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perf: generation-guarded inline cache for host-value protocol dispatch

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Host-value protocol dispatch (a protocol extended to Number/String/… called on a
host value) recomputed the candidate type-tag list and walked the registry — up
to ~15 map lookups plus an array alloc — on every call. It now caches
(most-specific-host-tag, protocol, method) -> fn, guarded by a registry
generation that bumps in register-protocol-method; re-extending a protocol
invalidates the cache. The single-lookup fast paths (deftype protocol dispatch,
direct multimethod dispatch) are untouched — they're already cheap, so no cache
overhead there. The multimethod hierarchy-fallback cache (narrower: only
derive-based dispatch) is deferred.

Test in test/integration/dispatch-cache-test.janet covers correctness under
re-extension in all three modes. Conformance 218/218; battery 3916.
This commit is contained in:
Yogthos 2026-06-06 17:38:44 -04:00
parent db08cecde8
commit 279c8e176b
3 changed files with 62 additions and 8 deletions
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33
test/integration/dispatch-cache-test.janet Normal file
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# Protocol host-value dispatch cache (jolt-4ay).
#
# Host-value protocol dispatch used to recompute the candidate type-tag list and
# walk the registry on every call. It's now a generation-guarded cache keyed by
# (most-specific-host-tag, protocol, method); registering a protocol impl bumps
# the registry generation and invalidates it. This pins correctness: the cache
# must never hide a re-extension.
(use ../../src/jolt/api)
(var failures 0)
(defn- check [label got want]
(unless (= got want)
(++ failures)
(printf "FAIL [%s] got %q want %q" label got want)))
(each mode [{:compile? true} {} {:aot-core? false}]
(def ctx (init mode))
(eval-string ctx "(defprotocol P (m [x]))")
(eval-string ctx "(extend-protocol P Number (m [x] (* x 2)))")
(check (string mode " host dispatch") (eval-string ctx "(m 5)") 10)
(check (string mode " cache hit (same class)") (eval-string ctx "(m 7)") 14)
# Re-extend: registry generation bumps, cache must invalidate.
(eval-string ctx "(extend-protocol P Number (m [x] (+ x 100)))")
(check (string mode " sees re-extension") (eval-string ctx "(m 5)") 105)
# Extending a different host class bumps gen too; number impl re-resolves.
(eval-string ctx "(extend-protocol P String (m [x] (str \"s:\" x)))")
(check (string mode " other class") (eval-string ctx "(m \"hi\")") "s:hi")
(check (string mode " number after other-class extend") (eval-string ctx "(m 3)") 103))
(if (pos? failures)
(do (printf "dispatch-cache: %d failure(s)" failures) (os/exit 1))
(print "dispatch-cache: all cases passed (compile, interpret, aot-core off)"))