commit cb8419dda3b8824e0f151f0eda0803e20090a027
Author: Joel Pittet <joel@pittet.ca>
Date:   Mon Mar 31 18:47:15 2014 -0700

    convert field__node__title

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 5eca228..d1d152f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2396,17 +2396,24 @@ function template_preprocess_field(&$variables, $hook) {
     $default_attributes = new Attribute;
   }
 
-  // Modules (e.g., rdf.module) can add field item attributes (to
-  // $item->_attributes) within hook_entity_prepare_view(). Some field
-  // formatters move those attributes into some nested formatter-specific
-  // element in order have them rendered on the desired HTML element (e.g., on
-  // the <a> element of a field item being rendered as a link). Other field
-  // formatters leave them within $element['#items'][$delta]['_attributes'] to
-  // be rendered on the item wrappers provided by field.html.twig.
+  // We want other preprocess functions and the theme implementation to have
+  // fast access to the field item render arrays. The item render array keys
+  // (deltas) should always be numerically indexed starting from 0, and looping
+  // on those keys is faster than calling element_children() or looping on all
+  // keys within $element, since that requires traversal of all element
+  // properties.
   $variables['items'] = array();
   $delta = 0;
   while (!empty($element[$delta])) {
     $variables['items'][$delta]['value'] = $element[$delta];
+
+    // Modules (e.g., rdf.module) can add field item attributes (to
+    // $item->_attributes) within hook_entity_prepare_view(). Some field
+    // formatters move those attributes into some nested formatter-specific
+    // element in order have them rendered on the desired HTML element (e.g., on
+    // the <a> element of a field item being rendered as a link). Other field
+    // formatters leave them within $element['#items'][$delta]['_attributes'] to
+    // be rendered on the item wrappers provided by field.html.twig.
     $variables['items'][$delta]['attributes'] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : clone($default_attributes);
     $delta++;
   }