diff --git a/includes/common.inc b/includes/common.inc
index ceac115..ef71ee8 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -5212,6 +5212,11 @@ function _drupal_bootstrap_full() {
   fix_gpc_magic();
   // Load all enabled modules
   module_load_all();
+  // Reset drupal_alter() and module_implements() static caches as these
+  // include implementations for vital modules only when called early on
+  // in the bootstrap.
+  drupal_static_reset('drupal_alter');
+  drupal_static_reset('module_implements');
   // Make sure all stream wrappers are registered.
   file_get_stream_wrappers();
   // Ensure mt_rand is reseeded, to prevent random values from one page load
diff --git a/includes/module.inc b/includes/module.inc
index 494924f..5f83744 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -712,6 +712,7 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   if ($reset) {
     $implementations = array();
     cache_set('module_implements', array(), 'cache_bootstrap');
+    drupal_static_reset('_module_implements_get_implementations');
     drupal_static_reset('module_hook_info');
     drupal_static_reset('drupal_alter');
     cache_clear_all('hook_info', 'cache_bootstrap');
@@ -720,13 +721,7 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
 
   // Fetch implementations from cache.
   if (empty($implementations)) {
-    $implementations = cache_get('module_implements', 'cache_bootstrap');
-    if ($implementations === FALSE) {
-      $implementations = array();
-    }
-    else {
-      $implementations = $implementations->data;
-    }
+    $implementations = _module_implements_get_implementations();
   }
 
   if (!isset($implementations[$hook])) {
@@ -775,6 +770,38 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
 }
 
 /**
+ * Helper function that gets the module implementations from cache.
+ *
+ * @return array
+ *   The module implementations.
+ */
+function _module_implements_get_implementations() {
+  $implementations_cached = &drupal_static(__FUNCTION__);
+  // Try to fetch the implementations from static cache.
+  if (isset($implementations_cached)) {
+    $implementations = $implementations_cached;
+  }
+  // Otherwise, get the implementations from the cache backend.
+  else {
+    $implementations = cache_get('module_implements', 'cache_bootstrap');
+    if ($implementations === FALSE) {
+      $implementations = array();
+    }
+    else {
+      // Set up the implementations variable.
+      $implementations = $implementations->data;
+
+      // Because we reset the "module_implements" static at the end of the
+      // bootstrap, we store the cache result into another static variable
+      // so as to prevent having a second cache_get() in case
+      // module_implements() is called early in the bootstrap.
+      // @see _drupal_bootstrap_full()
+      $implementations_cached = $implementations;
+    }
+  }
+}
+
+/**
  * Retrieves a list of hooks that are declared through hook_hook_info().
  *
  * @return
@@ -833,6 +860,11 @@ function module_hook_info() {
  * @see module_implements()
  */
 function module_implements_write_cache() {
+  // The list of implementations includes vital modules only before full
+  // bootstrap, so do not write cache if we are not fully bootstrapped yet.
+  if (drupal_get_bootstrap_phase() != DRUPAL_BOOTSTRAP_FULL) {
+    return;
+  }
   $implementations = &drupal_static('module_implements');
   if (isset($implementations['#write_cache'])) {
     unset($implementations['#write_cache']);
diff --git a/modules/simpletest/simpletest.info b/modules/simpletest/simpletest.info
index 7b139ba..1aec619 100644
--- a/modules/simpletest/simpletest.info
+++ b/modules/simpletest/simpletest.info
@@ -11,6 +11,7 @@ configure = admin/config/development/testing/settings
 files[] = tests/actions.test
 files[] = tests/ajax.test
 files[] = tests/batch.test
+files[] = tests/boot.test
 files[] = tests/bootstrap.test
 files[] = tests/cache.test
 files[] = tests/common.test
diff --git a/modules/simpletest/tests/boot.test b/modules/simpletest/tests/boot.test
new file mode 100644
index 0000000..562b082
--- /dev/null
+++ b/modules/simpletest/tests/boot.test
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * Perform early bootstrap tests.
+ */
+class EarlyBootstrapTestCase extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name'  => 'Early bootstrap test',
+      'description'  => 'Confirm that calling module_implements() during early bootstrap does not pollute the module_implements() cache.',
+      'group' => 'System',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('boot_test_1', 'boot_test_2');
+  }
+
+  /**
+   * Test hook_boot() on both regular and "early exit" pages.
+   */
+  public function testHookBoot() {
+    $paths = array('', 'early_exit');
+    foreach ($paths as $path) {
+      // Empty the module_implements() caches.
+      module_implements(NULL, FALSE, TRUE);
+      // Do a request to the front page, which will call module_implements()
+      // during hook_boot().
+      $this->drupalGet($path);
+      // Reset the static cache so we get implementation data from the persistent
+      // cache.
+      drupal_static_reset();
+      // Make sure we get a full list of all modules implementing hook_help().
+      $modules = module_implements('help');
+      $this->assertTrue(in_array('boot_test_2', $modules));
+    }
+  }
+}
diff --git a/modules/simpletest/tests/boot_test_1.info b/modules/simpletest/tests/boot_test_1.info
new file mode 100644
index 0000000..a6f2ffb
--- /dev/null
+++ b/modules/simpletest/tests/boot_test_1.info
@@ -0,0 +1,6 @@
+name = Early bootstrap tests
+description = A support module for hook_boot testing.
+core = 7.x
+package = Testing
+version = VERSION
+hidden = TRUE
diff --git a/modules/simpletest/tests/boot_test_1.module b/modules/simpletest/tests/boot_test_1.module
new file mode 100644
index 0000000..a452e28
--- /dev/null
+++ b/modules/simpletest/tests/boot_test_1.module
@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * @file
+ * Tests calling module_implements() during hook_boot() invocation.
+ */
+
+/**
+ * Implements hook_boot().
+ */
+function boot_test_1_boot() {
+  // Calling module_implements during hook_boot() will return "vital" modules
+  // only, and this list of modules will be statically cached.
+  module_implements('help');
+  // Define a special path to test that the static cache isn't written away
+  // if we exit before having completed the bootstrap.
+  if ($_GET['q'] == 'early_exit') {
+    module_implements_write_cache();
+    exit();
+  }
+}
diff --git a/modules/simpletest/tests/boot_test_2.info b/modules/simpletest/tests/boot_test_2.info
new file mode 100644
index 0000000..f421997
--- /dev/null
+++ b/modules/simpletest/tests/boot_test_2.info
@@ -0,0 +1,6 @@
+name = Early bootstrap tests
+description = A support module for hook_boot hook testing.
+core = 7.x
+package = Testing
+version = VERSION
+hidden = TRUE
diff --git a/modules/simpletest/tests/boot_test_2.module b/modules/simpletest/tests/boot_test_2.module
new file mode 100644
index 0000000..c3ab8d6
--- /dev/null
+++ b/modules/simpletest/tests/boot_test_2.module
@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Defines a hook_help() implementation in a non-"bootstrap" module.
+ */
+
+/**
+ * Implements hook_help().
+ */
+function boot_test_2_help($path, $arg) {
+  // Empty hook.
+}