A significant performance difference

[safety0ff via Digitalmars-d-learn]

The following D code runs over 2x faster than the C++ code 
(comparing dmd no options to g++ no options.) Its not a fair 
comparison because it changes the order of operations.

import core.stdc.stdio;

const uint H = 9, W = 12;

const uint[3][6] g = [[7, 0, H - 3],
                       [1 + (1 << H) + (1 << (2 * H)), 0, H - 1],
                       [3 + (1 << H), 0, H - 2],
                       [3 + (2 << H), 0, H - 2],
                       [1 + (1 << H) + (2 << H), 0, H - 2],
                       [1 + (1 << H) + (1 << (H - 1)), 1, H - 1]];

int main() {
     ulong p, i, k;
     ulong[uint] x, y;
     uint l;
     x[0] = 1;

     for (i = 0; i < W; ++i) {
         y = null;
         while (x.length)
         foreach (j; x.keys) {
             p = x[j];
             x.remove(j);

             for (k = 0; k < H; ++k)
                 if ((j & (1 << k)) == 0)
                     break;

             if (k == H)
                 y[j >> H] += p;
             else
                 for (l = 0; l < 6; ++l)
                     if (k >= g[l][1] && k <= g[l][2])
                         if ((j & (g[l][0] << k)) == 0)
                             x[j + (g[l][0] << k)] += p;
         }
         x = y;
     }

     printf("%lld\n", y[0]);
     return 0;
}