md.texi: Note problems using function calls to determine insn lengths and point readers...

        * doc/md.texi: Note problems using function calls to determine
        insn lengths and point readers to a potential workaround.

From-SVN: r218439

gcc/ChangeLog

+2014-12-05  Jeff Law  <law@redhat.com>
+
+	* doc/md.texi: Note problems using function calls to determine
+	insn lengths and point readers to a potential workaround.
+
 2014-12-05  Andreas Schwab  <schwab@linux-m68k.org>
 
 	* combine.c (is_parallel_of_n_reg_sets)

gcc/doc/md.texi

@@ -8345,6 +8345,12 @@ the number of vectors multiplied by the size of each vector.
 
 Lengths are measured in addressable storage units (bytes).
 
+Note that it is possible to call functions via the @code{symbol_ref}
+mechanism to compute the length of an insn.  However, if you use this
+mechanism you must provide dummy clauses to express the maximum length
+without using the function call.  You can an example of this in the
+@code{pa} machine description for the @code{call_symref} pattern.
+
 The following macros can be used to refine the length computation:
 
 @table @code