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Date: Tue, 27 May 2008 12:42:00 -0000
Subject: [Bug tree-optimization/36343]  New: Wrong code due to bad TBAA pruning of points-to-sets and use in call clobbering
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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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extern void abort (void);

void __attribute__((noinline))
bar (int **p)
{
  float *q = (float *)p;
  *q = 0.0;
}

float __attribute__((noinline))
foo (int b)
{
  int *i = 0;
  float f = 1.0;
  int **p;
  if (b)
    p = &i;
  else
    p = (int **)&f;
  bar (p);
  if (b)
    return **p;
  return f;
}

int main()
{
  if (foo(0) != 0.0)
    abort ();
  return 0;
}


We TBAA-prune the points-to-set of p due to the dereference *p but use
the pruned points-to-set for computing the call-clobber variables which
causes f not to be call-clobbered.  Thus we return 1.0 from foo.  Oops.


-- 
           Summary: Wrong code due to bad TBAA pruning of points-to-sets and
                    use in call clobbering
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Keywords: wrong-code, alias
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36343