25 Nov 2012 crhodes   » (Master)

In the last episode, we discovered some things about the implementation of discriminating functions in SBCL's CLOS, and also I discovered that I had actually documented some of it many years ago in the SBCL Internals manual: as well as the chapter on discriminating functions, there's some interesting stuff about how to make slot-value tolerably efficient. And so I sent Faré the functions for precompiling generic functions, and congratulated myself on a job well done.

And inevitably I got a reply, by return: “it looks like this doesn't work with eql-specializers.” Also “our generic functions have 417 eql-specializers between them.” Ah.

Faré is quite right, the code in the previous diary entry will signal an error on finding a method with eql specializers – and even if it were fixed to not have that problem, the underlying (cacheing) mechanism for efficient method calls, based on looking at the identity of the class-of each argument, fails to work when there are applicable methods with eql specializers (because the set of applicable methods for arguments of a class will vary on whether the argument matches an eql-specializer or not).

So, it is perhaps not surprising that there is an alternative optimized discriminating function mechanism which comes into play if a generic function has a substantial number of methods with eql-specializers: instead of a cacheing discriminating function, we generate and use a dispatching discriminating function, which generates a network of type tests to distinguish between all the possible cases of applicable methods, based on the actual types of the arguments – and importantly in this context, those types can include eql types.

The same sort of support exists in SBCL for constructing a dispatching discriminating function in advance as I presented for constructing a cacheing discriminating function. Something like the following:

(defun precompile-dispatching-gf (gf)
  (let* ((lock (sb-pcl::gf-lock gf)))
    (setf (sb-pcl::gf-precompute-dfun-and-emf-p (sb-pcl::gf-arg-info gf)) t)
    (multiple-value-bind (dfun cache info)
        (sb-pcl::make-final-dispatch-dfun gf)
      (sb-thread::call-with-recursive-system-lock ; or -SPINLOCK
       (lambda () 
         (sb-pcl::set-dfun gf dfun cache info)
         (sb-mop:set-funcallable-instance-function gf dfun))
       lock))))

(loop for x being the external-symbols of :cl
      initially (progn (fmakunbound 'bar) (fmakunbound 'baz)
                       (defgeneric bar (y z)) (defgeneric baz (y z)))
      do (eval `(defmethod bar ((y (eql ',x)) z) (list y z)))
         (eval `(defmethod baz ((y (eql ',x)) z) (list y z))))

(precompile-dispatching-gf #'bar)

(time (bar 'defmethod 3)) ; 6,250 processor cycles
(time (baz 'defmethod 3)) ; 1,098,642,785 processor cycles

I suppose that would make a noticeable difference to application startup times. Next up, unless I've made further oversights which need correction: automating this process, and some incidental thoughts.

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