bcache: make bset_search_tree() be more understandable (944a4f34) · Commits · 戴 / test

The purpose of following code in bset_search_tree() is to avoid a branch instruction, 994 if (likely(f->exponent != 127)) 995 n = j * 2 + (((unsigned int) 996 (f->mantissa - 997 bfloat_mantissa(search, f))) >> 31); 998 else 999 n = (bkey_cmp(tree_to_bkey(t, j), search) > 0) 1000 ? j * 2 1001 : j * 2 + 1; This piece of code is not very clear to understand, even when I tried to add code comment for it, I made mistake. This patch removes the implict bit operation and uses explicit branch to calculate next location in binary tree search. Signed-off-by:

Coly Li <colyli@suse.de> Signed-off-by:

Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bset.c

+11 −19

Original line number	Diff line number	Diff line
		@@ -975,25 +975,17 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
		j = n;
		f = &t->tree[j];

		/*
		* Similar bit trick, use subtract operation to avoid a branch
		* instruction.
		*
		* n = (f->mantissa > bfloat_mantissa())
		* ? j * 2
		* : j * 2 + 1;
		*
		* We need to subtract 1 from f->mantissa for the sign bit trick
		* to work - that's done in make_bfloat()
		*/
		if (likely(f->exponent != 127))
		n = j * 2 + (((unsigned int)
		(f->mantissa -
		bfloat_mantissa(search, f))) >> 31);
		if (likely(f->exponent != 127)) {
		if (f->mantissa >= bfloat_mantissa(search, f))
		n = j * 2;
		else
		n = (bkey_cmp(tree_to_bkey(t, j), search) > 0)
		? j * 2
		: j * 2 + 1;
		n = j * 2 + 1;
		} else {
		if (bkey_cmp(tree_to_bkey(t, j), search) > 0)
		n = j * 2;
		else
		n = j * 2 + 1;
		}
		} while (n < t->size);

		inorder = to_inorder(j, t);

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