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Subject: [Bug middle-end/40979] induct benchmark 60% slower when compiled with -fgraphite-identity
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40979

--- Comment #11 from Sebastian Pop <spop at gcc dot gnu.org> 2011-01-31 18:12:38 UTC ---
Here is a reduced testcase from induct.f90 for the first loop
not vectorized with -fgraphite-identity:

module mqc_m
integer, parameter, private :: longreal = selected_real_kind(15,90)
contains
      subroutine mutual_ind_quad_cir_coil (m, l12)
      real (kind = longreal), dimension(9), save :: w2gauss, w1gauss
      real (kind = longreal) :: l12_lower, numerator
      real (kind = longreal), dimension(3) :: current_vector, coil_current_vec
      w2gauss(1) = 16.0_longreal/81.0_longreal
      w1gauss(5) = 0.3302393550_longreal
      do i = 1, 2*m
          do j = 1, 9
              do k = 1, 9
                  numerator = w1gauss(j) * w2gauss(k) *                        
            &
                                                
dot_product(coil_current_vec,current_vector)
                  l12_lower = l12_lower + numerator
              end do
          end do
      end do
      l12 = l12_lower
      end subroutine mutual_ind_quad_cir_coil
end module mqc_m

The problem seems to be that graphite introduces a
Commutative_Associative_Reduction array that confuses the vectorizer.

I looked at how to improve translate_scalar_reduction_to_array in
order to avoid the creation of the temporary array, but it seems to be
difficult as the result is written to memory under a different type
than the reduction itself: l12_lower is a real whereas l12 is an
integer:

    l12_lower_200 = some_computation;
    # l12_lower_9 = PHI <l12_lower_16(D)(2), l12_lower_200(9)>
    D.1585_43 = (integer(kind=4)) l12_lower_9;
    # .MEM_48 = VDEF <.MEM_47>
    *l12_44(D) = D.1585_43;

so we cannot use *l12_44(D) as a data reference in the loop to perform
the reduction as it does not have the same precision as l12_lower: it
seems to me that we cannot avoid creating the temporary array.

The solution could be to clean up the temporary arrays after graphite.