Re: [PATCH] Speedup wc -l

[Pádraig Brady]

On 24/03/15 07:34, Bernhard Voelker wrote:
> On 03/23/2015 01:03 PM, Pádraig Brady wrote:
>> It just occurred to me that we could simplify from 3 to 2 loops,
>> while also making the code more adaptive to the input,
>> by simply determining the average line length per block.
> 
> 
> great idea!
> 
>> I'll push the attached later.
> 
> just 2 minor, almost readability notes:
> 
> As you introduced the 'end' variable, why not using it
> in the memchr() case, too?
> 
> @@ -290,7 +290,7 @@ wc (int fd, char const *file_x, struct fstatus *fstatus, 
> off_t current_pos)
>            else
>              {
>                /* memchr is more efficient with longer lines.  */
> -              while ((p = memchr (p, '\n', (buf + bytes_read) - p)))
> +              while ((p = memchr (p, '\n', end - p)))

Nice one.

>                  {
>                    ++p;
>                    ++lines;
> 
> I'd favor multiplication by 15 on the other side, thus avoiding
> to have to check for DIV-By-ZERO manually:
> 
> @@ -303,7 +303,7 @@ wc (int fd, char const *file_x, struct fstatus *fstatus, 
> off_t current_pos)
>               FIXME: This line length was determined in 2015, on both
>               x86_64 and ppc64, but it's worth re-evaluating in future with
>               newer compilers, CPUs, or memchr() implementations etc.  */
> -          if (lines == plines || (bytes_read / (lines - plines) > 15))
> +          if ((lines - plines) * 15 <= bytes_read)

Better thanks. Actually I'll divide both sides by 15 to
avoid any hint of overflow issues.

thanks!
Pádraig.