[Bug middle-end/46488] [4.5 regression] server/core_filters.c from apache httpd 2.2.17 miscompiled at -O3

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46488

--- Comment #33 from rguenther at suse dot de <rguenther at suse dot de> 2010-11-30 15:41:36 UTC ---
On Tue, 30 Nov 2010, ebotcazou at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46488
> 
> --- Comment #32 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2010-11-30 15:10:58 UTC ---
> The problem appears to be deeply rooted in the Ring construct, more precisely
> in the HEAD trick.  IIUC the idea is to "attach" a doubly-linked list to
> another structure by means of a "virtual" member overlaid on top of the
> structure; the only thing they actually share is a special APR_RING_ENTRY (the
> APR_RING_HEAD).  But this overlay fundamentally violates the aliasing rules
> even if one try to narrow the accesses to just the shared part.
> 
> Richard, is that how the aliasing rules are implemented in the 4.5.x series? 

Yes, also in 4.4 and 4.3, but maybe you need to be more lucky to trigger
the problem there.

> Has this been changed in 4.6.0?

No.  But with 4.6 we can ignore pointer types when doing copy-propagation
and thus we probably see that they must-alias (in which case we will
not apply TBAA to be more nice to our users).

Richard.