[PATCH] vect: Enhance condition check to use partial vectors in vectorizable_condition

[Richard Sandiford]

"Kewen.Lin" <linkw@linux.ibm.com> writes:
> Hi,
>
> This patch is derived from the review of vector with length patch series.
> The length-based partial vector approach doesn't support reduction so far,
> so we would like to disable vectorization with partial vectors explicitly
> for it in vectorizable_condition.  Otherwise, it will cause some unexpected
> failures for a few cases like gcc.dg/vect/pr65947-2.c.
>
> But if we disable it for the cases excepting for reduction_type equal to
> EXTRACT_LAST_REDUCTION, it cause one regression failure on aarch64:
>   gcc.target/aarch64/sve/reduc_8.c -march=armv8.2-a+sve
>
> The disabling makes the outer loop can't work with partial vectors, the
> check fails.  But the case is safe to adopt it.  As Richard S. pointed out
> in the review comments, the extra inactive lanes only matter for double
> reductions, so this patch is to permit vectorization with partial vectors
> for cases EXTRACT_LAST_REDUCTION or nested-cycle reduction.
>
> Testing is ongoing, is it ok for trunk if the testing goes well?
>
> BR,
> Kewen
> -----
> gcc/ChangeLog:
>
> 	* tree-vect-stmts.c (vectorizable_condition): Prohibit vectorization
> 	with partial vectors explicitly excepting for EXTRACT_LAST_REDUCTION
> 	or nested-cycle reduction.
>
> diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
> index 40e2664f93b..c23520aceab 100644
> --- a/gcc/tree-vect-stmts.c
> +++ b/gcc/tree-vect-stmts.c
> @@ -9968,11 +9968,16 @@ vectorizable_condition (vec_info *vinfo,
>  	  return false;
>  	}
>  
> -      if (loop_vinfo
> -	  && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo)
> -	  && reduction_type == EXTRACT_LAST_REDUCTION)
> -	vect_record_loop_mask (loop_vinfo, &LOOP_VINFO_MASKS (loop_vinfo),
> -			       ncopies * vec_num, vectype, NULL);
> +      if (loop_vinfo && for_reduction
> +	  && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo))
> +	{
> +	  if (reduction_type == EXTRACT_LAST_REDUCTION)
> +	    vect_record_loop_mask (loop_vinfo, &LOOP_VINFO_MASKS (loop_vinfo),
> +				   ncopies * vec_num, vectype, NULL);
> +	  /* Extra inactive lanes should be safe for vect_nested_cycle.  */
> +	  else if (STMT_VINFO_DEF_TYPE (reduc_info) != vect_nested_cycle)
> +	    LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;

We should print a dump message when setting this to false.  E.g.:

	  if (dump_enabled_p ())
	    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
			     "conditional reduction prevents the use"
			     " of partial vectors\n");

OK with that change, thanks.

Richard