[PATCH] stdlib: Optimize number of calls to comparison function

[Kuan-Wei Chiu]

On Wed, Mar 27, 2024 at 05:42:21PM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> On 27/03/24 16:59, Kuan-Wei Chiu wrote:
> > On Wed, Mar 27, 2024 at 04:45:35PM -0300, Adhemerval Zanella Netto wrote:
> >>
> >>
> >> On 16/02/24 04:08, Kuan-Wei Chiu wrote:
> >>> On Sun, Dec 03, 2023 at 05:48:39AM +0800, Kuan-Wei Chiu wrote:
> >>>> The current heapsort implementation in the siftdown function follows
> >>>> the standard textbook version, necessitating two comparisons at each
> >>>> level. Transitioning to the Bottom-up heapsort version allows us to
> >>>> decrease the required comparisons to just one per level. On average,
> >>>> this modification significantly reduces the comparison count from
> >>>> 2nlog2(n) - 3n + o(n) to nlog2(n) + 0.37*n + o(n).
> >>>>
> >>>> Refs:
> >>>>   BOTTOM-UP-HEAPSORT, a new variant of HEAPSORT beating, on an average,
> >>>>   QUICKSORT (if n is not very small)
> >>>>   Ingo Wegener
> >>>>   Theoretical Computer Science, 118(1); Pages 81-98, 13 September 1993
> >>>>   https://doi.org/10.1016/0304-3975(93)90364-Y
> >>>>
> >>>> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> >>>> ---
> >>>>  stdlib/qsort.c | 36 ++++++++++++++++++------------------
> >>>>  1 file changed, 18 insertions(+), 18 deletions(-)
> >>>>
> >>>> diff --git a/stdlib/qsort.c b/stdlib/qsort.c
> >>>> index be01fb5598..f5babef150 100644
> >>>> --- a/stdlib/qsort.c
> >>>> +++ b/stdlib/qsort.c
> >>>> @@ -132,26 +132,26 @@ static inline void
> >>>>  siftdown (void *base, size_t size, size_t k, size_t n,
> >>>>  	  enum swap_type_t swap_type, __compar_d_fn_t cmp, void *arg)
> >>>>  {
> >>>> -  /* There can only be a heap condition violation if there are
> >>>> -     children.  */
> >>>> -  while (2 * k + 1 <= n)
> >>>> -    {
> >>>> -      /* Left child.  */
> >>>> -      size_t j = 2 * k + 1;
> >>>> -      /* If the right child is larger, use it.  */
> >>>> -      if (j < n && cmp (base + (j * size), base + ((j + 1) * size), arg) < 0)
> >>>> -	j++;
> >>>> -
> >>>> -      /* If k is already >= to its children, we are done.  */
> >>>> -      if (j == k || cmp (base + (k * size), base + (j * size), arg) >= 0)
> >>>> -	break;
> >>>> +  size_t i, j;
> >>>> +
> >>>> +  /* Find the sift-down path all the way to the leaves. */
> >>>> +  for (i = k; j = 2 * i + 1, j + 1 <= n;)
> >>>> +    i = cmp (base + j * size, base + (j + 1) * size, arg) >= 0 ? j : (j + 1);
> >>>>  
> >>>> -      /* Heal the violation.  */
> >>>> -      do_swap (base + (size * j), base + (k * size), size, swap_type);
> >>>> +  /* Special case for the last leaf with no sibling. */
> >>>> +  if (j == n)
> >>>> +    i = j;
> >>>>  
> >>>> -      /* Swapping with j may have introduced a violation at j.  Fix
> >>>> -	 it in the next loop iteration.  */
> >>>> -      k = j;
> >>>> +  /* Backtrack to the correct location. */
> >>>> +  while (i != k && cmp (base + k * size, base + i * size, arg) > 0)
> >>>> +    i = (i - 1) / 2;
> >>>> +
> >>>> +  /* Shift the element into its correct place. */
> >>>> +  j = i;
> >>>> +  while (i != k)
> >>>> +    {
> >>>> +      i = (i - 1) / 2;
> >>>> +      do_swap (base + i * size, base + j * size, size, swap_type);
> >>>>      }
> >>>>  }
> >>>>  
> >>>> -- 
> >>>> 2.25.1
> >>>>
> >>>
> >>> Since we still retain heapsort as a fallback for mergesort, should we
> >>> reconsider applying this patch?
> >>>
> >>> Thanks,
> >>> Kuan-Wei
> >>
> >> Yes, I think it is work and it looks great to me.
> > 
> > Should I resend a new patch, or can you directly apply this one?
> 
> It applies as-is, so it should be ok. I will run some tests and apply, thanks.

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