[gcc r10-8693] tree-optimization/96854 - SLP reduction of two-operator is broken

[gcc r10-8693] tree-optimization/96854 - SLP reduction of two-operator is broken

[Richard Biener]

https://gcc.gnu.org/g:9f980cdba9e2fc0cc3f50c2c790f53b4dcd9dbe5

commit r10-8693-g9f980cdba9e2fc0cc3f50c2c790f53b4dcd9dbe5
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 31 13:36:09 2020 +0200

    tree-optimization/96854 - SLP reduction of two-operator is broken
    
    This fixes SLP reduction of two-operator operations by marking those
    not supported.  In fact any live lane out of such an operation cannot
    be code-generated correctly.
    
    2020-08-31  Richard Biener  <rguenther@suse.de>
    
            PR tree-optimization/96854
            * tree-vect-loop.c (vectorizable_live_operation): Disallow
            SLP_TREE_TWO_OPERATORS nodes.
    
            * gcc.dg/vect/pr96854.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/vect/pr96854.c | 20 ++++++++++++++++++++
 gcc/tree-vect-loop.c                |  5 +++++
 2 files changed, 25 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/vect/pr96854.c b/gcc/testsuite/gcc.dg/vect/pr96854.c
new file mode 100644
index 00000000000..e3980d41303
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr96854.c
@@ -0,0 +1,20 @@
+/* { dg-additional-options "-ffast-math" } */
+
+double _Complex __attribute__((noipa))
+foo (double _Complex acc, const double _Complex *x, const double _Complex* y, int N)
+{
+  for (int c = 0; c < N; ++c)
+    acc -= x[c] * y[c];
+  return acc;
+}
+
+int
+main()
+{
+  static const double _Complex y[] = { 1, 2, };
+  static const double _Complex x[] = { 1, 3, };
+  double _Complex ref = foo (0, x, y, 2);
+  if (__builtin_creal (ref) != -7.)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index b6c3faeae51..29071630327 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -7867,6 +7867,11 @@ vectorizable_live_operation (stmt_vec_info stmt_info,
 
   gcc_assert (STMT_VINFO_LIVE_P (stmt_info));
 
+  /* Due to how we generate code for SLP_TREE_TWO_OPERATORS we cannot
+     vectorize live operations out of it.  */
+  if (slp_node && SLP_TREE_TWO_OPERATORS (slp_node))
+    return false;
+
   /* If a stmt of a reduction is live, vectorize it via
      vect_create_epilog_for_reduction.  vectorizable_reduction assessed
      validity so just trigger the transform here.  */