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bitops: unroll while loop in find_next_bit()

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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: Peter Lieven <pl@kamp.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Peter Lieven 2013-03-26 10:58:34 +01:00 • committed by Juan Quintela
parent 56ded708ec
commit 49f676a00a
1 changed files with 17 additions and 1 deletions
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18
util/bitops.c
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@ -42,7 +42,23 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
size -= BITS_PER_LONG; size -= BITS_PER_LONG;
result += 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++))) { if ((tmp = *(p++))) {
goto found_middle; goto found_middle;
} }