[PATCH] tree-optimization/100778 - avoid cross-BB vectorization of trapping op

[Richard Biener]

This avoids vectorizing a possibly trapping operation when lanes
are handled in different BBs.  I spotted this when working on the
originally reported issue in PR100778.

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

2021-05-28  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/100778
        * tree-vect-slp.c (vect_build_slp_tree_1): Prevent possibly
        trapping ops in different BBs.

        * gcc.dg/vect/bb-slp-pr100778-1.c: New testcase.
---
 gcc/testsuite/gcc.dg/vect/bb-slp-pr100778-1.c | 18 ++++++++++++++++++
 gcc/tree-vect-slp.c                           |  4 ++--
 2 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/vect/bb-slp-pr100778-1.c

diff --git a/gcc/testsuite/gcc.dg/vect/bb-slp-pr100778-1.c 
b/gcc/testsuite/gcc.dg/vect/bb-slp-pr100778-1.c
new file mode 100644
index 00000000000..9f8b7eecef1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/bb-slp-pr100778-1.c
@@ -0,0 +1,18 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target vect_double } */
+
+double foo (int x, double *p)
+{
+  double res = p[0] + p[1];
+  double tem = p[0] / x;
+  if (x)
+    {
+      p[0] = tem;
+      p[1] /= x;
+    }
+  return res + tem;
+}
+
+/* We may not SLP vectorize the FP division because it can trap and it
+   is distributed between two basic-blocks.  */
+/* { dg-final { scan-tree-dump "Build SLP failed: different BB for PHI or 
possibly trapping operation in _\[0-9\]+ = _\[0-9\]+ / _\[0-9\]+;" "slp2" } } */
diff --git a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
index 0ec92b0f0ca..ca1539e63f2 100644
--- a/gcc/tree-vect-slp.c
+++ b/gcc/tree-vect-slp.c
@@ -1214,14 +1214,14 @@ vect_build_slp_tree_1 (vec_info *vinfo, unsigned char 
*swap,
                }
            }
 
-         if (phi_p
+         if ((phi_p || gimple_could_trap_p (stmt_info->stmt))
              && (gimple_bb (first_stmt_info->stmt)
                  != gimple_bb (stmt_info->stmt)))
            {
              if (dump_enabled_p ())
                dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                                 "Build SLP failed: different BB for PHI "
-                                "in %G", stmt);
+                                "or possibly trapping operation in %G", stmt);
              /* Mismatch.  */
              continue;
            }
-- 
2.26.2