[PATCH] stdlib: Optimize number of calls to comparison function
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Wed Mar 27 19:45:35 GMT 2024
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.
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