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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.
Date: Fri, 22 Mar 2013 13:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-03-22
             Blocks|                            |53947
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-22 13:29:57 UTC ---
The issue is that x is kept live by applying store-motion:

  <bb 3>:
  # prephitmp_26 = PHI <1(2), i.3_14(4)>
  # ivtmp_30 = PHI <700(2), ivtmp_15(4)>
  _5 = (integer(kind=8)) prephitmp_26;
  _6 = _5 + -1;
  _7 = my_data.x1[_6];
  _8 = my_data.y1[_6];
  x.1_9 = _7 - _8;
  _11 = my_data.t1[_6];
  _12 = x.1_9 * _11;
  my_data.z1[_6] = _12;
  i.3_14 = prephitmp_26 + 1;
  ivtmp_15 = ivtmp_30 - 1;
  if (ivtmp_15 == 0)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  goto <bb 3>;

  <bb 5>:
  # x_lsm.7_25 = PHI <x.1_9(3)>
  x = x_lsm.7_25;
  i = 701;
  return;

because it appears that 'save' makes all variables global ones.  This kind
of "reduction" is not handled by the vectorizer.  If would be handled
by a pass that re-materializes x_lsm.7_25 from memory and operations
after the loop.  Or by handling the "final" value properly by means
of vector extraction or in the epilogue loop, simply using it, or
forcing at least one iteration of the epilogue loop by adjusting the
number of iterations of the vectorized loop.

I like the last option most ;)