On 18.05.2011 11:57, Stefan Hajnoczi wrote:
Yes, optimizing is_not_zero() is good. The only additional thing I
suggest is adding a comment before the function to document the length
constraint.
OK, fixed.
On 18.05.2011 12:05, Kevin Wolf wrote:
A future bdrv_is_allocated() patch must make sure that the conversion
falls back to a simple is_not_zero() when a backing file is used.
Thanks, I'll take this into account.
Signed-off-by: Dmitry Konishchev <konishc...@gmail.com>
---
qemu-img.c | 30 +++++++++++++++++++++++++++---
1 files changed, 27 insertions(+), 3 deletions(-)
diff --git a/qemu-img.c b/qemu-img.c
index e825123..7665c2f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -496,14 +496,38 @@ 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)
+ len /= sizeof(long);
+
+ long d0;
+ long d1;
+ long d2;
+ long d3;
+
+ for(i = 0; i < len; i += 4) {
+ d0 = ((const long*) sector)[i + 0];
+ d1 = ((const long*) sector)[i + 1];
+ d2 = ((const long*) sector)[i + 2];
+ d3 = ((const long*) sector)[i + 3];
+
+ if (d0 || d1 || d2 || d3)
return 1;
}
+
return 0;
}
--
1.7.4.1
