5.4 Use of Declarations to Improve Efficiency

Declarations, especially type and function declarations, increase the efficiency of the compiled code. For example, for the following Lisp source file, with two Common-Lisp declarations added,

     (eval-when (compile)
       (proclaim '(function tak (fixnum fixnum fixnum) fixnum))
     
     (defun tak (x y z)
       (declare (fixnum x y z))
       (if (not (< y x))
           z
           (tak (tak (1- x) y z)
                (tak (1- y) z x)
                (tak (1- z) x y))))

The compiler generates the following C code:

     /*      local entry for function TAK                                  */
     static int LI1(register int V1,register int V2,register int V3)
     { VT3 VLEX3 CLSR3
     TTL:
             if (V2 < V1) {
             goto L2;}
             return(V3);
     L2:
             { int V5;
               V5 = LI1((V1)-1,V2,V3);
             { int V6;
               V6 = LI1((V2)-1,V3,V1);
               V3 = LI1((V3)-1,V1,V2);
               V2 = V6;
               V1 = V5;}}
             goto TTL;
     ;;; Note: Tail-recursive call of TAK was replaced by iteration.
     }