Cause:
The vectorizer does not support certain data types because there is no corresponding SIMD instruction. For example, the compiler supports REAL(16) arithmetic (quad precision) through a software implementation, but is unable to vectorize it:
subroutine d_15015_1(a,b,n)
implicit none
real(16), dimension(n), intent(in ) :: a
real(16), dimension(n), intent(out) :: b
integer, intent(in ) :: n
integer :: i
do i=1,n
b(i) = a(i)**3
enddo
end subroutine d_15015_1
By default, the compiler is unable to vectorize loops containing complex double precision variables, because an SSE register can only contain a single data element:
subroutine d_15015_2(a,b,n)
implicit none
complex(8), dimension(n), intent(in ) :: a
complex(8), dimension(n), intent(out) :: b
integer, intent(in ) :: n
integer :: i
do i=1,n
b(i) = 1. / sqrt(1.+a(i)**2)
enddo
end subroutine d_15015_2
Resolution: Only use quad precision variables in places where the extra precision is really required, such as in calculations where they may be large cancellations. The first example will vectorize if it is written in double precision. Loops containing double precision complex variables can be vectorized using instruction sets that support wider vectors. The second example can be vectorized for systems supporting Intel® Advanced Vector Extensions (Intel® AVX) by compiling with the option /QxAVX (Windows*) or -xavx (Linux* or OS X*):
> ifort -xavx -vec-report2 -c d_15015_2.f90
d_15015_2.f90(8): (col. 4) remark: LOOP WAS VECTORIZED
Back to the list of vectorization diagnostics for Intel Fortran