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Subject: [Bug tree-optimization/42108] [4.4/4.5 Regression] Vectorizer cannot deal with PAREN_EXPR gracefully, 50% performance regression
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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #12 from rguenth at gcc dot gnu dot org  2009-11-20 14:13 -------
The loop is not unrolled because the frontend presents us with very funny
obfuscated code:

      do  k=i,nnd,n
        temp=temp+(x(k)-x(k+jmini))**2
      end do

gets translated to

{
  character(kind=4) countm1.6;
  integer(kind=4) D.1551;
  integer(kind=4) D.1550;
  integer(kind=4) D.1549;

  D.1549 = i;
  D.1550 = *nnd;
  D.1551 = *n;
  k = D.1549;
  if (D.1551 > 0)
    {
      if (D.1550 < D.1549) goto L.6;, countm1.6 = (character(kind=4)) (D.1550 -
D.1549) / (character(kind=4)) D.1551;;
    }
  else
    {
      if (D.1550 > D.1549) goto L.6;, countm1.6 = (character(kind=4)) (D.1549 -
D.1550) / (character(kind=4)) -D.1551;;
    }
  while (1)
    {
        {
          real(kind=8) D.1556;
          real(kind=8) D.1555;

          D.1555 = (((*x)[(integer(kind=8)) k + -1] - (*x)[(integer(kind=8)) (k
+ jmini) + -1]));
          D.1556 = D.1555 * D.1555;
          temp = temp + D.1556;
        }
      L.5:;
      k = k + D.1551;
      if (countm1.6 == 0) goto L.6;
      countm1.6 = countm1.6 + 4294967295;
    }
  L.6:;
}


WTF!?

The funny conditional initialization of countm1.6 makes the analysis of
the number of iterations of this loop impossible (not to mention the
conversions to character(kind=4)).

Why does the frontend do induction variable "optimization" at all and
not simply generate a loop with a non-unit counting IV?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108