On Fri 24 Apr 2020 09:39:25 PM CEST, Eric Blake wrote:
>> + /* Update bitmap with the subclusters that were just written */
>> + if (has_subclusters(s)) {
>> + unsigned written_from = m->cow_start.offset;
>> + unsigned written_to = m->cow_end.offset + m->cow_end.nb_bytes ?:
>> + m->nb_clusters << s->cluster_bits;
>> + uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
>> + int sc;
>> + for (sc = 0; sc < s->subclusters_per_cluster; sc++) {
>> + int sc_off = i * s->cluster_size + sc * s->subcluster_size;
>> + if (sc_off >= written_from && sc_off < written_to) {
>> + l2_bitmap |= QCOW_OFLAG_SUB_ALLOC(sc);
>> + l2_bitmap &= ~QCOW_OFLAG_SUB_ZERO(sc);
>> + }
>> + }
>
> Are there more efficient ways to set this series of bits than iterating
> one bit at a time, while still remaining legible? For example, what if
> we had something like:
>
> l2_bitmap = get_l2_bitmap(...);
> int sc_from = OFFSET_TO_SC(written_from);
> int sc_to = OFFSET_TO_SC(written_to - 1);
> l2_bitmap |= QCOW_OFLAG_SUB_ALLOC_RANGE(sc_from, sc_to);
> l2_bitmap &= ~QCOW_OFLAG_SUB_ZERO_RANGE(sc_from, sc_to);
That's a very good suggestion, thanks!
Berto
