sub sms_WxH_PDL_conv2d ( $m, $w, $h ) {
    $m -> conv2d( ones( $w, $h )) 
       -> slice(
    [floor(($w-1)/2),floor(-($w+1)/2)],
    [floor(($h-1)/2),floor(-($h+1)/2)] )
}

__END__

    Time (s) vs. N (NxN submatrix, PDL: Double D [1500,1500] matrix)
      +-----------------------------------------------------------+
    1 |-+  +      +      +      +       +      +      +      +  +-|
      |                         A                                 |
      |                                                      E    |
      |                                                           |
      |                                                           |
      |                                                           |
  0.8 |-+                                                       +-|
      |                                                      B    |
      |                                                           |
      |                                                           |
      |                                                           |
  0.6 |-+                A                                      +-|
      |                                        E                  |
      |                                                           |
      |                                                           |
      |                                                           |
  0.4 |-+                                      B                +-|
      |                                 E                         |
      |                                                           |
      |           A                                               |
      |                         E       B                         |
  0.2 |-+                                                       +-|
      |                  E      B                                 |
      |        A  D                                               |
      |    D   D  E      D      D       D      D             D    |
      |    E   E  C      C      C       C      C             C    |
    0 |-+  B   C                                                +-|
      |    +      +      +      +       +      +      +      +    |
      +-----------------------------------------------------------+
           2      4      6      8       10     12     14     16
                         sms_WxH_PDL_naive    A
                      sms_WxH_pdlpp_4loops    B
                          sms_WxH_PDL_lags    C
                       sms_WxH_PDL_sliding    D
                        sms_WxH_PDL_conv2d    E
  +----+-------+-------+-------+-------+-------+
  | N  | A     | B     | C     | D     | E     |
  +----+-------+-------+-------+-------+-------+
  | 2  | 0.061 | 0.009 | 0.030 | 0.083 | 0.020 |
  | 3  | 0.120 | 0.019 | 0.013 | 0.073 | 0.042 |
  | 4  | 0.252 | 0.030 | 0.039 | 0.098 | 0.066 |
  | 6  | 0.566 | 0.078 | 0.028 | 0.078 | 0.141 |
  | 8  | 0.963 | 0.139 | 0.033 | 0.081 | 0.237 |
  | 10 |       | 0.248 | 0.031 | 0.078 | 0.366 |
  | 12 |       | 0.388 | 0.033 | 0.078 | 0.531 |
  | 16 |       | 0.728 | 0.041 | 0.069 | 0.928 |
  +----+-------+-------+-------+-------+-------+