[Bug middle-end/68785] [6 Regression] valgrind reports issues with folding on x86_64

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68785

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-12-08
                 CC|                            |rguenth at gcc dot gnu.org
          Component|target                      |middle-end
   Target Milestone|---                         |6.0
     Ever confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So it folds

# VUSE <.MEM_30>
# rhs access alignment 32+0
_92 = MEM[(u32 *)path_7];

but path_7 is know to point to "".  location of the stmt above is
drivers/acpi/acpica/nsaccess.c:562:36  I guess that's

  *(u32 *)(void *)(&simple_name) = *(u32 *)(void *)(path);

eventually jump-threaded from the

 if (!pathname) {



  num_segments = 0;
  this_node = acpi_gbl_root_node;
  path = "";

case .  Yeah, quite obvious.

We avoid doing the work to zero the "undefined" area given the program does
not invoke undefined behavior only if the uninitialized bits of the result
are not used (like masked out or so).

One could silence valgrind with some annotation I guess.

Patch to make it trigger as ICE:

Index: gcc/gimple-fold.c
===================================================================
--- gcc/gimple-fold.c   (revision 231355)
+++ gcc/gimple-fold.c   (working copy)
@@ -5495,9 +5492,13 @@ fold_ctor_reference (tree type, tree cto
       && size <= MAX_BITSIZE_MODE_ANY_MODE)
     {
       unsigned char buf[MAX_BITSIZE_MODE_ANY_MODE / BITS_PER_UNIT];
-      if (native_encode_expr (ctor, buf, size / BITS_PER_UNIT,
-                             offset / BITS_PER_UNIT) > 0)
-       return native_interpret_expr (type, buf, size / BITS_PER_UNIT);
+      int elen;
+      if ((elen = native_encode_expr (ctor, buf, size / BITS_PER_UNIT,
+                             offset / BITS_PER_UNIT)) > 0)
+       {
+         gcc_assert (elen >= size / BITS_PER_UNIT);
+         return native_interpret_expr (type, buf, size / BITS_PER_UNIT);
+       }
     }
   if (TREE_CODE (ctor) == CONSTRUCTOR)
     {