Re: Make fix for PR 83965 handle SLP reduction chains

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Richard Biener <richard.guenther@gmail.com> writes:
> 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.

It's a GCC 8 regression, so OK for stage4?

Richard

> 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;
>> +}