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bcache: make bset_search_tree() be more understandable 

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>
This commit is contained in:
Coly Li 2019-06-28 19:59:41 +08:00 committed by Jens Axboe
parent 68a53c95a0
commit 944a4f340a
1 changed files with 11 additions and 19 deletions
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30
drivers/md/bcache/bset.c
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@ -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);
else
n = (bkey_cmp(tree_to_bkey(t, j), search) > 0)
? j * 2
: j * 2 + 1;
if (likely(f->exponent != 127)) {
if (f->mantissa >= bfloat_mantissa(search, f))
n = j * 2;
else
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);