Diagnostic 13379: loop was not vectorized with "simd"

Product Version:  Intel® Fortran Compiler 15.0 and above

Cause:

When  a loop contains a conditional statement which controls the assignment of a scalar value AND the scalar value is referenced AFTER the loop exits. 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 f13379( a, b, n )
implicit none
integer :: a(n), b(n), n

integer :: i, x=10

!dir$ simd
do i=1,n
  if( a(i) > 0 ) then
     x = i  !...here is the conditional assignment
  end if
  b(i) = x
end do
!... reference the scalar outside of the loop
write(*,*) "last value of x: ", x
end subroutine f13379

ifort -c /O2 /Qopt-report:2 /Qopt-report-phase:vec /Qopt-report-file:stdout f13379.f90

Begin optimization report for: F13379

    Report from: Vector optimizations [vec]

LOOP BEGIN at f13379.f90(8,1)
    ....
   remark #13379: loop was not vectorized with "simd"
LOOP END

Resolution:

The reference of the scalar after the loop requires that the value coming out of the loop is "correct", meaning that the loop iterations were executed strictly in-order and sequentially.  IF the scalar is NOT referenced outside of the loop, the compiler can can vectorize this loop since the order of that the iterations are evaluated does not matter - without reference outside the loop the final value of the scalar does not matter since it is no longer referenced.

Example

subroutine f13379( a, b, n )
implicit none
integer :: a(n), b(n), n

integer :: i, x=10

!dir$ simd
do i=1,n
  if( a(i) > 0 ) then
     x = i  !...here is the conditional assignment
  end if
  b(i) = x
end do
!... no reference to scalar X outside of the loop
!... removed the WRITE statment for X
end subroutine f13379

Begin optimization report for: F13379
    Report from: Vector optimizations [vec]

LOOP BEGIN at f13379.f90(8,1)
f13379.f90(8,1):remark #15301: SIMD LOOP WAS VECTORIZED
LOOP END

See also:

Requirements for Vectorizable Loops

Vectorization Essentials

Vectorization and Optimization Reports

Back to the list of vectorization diagnostics for Intel® Fortran