From a98cef7e6c42d40c8d77640030d3eb2697ae647b Mon Sep 17 00:00:00 2001 From: Keisuke Nishida Date: Tue, 22 Aug 2000 15:54:19 +0000 Subject: Initial revision --- doc/goops.mail | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 78 insertions(+) create mode 100644 doc/goops.mail (limited to 'doc/goops.mail') diff --git a/doc/goops.mail b/doc/goops.mail new file mode 100644 index 000000000..305e80403 --- /dev/null +++ b/doc/goops.mail @@ -0,0 +1,78 @@ +From: Mikael Djurfeldt +Subject: Re: After GOOPS integration: Computation with native types! +To: Keisuke Nishida +Cc: djurfeldt@nada.kth.se, guile@sourceware.cygnus.com +Cc: djurfeldt@nada.kth.se +Date: 17 Aug 2000 03:01:13 +0200 + +Keisuke Nishida writes: + +> Do I need to include some special feature in my VM? Hmm, but maybe +> I shouldn't do that now... + +Probably not, so I probably shouldn't answer, but... :) + +You'll need to include some extremely efficient mechanism to do +multi-method dispatch. The SCM_IM_DISPATCH form, with its +implementation at line 2250 in eval.c, is the current basis for +efficient dispatch in GOOPS. + +I think we should develop a new instruction for the VM which +corresponds to the SCM_IM_DISPATCH form. + +This form serves both the purpose to map argument types to the correct +code, and as a cache of compiled methods. + +Notice that I talk about cmethods below, not methods. In GOOPS, the +GF has a set of methods, but each method has a "code-table" mapping +argument types to code compiled for those particular concrete types. +(So, in essence, GOOPS methods abstractly do a deeper level of type +dispatch.) + +The SCM_IM_DISPATCH form has two shapes, depending on whether we use +sequential search (few cmethods) or hashed lookup (many cmethods). + +Shape 1: + + (#@dispatch args N-SPECIALIZED #((TYPE1 ... ENV FORMALS FORM1 ...) ...) GF) + +Shape 2: + + (#@dispatch args N-SPECIALIZED HASHSET MASK + #((TYPE1 ... ENV FORMALS FORM1 ...) ...) + GF) + +`args' is (I hope!) a now historic obscure optimization. + +N-SPECIALIZED is the maximum number of arguments t do type checking +on. This is used early termination of argument checking where the +already checked arguments are enough to pick out the cmethod. + +The vector is the cache proper. + +During sequential search the argument types are simply checked against +each entry. + +The method for hashed dispatch is described in: + +http://www.parc.xerox.com/csl/groups/sda/publications/papers/Kiczales-Andreas-PCL + +In this method, each class has a hash code. Dispatch means summing +the hash codes for all arguments (up til N-SPECIALIZED) and using the +sum to pick a location in the cache. The cache is sequentially +searched for an argument type match from that point. + +Kiczales introduced a clever method to maximize the probability of a +direct cache hit. We actually have 8 separate sets of hash codes for +all types. The hash set to use is selected specifically per GF and is +optimized to give fastest average hit. + + +What we could try to do as soon as the VM is complete enough is to +represent the cmethods as chunks of byte code. In the current GOOPS +code, the compilation step (which is currently empty) is situated in +`compile-cmethod' in guile-oops/compile.scm. [Apologies for the +terrible code. That particular part was written at Arlanda airport +after a sleepless night (packing luggage, not coding), on my way to +visit Marius (who, BTW, didn't take GOOPS seriously. ;-)] + -- cgit v1.2.1