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

[Kent Overstreet]

On Sun, Jan 21, 2024 at 11:36:46PM +0800, Kuan-Wei Chiu wrote:
> 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.

This is a good commit message - but it would be even better if it was a
function comment on parent()

> 
> 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
>