[Qemu-devel] [PATCHv4 4/9] bitops: use vector algorithm to optimize find_next_bit()

[Peter Lieven]

this patch adds the usage of buffer_find_nonzero_offset()
to skip large areas of zeroes.

compared to loop unrolling presented in an earlier
patch this adds another 50% performance benefit for
skipping large areas of zeroes. loop unrolling alone
added close to 100% speedup.

Signed-off-by: Peter Lieven <p...@kamp.de>
Reviewed-by: Eric Blake <ebl...@redhat.com>
---
 util/bitops.c |   24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/util/bitops.c b/util/bitops.c
index e72237a..9bb61ff 100644
--- a/util/bitops.c
+++ b/util/bitops.c
@@ -42,10 +42,28 @@ 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)) {
-        if ((tmp = *(p++))) {
-            goto found_middle;
+    while (size >= BITS_PER_LONG) {
+        tmp = *p;
+        if (tmp) {
+             goto found_middle;
+        }
+        if (can_use_buffer_find_nonzero_offset(p, size / BITS_PER_BYTE)) {
+            size_t tmp2 =
+                buffer_find_nonzero_offset(p, size / BITS_PER_BYTE);
+            result += tmp2 * BITS_PER_BYTE;
+            size -= tmp2 * BITS_PER_BYTE;
+            p += tmp2 / sizeof(unsigned long);
+            if (!size) {
+                return result;
+            }
+            if (tmp2) {
+                tmp = *p;
+                if (tmp) {
+                    goto found_middle;
+                }
+            }
         }
+        p++;
         result += BITS_PER_LONG;
         size -= BITS_PER_LONG;
     }
-- 
1.7.9.5