Commit 49f676a0 authored by Peter Lieven's avatar Peter Lieven Committed by Juan Quintela
Browse files

bitops: unroll while loop in find_next_bit() 



this patch adopts the loop unrolling idea of bitmap_is_zero() to
speed up the skipping of large areas with zeros in find_next_bit().

this routine is extensively used to find dirty pages in
live migration.

testing only the find_next_bit performance on a zeroed bitfield
the loop onrolling decreased executing time by approx. 50% on x86_64.

Signed-off-by: default avatarPeter Lieven <pl@kamp.de>
Signed-off-by: default avatarJuan Quintela <quintela@redhat.com>
parent 56ded708

util/bitops.c

+17 −1
Original line number Diff line number Diff line
@@ -42,7 +42,23 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size, 
        size -= BITS_PER_LONG;

        result += BITS_PER_LONG;

    }

    while (size & ~(BITS_PER_LONG-1)) {

    while (size >= 4*BITS_PER_LONG) {

        unsigned long d1, d2, d3;

        tmp = *p;

        d1 = *(p+1);

        d2 = *(p+2);

        d3 = *(p+3);

        if (tmp) {

            goto found_middle;

        }

        if (d1 | d2 | d3) {

            break;

        }

        p += 4;

        result += 4*BITS_PER_LONG;

        size -= 4*BITS_PER_LONG;

    }

    while (size >= BITS_PER_LONG) {

        if ((tmp = *(p++))) {

            goto found_middle;

        }