[PATCH] lib/sort: Optimize number of calls to comparison function

[PATCH] lib/sort: Optimize number of calls to comparison function

[Kuan-Wei Chiu]

The current implementation continues the loop when the comparison
function returns a non-negative value, which includes the case when it
returns 0. However, in scenarios where the comparison function returns
0, further comparisons are unnecessary. By making this adjustment, we
can potentially reduce the number of comparisons by approximately 50%
in extreme cases where all elements in the array are equal, and the
array size is sufficiently large.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 lib/sort.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/sort.c b/lib/sort.c
index b399bf10d675..1e98a62bb2f3 100644
--- a/lib/sort.c
+++ b/lib/sort.c
@@ -267,7 +267,7 @@ void sort_r(void *base, size_t num, size_t size,
 			b = c;
 
 		/* Now backtrack from "b" to the correct location for "a" */
-		while (b != a && do_cmp(base + a, base + b, cmp_func, priv) >= 0)
+		while (b != a && do_cmp(base + a, base + b, cmp_func, priv) > 0)
 			b = parent(b, lsbit, size);
 		c = b;			/* Where "a" belongs */
 		while (b != a) {	/* Shift it into place */
-- 
2.25.1

Re: [PATCH] lib/sort: Optimize number of calls to comparison function

[Kuan-Wei Chiu]

On Sun, Dec 03, 2023 at 12:37:17AM +0800, Kuan-Wei Chiu wrote:
> The current implementation continues the loop when the comparison
> function returns a non-negative value, which includes the case when it
> returns 0. However, in scenarios where the comparison function returns
> 0, further comparisons are unnecessary. By making this adjustment, we
> can potentially reduce the number of comparisons by approximately 50%
> in extreme cases where all elements in the array are equal, and the
> array size is sufficiently large.
> 
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
>  lib/sort.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/sort.c b/lib/sort.c
> index b399bf10d675..1e98a62bb2f3 100644
> --- a/lib/sort.c
> +++ b/lib/sort.c
> @@ -267,7 +267,7 @@ void sort_r(void *base, size_t num, size_t size,
>  			b = c;
>  
>  		/* Now backtrack from "b" to the correct location for "a" */
> -		while (b != a && do_cmp(base + a, base + b, cmp_func, priv) >= 0)
> +		while (b != a && do_cmp(base + a, base + b, cmp_func, priv) > 0)
>  			b = parent(b, lsbit, size);
>  		c = b;			/* Where "a" belongs */
>  		while (b != a) {	/* Shift it into place */
> -- 
> 2.25.1
>

While the patch decreases the number of comparisons, it simultaneously
leads to an increase in the number of swaps. As a result, the overall
performance improvement may not be guaranteed.

Therefore, I believe it would be more prudent to drop this patch.
I apologize for any disruption caused on the mailing list.

Best regards,
Kuan-Wei Chiu