[Bug tree-optimization/47575] New: store-motion removes global stores from endless loops

[Bug tree-optimization/47575] New: store-motion removes global stores from endless loops

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47575

           Summary: store-motion removes global stores from endless loops
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org


void foo (int *k) 
{
  for (;;)
    *k = 0;
}

is optimized to

foo (int * k)
{
  int k__lsm.2;

<bb 2>:
  k__lsm.2_4 = MEM[(int *)k_1(D)];

<bb 3>:
  k__lsm.2_6 = 0;

<bb 4>:
  goto <bb 3>;

}

by LIM, removing the global store.  We can use is_hidden_global_store
or friends and require at least one exit if that is true for a sm ref.

[Bug tree-optimization/47575] store-motion removes global stores from endless loops

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47575

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.02.01 16:25:49
                 CC|                            |rakdver at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-02-01 16:25:49 UTC ---
Something like

Index: tree-ssa-loop-im.c
===================================================================
--- tree-ssa-loop-im.c  (revision 169487)
+++ tree-ssa-loop-im.c  (working copy)
@@ -2329,7 +2329,10 @@ can_sm_ref_p (struct loop *loop, mem_ref
   base = get_base_address (ref->mem);
   if ((tree_could_trap_p (ref->mem)
        || (DECL_P (base) && TREE_READONLY (base)))
-      && !ref_always_accessed_p (loop, ref, true))
+      && (!ref_always_accessed_p (loop, ref, true))
+          || (TREE_CODE (base) == MEM_REF
+             && ptr_deref_may_alias_global_p (TREE_OPERAND (base, 0))
+             && !loop->exits->e))
     return false;

   /* And it must be independent on all other memory references

minus introducing a new predicate loop_has_no_exit_p ().

[Bug tree-optimization/47575] store-motion removes global stores from endless loops

[rakdver at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47575

--- Comment #2 from Zdenek Dvorak <rakdver at gcc dot gnu.org> 2011-02-01 16:56:57 UTC ---
There is nothing in C standard requiring us to preserve global stores that are
unused, unless they are volatile.  Store motion is by far not the only
optimization in gcc that affects global stores.  That this happens in an
endless loop is not important.

[Bug tree-optimization/47575] store-motion removes global stores from endless loops

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47575

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Indeed, nothing to see here.