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Re: svn commit: r1001413 - in /subversion/branches/performance/subversion: include/svn_string.h libsvn_subr/svn_string.c

From: Hyrum K. Wright <hyrum_wright_at_mail.utexas.edu>
Date: Mon, 27 Sep 2010 16:05:00 -0500

+1 to merge to trunk.

On Sun, Sep 26, 2010 at 6:01 AM, <stefan2_at_apache.org> wrote:
> Author: stefan2
> Date: Sun Sep 26 11:01:03 2010
> New Revision: 1001413
>
> URL: http://svn.apache.org/viewvc?rev=1001413&view=rev
> Log:
> Extensively document the benefits of svn_stringbuf_appendbyte and
> the rationals behind its implementation. To simplify the explanations,
> one statement had to be moved.
>
> * subversion/include/svn_string.h
>  (svn_stringbuf_appendbyte): extend docstring to indicate that this method
>  is cheaper to call and execute
> * subversion/libsvn_subr/svn_string.c
>  (svn_stringbuf_appendbyte): reorder statements for easier description;
>  add extensive description about the optimizations done
>
> Modified:
>    subversion/branches/performance/subversion/include/svn_string.h
>    subversion/branches/performance/subversion/libsvn_subr/svn_string.c
>
> Modified: subversion/branches/performance/subversion/include/svn_string.h
> URL: http://svn.apache.org/viewvc/subversion/branches/performance/subversion/include/svn_string.h?rev=1001413&r1=1001412&r2=1001413&view=diff
> ==============================================================================
> --- subversion/branches/performance/subversion/include/svn_string.h (original)
> +++ subversion/branches/performance/subversion/include/svn_string.h Sun Sep 26 11:01:03 2010
> @@ -259,6 +259,10 @@ void
>  svn_stringbuf_fillchar(svn_stringbuf_t *str, unsigned char c);
>
>  /** Append a single character @a byte onto @a targetstr.
> + * This is an optimized version of @ref svn_stringbuf_appendbytes
> + * that is much faster to call and execute. Gains vary with the ABI.
> + * The advantages extend beyond the actual call because the reduced
> + * register pressure allows for more optimization within the caller.
>  *
>  * reallocs if necessary. @a targetstr is affected, nothing else is.
>  * @since New in 1.7.
>
> Modified: subversion/branches/performance/subversion/libsvn_subr/svn_string.c
> URL: http://svn.apache.org/viewvc/subversion/branches/performance/subversion/libsvn_subr/svn_string.c?rev=1001413&r1=1001412&r2=1001413&view=diff
> ==============================================================================
> --- subversion/branches/performance/subversion/libsvn_subr/svn_string.c (original)
> +++ subversion/branches/performance/subversion/libsvn_subr/svn_string.c Sun Sep 26 11:01:03 2010
> @@ -393,28 +393,60 @@ svn_stringbuf_ensure(svn_stringbuf_t *st
>  }
>
>
> +/* WARNING - Optimized code ahead!
> + * This function has been hand-tuned for performance. Please read
> + * the comments below before modifying the code.
> + */
>  void
>  svn_stringbuf_appendbyte(svn_stringbuf_t *str, char byte)
>  {
> +  char *dest;
> +  apr_size_t old_len = str->len;
> +
>   /* In most cases, there will be pre-allocated memory left
>    * to just write the new byte at the end of the used section
>    * and terminate the string properly.
>    */
> -  apr_size_t old_len = str->len;
> -  if (str->blocksize > old_len + 1)
> +  if (str->blocksize < old_len + 1)
>     {
> -      char *dest = str->data;
> +      /* The following read does not depend this write, so we
> +       * can issue the write first to minimize register pressure:
> +       * The value of old_len+1 is no longer needed; on most processors,
> +       * dest[old_len+1] will be calculated implicitly as part of
> +       * the addressing scheme.
> +       */
> +      str->len = old_len+1;
> +
> +      /* Since the compiler cannot be sure that *src->data and *src
> +       * don't overlap, we read src->data *once* before writing
> +       * to *src->data. Replacing dest with str->data would force
> +       * the compiler to read it again after the first byte.
> +       */
> +      dest = str->data;
>
> +      /* If not already available in a register as per ABI, load
> +       * "byte" into the register (e.g. the one freed from old_len+1),
> +       * then write it to the string buffer and terminate it properly.
> +       *
> +       * Including the "byte" fetch, all operations so far could be
> +       * issued at once and be scheduled at the CPU's descression.
> +       * Most likely, no-one will soon depend on the data that will be
> +       * written in this function. So, no stalls there, either.
> +       */
>       dest[old_len] = byte;
>       dest[old_len+1] = '\0';
> -
> -      str->len = old_len+1;
>     }
>   else
>     {
>       /* we need to re-allocate the string buffer
>        * -> let the more generic implementation take care of that part
>        */
> +
> +      /* Depending on the ABI, "byte" is a register value. If we were
> +       * to take its address directly, the compiler might decide to
> +       * put in on the stack *unconditionally*, even if that would
> +       * only be necessary for this block.
> +       */
>       char b = byte;
>       svn_stringbuf_appendbytes(str, &b, 1);
>     }
>
>
>
Received on 2010-09-27 23:05:41 CEST

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