Fortran Multidimensional Array Performance

Question

When manipulating and assigning subarrays to multidimensional arrays in Fortran90, I came across an interesting performance feature.

Fortran90 introduced the ability to manipulate subsections of arrays, and I saw several places that recommend performing array operations using this “clipping” method instead of loops. For example, if I need to add two arrays, aand with a bsize of 10, it is better to write:

c(1:10) = a(1:10) + b(1:10)

or

c = a + b

Instead

do i = 1, 10
    c(i) = a(i) + b(i)
end do

I tried this method for simple one-dimensional and two-dimensional arrays and found that it is faster with "slice" notation. However, when assigning such results in multidimensional arrays, it became a little more interesting.

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program main
    implicit none

    integer, parameter :: mSize = 10000
    integer :: i, j
    integer :: pCnt, nCnt, cntRt, cntMx
    integer, dimension(mSize, mSize) :: a, b
    integer, dimension(mSize, mSize, 3) :: c

    pCnt = 0
    call SYSTEM_CLOCK(nCnt, cntRt, cntMx)
    print *, "First call: ", nCnt-pCnt
    pCnt = nCnt

    do j = 1, mSize
        do i = 1, mSize
            a(i, j) = i*j
            b(i, j) = i+j
        end do
    end do

    call SYSTEM_CLOCK(nCnt, cntRt, cntMx)
    print *, "Created Matrices: ", nCnt-pCnt
    pCnt = nCnt

    ! OPERATIONS BY SLICING NOTATION
    !c(1:mSize, 1:mSize, 1) = a + b
    !c(1:mSize, 1:mSize, 2) = a - b
    !c(1:mSize, 1:mSize, 3) = a * b

    ! OPERATIONS WITH LOOP
    do j = 1, mSize
        do i = 1, mSize
            c(i, j, 1) = a(i, j) + b(i, j)
            c(i, j, 2) = a(i, j) - b(i, j)
            c(i, j, 3) = a(i, j) * b(i, j)
        end do
    end do

    call SYSTEM_CLOCK(nCnt, cntRt, cntMx)
    print *, "Added Matrices: ", nCnt-pCnt
    pCnt = nCnt
end program main

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-O3 GNU Fortran 4.8.4 Ubuntu 14.04

• -O3

  .

       5 Runs - 843, 842, 842, 841, 859
  
       Average - 845.4
  

  .

       5 Runs - 1713, 1713, 1723, 1711, 1713
  
       Average - 1714.6
  

• -O3

  .

       5 Runs - 545, 545, 544, 544, 548
  
       Average - 545.2
  

  .

       5 Runs - 479, 477, 475, 472, 472
  
       Average - 475
  

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Answer 1

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Answer 2

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do j = 1, mSize
    do i = 1, mSize
        c(i, j, 1) = a(i, j) + b(i, j)
    end do
end do
do j = 1, mSize
    do i = 1, mSize
        c(i, j, 2) = a(i, j) - b(i, j)
    end do
end do
do j = 1, mSize
    do i = 1, mSize
        c(i, j, 3) = a(i, j) * b(i, j)
    end do
end do

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