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.

(cherry picked from commit 53d6f57c1b20c6da52aefce737fb7d5263686ba3)

diff --git a/gcc/testsuite/gcc.dg/vect/pr110381.c b/gcc/testsuite/gcc.dg/vect/pr110381.c
new file mode 100644 (file)
index 0000000..278f442
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr110381.c
@@ -0,0 +1,44 @@
+/* { dg-require-effective-target vect_float_strict } */
+
+#include "tree-vect.h"
+
+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];
+
+  check_vect ();
+
+  __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 54e6a9e4224f916a5647f5584874341a4af8845f..19cab93761c98553ddf800e5a9899b41b69fa644 100644 (file)
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -3733,9 +3733,8 @@ vect_optimize_slp (vec_info *vinfo)
       vertices[idx].perm_out = perms.length () - 1;
     }
 
-  /* In addition to the above we have to mark outgoing permutes 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.  */
   for (slp_instance instance : vinfo->slp_instances)
     if (SLP_INSTANCE_KIND (instance) == slp_inst_kind_ctor)
       {
@@ -3744,6 +3743,20 @@ vect_optimize_slp (vec_info *vinfo)
        vertices[SLP_INSTANCE_TREE (instance)->vertex].perm_in = 0;
        vertices[SLP_INSTANCE_TREE (instance)->vertex].perm_out = 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 (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;
+           vertices[node_i].perm_in = 0;
+           vertices[node_i].perm_out = 0;
+         }
+      }
 
   /* Propagate permutes along the graph and compute materialization points.  */
   bool changed;