Re: Make fix for PR 83965 handle SLP reduction chains

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Feb 20, 2018 at 5:53 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> This patch prevents pattern-matching of fold-left SLP reduction chains,
> which the previous patch for 83965 didn't handle properly.  It only
> stops the last statement in the group from being matched, but that's
> enough to cause the group to be dissolved later.
>
> A better fix would be to put all the information about the reduction
> on the the first statement in the reduction chain, so that every
> statement in the group can tell what the group is doing.  That doesn't
> seem like stage 4 material though.
>
> As it stands, things seem to be a bit of a mess.  In
> vect_force_simple_reduction we attach the reduction type and
> phi pointer to the last statement in a reduction chain:
>
>       reduc_def_info = vinfo_for_stmt (def);
>       STMT_VINFO_REDUC_TYPE (reduc_def_info) = v_reduc_type;
>       STMT_VINFO_REDUC_DEF (reduc_def_info) = phi;
>
> and mark it as vect_reduction_type in vect_analyze_scalar_cycles_1:
>
>                   STMT_VINFO_DEF_TYPE (vinfo_for_stmt (reduc_stmt)) =
>                                                            vect_reduction_def;
>
> This code in vectorizable_reduction gave the impression that
> everything really is keyed off the last statement:
>
>   /* In case of reduction chain we switch to the first stmt in the chain, but
>      we don't update STMT_INFO, since only the last stmt is marked as reduction
>      and has reduction properties.  */
>   if (GROUP_FIRST_ELEMENT (stmt_info)
>       && GROUP_FIRST_ELEMENT (stmt_info) != stmt)
>     {
>       stmt = GROUP_FIRST_ELEMENT (stmt_info);
>       first_p = false;
>     }
>
> But this code is dead these days.  GROUP_FIRST_ELEMENT is only nonnull
> for SLP reduction chains, since we dissolve the group if SLP fails.
> And SLP only analyses the first statement in the group, not the last:
>
>   stmt = SLP_TREE_SCALAR_STMTS (node)[0];
>   stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
>   [...]
>   bool res = vect_analyze_stmt (stmt, &dummy, node, node_instance);
>
> So from that point of view the DEF_TYPE, REDUC_TYPE and REDUC_DEF
> are being attached to the wrong statement, since we only analyse
> the first one.  But it turns out that REDUC_TYPE and REDUC_DEF
> don't matter for the first statement in the group, since that
> takes the phi as an input, and when the phi is a direct input,
> we use *its* REDUC_TYPE and REDUC_DEF instead of the statement's
> own info.  The DEF_TYPE problem is handled by:
>
>       /* Mark the first element of the reduction chain as reduction to properly
>          transform the node.  In the reduction analysis phase only the last
>          element of the chain is marked as reduction.  */
>       if (!STMT_VINFO_GROUPED_ACCESS (vinfo_for_stmt (stmt)))
>         STMT_VINFO_DEF_TYPE (vinfo_for_stmt (stmt)) = vect_reduction_def;
>
> in vect_analyze_slp_instance (cancelled by:
>
>                 STMT_VINFO_DEF_TYPE (vinfo_for_stmt (first_element))
>                   = vect_internal_def;
>
> in vect_analyze_slp on failure), with the operation being repeated
> in vect_schedule_slp_instance (redundantly AFAICT):
>
>   /* Mark the first element of the reduction chain as reduction to properly
>      transform the node.  In the analysis phase only the last element of the
>      chain is marked as reduction.  */
>   if (GROUP_FIRST_ELEMENT (stmt_info) && !STMT_VINFO_GROUPED_ACCESS (stmt_info)
>       && GROUP_FIRST_ELEMENT (stmt_info) == stmt)
>     {
>       STMT_VINFO_DEF_TYPE (stmt_info) = vect_reduction_def;
>       STMT_VINFO_TYPE (stmt_info) = reduc_vec_info_type;
>     }
>
> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64-linux-gnu.
> OK to install?

Ok for stage1.

Richard.

