[15/n] Consider building nodes from scalars in vect_slp_analyze_node_operations

[Richard Sandiford <richard.sandiford@arm.com>]

If the statements in an SLP node aren't similar enough to be vectorised,
or aren't something the vectoriser has code to handle, the BB vectoriser
tries building the vector from scalars instead.  This patch does the
same thing if we're able to build a viable-looking tree but fail later
during the analysis phase, e.g. because the target doesn't support a
particular vector operation.

This is needed to avoid regressions with a later patch.


2019-10-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vect_contains_pattern_stmt_p): New function.
	(vect_slp_convert_to_external): Likewise.
	(vect_slp_analyze_node_operations): If analysis fails, try building
	the node from scalars instead.

gcc/testsuite/
	* gcc.dg/vect/bb-slp-div-2.c: New test.

Index: gcc/tree-vect-slp.c
===================================================================
--- gcc/tree-vect-slp.c	2019-10-29 17:01:46.000000000 +0000
+++ gcc/tree-vect-slp.c	2019-10-29 17:02:06.355512105 +0000
@@ -225,6 +225,19 @@ vect_free_oprnd_info (vec<slp_oprnd_info
 }
 
 
+/* Return true if STMTS contains a pattern statement.  */
+
+static bool
+vect_contains_pattern_stmt_p (vec<stmt_vec_info> stmts)
+{
+  stmt_vec_info stmt_info;
+  unsigned int i;
+  FOR_EACH_VEC_ELT (stmts, i, stmt_info)
+    if (is_pattern_stmt_p (stmt_info))
+      return true;
+  return false;
+}
+
 /* Find the place of the data-ref in STMT_INFO in the interleaving chain
    that starts from FIRST_STMT_INFO.  Return -1 if the data-ref is not a part
    of the chain.  */
@@ -2630,6 +2643,39 @@ vect_slp_analyze_node_operations_1 (vec_
   return vect_analyze_stmt (stmt_info, &dummy, node, node_instance, cost_vec);
 }
 
+/* Try to build NODE from scalars, returning true on success.
+   NODE_INSTANCE is the SLP instance that contains NODE.  */
+
+static bool
+vect_slp_convert_to_external (vec_info *vinfo, slp_tree node,
+			      slp_instance node_instance)
+{
+  stmt_vec_info stmt_info;
+  unsigned int i;
+
+  if (!is_a <bb_vec_info> (vinfo)
+      || node == SLP_INSTANCE_TREE (node_instance)
+      || vect_contains_pattern_stmt_p (SLP_TREE_SCALAR_STMTS (node)))
+    return false;
+
+  if (dump_enabled_p ())
+    dump_printf_loc (MSG_NOTE, vect_location,
+		     "Building vector operands from scalars instead\n");
+
+  /* Don't remove and free the child nodes here, since they could be
+     referenced by other structures.  The analysis and scheduling phases
+     (need to) ignore child nodes of anything that isn't vect_internal_def.  */
+  unsigned int group_size = SLP_TREE_SCALAR_STMTS (node).length ();
+  SLP_TREE_DEF_TYPE (node) = vect_external_def;
+  SLP_TREE_SCALAR_OPS (node).safe_grow (group_size);
+  FOR_EACH_VEC_ELT (SLP_TREE_SCALAR_STMTS (node), i, stmt_info)
+    {
+      tree lhs = gimple_get_lhs (vect_orig_stmt (stmt_info)->stmt);
+      SLP_TREE_SCALAR_OPS (node)[i] = lhs;
+    }
+  return true;
+}
+
 /* Analyze statements contained in SLP tree NODE after recursively analyzing
    the subtree.  NODE_INSTANCE contains NODE and VINFO contains INSTANCE.
 
@@ -2656,6 +2702,13 @@ vect_slp_analyze_node_operations (vec_in
     {
       SLP_TREE_NUMBER_OF_VEC_STMTS (node)
 	= SLP_TREE_NUMBER_OF_VEC_STMTS (*leader);
+      /* Cope with cases in which we made a late decision to build the
+	 node from scalars.  */
+      if (SLP_TREE_DEF_TYPE (*leader) == vect_external_def
+	  && vect_slp_convert_to_external (vinfo, node, node_instance))
+	;
+      else
+	gcc_assert (SLP_TREE_DEF_TYPE (node) == SLP_TREE_DEF_TYPE (*leader));
       return true;
     }
 
@@ -2715,6 +2768,11 @@ vect_slp_analyze_node_operations (vec_in
     if (SLP_TREE_SCALAR_STMTS (child).length () != 0)
       STMT_VINFO_DEF_TYPE (SLP_TREE_SCALAR_STMTS (child)[0]) = dt[j];
 
+  /* If this node can't be vectorized, try pruning the tree here rather
+     than felling the whole thing.  */
+  if (!res && vect_slp_convert_to_external (vinfo, node, node_instance))
+    res = true;
+
   return res;
 }
 
Index: gcc/testsuite/gcc.dg/vect/bb-slp-div-2.c
===================================================================
--- /dev/null	2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.dg/vect/bb-slp-div-2.c	2019-10-29 17:02:06.351512133 +0000
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+
+int x[4], y[4], z[4];
+
+void
+f (void)
+{
+  x[0] += y[0] / z[0] * 2;
+  x[1] += y[1] / z[1] * 2;
+  x[2] += y[2] / z[2] * 2;
+  x[3] += y[3] / z[3] * 2;
+}
+
+/* { dg-final { scan-tree-dump "basic block vectorized" "slp2" { target vect_int } } } */