Sealable Metaobjects

1Introduction

We present an extension of the Common Lisp Object System (CLOS) that allowsa compiler to inline a generic function under certain conditions.

We should note that moving parts of the callee into the caller is usually a very bad idea. It prevents safe and efficient function redefinition and inflates the amount of generated machine code at the call site. Most severely, when moving parts of a generic function to the caller, we lose the ability to redefine or extend some of the involved objects and metaobjects.

Nevertheless, there are two cases where the aforementioned drawbacks are tolerable. The one case is when passing built-in Common Lisp objects to specified functions. The other case is for user code that has such extreme performance demands that the alternative of using this technique would be to refrain from using generic functions altogether.

2The Technique

The goal is to inline a generic function under certain circumstances.These circumstances are:

1. It is possible to statically determine the generic function beingcalled.
2. This generic function is sealed, i.e., it is an instance ofSEALABLE-GENERIC-FUNCTION that has previously been passed to thefunction SEAL-GENERIC-FUNCTION.
3. This sealed generic function has at least one sealed method, i.e., amethod of type POTENTIALLY-SEALABLE-METHOD that specializes, on eachrelevant argument, on a built-in or sealed class, or an eql specializerwhose object is an instance of a built-in or sealed class.
4. It must be possible to determine, statically, that the types of allarguments in a specializing position uniquely determine the list ofapplicable methods.

3Examples

The following examples illustrate how sealable metaobjects can be used.Each example code can be evaluated as-is. However, for actual use, werecommend the following practices:

• Sealable generic functions should be defined in a separate file that isloaded early. If this is not done, its methods may not use the correctmethod-class. (An alternative is to specify the method class of eachmethod explicitly).
• Metaobject sealing should be the very last step when loading a project.Ideally, all calls to SEAL-GENERIC-FUNCTION should be in a separate filethat ASDF loads last. This way, sealing can also be disabledconveniently, e.g., to measure whether sealing actually improvesperformance (Which you should do!).

3.0.1Generic Plus

This example shows how one can implement a generic version of cl:+.

(defgeneric generic-binary-+ (a b)
  (:generic-function-class sealable-metaobjects:fast-generic-function))

(defmethod generic-binary-+ ((a number) (b number))
  (+ a b))

(defmethod generic-binary-+ ((a character) (b character))
  (+ (char-code a)
     (char-code b)))

(sealable-metaobjects:seal-domain #'generic-binary-+ '(number number))
(sealable-metaobjects:seal-domain #'generic-binary-+ '(character character))

(defun generic-+ (&rest things)
  (cond ((null things) 0)
        ((null (rest things)) (first things))
        (t (reduce #'generic-binary-+ things))))

(define-compiler-macro generic-+ (&rest things)
  (cond ((null things) 0)
        ((null (rest things)) (first things))
        (t
         (flet ((symbolic-generic-binary-+ (a b)
                  `(generic-binary-+ ,a ,b)))
           (reduce #'symbolic-generic-binary-+ things)))))

You can quickly verify that this new operator is as efficient as cl:+:

(defun triple-1 (x)
  (declare (single-float x))
  (+ x x x))

(defun triple-2 (x)
  (declare (single-float x))
  (generic-+ x x x))

;;; Both functions should compile to the same assembler code.
(disassemble #'triple-1)
(disassemble #'triple-2)

Yet, other than cl:+, generic-+ can be extended by the user, just like a regular generic function. The only restriction is that new methods must not interfere with the behavior of methods that specialize on sealed types only.

3.0.2Generic Find

This example illustrates how one can implement a fast, generic version of cl:find.

(defgeneric generic-find (item sequence &key test)
  (:generic-function-class sealable-metaobjects:fast-generic-function))

(defmethod generic-find (elt (list list) &key (test #'eql))
  (and (member elt list :test test)
       t))

(defmethod generic-find (elt (vector vector) &key (test #'eql))
  (cl:find elt vector :test test))

(sealable-metaobjects:seal-domain #'generic-find '(t list))
(sealable-metaobjects:seal-domain #'generic-find '(t vector))

(defun small-prime-p (x)
  (generic-find x '(2 3 5 7 11)))

;; The call to GENERIC-FIND should have been replaced by a direct call to
;; the appropriate effective method.
(disassemble #'small-prime-p)

4Related Work

• https://github.com/guicho271828/inlined-generic-function
• https://opendylan.org/books/drm/Define_Sealed_Domain
• https://github.com/markcox80/specialization-store
• http://home.pipeline.com/~hbaker1/CLOStrophobia.html
• Generic Function Sealing by Paul Khuong (unpublished)