+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="codegen">Code Generator backend info</a></div>
+<!-- *********************************************************************** -->
+
+<p>Expressions used by code generator to describe instructions and isel
+patterns:</p>
+
+<div class="doc_text">
+
+<dt><tt>(implicit a)</tt></dt>
+ <dd>an implicitly defined physical register. This tells the dag instruction
+ selection emitter the input pattern's extra definitions matches implicit
+ physical register definitions.</dd>
+<dt><tt>(parallel (a), (b))</tt></dt>
+ <dd>a list of dags specifying parallel operations which map to the same
+ instruction.</dd>
+
+</div>
+