[Bug tree-optimization/52865] GCC can't vectorize fortran loop but able to vectorize similar c-loop

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52865

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
                 CC|                            |burnus at gcc dot gnu.org
          Component|fortran                     |tree-optimization

--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-04-04 
14:00:30 UTC ---
(In reply to comment #2)
>   DOUBLE PRECISION Dx(*) , Dy(*)
> and
>   double X[1000], Y[1000]
> are not at all the same.

But one still gets the same result if one uses:

  void daxpy(int m, int n, double X[], double Y[], double z)

which should be close to what one gets with Fortran.


 * * *

For the Fortran loop, -ftree-vectorizer-verbose=3 shows:

14: ===== analyze_loop_nest =====
14: === vect_analyze_loop_form ===
14: not vectorized: unexpected loop form.
14: bad loop form.


For the C loop:
6: Profitability threshold is 2 loop iterations.
6: created 1 versioning for alias checks.
6: vectorizing stmts using SLP.
6: LOOP VECTORIZED.


For the Fortran loop, using ifort 12.1:
(15): (col. 19) remark: BLOCK WAS VECTORIZED.
(14): (col. 16) remark: loop was not vectorized: not inner loop.



Original dump for the Fortran loop (-fdump-tree-original):

    D.1862 = mp1;
    D.1863 = *n;
    i = D.1862;
    if (D.1863 < D.1862) goto L.2;
    countm1.0 = (unsigned int) (NON_LVALUE_EXPR <D.1863>
                                - NON_LVALUE_EXPR <D.1862>) / 4;
    while (1)
      {
        (*dy)[(integer(kind=8)) i + -1] = (*dy)[(integer(kind=8)) i + -1]
                                  + *da * (*dx)[(integer(kind=8)) i + -1];
        (*dy)[(integer(kind=8)) (i + 1) + -1]
                   = (*dy)[(integer(kind=8)) (i + 1) + -1]
             + *da * (*dx)[(integer(kind=8)) (i + 1) + -1];
        (*dy)[(integer(kind=8)) (i + 2) + -1]
                   = (*dy)[(integer(kind=8)) (i + 2) + -1]
             + *da * (*dx)[(integer(kind=8)) (i + 2) + -1];
        (*dy)[(integer(kind=8)) (i + 3) + -1]
                   = (*dy)[(integer(kind=8)) (i + 3) + -1]
             + *da * (*dx)[(integer(kind=8)) (i + 3) + -1];
        L.1:;
        i = i + 4;
        if (countm1.0 == 0) goto L.2;
        countm1.0 = countm1.0 + 4294967295;
      }
    L.2:;