[Bug tree-optimization/67683] Missed vectorization: shifts of an induction variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67683

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-09-23
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is the unsupported induction (the PHI) of val

  <bb 5>:
  # val_16 = PHI <52719(4), val_11(7)>
  # i_17 = PHI <0(4), i_12(7)>
  _6 = (sizetype) i_17;
  _8 = data_7(D) + _6;
  _9 = (unsigned char) val_16;
  *_8 = _9;
  val_11 = val_16 >> 1;
  i_12 = i_17 + 1;
  if (max_5(D) > i_12)

SCEV already fails to analyze the evolution of val_16 so it doesn't appear
as "induction" to the vectorizer.  Reductions with shifts are not handled
either (but reduction uses inside the loop are not supported by the vectorizer
anyway).