(arm_volatile_func): Add comment describing purpose.

From-SVN: r9180

gcc/config/arm/arm.c

@@ -2719,6 +2719,13 @@ output_return_instruction (operand, really_return)
   return "";
 }
 
+/* Return nonzero if optimizing and the current function is volatile
+   (ie: `noreturn').
+   Since such functions don't return call-saved registers don't need to
+   be saved.  Making such an optimization may seem dubious at first glance.
+   It was put in for a particular user who was using them to do context
+   switching and obviously wanted it to be fast.  */
+
 int
 arm_volatile_func ()
 {