mainline inclusion from mainline-5.3-rc1 commit 944a4f340a65c21ee311d2d3e617034bef9d0b25 category: backport
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 | 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);