fb_comp
val = fb_comp(val >> 1, val << 1, ~0UL / 3);
val = fb_comp(val >> 2, val << 2, ~0UL / 5);
return fb_comp(val >> 4, val << 4, ~0UL / 17);
fb_write_offset(fb_comp(val, fb_read_offset(offset, dst), mask), offset, dst);
fb_write_offset(fb_comp(val, old, mask), offset, dst);