[2/3,vect] Consider outside costs earlier for epilogue loops

Hi,

This patch changes the order in which we check outside and inside costs 
for epilogue loops, this is to ensure that a predicated epilogue is more 
likely to be picked over an unpredicated one, since it saves having to 
enter a scalar epilogue loop.

gcc/ChangeLog:

         * tree-vect-loop.c (vect_better_loop_vinfo_p): Change how 
epilogue loop costs are compared.

Hi,

I completely forgot I still had this patch out as well, I grouped it 
together with the unrolling because it was what motivated the change, 
but it is actually wider applicable and can be reviewed separately.

On 17/09/2021 16:32, Andre Vieira (lists) via Gcc-patches wrote:
> Hi,
>
> This patch changes the order in which we check outside and inside 
> costs for epilogue loops, this is to ensure that a predicated epilogue 
> is more likely to be picked over an unpredicated one, since it saves 
> having to enter a scalar epilogue loop.
>
> gcc/ChangeLog:
>
>         * tree-vect-loop.c (vect_better_loop_vinfo_p): Change how 
> epilogue loop costs are compared.

"Andre Vieira (lists) via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
> Hi,
>
> This patch changes the order in which we check outside and inside costs 
> for epilogue loops, this is to ensure that a predicated epilogue is more 
> likely to be picked over an unpredicated one, since it saves having to 
> enter a scalar epilogue loop.
>
> gcc/ChangeLog:
>
>          * tree-vect-loop.c (vect_better_loop_vinfo_p): Change how 
> epilogue loop costs are compared.

OK, thanks.  Sorry for the slow review.

Richard

> diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
> index 14f8150d7c262b9422784e0e997ca4387664a20a..038af13a91d43c9f09186d042cf415020ea73a38 100644
> --- a/gcc/tree-vect-loop.c
> +++ b/gcc/tree-vect-loop.c
> @@ -2881,17 +2881,75 @@ vect_better_loop_vinfo_p (loop_vec_info new_loop_vinfo,
>  	return new_simdlen_p;
>      }
>  
> +  loop_vec_info main_loop = LOOP_VINFO_ORIG_LOOP_INFO (old_loop_vinfo);
> +  if (main_loop)
> +    {
> +      poly_uint64 main_poly_vf = LOOP_VINFO_VECT_FACTOR (main_loop);
> +      unsigned HOST_WIDE_INT main_vf;
> +      unsigned HOST_WIDE_INT old_factor, new_factor, old_cost, new_cost;
> +      /* If we can determine how many iterations are left for the epilogue
> +	 loop, that is if both the main loop's vectorization factor and number
> +	 of iterations are constant, then we use them to calculate the cost of
> +	 the epilogue loop together with a 'likely value' for the epilogues
> +	 vectorization factor.  Otherwise we use the main loop's vectorization
> +	 factor and the maximum poly value for the epilogue's.  If the target
> +	 has not provided with a sensible upper bound poly vectorization
> +	 factors are likely to be favored over constant ones.  */
> +      if (main_poly_vf.is_constant (&main_vf)
> +	  && LOOP_VINFO_NITERS_KNOWN_P (main_loop))
> +	{
> +	  unsigned HOST_WIDE_INT niters
> +	    = LOOP_VINFO_INT_NITERS (main_loop) % main_vf;
> +	  HOST_WIDE_INT old_likely_vf
> +	    = estimated_poly_value (old_vf, POLY_VALUE_LIKELY);
> +	  HOST_WIDE_INT new_likely_vf
> +	    = estimated_poly_value (new_vf, POLY_VALUE_LIKELY);
> +
> +	  /* If the epilogue is using partial vectors we account for the
> +	     partial iteration here too.  */
> +	  old_factor = niters / old_likely_vf;
> +	  if (LOOP_VINFO_USING_PARTIAL_VECTORS_P (old_loop_vinfo)
> +	      && niters % old_likely_vf != 0)
> +	    old_factor++;
> +
> +	  new_factor = niters / new_likely_vf;
> +	  if (LOOP_VINFO_USING_PARTIAL_VECTORS_P (new_loop_vinfo)
> +	      && niters % new_likely_vf != 0)
> +	    new_factor++;
> +	}
> +      else
> +	{
> +	  unsigned HOST_WIDE_INT main_vf_max
> +	    = estimated_poly_value (main_poly_vf, POLY_VALUE_MAX);
> +
> +	  old_factor = main_vf_max / estimated_poly_value (old_vf,
> +							   POLY_VALUE_MAX);
> +	  new_factor = main_vf_max / estimated_poly_value (new_vf,
> +							   POLY_VALUE_MAX);
> +
> +	  /* If the loop is not using partial vectors then it will iterate one
> +	     time less than one that does.  It is safe to subtract one here,
> +	     because the main loop's vf is always at least 2x bigger than that
> +	     of an epilogue.  */
> +	  if (!LOOP_VINFO_USING_PARTIAL_VECTORS_P (old_loop_vinfo))
> +	    old_factor -= 1;
> +	  if (!LOOP_VINFO_USING_PARTIAL_VECTORS_P (new_loop_vinfo))
> +	    new_factor -= 1;
> +	}
> +
> +      /* Compute the costs by multiplying the inside costs with the factor and
> +	 add the outside costs for a more complete picture.  The factor is the
> +	 amount of times we are expecting to iterate this epilogue.  */
> +      old_cost = old_loop_vinfo->vec_inside_cost * old_factor;
> +      new_cost = new_loop_vinfo->vec_inside_cost * new_factor;
> +      old_cost += old_loop_vinfo->vec_outside_cost;
> +      new_cost += new_loop_vinfo->vec_outside_cost;
> +      return new_cost < old_cost;
> +    }
> +
>    /* Limit the VFs to what is likely to be the maximum number of iterations,
>       to handle cases in which at least one loop_vinfo is fully-masked.  */
> -  HOST_WIDE_INT estimated_max_niter;
> -  loop_vec_info main_loop = LOOP_VINFO_ORIG_LOOP_INFO (old_loop_vinfo);
> -  unsigned HOST_WIDE_INT main_vf;
> -  if (main_loop
> -      && LOOP_VINFO_NITERS_KNOWN_P (main_loop)
> -      && LOOP_VINFO_VECT_FACTOR (main_loop).is_constant (&main_vf))
> -    estimated_max_niter = LOOP_VINFO_INT_NITERS (main_loop) % main_vf;
> -  else
> -    estimated_max_niter = likely_max_stmt_executions_int (loop);
> +  HOST_WIDE_INT estimated_max_niter = likely_max_stmt_executions_int (loop);
>    if (estimated_max_niter != -1)
>      {
>        if (known_le (estimated_max_niter, new_vf))