Diagnostic 15516: Loop Was Not Vectorized: Cost Model Has Chosen vectorlength Of 1 -- Maybe Possible To Override Via Pragma/directive With Vectorlength Clause

Product Version: Intel® Fortran Compiler 15.0 and a later version  

Cause:

The vectorization report generated using Intel® Fortran Compiler's optimization and vectorization report options includes non-vectorized loop instance:

Windows* OS:  /O2  /Qopt-report:2  /Qopt-report-phase:vec    

Linux OS or OS X:  -O2 -qopt-report2  -qopt-report-phase=vec

Example:

An example below will generate the following  remark in optimization report:

subroutine f15516(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 f15516

ifort -c /O2 /Qopt-report:2 /Qopt-report-phase:vec /Qopt-report-file:stdout f15516.f90
 
Begin optimization report for: F15516

    Report from: Vector optimizations [vec]

LOOP BEGIN at f15516.f90(8,4)
   remark #15516: loop was not vectorized: cost model has chosen vectorlength of 1 -- maybe possible to override via pragma/directive with vectorlength clause
LOOP END

Resolution: 

See also:

Requirements for Vectorizable Loops

Vectorization Essentials

Vectorization and Optimization Reports

Back to the list of vectorization diagnostics for Intel® Fortran