[Bug tree-optimization/47621] New: Missed dependencies in address-taken optimization

[uweigand at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47621

           Summary: Missed dependencies in address-taken optimization
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: uweig...@gcc.gnu.org
                CC: rguent...@suse.de


The following program is mis-optimized with 4.6 (not earlier versions) at -O or
higher optimization levels:

int
main (void)
{
  int data = 1;

  struct ptr
    {
      int val;
    } *ptr = (struct ptr *) &data;

  ptr->val = 0;

  return data;
}

This program should return 0, but actually returns 1.

After the "address-taken" optimization pass, we have:

  dataD.1975_4 = 1;
  MEM[(struct ptr *)&dataD.1975].valD.1977 = 0;
  D.3453_2 = dataD.1975_4;
  return D.3453_2;

so the dependency between the assignment to ptr->val and the use of data is
lost.

See this post for more information:
http://gcc.gnu.org/ml/gcc/2011-02/msg00079.html

H.J. Lu identified the following patch as introducing the regression:
http://gcc.gnu.org/ml/gcc-patches/2010-10/msg00788.html