bcache: remove unnecessary prefetch() in bset_search_tree()

In function bset_search_tree(), when p >= t->size, t->tree[0] will be
prefetched by the following code piece,
 974                 unsigned int p = n << 4;
 975
 976                 p &= ((int) (p - t->size)) >> 31;
 977
 978                 prefetch(&t->tree[p]);

The purpose of the above code is to avoid a branch instruction, but
when p >= t->size, prefetch(&t->tree[0]) has no positive performance
contribution at all. This patch avoids the unncessary prefetch by only
calling prefetch() when p < t->size.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index 268f1b6..e36a108 100644 (file)
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -970,22 +970,10 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
        unsigned int inorder, j, n = 1;
 
        do {
-               /*
-                * A bit trick here.
-                * If p < t->size, (int)(p - t->size) is a minus value and
-                * the most significant bit is set, right shifting 31 bits
-                * gets 1. If p >= t->size, the most significant bit is
-                * not set, right shifting 31 bits gets 0.
-                * So the following 2 lines equals to
-                *      if (p >= t->size)
-                *              p = 0;
-                * but a branch instruction is avoided.
-                */
                unsigned int p = n << 4;
 
-               p &= ((int) (p - t->size)) >> 31;
-
-               prefetch(&t->tree[p]);
+               if (p < t->size)
+                       prefetch(&t->tree[p]);
 
                j = n;
                f = &t->tree[j];