Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -1170,7 +1170,7 @@
 def GNUInline : InheritableAttr {
   let Spellings = [GCC<"gnu_inline">];
   let Subjects = SubjectList<[Function]>;
-  let Documentation = [Undocumented];
+  let Documentation = [GnuInlineDocs];
 }
 
 def Hot : InheritableAttr {
Index: include/clang/Basic/AttrDocs.td
===================================================================
--- include/clang/Basic/AttrDocs.td
+++ include/clang/Basic/AttrDocs.td
@@ -3505,3 +3505,22 @@
 invoking clang with -fno-c++-static-destructors.
   }];
 }
+
+def GnuInlineDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``gnu_inline`` attribute specifies that a function marked ``extern inline``
+should follow the GNU inline extension that if the function cannot be inlined
+(possibly due to being referred to by function pointer), then no out of line
+definition will be emitted (instead of c99's behaviour of always emitting an
+out of line definition).
+
+If ``__GNUC_STDC_INLINE__`` is defined, then the current translation unit is
+not being compiled with ``gnu_inline`` semantics, and the ``gnu_inline``
+function attribute can be used to get GNU inline semantics on a per function basis.
+If ``__GNUC_STDC_INLINE__`` is not defined, then the translation unit is
+already being compiled with GNU inline semantics as the implied default.
+
+This is the default behavior with ``-std=gnu89`` or ``-fgnu89-inline``.
+  }];
+}