[Bug rtl-optimization/21450] New: [4.0/4.1 regression] -fstrict-aliasing causes miscompilation

["belyshev at depni dot sinp dot msu dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

This C++ code snippet aborts on amd64 and i386 if compiled with -O[23]:
(Note: tree dumps are correct)

-------------------------------------------------------------------------------
extern "C" void abort (void);

struct T {};
struct U : T {};

int main ()
{
  int i;
  U *p;
  T **q;

  for (i = 0; i < 2; i++)
    {
      q = (T **) &p;
      *q = 0;
      if (p)
	abort ();
    }
  return 0;
}
-------------------------------------------------------------------------------

The above bug shows as Mozilla Firefox segfault in function
imgRequest::NotifyProxyListener(imgRequestProxy *) when pressing 'reload'
button, and the testcase was reduced from function
imgCacheValidator::OnStartRequest(nsIRequest *, nsISupports *), here is part of
original code:

NS_IMETHODIMP imgCacheValidator::OnStartRequest(nsIRequest *aRequest,
nsISupports *ctxt)
{
...
      for (PRInt32 i = count-1; i>=0; i--) {
        imgRequestProxy *proxy;
        mProxies.GetElementAt(i, (nsISupports**)&proxy);
        mRequest->NotifyProxyListener(proxy);
        NS_RELEASE(proxy);
      }
...

-- 
           Summary: [4.0/4.1 regression] -fstrict-aliasing causes
                    miscompilation
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: critical
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: belyshev at depni dot sinp dot msu dot ru
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21450