[PATCH 06/15] bcache: remove unnecessary prefetch() in bset_search_tree()

[Coly Li <colyli@suse.de>]

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>
---
 drivers/md/bcache/bset.c | 16 ++--------------
 1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index 8f07fa6e1739..aa2e4ab0fab9 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -960,22 +960,10 @@ 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];
-- 
2.16.4