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

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

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.