[PATCH 08/37] bcache: remove unnecessary prefetch() in bset_search_tree()
From: Coly Li <hidden>
Date: 2019-06-28 12:00:43
Subsystem:
bcache (block layer cache), the rest · Maintainers:
Coly Li, Kent Overstreet, Linus Torvalds
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 <redacted> --- 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 268f1b685084..e36a108d3648 100644
--- 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];
--
2.16.4