Seal your generic functions for an extra boost in performance.
This library introduces fast generic functions, i.e., functions that behave just like regular generic functions, except that the can be sealed on certain domains. If the compiler can then statically detect that the arguments to a fast generic function fall within such a domain, it will perform a variety of optimizations.
1Example 1 - Generic Find
This example illustrates how one can define a (hopefully) fast method for finding items in a sequence.
The first step is to define a generic function whose generic function class
(defgeneric generic-find (item sequence &key test) (:generic-function-class fast-generic-functions:fast-generic-function))
Once this definition is loaded (and only then, so you shouldn't put the next snippets in the same file as the defgeneric form), it is possible to add methods to it in the usual way.
(defmethod generic-find (item (list list) &key (test #'eql)) (and (member item list :test test) t)) (defmethod generic-find (item (vector vector) &key (test #'eql)) (cl:find item vector :test test)) (seal-domain #'generic-find '(t list)) (seal-domain #'generic-find '(t vector))
The novelty are the two calls to
seal-domain. These calls seal the
specified part of the function domain, and at the same time install
compiler optimizations for calls to that generic function.
Whenever the compiler can detect that the arguments of a call to a fast generic function fall within such a sealed domain, the entire call can be optimized in a variety of ways. By default, the call to the fast generic function's discriminating function will be replaced by a direct call to a custom effective method function. This means that there will be zero overhead for determining the generic function's behavior. The following example illustrates this:
(defun small-prime-p (x) (generic-find x '(2 3 5 7 11))) ;; The call to GENERIC-FIND should have been replaced by a direct call to ;; the appropriate effective method function. (disassemble #'small-prime-p)
It is even possible to inline the entire effective method into the call site. However, to avoid code bloat, this feature is disabled by default. To enable it, each method withing the sealed domain must contain an appropriate declaration, as shown in the next example.
2Example 2 - Extensible Number Functions
(defgeneric binary-+ (x y) (:generic-function-class fast-generic-function)) (defmethod binary-+ ((x number) (y number)) (declare (method-properties inlineable)) (+ x y)) (seal-domain #'binary-+ '(number number))
It is easy to generalize such a binary function to a function that accepts any number of arguments:
(defun generic-+ (&rest things) (cond ((null things) 0) ((null (rest things)) (first things)) (t (reduce #'binary-+ things)))) (define-compiler-macro generic-+ (&rest things) (cond ((null things) 0) ((null (rest things)) (first things)) (t (reduce (lambda (a b) `(binary-+ ,a ,b)) things))))
With all this in place, we can use our
generic-+ function much like
Common Lisp's built-in
+ without worrying about performance. The next
code snippet shows that in fact, each call to
generic-+ is inlined and
turned into a single
(disassemble (compile nil '(lambda (x y z) (declare (single-float x y z)) (generic-+ x y z)))) ;; disassembly for (lambda (x y z)) ;; Size: 38 bytes. Origin: #x52FD9354 ;; 54: 498B4510 mov RAX, [R13+16] ;; 58: 488945F8 mov [RBP-8], RAX ;; 5C: 0F28CC movaps XMM1, XMM4 ;; 5F: F30F58CB addss XMM1, XMM3 ;; 63: F30F58CA addss XMM1, XMM2 ;; 67: 660F7ECA movd EDX, XMM1 ;; 6B: 48C1E220 shl RDX, 32 ;; 6F: 80CA19 or DL, 25 ;; 72: 488BE5 mov RSP, RBP ;; 75: F8 clc ;; 76: 5D pop RBP ;; 77: C3 ret ;; 78: CC10 int3 16
Once a fast generic function has been sealed, it is not possible to add, remove, or redefine methods within the sealed domain. However, outside of the sealed domain, it behaves just like a standard generic function. That means we can extend its behavior, e.g., to allow addition of strings:
(defmethod binary-+ ((x string) (y string)) (concatenate 'string x y)) (generic-+ "foo" "bar" "baz") ;; => "foobarbaz"
3Specializing on a User-Defined Class
By default, only built-in classes and structure classes can appear as specializers of a method within a sealed domain of a fast generic function. However, it is also possible to define custom sealable classes. This example illustrates how.
Since this example has plenty of dependencies (metaobject definition and use, generic function definition and method defintion, sealing and use of a sealed function), each of the following snippets of code should be put into its own file.
In the first snippet, we define sealable standard class, that is both a sealable class and a standard class.
(defclass sealable-standard-class (sealable-metaobjects:sealable-class standard-class) ()) (defmethod validate-superclass ((class sealable-standard-class) (superclass standard-class)) t)
In the next snippet, we define a class
foo whose metaclass is our newly
sealable-standard-class. Because the implementation of fast
generic functions uses literal instances to find an optimized effective
method function at load time, each sealable class must also have a suitable
(defclass foo () ((x :reader x :initarg :x)) (:metaclass sealable-standard-class)) (defmethod make-load-form ((foo foo) &optional env) (make-load-form-saving-slots foo :slot-names '(x) :environment env))
In the next snippet, we define a fast generic function
(defgeneric op (foo) (:generic-function-class fast-generic-functions:fast-generic-function))
Once we have loaded the definition of
op, we can add individual methods
and seal some of them. In particular, we can add a method that specializes
We could also have defined this method without
foo being a sealable
class, but then the call to
seal-domain would have signaled an error.
(defmethod op ((foo foo)) (* 2 (x foo))) (sealable-metaobjects:seal-domain #'op '(foo))
Finally, we have everything in place for having optimized calls to
the case where its argument is of type
(defun bar () (let ((foo (make-instance 'foo :x 42))) (declare (foo foo)) (op foo)))
If this example is too intimidating for you, please remember that you can
always specialize fast methods on built-in classes (like integer and
simple-vector) or structure classes (everything defined via