Devirt: cache the resolved impl in a per-site cell (inline cache) (#237)

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A devirtualized protocol call resolved its impl with devirt-resolve on EVERY call
— but the tag/proto/method are compile-time constants, so the resolved fn is a
runtime constant (closed world). That per-call find-protocol-method (three
hashtable lookups) was the cost: on mono-dispatch, dispatch was ~75% of the time
(ablation: same arithmetic direct-call 166ms vs dispatch 673ms).

Resolve once. When emitting a direct-link def, each devirt site gets a fresh cache
cell, bound to #f in a let wrapping the def (so it persists across calls and is
shared by every invocation); the site resolves into it on first use ((or cell
(let ((_f (devirt-resolve ..))) (set! cell _f) _f))) and reuses it after — the
inline cache the JVM gets for free. First call still passes the real receiver, so
the Object/host-tag fallback (devirt-resolve) is unchanged.

mono-dispatch 673ms -> 214ms (~3.15x), 47.5x -> ~15x JVM, near the 166ms
direct-call floor. run-devirt.ss gains the cached-path checks (cell present, 1st
call caches + 2nd reuses, both == dispatch). make test / shakesmoke green, selfhost
holds, 0 new divergences.

Co-authored-by: Yogthos <yogthos@gmail.com>

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Dmitri Sotnikov 2026-06-26 18:34:13 +00:00 • committed by GitHub

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