[PATCH] Fix no-strict-aliasing condition in VN

[Richard Biener]

There's a path I added where get_alias_set () != 0 was meant to
cover -fno-strict-aliasing but it doesn't since quite some time
(oops).  Fixed as follows - preprartory for PR91091 where we'd
otherwise break the testcase below.

Bootstrap / regtest running on x86_64-unknown-linux-gnu.

I'll also backport this to the GCC 8/9 branches where the issue
is latent.

Richard.

2019-07-05  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/91091
        * tree-ssa-sccvn.c (vn_reference_lookup_3): Overlap of
        accesses can happen with -fno-strict-aliasing.

        * gcc.dg/tree-ssa/pr91091-1.c: New testcase.

Index: gcc/tree-ssa-sccvn.c
===================================================================
--- gcc/tree-ssa-sccvn.c        (revision 273133)
+++ gcc/tree-ssa-sccvn.c        (working copy)
@@ -1996,7 +1996,8 @@ vn_reference_lookup_3 (ao_ref *ref, tree
             activation of a union member via a store makes the
             values of untouched bytes unspecified.  */
          && (known_eq (ref->size, BITS_PER_UNIT)
-             || (get_alias_set (lhs) != 0
+             || (flag_strict_aliasing
+                 && get_alias_set (lhs) != 0
                  && ao_ref_alias_set (ref) != 0)))
        {
          tree *saved_last_vuse_ptr = data->last_vuse_ptr;
Index: gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c   (nonexistent)
+++ gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c   (working copy)
@@ -0,0 +1,23 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -fno-strict-aliasing" } */
+
+struct s { int x; } __attribute__((packed));
+struct t { int x; };
+
+void __attribute__((noinline,noipa))
+swap(struct s* p, struct t* q)
+{
+  p->x = q->x;
+  q->x = p->x;
+}
+
+int main()
+{    
+  struct t a[2];
+  a[0].x = 0x12345678;
+  a[1].x = 0x98765432;
+  swap ((struct s *)((char *)a + 1), a);
+  if (a[0].x != 0x12345678)
+    __builtin_abort ();
+  return 0;
+}