From f6a00aa150ec685966ca9453601328fdb2f87f34 Mon Sep 17 00:00:00 2001 From: Dmitry Konishchev Date: Wed, 18 May 2011 15:03:59 +0400 Subject: [PATCH] qemu_img: is_not_zero() optimization I run qemu-img under profiler and realized, that most of CPU time is consumed by is_not_zero() function. I had made a couple of optimizations on it and got the following output for `time qemu-img convert -O qcow2 volume.qcow2 snapshot.qcow2`: Original qemu-img: real 0m56.159s user 0m34.670s sys 0m12.079s Patched qemu-img: real 0m34.805s user 0m18.445s sys 0m12.552s Signed-off-by: Dmitry Konishchev Signed-off-by: Kevin Wolf --- qemu-img.c | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) diff --git a/qemu-img.c b/qemu-img.c index 1da5484ff0..4f162d1abe 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -496,14 +496,37 @@ static int img_commit(int argc, char **argv) return 0; } +/* + * Checks whether the sector is not a zero sector. + * + * Attention! The len must be a multiple of 4 * sizeof(long) due to + * restriction of optimizations in this function. + */ static int is_not_zero(const uint8_t *sector, int len) { + /* + * Use long as the biggest available internal data type that fits into the + * CPU register and unroll the loop to smooth out the effect of memory + * latency. + */ + int i; - len >>= 2; - for(i = 0;i < len; i++) { - if (((uint32_t *)sector)[i] != 0) + long d0, d1, d2, d3; + const long * const data = (const long *) sector; + + len /= sizeof(long); + + for(i = 0; i < len; i += 4) { + d0 = data[i + 0]; + d1 = data[i + 1]; + d2 = data[i + 2]; + d3 = data[i + 3]; + + if (d0 || d1 || d2 || d3) { return 1; + } } + return 0; }