One correction (in the comments -- which are great, BTW):
On Mon, Sep 27, 2010 at 5:05 PM, Hyrum K. Wright
> +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
>> 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
>> 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.
>> + */
>> 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;
>> /* 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 2010-09-28 22:00:53 CEST