[PATCH] tree-optimization/115254 - don't account single-lane SLP against discovery limit

[Richard Biener]

The following avoids accounting single-lane SLP to the discovery
limit.  As the two testcases show this makes discovery fail,
unfortunately even not the same across targets.  The following
should fix two FAILs for GCN as a side-effect.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

        PR tree-optimization/115254
        * tree-vect-slp.cc (vect_build_slp_tree): Only account
        multi-lane SLP to limit.

        * gcc.dg/vect/slp-cond-2-big-array.c: Expect 4 times SLP.
        * gcc.dg/vect/slp-cond-2.c: Likewise.
---
 .../gcc.dg/vect/slp-cond-2-big-array.c        |  2 +-
 gcc/testsuite/gcc.dg/vect/slp-cond-2.c        |  2 +-
 gcc/tree-vect-slp.cc                          | 31 +++++++++++--------
 3 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/vect/slp-cond-2-big-array.c 
b/gcc/testsuite/gcc.dg/vect/slp-cond-2-big-array.c
index cb7eb94b3a3..9a9f63c0b8d 100644
--- a/gcc/testsuite/gcc.dg/vect/slp-cond-2-big-array.c
+++ b/gcc/testsuite/gcc.dg/vect/slp-cond-2-big-array.c
@@ -128,4 +128,4 @@ main ()
   return 0;
 }
 
-/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 3 "vect" } 
} */
+/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 4 "vect" } 
} */
diff --git a/gcc/testsuite/gcc.dg/vect/slp-cond-2.c 
b/gcc/testsuite/gcc.dg/vect/slp-cond-2.c
index 1dcee46cd95..08bbb3dbec6 100644
--- a/gcc/testsuite/gcc.dg/vect/slp-cond-2.c
+++ b/gcc/testsuite/gcc.dg/vect/slp-cond-2.c
@@ -128,4 +128,4 @@ main ()
   return 0;
 }
 
-/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 3 "vect" } 
} */
+/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 4 "vect" } 
} */
diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 0dd9a4daf6a..bbfde8849c1 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -1725,21 +1725,26 @@ vect_build_slp_tree (vec_info *vinfo,
   SLP_TREE_SCALAR_STMTS (res) = stmts;
   bst_map->put (stmts.copy (), res);
 
-  if (*limit == 0)
+  /* Single-lane SLP doesn't have the chance of run-away, do not account
+     it to the limit.  */
+  if (stmts.length () > 1)
     {
-      if (dump_enabled_p ())
-       dump_printf_loc (MSG_NOTE, vect_location,
-                        "SLP discovery limit exceeded\n");
-      /* Mark the node invalid so we can detect those when still in use
-        as backedge destinations.  */
-      SLP_TREE_SCALAR_STMTS (res) = vNULL;
-      SLP_TREE_DEF_TYPE (res) = vect_uninitialized_def;
-      res->failed = XNEWVEC (bool, group_size);
-      memset (res->failed, 0, sizeof (bool) * group_size);
-      memset (matches, 0, sizeof (bool) * group_size);
-      return NULL;
+      if (*limit == 0)
+       {
+         if (dump_enabled_p ())
+           dump_printf_loc (MSG_NOTE, vect_location,
+                            "SLP discovery limit exceeded\n");
+         /* Mark the node invalid so we can detect those when still in use
+            as backedge destinations.  */
+         SLP_TREE_SCALAR_STMTS (res) = vNULL;
+         SLP_TREE_DEF_TYPE (res) = vect_uninitialized_def;
+         res->failed = XNEWVEC (bool, group_size);
+         memset (res->failed, 0, sizeof (bool) * group_size);
+         memset (matches, 0, sizeof (bool) * group_size);
+         return NULL;
+       }
+      --*limit;
     }
-  --*limit;
 
   if (dump_enabled_p ())
     dump_printf_loc (MSG_NOTE, vect_location,
-- 
2.35.3