Re: [PATCH 2/2] vect: Enhance cost evaluation in vect_transform_slp_perm_load_1

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Richi,

Thanks for the review!

on 2023/5/22 21:44, Richard Biener wrote:
> On Wed, May 17, 2023 at 8:15 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>>
>> Hi,
>>
>> Following Richi's suggestion in [1], I'm working on deferring
>> cost evaluation next to the transformation, this patch is
>> to enhance function vect_transform_slp_perm_load_1 which
>> could under-cost for vector permutation, since the costing
>> doesn't try to consider nvectors_per_build, it's inconsistent
>> with the transformation part.
>>
>> Bootstrapped and regtested on x86_64-redhat-linux,
>> aarch64-linux-gnu and powerpc64{,le}-linux-gnu.
>>
>> Is it ok for trunk?
>>
>> [1] https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563624.html
>>
>> BR,
>> Kewen
>> -----
>> gcc/ChangeLog:
>>
>>         * tree-vect-slp.cc (vect_transform_slp_perm_load_1): Adjust the
>>         calculation on n_perms by considering nvectors_per_build.
>>
>> gcc/testsuite/ChangeLog:
>>
>>         * gcc.dg/vect/costmodel/ppc/costmodel-slp-perm.c: New test.
>> ---
>>  .../vect/costmodel/ppc/costmodel-slp-perm.c   | 23 +++++++
>>  gcc/tree-vect-slp.cc                          | 66 ++++++++++---------
>>  2 files changed, 57 insertions(+), 32 deletions(-)
>>  create mode 100644 gcc/testsuite/gcc.dg/vect/costmodel/ppc/costmodel-slp-perm.c
>>
>> diff --git a/gcc/testsuite/gcc.dg/vect/costmodel/ppc/costmodel-slp-perm.c b/gcc/testsuite/gcc.dg/vect/costmodel/ppc/costmodel-slp-perm.c
>> new file mode 100644
>> index 00000000000..e5c4dceddfb
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.dg/vect/costmodel/ppc/costmodel-slp-perm.c
>> @@ -0,0 +1,23 @@
>> +/* { dg-do compile } */
>> +/* { dg-require-effective-target vect_int } */
>> +/* { dg-require-effective-target powerpc_p9vector_ok } */
>> +/* Specify power9 to ensure the vectorization is profitable
>> +   and test point stands, otherwise it could be not profitable
>> +   to vectorize.  */
>> +/* { dg-additional-options "-mdejagnu-cpu=power9 -mpower9-vector" } */
>> +
>> +/* Verify we cost the exact count for required vec_perm.  */
>> +
>> +int x[1024], y[1024];
>> +
>> +void
>> +foo ()
>> +{
>> +  for (int i = 0; i < 512; ++i)
>> +    {
>> +      x[2 * i] = y[1023 - (2 * i)];
>> +      x[2 * i + 1] = y[1023 - (2 * i + 1)];
>> +    }
>> +}
>> +
>> +/* { dg-final { scan-tree-dump-times "2 times vec_perm" 1 "vect" } } */
>> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
>> index e5c9d7e766e..af9a6dd4fa9 100644
>> --- a/gcc/tree-vect-slp.cc
>> +++ b/gcc/tree-vect-slp.cc
>> @@ -8115,12 +8115,12 @@ vect_transform_slp_perm_load_1 (vec_info *vinfo, slp_tree node,
>>
>>    mode = TYPE_MODE (vectype);
>>    poly_uint64 nunits = TYPE_VECTOR_SUBPARTS (vectype);
>> +  unsigned int nstmts = SLP_TREE_NUMBER_OF_VEC_STMTS (node);
>>
>>    /* Initialize the vect stmts of NODE to properly insert the generated
>>       stmts later.  */
>>    if (! analyze_only)
>> -    for (unsigned i = SLP_TREE_VEC_STMTS (node).length ();
>> -        i < SLP_TREE_NUMBER_OF_VEC_STMTS (node); i++)
>> +    for (unsigned i = SLP_TREE_VEC_STMTS (node).length (); i < nstmts; i++)
>>        SLP_TREE_VEC_STMTS (node).quick_push (NULL);
>>
>>    /* Generate permutation masks for every NODE. Number of masks for each NODE
>> @@ -8161,7 +8161,10 @@ vect_transform_slp_perm_load_1 (vec_info *vinfo, slp_tree node,
>>          (b) the permutes only need a single vector input.  */
>>        mask.new_vector (nunits, group_size, 3);
>>        nelts_to_build = mask.encoded_nelts ();
>> -      nvectors_per_build = SLP_TREE_VEC_STMTS (node).length ();
>> +      /* It's possible to obtain zero nstmts during analyze_only, so make
>> +        it at least one to ensure the later computation for n_perms
>> +        proceed.  */
>> +      nvectors_per_build = nstmts > 0 ? nstmts : 1;
>>        in_nlanes = DR_GROUP_SIZE (stmt_info) * 3;
>>      }
>>    else
>> @@ -8252,40 +8255,39 @@ vect_transform_slp_perm_load_1 (vec_info *vinfo, slp_tree node,
>>                   return false;
>>                 }
>>
>> -             ++*n_perms;
>> -
>> +             tree mask_vec = NULL_TREE;
>>               if (!analyze_only)
>> -               {
>> -                 tree mask_vec = vect_gen_perm_mask_checked (vectype, indices);
>> +               mask_vec = vect_gen_perm_mask_checked (vectype, indices);
>>
>> -                 if (second_vec_index == -1)
>> -                   second_vec_index = first_vec_index;
>> +             if (second_vec_index == -1)
>> +               second_vec_index = first_vec_index;
>>
>> -                 for (unsigned int ri = 0; ri < nvectors_per_build; ++ri)
>> +             for (unsigned int ri = 0; ri < nvectors_per_build; ++ri)
>> +               {
>> +                 ++*n_perms;
> 
> So the "real" change is doing
> 
>       *n_perms += nvectors_per_build;
> 
> and *n_perms was unused when !analyze_only?  And since at

Yes, although both !analyze_only and analyze_only calls pass n_perms in, now
only the call sites with analyze_only will use the returned n_perms further.

