Re: [PATCH] tree-optimization/110742 - fix latent issue with permuting existing vectors

[Richard Biener via Gcc-patches]

> Am 20.07.2023 um 18:59 schrieb Richard Sandiford <richard.sandif...@arm.com>:
> 
> Richard Biener <rguent...@suse.de> writes:
>>>> Am 20.07.2023 um 16:09 schrieb Richard Sandiford 
>>>> <richard.sandif...@arm.com>:
>>> 
>>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>>> When we materialize a layout we push edge permutes to constant/external
>>>> defs without checking we can actually do so.  For externals defined
>>>> by vector stmts rather than scalar components we can't.
>>>> 
>>>> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>>>> 
>>>> OK?
>>>> 
>>>> Thanks,
>>>> Richard.
>>>> 
>>>>   PR tree-optimization/110742
>>>>   * tree-vect-slp.cc (vect_optimize_slp_pass::get_result_with_layout):
>>>>   Do not materialize an edge permutation in an external node with
>>>>   vector defs.
>>>>   (vect_slp_analyze_node_operations_1): Guard purely internal
>>>>   nodes better.
>>>> 
>>>>   * g++.dg/torture/pr110742.C: New testcase.
>>>> ---
>>>> gcc/testsuite/g++.dg/torture/pr110742.C | 47 +++++++++++++++++++++++++
>>>> gcc/tree-vect-slp.cc                    |  8 +++--
>>>> 2 files changed, 53 insertions(+), 2 deletions(-)
>>>> create mode 100644 gcc/testsuite/g++.dg/torture/pr110742.C
>>>> 
>>>> diff --git a/gcc/testsuite/g++.dg/torture/pr110742.C 
>>>> b/gcc/testsuite/g++.dg/torture/pr110742.C
>>>> new file mode 100644
>>>> index 00000000000..d41ac0479d2
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/g++.dg/torture/pr110742.C
>>>> @@ -0,0 +1,47 @@
>>>> +// { dg-do compile }
>>>> +
>>>> +struct HARD_REG_SET {
>>>> +  HARD_REG_SET operator~() const {
>>>> +    HARD_REG_SET res;
>>>> +    for (unsigned int i = 0; i < (sizeof(elts) / sizeof((elts)[0])); ++i)
>>>> +      res.elts[i] = ~elts[i];
>>>> +    return res;
>>>> +  }
>>>> +  HARD_REG_SET operator&(const HARD_REG_SET &other) const {
>>>> +    HARD_REG_SET res;
>>>> +    for (unsigned int i = 0; i < (sizeof(elts) / sizeof((elts)[0])); ++i)
>>>> +      res.elts[i] = elts[i] & other.elts[i];
>>>> +    return res;
>>>> +  }
>>>> +  unsigned long elts[4];
>>>> +};
>>>> +typedef const HARD_REG_SET &const_hard_reg_set;
>>>> +inline bool hard_reg_set_subset_p(const_hard_reg_set x, 
>>>> const_hard_reg_set y) {
>>>> +  unsigned long bad = 0;
>>>> +  for (unsigned int i = 0; i < (sizeof(x.elts) / sizeof((x.elts)[0])); 
>>>> ++i)
>>>> +    bad |= (x.elts[i] & ~y.elts[i]);
>>>> +  return bad == 0;
>>>> +}
>>>> +inline bool hard_reg_set_empty_p(const_hard_reg_set x) {
>>>> +  unsigned long bad = 0;
>>>> +  for (unsigned int i = 0; i < (sizeof(x.elts) / sizeof((x.elts)[0])); 
>>>> ++i)
>>>> +    bad |= x.elts[i];
>>>> +  return bad == 0;
>>>> +}
>>>> +extern HARD_REG_SET rr[2];
>>>> +extern int t[2];
>>>> +extern HARD_REG_SET nn;
>>>> +static HARD_REG_SET mm;
>>>> +void setup_reg_class_relations(void) {
>>>> +  HARD_REG_SET intersection_set, union_set, temp_set2;
>>>> +  for (int cl2 = 0; cl2 < 2; cl2++) {
>>>> +    temp_set2 = rr[cl2] & ~nn;
>>>> +    if (hard_reg_set_empty_p(mm) && hard_reg_set_empty_p(temp_set2)) {
>>>> +      mm = rr[0] & nn;
>>>> +      if (hard_reg_set_subset_p(mm, intersection_set))
>>>> +        if (!hard_reg_set_subset_p(mm, temp_set2) ||
>>>> +            hard_reg_set_subset_p(rr[0], rr[t[cl2]]))
>>>> +          t[cl2] = 0;
>>>> +    }
>>>> +  }
>>>> +}
>>>> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
>>>> index 693621ca990..1d79c77e8ce 100644
>>>> --- a/gcc/tree-vect-slp.cc
>>>> +++ b/gcc/tree-vect-slp.cc
>>>> @@ -5198,7 +5198,10 @@ vect_optimize_slp_pass::get_result_with_layout 
>>>> (slp_tree node,
>>>>    return result;
>>>> 
>>>>  if (SLP_TREE_DEF_TYPE (node) == vect_constant_def
>>>> -      || SLP_TREE_DEF_TYPE (node) == vect_external_def)
>>>> +      || (SLP_TREE_DEF_TYPE (node) == vect_external_def
>>>> +      && (to_layout_i == 0
>>>> +          /* We can't permute vector defs.  */
>>>> +          || SLP_TREE_VEC_DEFS (node).is_empty ())))
>>> 
>>> Guess it's personal preference, but IMO it's easier to follow without the
>>> to_layout_i condition, so that it ties directly to the create_partitions
>>> test.
>> 
>> I don’t understand- in the code guarding this we seem to expect to_layout_i 
>> == 0 and that’s the case we can handle as noop.  I didn’t understand why the 
>> function doesn’t always just do nothing in this case though, so I must have 
>> missed something.
> 
> OK, so I guess that disproves that my way is easier to understand :)
> 
> I think logically, the code is doing the equivalent of:
> 
>  int partition_i = m_vertices[node->vertex].partition;
>  if (partition < 0)
>    {
>      /* If the vector is uniform or unchanged, there's nothing to do.  */
>      ...      
>    }
>  else
>    {
>      ... Return node if to_layout_i matches this partition's chosen layout...
>    }
> 
> And I guess I should have written it that way.
> 
> So when there is no partition, we have a constant or external def
> built from individual scalars.  We can use the node as-is if the
> caller wants an unpermuted node or if all elements are equal
> (so that the permutation doesn't matter).  Otherwise we need
> to permute the scalars.
> 
> When there is a partition, we can use the node as-is if the caller
> wants the layout that was chosen for that partition.  Otherwise we
> need a new VEC_PERM_EXPR node.
> 
> In the particular case of external defs built from vectors, we're
> guaranteed that the node's chosen layout is 0 (i.e. the original layout),
> and so both ways work.

Hmm, but I arrived here with that not being the case … (the chosen Lay-out not 
zero)


>  But in principle this case fits the "else" arm
> better than the "then" arm, because we're dealing with a node that is in
> a partition, and that is not built from scalars.
> 
> Thanks,
> Richard