linux-kernel - Re: [PATCH 2/5] bcachefs: Introduce parent function for sort_cmp

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Date: Mon, 22 Jan 2024 01:05:28 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: colyli@...e.de, bfoster@...hat.com, jserv@...s.ncku.edu.tw,
	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH 2/5] bcachefs: Introduce parent function for
 sort_cmp_size()

On Sun, Jan 21, 2024 at 11:17:30AM -0500, Kent Overstreet wrote:
> 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()
>
Sure, however, it seems that sort_cmp_size() can be directly replaced
with the sort function from include/linux. Once we decide on the
cleanup tasks, if we still choose to retain this patch, I will make the
adjustments.
> > 
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> > ---
> > 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
> >