> analysis time we (sometimes?) have zero nvectors you have to
> fixup above?  Which cases are that?

Yes, the fixup is to avoid to result in unexpected n_perms in function
vect_optimize_slp_pass::internal_node_cost。  One typical case is
gcc.dg/vect/bb-slp-50.c, without special casing zero, slp2 fails to optimize
out one more vec_perm unexpectedly.

In vect_optimize_slp_pass::internal_node_cost, it checks if the returned n_perms
is zero or not (vec_perm not needed or needed).

      if (!vect_transform_slp_perm_load_1 (m_vinfo, node, tmp_perm, vNULL,
					   nullptr, vf, true, false, &n_perms))
	{
	  auto rep = SLP_TREE_REPRESENTATIVE (node);
	  if (out_layout_i == 0)
	    {
	      /* Use the fallback cost if the load is an N-to-N permutation.
		 Otherwise assume that the node will be rejected later
		 and rebuilt from scalars.  */
	      if (STMT_VINFO_GROUPED_ACCESS (rep)
		  && (DR_GROUP_SIZE (DR_GROUP_FIRST_ELEMENT (rep))
		      == SLP_TREE_LANES (node)))
		return fallback_cost;
	      return 0;
	    }
	  return -1;
	}

      /* See the comment above the corresponding VEC_PERM_EXPR handling.  */
      return n_perms == 0 ? 0 : 1;

In vect_optimize_slp_pass::forward_pass (), it only considers the case that
factor > 0 (there is some vec_perm needed).

	      /* Accumulate the cost of using LAYOUT_I within NODE,
		 both for the inputs and the outputs.  */
	      int factor = internal_node_cost (vertex.node, layout_i,
					       layout_i);
	      if (factor < 0)
		{
		  is_possible = false;
		  break;
		}
	      else if (factor)
		layout_costs.internal_cost.add_serial_cost
		  ({ vertex.weight * factor, m_optimize_size });

BR,
Kewen

> 
> In principle the patch looks good to me.
> 
> Richard.
> 
>> +                 if (analyze_only)
>> +                   continue;
>> +                 /* Generate the permute statement if necessary.  */
>> +                 tree first_vec = dr_chain[first_vec_index + ri];
>> +                 tree second_vec = dr_chain[second_vec_index + ri];
>> +                 gassign *stmt = as_a<gassign *> (stmt_info->stmt);
>> +                 tree perm_dest
>> +                   = vect_create_destination_var (gimple_assign_lhs (stmt),
>> +                                                  vectype);
>> +                 perm_dest = make_ssa_name (perm_dest);
>> +                 gimple *perm_stmt
>> +                   = gimple_build_assign (perm_dest, VEC_PERM_EXPR, first_vec,
>> +                                          second_vec, mask_vec);
>> +                 vect_finish_stmt_generation (vinfo, stmt_info, perm_stmt,
>> +                                              gsi);
>> +                 if (dce_chain)
>>                     {
>> -                     /* Generate the permute statement if necessary.  */
>> -                     tree first_vec = dr_chain[first_vec_index + ri];
>> -                     tree second_vec = dr_chain[second_vec_index + ri];
>> -                     gassign *stmt = as_a<gassign *> (stmt_info->stmt);
>> -                     tree perm_dest
>> -                       = vect_create_destination_var (gimple_assign_lhs (stmt),
>> -                                                      vectype);
>> -                     perm_dest = make_ssa_name (perm_dest);
>> -                     gimple *perm_stmt
>> -                       = gimple_build_assign (perm_dest, VEC_PERM_EXPR,
>> -                                              first_vec, second_vec, mask_vec);
>> -                     vect_finish_stmt_generation (vinfo, stmt_info, perm_stmt,
>> -                                                  gsi);
>> -                     if (dce_chain)
>> -                       {
>> -                         bitmap_set_bit (used_defs, first_vec_index + ri);
>> -                         bitmap_set_bit (used_defs, second_vec_index + ri);
>> -                       }
>> -
>> -                     /* Store the vector statement in NODE.  */
>> -                     SLP_TREE_VEC_STMTS (node) [vect_stmts_counter++]
>> -                       = perm_stmt;
>> +                     bitmap_set_bit (used_defs, first_vec_index + ri);
>> +                     bitmap_set_bit (used_defs, second_vec_index + ri);
>>                     }
>> +
>> +                 /* Store the vector statement in NODE.  */
>> +                 SLP_TREE_VEC_STMTS (node)[vect_stmts_counter++] = perm_stmt;
>>                 }
>>             }
>>           else if (!analyze_only)
>> --
>> 2.39.1