Improve strtok(_r) performance

  ping

From: Wilco Dijkstra
Sent: 28 October 2016 12:35
To: libc-alpha@sourceware.org
Cc: nd
Subject: [PATCH] Improve strtok(_r) performance
    
Improve strtok(_r) performance.  Instead of calling strpbrk which calls
strcspn, call strcspn directly so we get the end of the token without
an extra call to rawmemchr.  Also avoid an unnecessary call to strcspn after
the last token by adding an early exit for an empty string.  The result
is a ~2x speedup of strtok on most inputs in bench-strtok.

Passes regression tests, OK for commit?

ChangeLog:
2015-10-28  Wilco Dijkstra  <wdijkstr@arm.com>

        * string/strtok.c (STRTOK): Optimize for performance.
        * string/strtok_r.c (__strtok_r): Likewise.
--
  

https://sourceware.org/ml/libc-alpha/2016-11/msg00150.html

On 14/11/2016 10:20, Wilco Dijkstra wrote:
> 
> ping
> 
> From: Wilco Dijkstra
> Sent: 28 October 2016 12:35
> To: libc-alpha@sourceware.org
> Cc: nd
> Subject: [PATCH] Improve strtok(_r) performance
>     
> Improve strtok(_r) performance.  Instead of calling strpbrk which calls
> strcspn, call strcspn directly so we get the end of the token without
> an extra call to rawmemchr.  Also avoid an unnecessary call to strcspn after
> the last token by adding an early exit for an empty string.  The result
> is a ~2x speedup of strtok on most inputs in bench-strtok.
> 
> Passes regression tests, OK for commit?
> 
> ChangeLog:
> 2015-10-28  Wilco Dijkstra  <wdijkstr@arm.com>
> 
>         * string/strtok.c (STRTOK): Optimize for performance.
>         * string/strtok_r.c (__strtok_r): Likewise.
> --
> 
> diff --git a/string/strtok.c b/string/strtok.c
> index 7a4574db5c80501e47d045ad4347e8a287b32191..b1ed48c24c8d20706b7d05481a138b18a01ff802 100644
> --- a/string/strtok.c
> +++ b/string/strtok.c
> @@ -38,11 +38,18 @@ static char *olds;
>  char *
>  STRTOK (char *s, const char *delim)
>  {
> -  char *token;
> +  char *end;
>  
>    if (s == NULL)
>      s = olds;
>  
> +  /* Return immediately at end of string.  */
> +  if (*s == '\0')
> +    {
> +      olds = s;
> +      return NULL;
> +    }
> +
>    /* Scan leading delimiters.  */
>    s += strspn (s, delim);
>    if (*s == '\0')
> @@ -52,16 +59,15 @@ STRTOK (char *s, const char *delim)
>      }
>  
>    /* Find the end of the token.  */
> -  token = s;
> -  s = strpbrk (token, delim);
> -  if (s == NULL)
> -    /* This token finishes the string.  */
> -    olds = __rawmemchr (token, '\0');
> -  else
> +  end = s + strcspn (s, delim);
> +  if (*end == '\0')
>      {
> -      /* Terminate the token and make OLDS point past it.  */
> -      *s = '\0';
> -      olds = s + 1;
> +      olds = end;
> +      return s;
>      }
> -  return token;
> +
> +  /* Terminate the token and make OLDS point past it.  */
> +  *end = '\0';
> +  olds = end + 1;
> +  return s;
>  }
> diff --git a/string/strtok_r.c b/string/strtok_r.c
> index f351304766108dad2c1cff881ad3bebae821b2a0..e049a5c82e026a3b6c1ba5da16ce81743717805e 100644
> --- a/string/strtok_r.c
> +++ b/string/strtok_r.c
> @@ -45,11 +45,17 @@
>  char *
>  __strtok_r (char *s, const char *delim, char **save_ptr)
>  {
> -  char *token;
> +  char *end;
>  
>    if (s == NULL)
>      s = *save_ptr;
>  
> +  if (*s == '\0')
> +    {
> +      *save_ptr = s;
> +      return NULL;
> +    }
> +
>    /* Scan leading delimiters.  */
>    s += strspn (s, delim);
>    if (*s == '\0')
> @@ -59,18 +65,17 @@ __strtok_r (char *s, const char *delim, char **save_ptr)
>      }
>  
>    /* Find the end of the token.  */
> -  token = s;
> -  s = strpbrk (token, delim);
> -  if (s == NULL)
> -    /* This token finishes the string.  */
> -    *save_ptr = __rawmemchr (token, '\0');
> -  else
> +  end = s + strcspn (s, delim);
> +  if (*end == '\0')
>      {
> -      /* Terminate the token and make *SAVE_PTR point past it.  */
> -      *s = '\0';
> -      *save_ptr = s + 1;
> +      *save_ptr = end;
> +      return s;
>      }
> -  return token;
> +
> +  /* Terminate the token and make *SAVE_PTR point past it.  */
> +  *end = '\0';
> +  *save_ptr = end + 1;
> +  return s;
>  }
>  #ifdef weak_alias
>  libc_hidden_def (__strtok_r)
>     
>
  

Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> Why not aim for simplicity and just use strtok_r and strtok? I should
> be a tail call in most architecture and performance loss should be
> minimum.

You mean avoiding the duplication of code? Say inlining or tailcalling
strtok_r in string/strtok_r.c?

Wilco
  

On 14/11/2016 10:50, Wilco Dijkstra wrote:
> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
>> Why not aim for simplicity and just use strtok_r and strtok? I should
>> be a tail call in most architecture and performance loss should be
>> minimum.
> 
> You mean avoiding the duplication of code? Say inlining or tailcalling
> strtok_r in string/strtok_r.c?
> 
> Wilco

Yes.