From b981d5c60b8ef78e2adecd6b5d7e36f9e5e61c54 Mon Sep 17 00:00:00 2001
From: Richard Biener <rguenther@suse.de>
Date: Wed, 17 Jan 2024 14:05:42 +0100
Subject: [PATCH] tree-optimization/113431 - wrong dependence with invariant
 load

The vectorizer dependence analysis is confused with invariant loads
when figuring whether the circumstances are so that we preserve
scalar stmt execution order.  The following rectifies this.

	PR tree-optimization/113431
	* tree-vect-data-refs.cc (vect_preserves_scalar_order_p):
	When there is an invariant load we might not preserve
	scalar order.

	* gcc.dg/vect/pr113431.c: New testcase.
---
 gcc/testsuite/gcc.dg/vect/pr113431.c | 18 ++++++++++++++++++
 gcc/tree-vect-data-refs.cc           |  6 ++++++
 2 files changed, 24 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113431.c

diff --git a/gcc/testsuite/gcc.dg/vect/pr113431.c b/gcc/testsuite/gcc.dg/vect/pr113431.c
new file mode 100644
index 00000000000..04448d9dd81
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113431.c
@@ -0,0 +1,18 @@
+/* { dg-additional-options "-O3 -fdump-tree-slp1-details" } */
+
+#include "tree-vect.h"
+
+int a[2][9];
+int b;
+int main()
+{
+  check_vect ();
+  for (b = 0; b < 2; b++)
+    for (long e = 8; e > 0; e--)
+      a[b][e] = a[0][1] == 0;
+  if (a[1][1] != 0)
+    __builtin_abort ();
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "optimized: basic block part vectorized" 2 "slp1" { target vect_int } } } */
diff --git a/gcc/tree-vect-data-refs.cc b/gcc/tree-vect-data-refs.cc
index 0495842b350..f592aeb8028 100644
--- a/gcc/tree-vect-data-refs.cc
+++ b/gcc/tree-vect-data-refs.cc
@@ -282,6 +282,12 @@ vect_preserves_scalar_order_p (dr_vec_info *dr_info_a, dr_vec_info *dr_info_b)
       && !STMT_VINFO_GROUPED_ACCESS (stmtinfo_b))
     return true;
 
+  /* If there is a loop invariant read involved we might vectorize it in
+     the prologue, breaking scalar oder with respect to the in-loop store.  */
+  if ((DR_IS_READ (dr_info_a->dr) && integer_zerop (DR_STEP (dr_info_a->dr)))
+      || (DR_IS_READ (dr_info_b->dr) && integer_zerop (DR_STEP (dr_info_b->dr))))
+    return false;
+
   /* STMT_A and STMT_B belong to overlapping groups.  All loads are
      emitted at the position of the first scalar load.
      Stores in a group are emitted at the position of the last scalar store.