http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2010.12.03 13:55:38
Target Milestone|--- |4.6.0
Ever Confirmed|0 |1
--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-03
13:55:38 UTC ---
Confirmed. GCC 4.5 tree DSE removes the first store to p:
fun (void * * q)
{
void * r;
void * * p.1;
<bb 2>:
p.1_1 = p;
p = 0B;
r_3 = *q_2(D);
p = p.1_1;
return r_3;
and then PRE figures out the redundant store.
4.6 correctly preserves the store to p as the read from *q aliases it
(4.5 type-based aliasing concludes that void * doesn't alias void **,
something that people regularly get wrong thus I dumbed down TBAA).
Where did you get this testcase from?
To illustrate the difference, the following testcase is not optimized
by GCC 4.5 either (and that's required):
extern void **p;
void **fun (void ***q)
{
void **t;
void **r;
t = p;
p = 0;
r = *q;
p = t;
return r;
}
The observed change is caused by:
2010-08-25 Richard Guenther <rguent...@suse.de>
* alias.c (get_alias_set): Assign a single alias-set to all pointers.
* gimple.c (gimple_get_alias_set): Remove special handling
for pointers.
