[gcc r14-2095] tree-optimization/110381 - preserve SLP permutation with in-order reductions

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:53d6f57c1b20c6da52aefce737fb7d5263686ba3

commit r14-2095-g53d6f57c1b20c6da52aefce737fb7d5263686ba3
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Jun 26 12:51:37 2023 +0200

    tree-optimization/110381 - preserve SLP permutation with in-order reductions
    
    The following fixes a bug that manifests itself during fold-left
    reduction transform in picking not the last scalar def to replace
    and thus double-counting some elements.  But the underlying issue
    is that we merge a load permutation into the in-order reduction
    which is of course wrong.
    
    Now, reduction analysis has not yet been performend when optimizing
    permutations so we have to resort to check that ourselves.
    
            PR tree-optimization/110381
            * tree-vect-slp.cc (vect_optimize_slp_pass::start_choosing_layouts):
            Materialize permutes before fold-left reductions.
    
            * gcc.dg/vect/pr110381.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/vect/pr110381.c | 40 ++++++++++++++++++++++++++++++++++++
 gcc/tree-vect-slp.cc                 | 18 ++++++++++++++--
 2 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/vect/pr110381.c b/gcc/testsuite/gcc.dg/vect/pr110381.c
new file mode 100644
index 00000000000..2313dbf11ca
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr110381.c
@@ -0,0 +1,40 @@
+/* { dg-do run } */
+
+struct FOO {
+   double a;
+   double b;
+   double c;
+};
+
+double __attribute__((noipa))
+sum_8_foos(const struct FOO* foos)
+{
+  double sum = 0;
+
+  for (int i = 0; i < 8; ++i)
+    {
+      struct FOO foo = foos[i];
+
+      /* Need to use an in-order reduction here, preserving
+         the load permutation.  */
+      sum += foo.a;
+      sum += foo.c;
+      sum += foo.b;
+    }
+
+  return sum;
+}
+
+int main()
+{
+  struct FOO foos[8];
+
+  __builtin_memset (foos, 0, sizeof (foos));
+  foos[0].a = __DBL_MAX__;
+  foos[0].b = 5;
+  foos[0].c = -__DBL_MAX__;
+
+  if (sum_8_foos (foos) != 5)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index ab89a82f1b3..fee992d0171 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -4673,14 +4673,28 @@ vect_optimize_slp_pass::start_choosing_layouts ()
   m_partition_layout_costs.safe_grow_cleared (m_partitions.length ()
 					      * m_perms.length ());
 
-  /* We have to mark outgoing permutations facing non-reduction graph
-     entries that are not represented as to be materialized.  */
+  /* We have to mark outgoing permutations facing non-associating-reduction
+     graph entries that are not represented as to be materialized.
+     slp_inst_kind_bb_reduc currently only covers associatable reductions.  */
   for (slp_instance instance : m_vinfo->slp_instances)
     if (SLP_INSTANCE_KIND (instance) == slp_inst_kind_ctor)
       {
 	unsigned int node_i = SLP_INSTANCE_TREE (instance)->vertex;
 	m_partitions[m_vertices[node_i].partition].layout = 0;
       }
+    else if (SLP_INSTANCE_KIND (instance) == slp_inst_kind_reduc_chain)
+      {
+	stmt_vec_info stmt_info
+	  = SLP_TREE_REPRESENTATIVE (SLP_INSTANCE_TREE (instance));
+	stmt_vec_info reduc_info = info_for_reduction (m_vinfo, stmt_info);
+	if (needs_fold_left_reduction_p (TREE_TYPE
+					   (gimple_get_lhs (stmt_info->stmt)),
+					 STMT_VINFO_REDUC_CODE (reduc_info)))
+	  {
+	    unsigned int node_i = SLP_INSTANCE_TREE (instance)->vertex;
+	    m_partitions[m_vertices[node_i].partition].layout = 0;
+	  }
+      }
 
   /* Check which layouts each node and partition can handle.  Calculate the
      weights associated with inserting layout changes on edges.  */