U64 gcm4( U64 max, U64 n ) {
    U64 c, lcm;

    for( c = 1; ( ~n & 1 ) && ( c < 12 ); ++c ) n >>= 1;
    lcm = n * 4096;
    return ( max / lcm ) * lcm;
}

##</code><code>##

anonyM:  gcm for s=2147483648 & r=1 to 1073741824 took:33.850345514550  ### original
anonyM: gcm4 for s=2147483648 & r=1 to 1073741824 took:33.590549831120  ### my tweak