Thanks to MarkJaquith on #wordpress for this freaky idea, which works! With a different one-line change to do_action(), the performance probe becomes a plugin.

function do_action($tag, $arg = '') {
  if( $tag != 'do_action' ) do_action( 'do_action', $tag );

I also experimented with a non-invasive plugin using add_action( 'all', 'performance_probe' ); but this gets called for all filters too and so generates too much information to be useful.

Just for fun. Whilst activated, it puts a couple of smilies at the top-left of the page to indicate how hard WordPress had to work to make the page.

“Smiley” Peformance Monitor plugin for WordPress

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Upgrade Problem or Not?

This probably isn’t the upgrade problem, but it is certainly a problem one might encounter upon upgrade to a version that contains this inefficient search code. Using the WordPress Trac I can browse the source looking back through the versions to find out in which the offending code was introduced.

2.0.4

Contains at line 379

2.0.3

Contains at line 378

2.0.2

Contains at line 378

2.0.1

Contains at line 378

No, this isn’t an upgrade problem at all. It’s just a problem my blog already has.