[Bug middle-end/35085] [4.3 Regression] gcc.dg/vect/vect-iv-9.c fails

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #13 from ubizjak at gmail dot com  2008-02-06 14:11 -------
I think that following difference is the key:

In case vectorizer is able to vecotrize, we enter vectorizer pass with:

<bb 3>:
  # ivtmp.17_1 = PHI <ivtmp.17_2(4), 26(2)>
  # i_18 = PHI <i_10(4), 0(2)>
  # s_17 = PHI <s_9(4), X_3(D)(2)>
  D.2002_7 = a[i_18];
  D.2003_8 = i_18 + D.2002_7;
  s_9 = D.2003_8 + s_17;
  i_10 = i_18 + 1;
  ivtmp.17_2 = ivtmp.17_1 - 1;
  if (ivtmp.17_2 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

and in case vectorizer is not able to vectorize (your case):

<bb 3>:
  # ivtmp.17_1 = PHI <ivtmp.17_2(4), 26(2)>
  # s_18 = PHI <s_9(4), X_3(D)(2)>
  # i_17 = PHI <i_10(4), 0(2)>
  D.2020_7 = a[i_17];
  D.2021_8 = s_18 + i_17;
  s_9 = D.2021_8 + D.2020_7;
  i_10 = i_17 + 1;
  ivtmp.17_2 = ivtmp.17_1 - 1;
  if (ivtmp.17_2 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

In success case, we have:

vect-iv-9.c:15: note: detected reduction:D.2003_8 + s_17
vect-iv-9.c:15: note: Detected reduction.

in your case, vectorizer chokes with:

vect-iv-9.c:15: note: reduction: unknown pattern.D.2021_8 + D.2020_7

I don't know, why there is a difference between my and your tree dump...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35085