[Bug tree-optimization/115629] Inefficient if-convert of masked conditionals

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115629

--- Comment #6 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #5)
> > In this case, the second load is conditional on the first load mask,  which
> > means it's already done an AND.
> > And crucially inverting it means you also inverted both conditions.
> > 
> > So there are some superflous masking operations happening.  But I guess that's
> > a separate bug.  Shall I just add some tests here and close it and open a new
> > PR?
> 
> Not sure if that helps - do we fully understand this is a separate issue and
> not related to how we if-convert?
> 

if-convert looks ok to me:

  <bb 3> [local count: 955630226]:
  # i_28 = PHI <i_25(10), 0(21)>
  _1 = (long unsigned int) i_28;
  _2 = _1 * 4;
  _3 = a_16(D) + _2;
  _4 = *_3;
  _31 = _4 != 0;
  _55 = _54 + _2;
  _6 = (int *) _55;
  _56 = ~_31;
  _7 = .MASK_LOAD (_6, 32B, _56);
  _22 = _7 == 0;
  _34 = _22 & _56;
  _58 = _57 + _2;
  _9 = (int *) _58;
  iftmp.0_19 = .MASK_LOAD (_9, 32B, _34);
  _61 = _4 | _7;
  _35 = _61 != 0;
  _60 = _59 + _2;
  _8 = (int *) _60;
  iftmp.0_21 = .MASK_LOAD (_8, 32B, _35);
  iftmp.0_12 = _34 ? iftmp.0_19 : iftmp.0_21;
  _10 = res_23(D) + _2;
  *_10 = iftmp.0_12;
  i_25 = i_28 + 1;
  if (n_15(D) > i_25)
    goto <bb 10>; [89.00%]
  else
    goto <bb 11>; [11.00%]

I think what's missing here is that

  _7 = .MASK_LOAD (_6, 32B, _56);
  _22 = _7 == 0;
  _34 = _22 & _56;
  iftmp.0_19 = .MASK_LOAD (_9, 32B, _34);

in itself produces an AND. namely (_7 && _34) && _56 where _56 is the loop
mask.

my (probably poor understanding) is that the mask tracking in the vectorizer
attempts to prevent to keep masks and their inverse live at the same time.

but that this code in this case doesn't track masks introduced by nested
MASK_LOADS.  at least, that's my naive interpretation.