+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