diff --git a/glib/gstring.c b/glib/gstring.c
index 05c66d2ae..155b426c7 100644
--- a/glib/gstring.c
+++ b/glib/gstring.c
@@ -445,7 +445,7 @@ g_string_insert_len (GString     *string,
    * since ">=" and "<=" are only valid when val really is a substring.
    * In practice, it will work on modern archs.
    */
-  if (val >= string->str && val <= string->str + string->len)
+  if (G_UNLIKELY (val >= string->str && val <= string->str + string->len))
     {
       gsize offset = val - string->str;
       gsize precount = 0;
@@ -593,9 +593,6 @@ GString *
 g_string_append (GString     *string,
                  const gchar *val)
 {
-  g_return_val_if_fail (string != NULL, NULL);
-  g_return_val_if_fail (val != NULL, string);
-
   return g_string_insert_len (string, -1, val, -1);
 }
 
@@ -620,9 +617,6 @@ g_string_append_len (GString     *string,
                      const gchar *val,
                      gssize       len)
 {
-  g_return_val_if_fail (string != NULL, NULL);
-  g_return_val_if_fail (len == 0 || val != NULL, string);
-
   return g_string_insert_len (string, -1, val, len);
 }
 
@@ -679,9 +673,6 @@ GString *
 g_string_prepend (GString     *string,
                   const gchar *val)
 {
-  g_return_val_if_fail (string != NULL, NULL);
-  g_return_val_if_fail (val != NULL, string);
-
   return g_string_insert_len (string, 0, val, -1);
 }
 
@@ -706,9 +697,6 @@ g_string_prepend_len (GString     *string,
                       const gchar *val,
                       gssize       len)
 {
-  g_return_val_if_fail (string != NULL, NULL);
-  g_return_val_if_fail (val != NULL, string);
-
   return g_string_insert_len (string, 0, val, len);
 }
 
@@ -766,12 +754,6 @@ g_string_insert (GString     *string,
                  gssize       pos,
                  const gchar *val)
 {
-  g_return_val_if_fail (string != NULL, NULL);
-  g_return_val_if_fail (val != NULL, string);
-
-  if (pos >= 0)
-    g_return_val_if_fail (pos <= string->len, string);
-
   return g_string_insert_len (string, pos, val, -1);
 }