Re: [PATCH][PING] Improve stpncpy performance

[OndÅej BÃlka <neleai at seznam dot cz>]

On Mon, Jul 06, 2015 at 12:29:24PM +0100, Wilco Dijkstra wrote:
> > Wilco wrote:
> > ping
> > 
> > > > -----Original Message-----
> > > > From: Wilco Dijkstra [mailto:wdijkstr@arm.com]
> > > > Sent: 12 January 2015 15:09
> > > > To: 'libc-alpha@sourceware.org'
> > > > Subject: [PATCH] Improve stpncpy performance
> > > >
> > > > Like strncpy, this patch improves stpncpy performance by using strnlen/memcpy/memset
> > rather
> > > > than a byte loop. Performance on bench-stpncpy is ~2x faster on average.
> > > >
> > > > +  size_t size = __strnlen (src, n);
> > > > +  memcpy (dest, src, size);
> > > > +  dest += size;
> > > > +  if (size == n)
> > > >      return dest;
> > > > -
> > > > -  for (;;)
> > > > -    {
> > > > -      c = *src++;
> > > > -      --n;
> > > > -      *dest++ = c;
> > > > -      if (c == '\0')
> > > > -	break;
> > > > -      if (n == 0)
> > > > -	return dest;
> > > > -    }
> > > > -
> > > > - zero_fill:
> > > > -  while (n-- > 0)
> > > > -    dest[n] = '\0';
> > > > -
> > > > -  return dest - 1;
> > > > +  return memset (dest, '\0', n - size);
> > > >  }
> > > >  #ifdef weak_alias
> > > >  libc_hidden_def (__stpncpy)
> > > > --
> > > > 1.9.1
> 
You don't have to use special case

if (size == n)
  return dest;

as it should be handled by

return memset (dest, '\0', 0);

That could improve performance a bit if its rare case. That doesn't
matter much as memset makes that function slow and it shouldn't be 
used in performance sensitive code.

Otherwise ok for me.