[Bug middle-end/36201] NVR in the front-end causes missed optimization later on (<retval> thought to alias arguments)

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from rguenth at gcc dot gnu dot org  2008-05-13 08:43 -------
Confirmed on 32bit targets.

SMT.6, UID D.1708, struct a, is addressable, is global, score: 320004, direct
reads: 0, direct writes: 0, indirect reads: 2, indirect writes: 0, call
clobbered (is global var, is incoming pointer), may aliases: { <retval> }
<retval>, UID D.1678, struct a, is addressable, score: 1280052, direct reads:
2, direct writes: 2, indirect reads: 2, indirect writes: 0, call clobbered (is
global var, is incoming pointer)

It's easy to hack flow-insensitive aliasing to say that RESULT_DECL isn't
really
addressable (which is the difference here I believe) - but it is harder to
argue that this doesn't ever go wrong ;)

With that we get

a f(a&, a&) (struct a & g, struct a & h)
{
  long long int pretmp.12;

<bb 2>:
  pretmp.12 = h->b;
  <retval>.b = (pretmp.12 + g->b) + pretmp.12 * 1023;
  return <retval>;

}

Now onto why the following is safe...


Index: tree-ssa-alias.c
===================================================================
--- tree-ssa-alias.c    (revision 135255)
+++ tree-ssa-alias.c    (working copy)
@@ -3338,6 +3338,10 @@ debug_may_aliases_for (tree var)
 bool
 may_be_aliased (tree var)
 {
+  /* Nothing can alias the result decl.  */
+  if (TREE_CODE (var) == RESULT_DECL)
+    return false;
+
   /* Obviously.  */
   if (TREE_ADDRESSABLE (var))
     return true;


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
 GCC target triplet|                            |i?86-*-*
   Last reconfirmed|0000-00-00 00:00:00         |2008-05-13 08:43:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36201