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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/57359] wrong code for union access at -O3 on x86_64-linux
Date: Fri, 31 May 2013 08:48:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57359

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |WAITING
   Last reconfirmed|                            |2013-05-31
                 CC|                            |rguenth at gcc dot gnu.org
         Resolution|INVALID                     |---
     Ever confirmed|0                           |1

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Dara Hazeghi from comment #5)
> Sorry to ask again on this, but after re-reading, I'm not sure I understand
> the type-punning argument here:
> 
> **ppll = ll; // write to u.ll
> *k = 0;      // write to u.i
> 
> j = *ppa;     // u not touched
> ia[0][0] = *j; // u not touched
> 
> printf("%d\n", u.i); // read from u.i
> 
> 
> From what I can tell, this is in fact reading the last member written to (so
> not falling under the unspecified behaviors listed in annex J / 6.2.6.1). 
> Perhaps there is an additional restriction I am missing?

If the above is what really happens (the testcase is quite obfuscated ;))
then the testcase is valid.

Can you try to un-obfuscate it somewhat?