@@ -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];
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(-)