Index: src/wp-admin/includes/image-edit.php
===================================================================
--- src/wp-admin/includes/image-edit.php	(revision 38301)
+++ src/wp-admin/includes/image-edit.php	(working copy)
@@ -700,13 +700,11 @@
  * Saves image to post along with enqueued changes
  * in $_REQUEST['history']
  *
- * @global array $_wp_additional_image_sizes
- *
  * @param int $post_id
  * @return \stdClass
  */
 function wp_save_image( $post_id ) {
-	global $_wp_additional_image_sizes;
+	$_wp_additional_image_sizes = wp_get_additional_image_sizes();
 
 	$return = new stdClass;
 	$success = $delete = $scaled = $nocrop = false;
Index: src/wp-admin/includes/image.php
===================================================================
--- src/wp-admin/includes/image.php	(revision 38301)
+++ src/wp-admin/includes/image.php	(working copy)
@@ -67,8 +67,6 @@
  *
  * @since 2.1.0
  *
- * @global array $_wp_additional_image_sizes
- *
  * @param int $attachment_id Attachment Id to process.
  * @param string $file Filepath of the Attached image.
  * @return mixed Metadata for attachment.
@@ -87,23 +85,34 @@
 		$metadata['file'] = _wp_relative_upload_path($file);
 
 		// Make thumbnails and other intermediate sizes.
-		global $_wp_additional_image_sizes;
+		$_wp_additional_image_sizes = wp_get_additional_image_sizes();
 
 		$sizes = array();
 		foreach ( get_intermediate_image_sizes() as $s ) {
 			$sizes[$s] = array( 'width' => '', 'height' => '', 'crop' => false );
-			if ( isset( $_wp_additional_image_sizes[$s]['width'] ) )
-				$sizes[$s]['width'] = intval( $_wp_additional_image_sizes[$s]['width'] ); // For theme-added sizes
-			else
-				$sizes[$s]['width'] = get_option( "{$s}_size_w" ); // For default sizes set in options
-			if ( isset( $_wp_additional_image_sizes[$s]['height'] ) )
-				$sizes[$s]['height'] = intval( $_wp_additional_image_sizes[$s]['height'] ); // For theme-added sizes
-			else
-				$sizes[$s]['height'] = get_option( "{$s}_size_h" ); // For default sizes set in options
-			if ( isset( $_wp_additional_image_sizes[$s]['crop'] ) )
-				$sizes[$s]['crop'] = $_wp_additional_image_sizes[$s]['crop']; // For theme-added sizes
-			else
-				$sizes[$s]['crop'] = get_option( "{$s}_crop" ); // For default sizes set in options
+			if ( isset( $_wp_additional_image_sizes[$s]['width'] ) ) {
+				// For theme-added sizes
+				$sizes[$s]['width'] = intval( $_wp_additional_image_sizes[$s]['width'] );
+			} else {
+				// For default sizes set in options
+				$sizes[$s]['width'] = get_option( "{$s}_size_w" );
+			}
+
+			if ( isset( $_wp_additional_image_sizes[$s]['height'] ) ) {
+				// For theme-added sizes
+				$sizes[$s]['height'] = intval( $_wp_additional_image_sizes[$s]['height'] );
+			} else {
+				// For default sizes set in options
+				$sizes[$s]['height'] = get_option( "{$s}_size_h" );
+			}
+
+			if ( isset( $_wp_additional_image_sizes[$s]['crop'] ) ) {
+				// For theme-added sizes
+				$sizes[$s]['crop'] = $_wp_additional_image_sizes[$s]['crop'];
+			} else {
+				// For default sizes set in options
+				$sizes[$s]['crop'] = get_option( "{$s}_crop" );
+			}
 		}
 
 		/**
Index: src/wp-admin/includes/post.php
===================================================================
--- src/wp-admin/includes/post.php	(revision 38301)
+++ src/wp-admin/includes/post.php	(working copy)
@@ -1367,14 +1367,12 @@
  *
  * @since 2.9.0
  *
- * @global array $_wp_additional_image_sizes
- *
  * @param int $thumbnail_id ID of the attachment used for thumbnail
  * @param mixed $post The post ID or object associated with the thumbnail, defaults to global $post.
  * @return string html
  */
 function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
-	global $_wp_additional_image_sizes;
+	$_wp_additional_image_sizes = wp_get_additional_image_sizes();
 
