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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/53676] [4.7/4.8 regression] empty loop is not always removed now
Date: Tue, 19 Jun 2012 13:18:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53676

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-19 13:18:19 UTC ---
First of all confirmed.  We rely on SCCP to compute the overall loop effect
which does not happen anymore.  The loops are different - 4.6 has

<bb 14>:
  # result_81 = PHI <result_44(13), 0(21)>
  # n_78 = PHI <n_46(13), 0(21)>
  result.7_42 = (unsigned char) result_81;
  D.7248_43 = result.7_42 + 2;
  result_44 = (signed char) D.7248_43;
  n_46 = n_78 + 1;
  if (n_46 != 8000)
    goto <bb 13>;
  else
    goto <bb 15>;

but with 4.7 we see

<bb 16>:
  # result_76 = PHI <result_50(15), 0(22)>
  # n_77 = PHI <n_52(15), 0(22)>
  D.7583_48 = (int) result_76;
  D.7582_49 = D.7583_48 + 2;
  result_50 = (signed char) D.7582_49;
  n_52 = n_77 + 1;
  if (n_52 != 8000)
    goto <bb 15>;
  else
    goto <bb 17>;

thus 4.6 performed the premature shortening optimization that 4.7 no longer
performs.

Testcase that nobody handles because it has the operation explicitely in int:

int main()
{
  int i;
  signed char result = 0;
  for (i = 0; i != 8000; ++i)
    {
      int tem = result;
      tem = tem + 2;
      result = tem;
    }
  if (__builtin_abs ((int)(signed char)((unsigned char ) result + 128)) != 0)
    __builtin_abort ();
  return 0;
}

Proper action could be to implement that shortening inside forwprop or
to teach the trick to SCEV analysis.