogue))
> +    min_profitable_iters = vf + peel_iters_prologue + peel_iters_epilogue;
>
>    if (dump_enabled_p ())
>      dump_printf_loc (MSG_NOTE, vect_location,