On Fri, Jul 21, 2023 at 8:08 AM Kewen.Lin <li...@linux.ibm.com> wrote:
>
> Hi,
>
> The function vect_update_epilogue_niters which has been
> removed by r14-2281 has some code taking care of that if
> there is only one scalar iteration left for epilogue then
> we won't try to vectorize it any more.
>
> Although costing should be able to care about it eventually,
> I think we still want this special casing without costing
> enabled, so this patch is to add it back in function
> vect_analyze_loop_costing, and make it more general for
> both main and epilogue loops as Richi suggested, it can fix
> some exposed failures on Power10:
>
> - gcc.target/powerpc/p9-vec-length-epil-{1,8}.c
> - gcc.dg/vect/slp-perm-{1,5,6,7}.c
>
> Bootstrapped and regtested on x86_64-redhat-linux,
> aarch64-linux-gnu, powerpc64-linux-gnu P8/P9 and
> powerpc64le-linux-gnu P9/P10.
>
> Is it ok for trunk?
OK.
Thanks,
Richard.
> BR,
> Kewen
> -----
> PR tree-optimization/110740
>
> gcc/ChangeLog:
>
> * tree-vect-loop.cc (vect_analyze_loop_costing): Do not vectorize a
> loop with a single scalar iteration.
> ---
> gcc/tree-vect-loop.cc | 55 ++++++++++++++++++++++++++-----------------
> 1 file changed, 34 insertions(+), 21 deletions(-)
>
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index b44fb9c7712..92d2abde094 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -2158,8 +2158,7 @@ vect_analyze_loop_costing (loop_vec_info loop_vinfo,
> epilogue we can also decide whether the main loop leaves us
> with enough iterations, prefering a smaller vector epilog then
> also possibly used for the case we skip the vector loop. */
> - if (!LOOP_VINFO_USING_PARTIAL_VECTORS_P (loop_vinfo)
> - && LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
> + if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
> {
> widest_int scalar_niters
> = wi::to_widest (LOOP_VINFO_NITERSM1 (loop_vinfo)) + 1;
> @@ -2182,32 +2181,46 @@ vect_analyze_loop_costing (loop_vec_info loop_vinfo,
> % lowest_vf + gap);
> }
> }
> -
> - /* Check that the loop processes at least one full vector. */
> - poly_uint64 vf = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
> - if (known_lt (scalar_niters, vf))
> + /* Reject vectorizing for a single scalar iteration, even if
> + we could in principle implement that using partial vectors. */
> + unsigned peeling_gap = LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo);
> + if (scalar_niters <= peeling_gap + 1)
> {
> if (dump_enabled_p ())
> dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> - "loop does not have enough iterations "
> - "to support vectorization.\n");
> + "not vectorized: loop only has a single "
> + "scalar iteration.\n");
> return 0;
> }
>
> - /* If we need to peel an extra epilogue iteration to handle data
> - accesses with gaps, check that there are enough scalar iterations
> - available.
> -
> - The check above is redundant with this one when peeling for gaps,
> - but the distinction is useful for diagnostics. */
> - if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> - && known_le (scalar_niters, vf))
> + if (!LOOP_VINFO_USING_PARTIAL_VECTORS_P (loop_vinfo))
> {
> - if (dump_enabled_p ())
> - dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> - "loop does not have enough iterations "
> - "to support peeling for gaps.\n");
> - return 0;
> + /* Check that the loop processes at least one full vector. */
> + poly_uint64 vf = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
> + if (known_lt (scalar_niters, vf))
> + {
> + if (dump_enabled_p ())
> + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> + "loop does not have enough iterations "
> + "to support vectorization.\n");
> + return 0;
> + }
> +
> + /* If we need to peel an extra epilogue iteration to handle data
> + accesses with gaps, check that there are enough scalar iterations
> + available.
> +
> + The check above is redundant with this one when peeling for gaps,
> + but the distinction is useful for diagnostics. */
> + if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> + && known_le (scalar_niters, vf))
> + {
> + if (dump_enabled_p ())
> + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> + "loop does not have enough iterations "
> + "to support peeling for gaps.\n");
> + return 0;
> + }
> }
> }
>
> --
> 2.39.3
