[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[rguenth at gcc dot gnu.org]

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 ;)