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