Index: gcc.fsf.doc/gcc/doc/extend.texi
===================================================================
--- gcc.fsf.doc/gcc/doc/extend.texi	(revision 119331)
+++ gcc.fsf.doc/gcc/doc/extend.texi	(working copy)
@@ -1616,9 +1616,12 @@
 
 @item always_inline
 @cindex @code{always_inline} function attribute
-Generally, functions are not inlined unless optimization is specified.
-For functions declared inline, this attribute inlines the function even
-if no optimization level was specified.
+Always inline this function; never emit it standalone.
+@code{always_inline} requires @option{-funit-at-a-time}; in turn,
+this is enabled by @option{-Os}, @option{-O2}, and @option{-O3}.
+If @option{-funit-at-a-time} is off (e.g. @option{-O0} and @option{-O1}),
+calls to @code{always_inline} functions won't reliably be inlined,
+and failure is a hard error.
 
 @item gnu_inline
 @cindex @code{gnu_inline} function attribute