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@xxxxxxx>
---
drivers/md/bcache/bset.c | 30 +++++++++++-------------------
1 file changed, 11 insertions(+), 19 deletions(-)
diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index 8af9509e78bd..08768796b543 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -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);
--
2.16.4
