tree-optimization/98854 - avoid some PHI BB vectorization

This avoids cases of PHI node vectorization that just causes us
to insert vector CTORs inside loops for values only required
outside of the loop.

2021-01-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98854
	* tree-vect-slp.c (vect_build_slp_tree_2): Also build
	PHIs from scalars when the number of CTORs matches the
	number of children.

	* gcc.dg/vect/bb-slp-pr98854.c: New testcase.

diff --git a/gcc/testsuite/gcc.dg/vect/bb-slp-pr98854.c b/gcc/testsuite/gcc.dg/vect/bb-slp-pr98854.c
new file mode 100644 (file)
index 0000000..0c8141e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/bb-slp-pr98854.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+
+double a[1024];
+
+int bar();
+void foo (int n)
+{
+  double x = 0, y = 0;
+  int i = 1023;
+  do
+    {
+      x += a[i] + a[i+1];
+      y += a[i] / a[i+1];
+      if (bar ())
+        break;
+    }
+  while (--i);
+  /* We want to avoid vectorizing the LC PHI and insert vector CTORs
+     inside of the loop where it is only needed here.  */
+  a[0] = x;
+  a[1] = y;
+}
+
+/* { dg-final { scan-tree-dump-not "vectorizing SLP node starting from: ._\[0-9\]+ = PHI" "slp1" } } */

diff --git a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
index 4465cf7494ee1608e34ed1091ac2c0428cd4de2e..10b876ff5ede8896eb95abd22f716e458c99416d 100644 (file)
--- a/gcc/tree-vect-slp.c
+++ b/gcc/tree-vect-slp.c
@@ -1896,7 +1896,10 @@ fail:
                n_vector_builds++;
            }
        }
-      if (all_uniform_p || n_vector_builds > 1)
+      if (all_uniform_p
+         || n_vector_builds > 1
+         || (n_vector_builds == children.length ()
+             && is_a <gphi *> (stmt_info->stmt)))
        {
          /* Roll back.  */
          matches[0] = false;