Performance-Aware Loops

Counted Work

Bound the Loop

Performance-aware code starts by making the amount of loop work explicit, before any timing or tuning claims.

Program

Play the program to choose how many values are included and watch the operation count stay visible.

counted_work.f90 
program counted_work_demo
    implicit none
    integer :: values(5)
    integer :: limit
    integer :: i
    integer :: total
    integer :: operations

    values = [2, 4, 6, 8, 10]
    limit = 
    total = 0
    operations = 0
    do i = 1, limit
        total = total + values(i)
        operations = operations + 1
    end do
    print '(I0, 1X, I0)', total, operations
end program counted_work_demo

program counted_work_demo
    implicit none
    integer :: values(5)
    integer :: limit
    integer :: i
    integer :: total
    integer :: operations

    values = [2, 4, 6, 8, 10]
    limit = 
    total = 0
    operations = 0
    do i = 1, limit
        total = total + values(i)
        operations = operations + 1
    end do
    print '(I0, 1X, I0)', total, operations
end program counted_work_demo

program counted_work_demo
    implicit none
    integer :: values(5)
    integer :: limit
    integer :: i
    integer :: total
    integer :: operations

    values = [2, 4, 6, 8, 10]
    limit = 
    total = 0
    operations = 0
    do i = 1, limit
        total = total + values(i)
        operations = operations + 1
    end do
    print '(I0, 1X, I0)', total, operations
end program counted_work_demo
work bound `limit` makes the loop bound visible and easy to reason about.
operation count `operations` records how many iterations actually ran.
work shape Counting work shows the performance shape without measuring elapsed time.