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 <[email protected]> --- 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
