[Bug fortran/83017] DO CONCURRENT not parallelizing

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83017

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
We fail to analyze dependeces here

Considering loop 6
loop is innermost
Creating dr for MEM[(real(kind=4)[0:D.3539] *)&A.7][_85]
analyze_innermost: Applying pattern match.pd:84, generic-match.c:11356
failed: evolution of offset is not affine.
        base_address:
        offset from base address:
        constant offset from base address:
        step:
        aligned to:
        base_object: MEM[(real(kind=4)[0:D.3539] *)&A.7]
        Access function 0: scev_not_known;
Creating dr for MEM[(real(kind=4)[0:D.3539] *)&A.7][_85]
analyze_innermost: Applying pattern match.pd:84, generic-match.c:11356
failed: evolution of offset is not affine.
        base_address:
        offset from base address:
        constant offset from base address:
        step:
        aligned to:
        base_object: MEM[(real(kind=4)[0:D.3539] *)&A.7]
        Access function 0: scev_not_known;
(compute_affine_dependence
  stmt_a: _90 = MEM[(real(kind=4)[0:D.3539] *)&A.7][_85];
  stmt_b: MEM[(real(kind=4)[0:D.3539] *)&A.7][_85] = _99;
Data ref a:
#(Data Ref:
#  bb: 7
#  stmt: _90 = MEM[(real(kind=4)[0:D.3539] *)&A.7][_85];
#  ref: MEM[(real(kind=4)[0:D.3539] *)&A.7][_85];
#  base_object: MEM[(real(kind=4)[0:D.3539] *)&A.7];
#  Access function 0: scev_not_known;
#)
Data ref b:
#(Data Ref:
#  bb: 7
#  stmt: MEM[(real(kind=4)[0:D.3539] *)&A.7][_85] = _99;
#  ref: MEM[(real(kind=4)[0:D.3539] *)&A.7][_85];
#  base_object: MEM[(real(kind=4)[0:D.3539] *)&A.7];
#  Access function 0: scev_not_known;
#)
affine dependence test not usable: access function not affine or constant.
) -> dependence analysis failed

...

this is because

           k = mod( j, nsplit ) + 1

makes the k index not an affine function.

  <bb 7> [85.00%]:
  # j_205 = PHI <_87(8), _82(6)>
  _85 = j_205 % 4;
  _87 = j_205 + 1;
  _90 = MEM[(real(kind=4)[0:D.3539] *)&A.7][_85];
  _91 = _87 << 1;
  _92 = _91 & 2;
  _93 = 1 - _92;
  _94 = (real(kind=4)) _93;
  _95 = j_205 * 2;
  _96 = _95 + -1;
  _97 = (real(kind=4)) _96;
  _98 = _94 / _97;
  _99 = _90 + _98;
  MEM[(real(kind=4)[0:D.3539] *)&A.7][_85] = _99;
  if (_83 < _87)
    goto <bb 10>; [15.00%]
  else
    goto <bb 8>; [85.00%]

what the middle-end could do is remove the modulo and instead build an
inner loop (which might then be unrolled as your commented code shows).