[Bug tree-optimization/53855] New: Emitting a warning for some dangling references.

[ed at edrosten dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53855

             Bug #: 53855
           Summary: Emitting a warning for some dangling references.
    Classification: Unclassified
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: e...@edrosten.com


A dangling reference is created in the following code:


struct IntRef
{
    IntRef(const int& v_)
    :v(v_)
    {
    }

    const int& v;
    double get() const
    {
        return v;
    }
};



struct BRef
{
    BRef(const IntRef& v_)
    :v(v_)
    {
    }

    const IntRef& v;
    double get() const
    {
        return v.get();
    }
};


double test(const int& a)
{
    const BRef c(a);
    return c.get();
}

After the first stage of copy propagation, the representation of test is (from
test.cc.029t.copyprop1):


;; Function double test(const int&) (_Z4testRKi, funcdef_no=8, decl_uid=2230,
cgraph_uid=8)

double test(const int&) (const int & a)
{
  const struct IntRef & c$v;
  const struct IntRef & D.2286;
  int D.2284;
  const int & D.2283;
  double D.2282;
  const struct BRef c;
  const struct IntRef D.2251;

<bb 2>:
  D.2251.v = a_1(D);
  D.2251 ={v} {CLOBBER};
  D.2283_9 = D.2251.v;
  D.2284_10 = *D.2283_9;
  D.2282_11 = (double) D.2284_10;
  return D.2282_11;

}

at which point is is quite obvious that D.2251 is used after it is CLOBBER'd.
Is it possible to add a feature to generate a warning in this case?


I'm using gcc version r189256 from SVN, but things are much the same in 4.7.