[PATCH 2/5] bcachefs: Introduce parent function for sort_cmp_size()

[Kuan-Wei Chiu <visitorckw@xxxxxxxxx>]

When dealing with array indices, the parent's index can be obtained
using the formula (i - 1) / 2. However, when working with byte offsets,
this approach is not straightforward. To address this, we have
introduced a branch-free parent function that does not require any
division operations to calculate the parent's byte offset.

Signed-off-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
---
This patch has undergone unit testing using the following code [1].

[1]:
static int test(void)
{
    size_t i, p, size, lsbit;

    for (i = 0; i < 10000; i++) {
        size = get_random_u32() % (1U << 10);
        lsbit = size & -size;
        i = get_random_u32() % (1U << 20) * size + size;
        p = parent(i, lsbit, size);
        if (p != (i / size - 1) / 2 * size)
            return -1;
    }

    return 0;
}

 fs/bcachefs/util.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/bcachefs/util.c b/fs/bcachefs/util.c
index bbc83b43162e..f5bbf96df2ce 100644
--- a/fs/bcachefs/util.c
+++ b/fs/bcachefs/util.c
@@ -907,6 +907,13 @@ static inline void do_swap(void *base, size_t n, size_t size,
 		  size);
 }
 
+static inline size_t parent(size_t i, size_t lsbit, size_t size)
+{
+	i -= size;
+	i -= size & -(i & lsbit);
+	return i >> 1;
+}
+
 void eytzinger0_sort(void *base, size_t n, size_t size,
 		     int (*cmp_func)(const void *, const void *, size_t),
 		     void (*swap_func)(void *, void *, size_t))
-- 
2.25.1