 	$post               = get_post( $post );
 	$post_type_object   = get_post_type_object( $post->post_type );
Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 38301)
+++ src/wp-includes/media.php	(working copy)
@@ -7,6 +7,21 @@
  */
 
 /**
+ * Retrieve additional image sizes.
+ *
+ * @since 4.7.0
+ *
+ * @global array $_wp_additional_image_sizes
+ */
+function wp_get_additional_image_sizes() {
+	global $_wp_additional_image_sizes;
+	if ( ! $_wp_additional_image_sizes ) {
+		$_wp_additional_image_sizes = array();
+	}
+	return $_wp_additional_image_sizes;
+}
+
+/**
  * Scale down the default size of an image.
  *
  * This is so that the image is a better fit for the editor and theme.
@@ -27,7 +42,6 @@
  * @since 2.5.0
  *
  * @global int   $content_width
- * @global array $_wp_additional_image_sizes
  *
  * @param int          $width   Width of the image in pixels.
  * @param int          $height  Height of the image in pixels.
@@ -39,8 +53,10 @@
  * @return array Width and height of what the result image should resize to.
  */
 function image_constrain_size_for_editor( $width, $height, $size = 'medium', $context = null ) {
-	global $content_width, $_wp_additional_image_sizes;
+	global $content_width;
 
+	$_wp_additional_image_sizes = wp_get_additional_image_sizes();
+
 	if ( ! $context )
 		$context = is_admin() ? 'edit' : 'display';
 
@@ -82,11 +98,13 @@
 		if ( intval($content_width) > 0 ) {
 			$max_width = min( intval($content_width), $max_width );
 		}
-	} elseif ( isset( $_wp_additional_image_sizes ) && count( $_wp_additional_image_sizes ) && in_array( $size, array_keys( $_wp_additional_image_sizes ) ) ) {
+	} elseif ( ! empty( $_wp_additional_image_sizes ) && in_array( $size, array_keys( $_wp_additional_image_sizes ) ) ) {
 		$max_width = intval( $_wp_additional_image_sizes[$size]['width'] );
 		$max_height = intval( $_wp_additional_image_sizes[$size]['height'] );
-		if ( intval($content_width) > 0 && 'edit' == $context ) // Only in admin. Assume that theme authors know what they're doing.
-			$max_width = min( intval($content_width), $max_width );
+		// Only in admin. Assume that theme authors know what they're doing.
+		if ( intval( $content_width ) > 0 && 'edit' === $context ) {
+			$max_width = min( intval( $content_width ), $max_width );
+		}
 	}
 	// $size == 'full' has no constraint
 	else {
@@ -258,15 +276,12 @@
  *
  * @since 3.9.0
  *
- * @global array $_wp_additional_image_sizes
- *
  * @param string $name The image size to check.
  * @return bool True if the image size exists, false if not.
  */
 function has_image_size( $name ) {
-	global $_wp_additional_image_sizes;
-
-	return isset( $_wp_additional_image_sizes[ $name ] );
+	$sizes = wp_get_additional_image_sizes();
+	return isset( $sizes[ $name ] );
 }
 
 /**
@@ -748,15 +763,14 @@
  *
  * @since 3.0.0
  *
- * @global array $_wp_additional_image_sizes
- *
  * @return array Returns a filtered array of image size strings.
  */
 function get_intermediate_image_sizes() {
-	global $_wp_additional_image_sizes;
+	$_wp_additional_image_sizes = wp_get_additional_image_sizes();
 	$image_sizes = array('thumbnail', 'medium', 'medium_large', 'large'); // Standard sizes
-	if ( isset( $_wp_additional_image_sizes ) && count( $_wp_additional_image_sizes ) )
+	if ( ! empty( $_wp_additional_image_sizes ) ) {
 		$image_sizes = array_merge( $image_sizes, array_keys( $_wp_additional_image_sizes ) );
+	}
 
 	/**
 	 * Filters the list of intermediate image sizes.