Re: Question about vectorization limit

[Jakub Jelinek]

On Fri, May 31, 2013 at 03:48:59PM +0200, Toon Moene wrote:
> >If you rewrite the above into:
> >SUBROUTINE XYZ(A, B, N)
> >DIMENSION A(N), B(N)
> >DO I = 1, N
> >    C = B(I)
> >    IF (A(I)>  0.0) THEN
> >       A(I) = C / A(I)
> >    ELSE
> >       A(I) = C
> >    ENDIF
> >ENDDO
> >END
> >
> >then it is vectorized just fine.
> 
> But this "inner loop" has at least 3 basic blocks - so what does the
> "loop->num_nodes != 2" test exactly codify ?

With the above testcase it has just 2.
Before ifcvt pass it still has 4:
  <bb 4>:
  # i_1 = PHI <1(3), i_18(7)>
  _8 = (integer(kind=8)) i_1;
  _9 = _8 + -1;
  c_11 = *b_10(D)[_9];
  _13 = *a_12(D)[_9];
  if (_13 > 0.0)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 5>:
  _14 = c_11 / _13;

  <bb 6>:
  # cstore_17 = PHI <_14(5), c_11(4)>
  *a_12(D)[_9] = cstore_17;
  i_18 = i_1 + 1;
  if (i_1 == _7)
    goto <bb 8>;
  else
    goto <bb 7>;

  <bb 7>:
  goto <bb 4>;
but ifcvt transforms that into:
  <bb 4>:
  # i_1 = PHI <1(3), i_18(5)>
  _8 = (integer(kind=8)) i_1;
  _9 = _8 + -1;
  c_11 = *b_10(D)[_9];
  _13 = *a_12(D)[_9];
  _14 = c_11 / _13;
  cstore_17 = _13 > 0.0 ? _14 : c_11;
  *a_12(D)[_9] = cstore_17;
  i_18 = i_1 + 1;
  if (i_1 == _7)
    goto <bb 6>;
  else
    goto <bb 5>;

  <bb 5>:
  goto <bb 4>;
which is already generally vectorizable.  Guess ifcvt can be certainly
taught if it finds a possibly trapping statement to check if the same
statement isn't present in all possible branches, though the question is if
this is best done in ifcvt or some other pass.

        Jakub