reverted:
--- b/core/includes/theme.inc
+++ a/core/includes/theme.inc
@@ -120,23 +120,6 @@
 function drupal_find_theme_functions($cache, $prefixes) {
   $implementations = array();
   $functions = get_defined_functions();
-  static $lookup_table = NULL;
-  static $seen = array();
-
-  foreach ($functions['user'] as $function) {
-    if (isset($seen[$function])) {
-      continue;
-    }
-    $seen[$function] = TRUE;
-    $t = explode('_', $function);
-    $count = count($t);
-    $prefix = '';
-    for ($i = 0; $i < $count; $i++) {
-      $prefix .= $t[$i];
-      $lookup_table[$prefix][] = $function;
-      $prefix .= '_';
-    }
-  }
 
   foreach ($cache as $hook => $info) {
     foreach ($prefixes as $prefix) {
@@ -152,8 +135,8 @@
       // are found using the base hook's pattern, not a pattern from an
       // intermediary suggestion.
       $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
+      if (!isset($info['base hook']) && !empty($pattern)) {
+        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $functions['user']);
-      if (!isset($info['base hook']) && !empty($pattern) && isset($lookup_table[$prefix])) {
-        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $lookup_table[$prefix]);
         if ($matches) {
           foreach ($matches as $match) {
             $new_hook = substr($match, strlen($prefix) + 1);
@@ -169,7 +152,7 @@
       // Find theme functions that implement registered theme hooks and include
       // that in what is returned so that the registry knows that the theme has
       // this implementation.
+      if (function_exists($prefix . '_' . $hook)) {
-      if (isset($seen[$prefix . '_' . $hook])) {
         $implementations[$hook] = array(
           'function' => $prefix . '_' . $hook,
         );
diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -537,37 +537,38 @@
       // Merge the newly created theme hooks into the existing cache.
       $cache = $result + $cache;
     }
+
+    // Let themes have variable preprocessors even if they didn't register a
+    // template.
+    if ($type == 'theme' || $type == 'base_theme') {
+      foreach ($cache as $hook => $info) {
+        // Check only if not registered by the theme or engine.
+        if (empty($result[$hook])) {
+          if (!isset($info['preprocess functions'])) {
+            $cache[$hook]['preprocess functions'] = array();
+          }
+          // Only use non-hook-specific variable preprocessors for theme hooks
+          // implemented as templates. See _theme().
+          if (isset($info['template']) && function_exists($name . '_preprocess')) {
+            $cache[$hook]['preprocess functions'][] = $name . '_preprocess';
+          }
+          if (function_exists($name . '_preprocess_' . $hook)) {
+            $cache[$hook]['preprocess functions'][] = $name . '_preprocess_' . $hook;
+            $cache[$hook]['theme path'] = $path;
+          }
+        }
+      }
+    }
   }
 
   /**
    * This completes the theme registry adding missing functions and hooks.
    *
    * @param array $cache
-   *   The theme registry that will eventually be cached; It is an associative
-   *   array keyed by theme hooks, whose values are associative arrays
-   *   describing the hook:
-   *   - 'type': The passed-in $type.
-   *   - 'theme path': The passed-in $path.
-   *   - 'function': The name of the function generating output for this theme
-   *     hook. Either defined explicitly in hook_theme() or, if neither
-   *     'function' nor 'template' is defined, then the default theme function
-   *     name is used. The default theme function name is the theme hook
-   *     prefixed by either 'theme_' for modules or '$name_' for everything
-   *     else. If 'function' is defined, 'template' is not used.
-   *   - 'template': The filename of the template generating output for this
-   *     theme hook. The template is in the directory defined by the 'path' key
-   *     of hook_theme() or defaults to "$path/templates".
-   *   - 'variables': The variables for this theme hook as defined in
-   *     hook_theme(). If there is more than one implementation and 'variables'
-   *     is not specified in a later one, then the previous definition is kept.
-   *   - 'render element': The renderable element for this theme hook as defined
-   *     in hook_theme(). If there is more than one implementation and
-   *     'render element' is not specified in a later one, then the previous
-   *     definition is kept.
-   *   - 'preprocess functions': See _theme() for detailed documentation.
+   *  @see ::processExtension()
    * @param object $theme
    *   Current active theme.
-  */
+   */
   protected function postProcessExtension(array &$cache, $theme) {
 
     // Get all user defined functions.