[Bug c/50284] New: possible miscompilation with -fstrict-aliasing

[rafael.espindola at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50284

             Bug #: 50284
           Summary: possible miscompilation with -fstrict-aliasing
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rafael.espindola@gmail.com


I am not sure if the attached program is valid, but I think it is covered by
c99 6.5 p7. On irc pinkia points out that it might be invalid. His arguments
are

* upcast is undefined in general, 6.5 p7 is trying to allow downcasting.
* upcasting is defined when the type was originally that type.

Two followup observations are that

* If we read "z->data.XXX" as an access to the member (an not the full
structure), all the access in the program are of the correct type.
* On the implementation side, this "bug" show up when main is in a another
translation unit too, a case where gcc could not know if "the type was
originally that type".

Philip Taylor pointed me at
http://davmac.wordpress.com/2010/02/26/c99-revisited/ which has an interesting
discussion about "Does accessing an object constitute access to the containing
object"?

This bug is "fixed" on trunk by 160947, but since that is an optimization
change, it probably has just deactivated the code path that caused this
behavior.

For some context, this testcase is a reduction from:

http://hg.mozilla.org/mozilla-central/file/a351ae35f2c4/js/src/jscntxtinlines.h#l179