[PATCH] Fix PR 28807 - wrong code with may_alias and structs

[Andrew Pinski <pinskia at physics dot uc dot edu>]

The problem here is that access_can_touch_variable would return false
for accesses via a base which has an aliasing set of 0 which is
incorrect.  Anyways this fixes the problem by checking the aliasing set
to make sure that does not happen.

I added a testcase to the testsuite for a possible missed optimization
which Daniel Berlin had caught with an earlier version of this patch.

OK? Bootstrapped and tested on i686-linux-gnu with no regressions.

Thanks,
Andrew Pinski

PS. The testcase for testing if this works fails at "-O3 -g" currently
but it is fails with an ICE which was also there before my patch.  I
filed the bug under PR 28834 and also noticed it is a regression.

ChangeLog:
	* tree-ssa-operands.c (access_can_touch_variable): Don't say
	the access through a base which has an alias set of 0 cannot
	touch the variable.

testsuite/ChangeLog:
	* gcc.c-torture/execute/mayalias-2.c: New test.
	* gcc.dg/tree-ssa/alias-13.c: New test.

Index: testsuite/gcc.c-torture/execute/mayalias-2.c
===================================================================
--- testsuite/gcc.c-torture/execute/mayalias-2.c	(revision 0)
+++ testsuite/gcc.c-torture/execute/mayalias-2.c	(revision 0)
@@ -0,0 +1,17 @@
+struct S { short x; };
+typedef struct S __attribute__((__may_alias__)) test;
+
+int f() {
+  int a=10;
+  test *p=(test *)&a;
+  p->x = 1;
+  return a;
+}
+
+int main() {
+  if (f() == 10)
+    __builtin_abort();
+  return 0;
+}
+
+
Index: testsuite/gcc.dg/tree-ssa/alias-13.c
===================================================================
--- testsuite/gcc.dg/tree-ssa/alias-13.c	(revision 0)
+++ testsuite/gcc.dg/tree-ssa/alias-13.c	(revision 0)
@@ -0,0 +1,33 @@
+/* { dg-do link } */
+/* { dg-options "-O2" } */
+
+
+struct a
+{
+  char a1;
+};
+
+int *aa;
+
+void g(int *a)
+{
+  aa = a;
+  *a = 2;
+}
+
+int t(int i, struct a *b)
+{
+  g(&i);
+  b->a1 = 1;
+  i = 2;
+  if (b->a1 != 1)
+    link_failure ();
+}
+int main(void)
+{
+  struct a b;
+  t(1, &b);
+  return 0;
+}
+
+
Index: tree-ssa-operands.c
===================================================================
--- tree-ssa-operands.c	(revision 116342)
+++ tree-ssa-operands.c	(working copy)
@@ -1150,7 +1150,10 @@ access_can_touch_variable (tree ref, tre
 	       || TREE_CODE (TREE_TYPE (base)) != UNION_TYPE)
 	   && !AGGREGATE_TYPE_P (TREE_TYPE (alias))
 	   && TREE_CODE (TREE_TYPE (alias)) != COMPLEX_TYPE
-	   && !POINTER_TYPE_P (TREE_TYPE (alias)))
+	   && !POINTER_TYPE_P (TREE_TYPE (alias))
+	   /* When the struct has may_alias attached to it, we need not to
+	      return true.  */
+	   && get_alias_set (base))
     {
 #ifdef ACCESS_DEBUGGING
       fprintf (stderr, "Access to ");