Re: [PATCH] Improve performance of strncpy

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

On 20-08-2014 09:44, Wilco Dijkstra wrote:
> Hi,
>
> This patch improves strncpy performance by using memset to clear memory after the string when the
> buffer is much larger than the copied string. This is better as memset is significantly faster than
> a simple byte-loop. On bench-strncpy it is ~25% faster.

Hi, the patch looks ok. I also pushed a similar modification for powerpc based on same idea. 

>
> ChangeLog:
> 2014-08-20  Wilco Dijkstra  <wdijkstr@arm.com>
>
> 	* string/strncpy.c (strncpy): Improve performance by using memset.
> ---
>  string/strncpy.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/string/strncpy.c b/string/strncpy.c
> index 0915e03..604417c 100644
> --- a/string/strncpy.c
> +++ b/string/strncpy.c
> @@ -78,9 +78,12 @@ STRNCPY (char *s1, const char *s2, size_t n)
>    while (c != '\0');
>  
>   zero_fill:
> -  do
> -    *++s1 = '\0';
> -  while (--n > 0);
> +  if (n >= 8)
> +    memset (s1 + 1, '\0', n);
> +  else
> +    do
> +      *++s1 = '\0';
> +    while (--n > 0);

I wonder if this test is really worth, my opinion is just to keep it simple
and just call memset on both 'goto' in loop and after 'last_chars'.

>  
>    return s;
>  }
> -- 1.7.9.5