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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>
alistair/sunxi64-5.4-dsi
Coly Li 2019-06-28 19:59:31 +08:00 committed by Jens Axboe
parent 08ec1e6282
commit f960facb39
1 changed files with 2 additions and 14 deletions
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16
drivers/md/bcache/bset.c
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@ -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];