tree-optimization/103544 - SLP reduction chain as SLP reduction issue

When SLP reduction chain vectorization support added handling of
an outer conversion in the chain picking a failed reduction up
as SLP reduction that broke the invariant that the whole reduction
was forward reachable.  The following plugs that hole noting
a future enhancement possibility.

2021-12-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103544
	* tree-vect-slp.c (vect_analyze_slp): Only add a SLP reduction
	opportunity if the stmt in question is the reduction root.
	(dot_slp_tree): Add missing check for NULL child.

	* gcc.dg/vect/pr103544.c: New testcase.

2 files changed, 33 insertions, 3 deletions

diff --git a/gcc/testsuite/gcc.dg/vect/pr103544.c b/gcc/testsuite/gcc.dg/vect/pr103544.c
new file mode 100644
index 0000000..c8bdee8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr103544.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O3" } */
+/* { dg-additional-options "-march=haswell" { target x86_64-*-* i?86-*-* } } */
+
+int crash_me(char* ptr, unsigned long size)
+{
+  short result[16] = {0};
+
+  unsigned long no_iters = 0;
+  for(unsigned long i = 0; i < size - 12; i+= 13){
+      for(unsigned long j = 0; j < 12; j++){
+	  result[j] += ptr[i + j] - '0';
+      }
+      no_iters++;
+  }
+
+  int result_int = 0;
+  for(int j = 0; j < 12; j++){
+      int bit_value = result[j] > no_iters/2 ? 1 : 0;
+      result_int |= bit_value;
+  }
+
+  return result_int;
+}
diff --git a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
index bc22ffe..b912c35 100644
--- a/gcc/tree-vect-slp.c
+++ b/gcc/tree-vect-slp.c
@@ -2537,7 +2537,8 @@ dot_slp_tree (FILE *f, slp_tree node, hash_set<slp_tree> &visited)
     fprintf (f, "\"%p\" -> \"%p\";", (void *)node, (void *)child);
 
   for (slp_tree child : SLP_TREE_CHILDREN (node))
-    dot_slp_tree (f, child, visited);
+    if (child)
+      dot_slp_tree (f, child, visited);
 }
 
 DEBUG_FUNCTION void
@@ -3418,8 +3419,13 @@ vect_analyze_slp (vec_info *vinfo, unsigned max_tree_size)
 		vinfo = next;
 	      }
 	    STMT_VINFO_DEF_TYPE (first_element) = vect_internal_def;
-	    /* It can be still vectorized as part of an SLP reduction.  */
-	    loop_vinfo->reductions.safe_push (last);
+	    /* It can be still vectorized as part of an SLP reduction.
+	       ???  But only if we didn't skip a conversion around the group.
+	       In that case we'd have to reverse engineer that conversion
+	       stmt following the chain using reduc_idx and from the PHI
+	       using reduc_def.  */
+	    if (STMT_VINFO_DEF_TYPE (last) == vect_reduction_def)
+	      loop_vinfo->reductions.safe_push (last);
 	  }
 
       /* Find SLP sequences starting from groups of reductions.  */