Describe abi_tag mangling.

abi.html

@@ -4129,10 +4129,10 @@ <h5><a name="mangle.name">Names</a></h5>
                       ::= &lt;<a href="#mangle.template-param">template-param</a>&gt;                      # template template parameter
                       ::= &lt;<a href="#mangle.substitution">substitution</a>&gt;
 
-    &lt;<a name="mangle.unqualified-name">unqualified-name</a>&gt; ::= &lt;<a href="#mangle.operator-name">operator-name</a>&gt;
-                       ::= &lt;<a href="#mangle.ctor-dtor-name">ctor-dtor-name</a>&gt;  
-                       ::= &lt;<a href="#mangle.source-name">source-name</a>&gt;   
-                       ::= &lt;<a href="#mangle.unnamed-type-name">unnamed-type-name</a>&gt;   
+    &lt;<a name="mangle.unqualified-name">unqualified-name</a>&gt; ::= &lt;<a href="#mangle.operator-name">operator-name</a>&gt; [&lt;<a href="#mangle.abi-tags">abi-tags</a>&gt;]
+                       ::= &lt;<a href="#mangle.ctor-dtor-name">ctor-dtor-name</a>&gt; [&lt;<a href="#mangle.abi-tags">abi-tags</a>&gt;]
+                       ::= &lt;<a href="#mangle.source-name">source-name</a>&gt; [&lt;<a href="#mangle.abi-tags">abi-tags</a>&gt;]
+                       ::= &lt;<a href="#mangle.unnamed-type-name">unnamed-type-name</a>&gt; [&lt;<a href="#mangle.abi-tags">abi-tags</a>&gt;]
 
     &lt;<a name="mangle.source-name">source-name</a>&gt; ::= &lt;<i>positive length</i> <a href="#mangle.number">number</a>&gt; &lt;<a href="#mangle.identifier">identifier</a>&gt;
     &lt;<a name="mangle.identifier">identifier</a>&gt; ::= &lt;<i>unqualified source code identifier</i>>
@@ -4147,6 +4147,64 @@ <h5><a name="mangle.name">Names</a></h5>
 may be either a function or data object name when derived from &lt;<a href="#mangle.name">name</a>&gt;,
 or a class or enum name when derived from &lt;<a href="#mangle.type">type</a>&gt;.
 
+<h5>ABI tags</h5>
+
+<p>The GNU <code>abi_tag</code> attribute can be applied to a variable,
+function, inline namespace, class, or enumeration.  The
+&lt;<a href="#mangle.unqualified-name">unqualified-name</a>&gt; for a tagged
+variable, function, or type includes a representation of the tags on that
+entity, in alphabetical order:
+<pre><code><font color=blue>
+    &lt;<a name="mangle.abi-tags">abi-tags</a>&gt; ::= &lt;<a href="#mangle.abi-tag">abi-tag</a>&gt; [&lt;<a href="#mangle.abi-tags">abi-tags</a>&gt;]
+    &lt;<a name="mangle.abi-tag">abi-tag</a>&gt; ::= B &lt;<a href="#mangle.source-name">source-name</a>&gt;
+</font></code></pre>
+
+For example:
+<pre>
+  struct [[gnu::abi_tag ("foo","bar")]] A { }; // mangles as 1AB3bar3foo
+</pre>
+
+If a name that would use a built-in
+&lt;<a href="#mangle.substitution">substitution</a>&gt; has ABI tags, the tags
+are appended to the substitution; the result is a substitutable component.
+
+<pre>
+  namespace std
+  {
+    template &lt;class T> struct char_traits { /* ... */ };
+    template &lt;class T> struct allocator { /* ... */ };
+    template &lt;class T, class R = char_traits&lt;T>, class A = allocator&lt;T>>
+      struct [[gnu::abi_tag ("X")]] basic_string { /* ... */ };
+    using string = basic_string&lt;char>;
+  }
+
+  void f(std::string, std::string) { } // mangles as _Z1fSsB1XS_
+</pre>
+
+If part of a declaration's type is not represented in the mangling, i.e. the
+type of a variable or a return type that is not represented in the mangling of
+a function, any ABI tags on that type (or components of a compound type) that
+are not otherwise represented in the mangling of the declaration are applied to
+the name of the declaration.  Note that there is no similar tag propagation
+from members or bases to a class type, as that would be impossible for
+incomplete types.
+
+<p>If no arguments are specified for the attribute on an inline namespace, the
+namespace has its own name as a tag.  Tags on an inline namespace are not
+represented in the mangled name of the namespace, but they are subject to the
+above tag propagation.  For example:
+
+<pre>
+  inline namespace [[gnu::abi_tag]] Foo {
+    struct A {};
+    A f() { } // mangles as _ZN3Foo1fEv
+  }
+  template &lt;class T> struct B { };
+  typedef void (*fp)(B&lt;A>);
+  fp p;      // mangles as _Z1pB3Foo
+  A g(A) { } // mangles as _Z1gN3Foo1AE
+</pre>
+
 <h5><a name="mangle.number"> Numbers </a></h5>
 
 <pre><code><font color=blue>    &lt;<a name="mangle.number">number</a>&gt; ::= [n] &lt;<i>non-negative decimal integer</i>>
@@ -5756,6 +5814,8 @@ <h3><a href="#unwind"> 5.3 Unwind Table Location </a></h3>
 
 <h2><a name="revisions">Appendix R: Revision History</a></h2>
 <p> <hr>
+<p class="revision"><span class="date">[160602]</span>
+  Describe <code>abi_tag</code> mangling.</p>
 
 <p class="revision"><span class="date">[151021]</span>
   Support transaction-safe functions.</p>