bcache: remove unnecessary prefetch() in bset_search_tree() (f960facb) · Commits · 戴 / test

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>

drivers/md/bcache/bset.c

+2 −14

Original line number	Original line	Diff line number	Diff line
	@@ -970,21 +970,9 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
	unsigned int inorder, j, n = 1;		unsigned int inorder, j, n = 1;

	do {		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;		unsigned int p = n << 4;

	p &= ((int) (p - t->size)) >> 31;		if (p < t->size)

	prefetch(&t->tree[p]);		prefetch(&t->tree[p]);

	j = n;		j = n;

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