> Richard
>
>
> 2018-02-20  Richard Sandiford  <richard.sandiford@linaro.org>
>
> gcc/
>         PR tree-optimization/83965
>         * tree-vect-patterns.c (vect_reassociating_reduction_p): Assume
>         that grouped statements are part of a reduction chain.  Return
>         true if the statement is not marked as a reduction itself but
>         is part of a group.
>         (vect_recog_dot_prod_pattern): Don't check whether the statement
>         is part of a group here.
>         (vect_recog_sad_pattern): Likewise.
>         (vect_recog_widen_sum_pattern): Likewise.
>
> gcc/testsuite/
>         PR tree-optimization/83965
>         * gcc.dg/vect/pr83965-2.c: New test.
>
> Index: gcc/tree-vect-patterns.c
> ===================================================================
> --- gcc/tree-vect-patterns.c    2018-02-20 09:40:41.843451227 +0000
> +++ gcc/tree-vect-patterns.c    2018-02-20 16:28:55.636762056 +0000
> @@ -222,13 +222,16 @@ vect_recog_temp_ssa_var (tree type, gimp
>  }
>
>  /* Return true if STMT_VINFO describes a reduction for which reassociation
> -   is allowed.  */
> +   is allowed.  If STMT_INFO is part of a group, assume that it's part of
> +   a reduction chain and optimistically assume that all statements
> +   except the last allow reassociation.  */
>
>  static bool
>  vect_reassociating_reduction_p (stmt_vec_info stmt_vinfo)
>  {
>    return (STMT_VINFO_DEF_TYPE (stmt_vinfo) == vect_reduction_def
> -         && STMT_VINFO_REDUC_TYPE (stmt_vinfo) != FOLD_LEFT_REDUCTION);
> +         ? STMT_VINFO_REDUC_TYPE (stmt_vinfo) != FOLD_LEFT_REDUCTION
> +         : GROUP_FIRST_ELEMENT (stmt_vinfo) != NULL);
>  }
>
>  /* Function vect_recog_dot_prod_pattern
> @@ -350,8 +353,7 @@ vect_recog_dot_prod_pattern (vec<gimple
>      {
>        gimple *def_stmt;
>
> -      if (!vect_reassociating_reduction_p (stmt_vinfo)
> -         && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
> +      if (!vect_reassociating_reduction_p (stmt_vinfo))
>         return NULL;
>        oprnd0 = gimple_assign_rhs1 (last_stmt);
>        oprnd1 = gimple_assign_rhs2 (last_stmt);
> @@ -571,8 +573,7 @@ vect_recog_sad_pattern (vec<gimple *> *s
>      {
>        gimple *def_stmt;
>
> -      if (!vect_reassociating_reduction_p (stmt_vinfo)
> -         && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
> +      if (!vect_reassociating_reduction_p (stmt_vinfo))
>         return NULL;
>        plus_oprnd0 = gimple_assign_rhs1 (last_stmt);
>        plus_oprnd1 = gimple_assign_rhs2 (last_stmt);
> @@ -1256,8 +1257,7 @@ vect_recog_widen_sum_pattern (vec<gimple
>    if (gimple_assign_rhs_code (last_stmt) != PLUS_EXPR)
>      return NULL;
>
> -  if (!vect_reassociating_reduction_p (stmt_vinfo)
> -      && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
> +  if (!vect_reassociating_reduction_p (stmt_vinfo))
>      return NULL;
>
>    oprnd0 = gimple_assign_rhs1 (last_stmt);
> Index: gcc/testsuite/gcc.dg/vect/pr83965-2.c
> ===================================================================
> --- /dev/null   2018-02-19 19:34:42.906488063 +0000
> +++ gcc/testsuite/gcc.dg/vect/pr83965-2.c       2018-02-20 16:28:55.635762095 +0000
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +/* { dg-additional-options "-Ofast -ftrapv" } */
> +
> +int c;
> +unsigned char d;
> +int e (unsigned char *f)
> +{
> +  int g;
> +  for (int a; a; a++)
> +    {
> +      for (int b = 0; b < 6; b++)
> +       g += __builtin_abs (f[b] - d);
> +      f += c;
> +    }
> +  return g;
> +}