5.7.1 Block Compilation Semantics

The effect of block compilation can be envisioned as the compiler turning all the defuns in the block compilation into a single labels form:

(declaim (start-block fun1 fun3))

(defun fun1 ()
  ...)

(defun fun2 ()
  ...
  (fun1)
  ...)

(defun fun3 (x)
  (if x
      (fun1)
      (fun2)))

(declaim (end-block))

becomes:

(labels ((fun1 ()
           ...)
         (fun2 ()
           ...
           (fun1)
           ...)
         (fun3 (x)
           (if x
               (fun1)
               (fun2))))
  (setf (fdefinition 'fun1) #'fun1)
  (setf (fdefinition 'fun3) #'fun3))

Calls between the block compiled functions are local calls, so changing the global definition of fun1 will have no effect on what fun2 does; fun2 will keep calling the old fun1.

The entry points fun1 and fun3 are still installed in the symbol-function as the global definitions of the functions, so a full call to an entry point works just as before. However, fun2 is not an entry point, so it is not globally defined. In addition, fun2 is only called in one place, so it will be let converted.