5.8.3 The Maybe-Inline Declaration

The extensions:maybe-inline declaration is a CMUCL extension. It is similar to inline, but indicates that inline expansion may sometimes be desirable, rather than saying that inline expansion should almost always be done. When used in a global declaration, extensions:maybe-inline causes the expansion for the named functions to be recorded, but the functions aren’t actually inline expanded unless space is 0 or the function is eventually (perhaps locally) declared inline.

Use of the extensions:maybe-inline declaration followed by the defun is preferable to the standard idiom of:

(proclaim '(inline myfun))
(defun myfun () ...)
(proclaim '(notinline myfun))

;;; Any calls to myfun here are not inline expanded.

(defun somefun ()
  (declare (inline myfun))
  ;;
  ;; Calls to myfun here are inline expanded.
  ...)

The problem with using notinline in this way is that in Common Lisp it does more than just suppress inline expansion, it also forbids the compiler to use any knowledge of myfun until a later inline declaration overrides the notinline. This prevents compiler warnings about incorrect calls to the function, and also prevents block compilation.

The extensions:maybe-inline declaration is used like this:

(proclaim '(extensions:maybe-inline myfun))
(defun myfun () ...)

;;; Any calls to myfun here are not inline expanded.

(defun somefun ()
  (declare (inline myfun))
  ;;
  ;; Calls to myfun here are inline expanded.
  ...)

(defun someotherfun ()
  (declare (optimize (space 0)))
  ;;
  ;; Calls to myfun here are expanded semi-inline.
  ...)

In this example, the use of extensions:maybe-inline causes the expansion to be recorded when the defun for somefun is compiled, and doesn’t waste space through doing inline expansion by default. Unlike notinline, this declaration still allows the compiler to assume that the known definition really is the one that will be called when giving compiler warnings, and also allows the compiler to do semi-inline expansion when the policy is appropriate.

When the goal is merely to control whether inline expansion is done by default, it is preferable to use extensions:maybe-inline rather than notinline. The notinline declaration should be reserved for those special occasions when a function may be redefined at run-time, so the compiler must be told that the obvious definition of a function is not necessarily the one that will be in effect at the time of the call.