[Bug fortran/34721] Output statements fool the -Wuninitialized option

[manu at gcc dot gnu dot org]

------- Comment #1 from manu at gcc dot gnu dot org  2008-01-10 14:07 -------
This is an example that shows that small differences in code lead to quite
different SSA representations. Without print we have a PHI node, where one of
the operands is the uninitialized value, so we warn. However, with the print
statement, we don't generate such PHI node, so we don't warn:

* With the print statement:

  # BLOCK 7, starting at line 5
  # PRED: 5 (false) 6 (fallthru)
  # STORES:  { dt_parm.5D.883 }
  [pr34721.f90 : 5] dt_parm.5D.883.commonD.821.filenameD.743 ={v}
&"pr34721.f90"[1]{lb: 1 sz: 1};
  # STORES:  { dt_parm.5D.883 }
  [pr34721.f90 : 5] dt_parm.5D.883.commonD.821.lineD.744 ={v} 5;
  # STORES:  { dt_parm.5D.883 }
  [pr34721.f90 : 5] dt_parm.5D.883.commonD.821.flagsD.741 ={v} 128;
  # STORES:  { dt_parm.5D.883 }
  [pr34721.f90 : 5] dt_parm.5D.883.commonD.821.unitD.742 ={v} 6;
  [pr34721.f90 : 5] _gfortran_st_write (&dt_parm.5D.883);
  [pr34721.f90 : 5] _gfortran_transfer_integer (&dt_parm.5D.883, &nD.872, 4); 
<== PASSED BY REFERENCE!!
  [pr34721.f90 : 5] _gfortran_st_write_done (&dt_parm.5D.883);
  # LOADS:  { nD.872 }
  [pr34721.f90 : 6] D.884_31 = nD.872;    <===== NO PHI NODE!!
  [pr34721.f90 : 6] iD.871_32 = 1;
  [pr34721.f90 : 6] if (iD.871_32 <= D.884_31)
    goto <bb 8>;
  else
    goto <bb 9>;
  # SUCC: 8 (true) 9 (false)

* Without the print statement

  # BLOCK 7, starting at line 6
  # PRED: 5 (false) 6 (fallthru)
  # nD.872_2 = PHI <nD.872_26(D)(5), nD.872_32(6)>   <== PHI NODE WITH
UNINITIALIZED VALUE!!
  [pr34721.f90 : 6] D.883_33 = nD.872_2;
  [pr34721.f90 : 6] iD.871_34 = 1;
  [pr34721.f90 : 6] if (iD.871_34 <= D.883_33)
    goto <bb 8>;
  else
    goto <bb 9>;
  # SUCC: 8 (true) 9 (false)


I have no idea why "print" generates loads/stores but if the variable is passed
by reference to a function, it is very risky to warn, since the function may
initialize the variable.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-01-10 14:07:44
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34721