Diagnostic 15543: Loop Was Not Vectorized: Loop With Function Call Not Considered An Optimization Candidate. (Fortran)

Cause:

A function call inside the loop is preventing auto-vectorization.

Example:

Program foo 
    implicit none
    integer, parameter  :: nx = 100000000
    real(8)             :: x, xp, sumx
    integer             :: i
    interface
       real(8) function bar(x, xp) 
          real(8), intent(in) :: x, xp
       end
    end interface 
  
    sumx = 0.
    xp   = 1.

    do i = 1,nx
       x = 1.D-8*real(i,8)
       sumx = sumx + bar(x,xp)
    enddo

    print *, 'Sum =',sumx
      
end


real(8) function bar(x, xp) 
  implicit none
  real(8), intent(in) :: x, xp

  bar = 1. - 2.*(x-xp) + 3.*(x-xp)**2 - 1.5*(x-xp)**3  + 0.2*(x-xp)**4
  bar = bar / sqrt(x**2 + xp**2)
  
end  

LOOP BEGIN at foo.f90(18,5)
   remark #15543: loop was not vectorized: loop with function call not considered an optimization candidate.   [ foo.f90(17,22) ]
LOOP END

Resolution:

real(8) function bar(x, xp) 
!$OMP DECLARE SIMD (bar) UNIFORM(xp)
  implicit none
  real(8), intent(in) :: x, xp

  bar = 1. - 2.*(x-xp) + 3.*(x-xp)**2 - 1.5*(x-xp)**3  + 0.2*(x-xp)**4
  bar = bar / sqrt(x**2 + xp**2)
  
end  

remark #15301: FUNCTION WAS VECTORIZED   [ bar.f90(1,18) ]

   remark #15344: loop was not vectorized: vector dependence prevents vectorization. First dependence is shown below. Use level 5 report for details
   remark #15346: vector dependence: assumed OUTPUT dependence between  line 17 and  line 18
LOOP END

Program foo 
    implicit none
    integer, parameter  :: nx = 100000000
    real(8)             :: x, xp, sumx
    integer             :: i

    interface
       real(8) function bar(x, xp) 
       !$OMP DECLARE SIMD (bar) UNIFORM(xp)
          real(8), intent(in) :: x, xp
       end
    end interface 
  
    sumx = 0.
    xp   = 1. 

    !$OMP SIMD  private(x)  reduction(+:sumx)
    do i = 1,nx
       x = 1.D-8*real(i,8)
       sumx = sumx + bar(x,xp)
    enddo

    print *, 'Sum =',sumx
      
end  

LOOP BEGIN at foo.f90(17,5)
   remark #15301: OpenMP SIMD LOOP WAS VECTORIZED
LOOP END

The loop is now vectorized successfully; running and timing the program shows a speedup.

Note that if the DECLARE SIMD directive is omitted, the !$OMP SIMD directive will still cause the remaining parts of the loop in foo to be vectorized, but the call to bar() will be serialized, so any performance gain is likely to be small. In either case, the private and reduction clauses of this directive are mandatory; without them, the compiler will assume no loop-carried dependencies and results may be incorrect.

Back to the list of vectorization diagnostics for Intel Fortran