A faster "buildX(u32 m)". Odd multiples of 3 are written
with three 32-bits words in "x", starting at x[1],
a simple solution for the first word: x[0] = 0x9b4b3491;
In a similar way the next prime(s) 5,7.. could be marked,
but with deminishing returns.
/* old-ns-new old-s-new
primes < 25 47 47 p < 2^31 7.4 7.1
< 50 62 62 < 2^32 15.8 15.2
< 100 109 94
< 200 187 156
< 400 312 250
< 800 578 436
< 1600 1092 843
< 3200 2152 1700
< 6400 4274 3385 */
void buildX(u32 m) // mark odd composites
{
m -= m / ~0u; m += m & 1; m >>= 1;
u32 a = 1, b = 2, c = 3, d = m >> 5;
x.resize(d + 1); x[0] = 0x9b4b3491;
for (; a <= d; a += 3) x[a] = 0x24924924;
for (; b <= d; b += 3) x[b] = 0x49249249;
for (; c <= d; c += 3) x[c] = 0x92492492;
for (a = 5, b = 12, c = 8; b < m; a += 2, b += c += 4)
if ((x[a >> 6] & 1 << (a >> 1)) == 0)
for (d = b; d < m; d += a) x[d >> 5] |= 1 << d;
x[m >> 5] |= ~0u << m;
}
Latest version: https://pastebin.com/0F6fiEAD
