Index: lib/moodlelib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/lib/moodlelib.php,v
retrieving revision 1.1492
diff -u -r1.1492 moodlelib.php
--- lib/moodlelib.php	1 Nov 2010 12:20:39 -0000	1.1492
+++ lib/moodlelib.php	2 Nov 2010 11:22:01 -0000
@@ -9237,7 +9237,7 @@
  * @return array
  */
 function get_performance_info() {
-    global $CFG, $PERF, $DB;
+    global $CFG, $PERF, $DB, $PAGE;
 
     $info = array();
     $info['html'] = '';         // holds userfriendly HTML representation
@@ -9287,6 +9287,33 @@
         }
     }
 
+    $jsmodules = $PAGE->requires->get_included_modules();
+    if ($jsmodules) {
+        $yuicount = 0;
+        $othercount = 0;
+        $details = '';
+        foreach ($jsmodules as $module => $backtraces) {
+            if (strpos($module, 'yui') === 0) {
+                $yuicount += 1;
+            } else {
+                $othercount += 1;
+            }
+            $details .= "<p>$module</p>";
+            foreach ($backtraces as $backtrace) {
+                $details .= format_backtrace($backtrace);
+            }
+        }
+
+        $info['html'] .= "<span class='includedyuimodules'>Included YUI modules: $yuicount</span> ";
+        $info['txt'] .= "includedyuimodules: $yuicount ";
+        $info['html'] .= "<span class='includedjsmodules'>Other JavaScript modules: $othercount</span> ";
+        $info['txt'] .= "includedjsmodules: $othercount ";
+        // Slightly odd to output the details in a display: none div. The point
+        // Is that it takes a lot of space, and if you care you can reveal it
+        // using firebug.
+        $info['html'] .= "<div class='notifytiny' style='display: none;'>$details</div>";
+    }
+
     if (!empty($PERF->logwrites)) {
         $info['logwrites'] = $PERF->logwrites;
         $info['html'] .= '<span class="logwrites">Log DB writes '.$info['logwrites'].'</span> ';
Index: lib/outputrequirementslib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/lib/outputrequirementslib.php,v
retrieving revision 1.55
diff -u -r1.55 outputrequirementslib.php
--- lib/outputrequirementslib.php	1 Nov 2010 17:45:00 -0000	1.55
+++ lib/outputrequirementslib.php	2 Nov 2010 11:22:01 -0000
@@ -477,6 +477,21 @@
     }
 
     /**
+     * This is only used by the commented-out code below.
+     * @param string|array $module name of module, or full module specification.
+     * @return boolean whether this module has already been required.
+     */
+    protected function is_already_included($module) {
+        if (is_string($module)) {
+            $modulename = $module;
+        } else {
+            $modulename = $module['name'];
+        }
+        return array_key_exists($modulename, $this->M_yui_loader->modules) ||
+                array_key_exists($modulename, $this->extramodules);
+    }
+
+    /**
      * Append YUI3 module to default YUI3 JS loader.
      * The structure of module array is described at http://developer.yahoo.com/yui/3/yui/:
      * @param string|array $module name of module (details are autodetected), or full module specification as array
@@ -489,6 +504,28 @@
             throw new coding_exception('Missing YUI3 module name or full description.');
         }
 
+        /*
+        // Comment to anyone who reviews this: I think this is a small
+        // performance win, but I don't know if it would have undesirable
+        // side-effects.
+        if ($this->is_already_included($module)) {
+            if (debugging('', DEBUG_DEVELOPER)) {
+                if (is_string($module)) {
+                    $modulename = $module;
+                } else {
+                    $modulename = $module['name'];
+                }
+                if (array_key_exists($modulename, $this->M_yui_loader->modules)) {
+                    $this->M_yui_loader->modules[$modulename]['requiredfrom'][] = debug_backtrace();
+                } else {
+                    $this->extramodules[$modulename]['requiredfrom'][] = debug_backtrace();
+                }
+            }
+
+            return;
+        }
+        */
+
         if (is_string($module)) {
             $module = $this->find_module($module);
         }
@@ -509,11 +546,48 @@
         }
         unset($module['strings']);
 
+        if (debugging('', DEBUG_DEVELOPER)) {
+            $module['requiredfrom'][] = debug_backtrace();
+        }
+
         if ($this->headdone) {
             $this->extramodules[$module['name']] = $module;
         } else {
             $this->M_yui_loader->modules[$module['name']] = $module;
         }
+
+        /*
+        // Note to reviewers: This seems to be necessary, but again, I don't
+        // really understand it.
+        if (!empty($module['requires'])) {
+            foreach ($module['requires'] as $requirement) {
+                $this->js_module($requirement);
+            }
+        }
+        */
+    }
+
+    /**
+     * @return array of the js_modules that were included, and the stack traces
+     * of where they were included from.
+     */
+    public function get_included_modules() {
+        $modules = array();
+        foreach ($this->M_yui_loader->modules as $name => $module) {
+            $modules[$name] = array();
+            if (!empty($module['requiredfrom'])) {
+                $modules[$name] += $module['requiredfrom'];
+            }
+        }
+
+        foreach ($this->extramodules as $name => $module) {
+            $modules[$name] += array();
+            if (!empty($module['requiredfrom'])) {
+                $modules[$name] += $module['requiredfrom'];
+            }
+        }
+
+        return $modules;
     }
 
     /**