Patchwork is_not_zero() optimization in qemu-img

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Submitter Dmitry Konishchev
Date May 18, 2011, 11:03 a.m.
Message ID <1305716639-27846-1-git-send-email-konishchev@gmail.com>
Download mbox | patch
Permalink /patch/96151/
State New
Headers show

Comments

Dmitry Konishchev - May 18, 2011, 11:03 a.m.
Please sorry - I've sent the previous email via Thunderbird which promised that won't wrap the lines but it did. :(
Sending this via git send-email.

Signed-off-by: Dmitry Konishchev <konishchev@gmail.com>
---
 qemu-img.c |   29 ++++++++++++++++++++++++++---
 1 files changed, 26 insertions(+), 3 deletions(-)
Kevin Wolf - May 18, 2011, 12:14 p.m.
Am 18.05.2011 13:03, schrieb Dmitry Konishchev:
> Please sorry - I've sent the previous email via Thunderbird which promised that won't wrap the lines but it did. :(
> Sending this via git send-email.
> 
> Signed-off-by: Dmitry Konishchev <konishchev@gmail.com>

Thanks, applied to the block branch.

If you add a commit message that describes what the patch changes (and
why) next time, the patch would be perfect. ;-) For now I took some part
from your initial mail as the commit message.

Kevin

Patch

diff --git a/qemu-img.c b/qemu-img.c
index e825123..c849c6f 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;
 }