[gcc r11-1582] tree-optimization/95770 - fix SLP vectorized stmt placement compute

[Richard Biener]

https://gcc.gnu.org/g:cf07eea8429c923b7eb884ffc1b267c80a0a839c

commit r11-1582-gcf07eea8429c923b7eb884ffc1b267c80a0a839c
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Jun 22 12:14:54 2020 +0200

    tree-optimization/95770 - fix SLP vectorized stmt placement compute
    
    This fixes the vectorized stmt placement compute for the case of
    external defs.
    
    2020-06-22  Richard Biener  <rguenther@suse.de>
    
            PR tree-optimization/95770
            * tree-vect-slp.c (vect_schedule_slp_instance): Also consider
            external defs.
    
            * gcc.dg/pr95770.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/pr95770.c |  9 +++++++++
 gcc/tree-vect-slp.c            | 18 +++++++++++++++---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr95770.c b/gcc/testsuite/gcc.dg/pr95770.c
new file mode 100644
index 00000000000..06714ea1fa5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr95770.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O3" } */
+
+float *a;
+void b(float c, float d)
+{
+  a[0] = a[1] = 0.5f * (c - 2 + d);
+  a[2] = a[3] = 0.5f * (c + 2 + d);
+}
diff --git a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
index 5c169f37022..4031db4a9c6 100644
--- a/gcc/tree-vect-slp.c
+++ b/gcc/tree-vect-slp.c
@@ -4216,9 +4216,6 @@ vect_schedule_slp_instance (vec_info *vinfo,
 	 children vectorized defs.  */
       gimple *last_stmt = NULL;
       FOR_EACH_VEC_ELT (SLP_TREE_CHILDREN (node), i, child)
-	/* ???  With only external defs the following breaks.  Note
-	   external defs do not get all vector init stmts generated
-	   at the same place.  */
 	if (SLP_TREE_DEF_TYPE (child) == vect_internal_def)
 	  {
 	    /* We are emitting all vectorized stmts in the same place and
@@ -4232,6 +4229,21 @@ vect_schedule_slp_instance (vec_info *vinfo,
 		  || vect_stmt_dominates_stmt_p (last_stmt, vstmt))
 		last_stmt = vstmt;
 	  }
+	else
+	  {
+	    /* For externals we have to look at all defs since their
+	       insertion place is decided per vector.  */
+	    unsigned j;
+	    tree vdef;
+	    FOR_EACH_VEC_ELT (SLP_TREE_VEC_DEFS (child), j, vdef)
+	      if (TREE_CODE (vdef) == SSA_NAME)
+		{
+		  gimple *vstmt = SSA_NAME_DEF_STMT (vdef);
+		  if (!last_stmt
+		      || vect_stmt_dominates_stmt_p (last_stmt, vstmt))
+		    last_stmt = vstmt;
+		}
+	  }
       if (is_a <gphi *> (last_stmt))
 	si = gsi_after_labels (gimple_bb (last_stmt));
       else