Re: 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;
}