Devirtualize monomorphic protocol calls (#227)

When the inference proves a protocol call's receiver is one record type, the
back end resolves the impl by that static tag (find-protocol-method) instead of
routing through the protocol var -> jolt-invoke -> protocol-resolve, which
re-derives the tag and walks the type table. Same table lookup, minus the
var-deref, the rest-cons, and the receiver-type computation.

Fires only on a monomorphic site: a megamorphic receiver joins to :any and
carries no :devirt-type, so it keeps ordinary dispatch (the dispatch bench is
unaffected). The annotation comes from the whole-program fixpoint typing a
reduce/HOF element or a ctor return as a specific record.

Modest on the dispatch benchmarks (~6% on mono-dispatch) — float boxing in the
reduce accumulator dominates there, a separate numeric lever — but it removes
the dispatch overhead wherever a typed receiver is known.

run-devirt.ss gate: emitted form uses find-protocol-method, and evaluating it
matches ordinary dispatch for an inline impl, an extend-type impl, and the
non-devirt path. make test / shakesmoke green, 0 new divergences.

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

host/chez/run-devirt.ss

@@ -0,0 +1,71 @@
+;; run-devirt.ss — protocol-call devirtualization gate (backend_scheme emit).
+;;
+;; The inference annotates a monomorphic protocol call with :devirt-type/-proto/
+;; -method (jolt.passes.types); the back end then resolves the impl by that static
+;; tag. This gate pins both halves: the emitted form uses find-protocol-method, and
+;; evaluating it returns the same value the ordinary dispatch would — for a record's
+;; inline impl, an extend-type impl, and across distinct receiver types.
+;;
+;;   chez --script host/chez/run-devirt.ss
+(import (chezscheme))
+(load "host/chez/rt.ss")
+(set-chez-ns! "clojure.core")
+(load "host/chez/seed/prelude.ss")
+(load "host/chez/post-prelude.ss")
+(set-chez-ns! "user")
+(load "host/chez/host-contract.ss")
+(load "host/chez/seed/image.ss")
+(load "host/chez/compile-eval.ss")
+
+(define analyze (var-deref "jolt.analyzer" "analyze"))
+(define emit    (var-deref "jolt.backend-scheme" "emit"))
+(define kw      (lambda (n) (keyword #f n)))
+
+(define (evals src) (jolt-compile-eval (string-append "(do " src ")") "user"))
+;; define two record types implementing one protocol — Circle via an inline impl,
+;; Square via extend-type — plus instances to dispatch on.
+(evals "(defprotocol Shape (area [s]))")
+(evals "(defrecord Circle [r] Shape (area [s] (:r s)))")
+(evals "(defrecord Square [w])")
+(evals "(extend-type Square Shape (area [s] (* (:w s) (:w s))))")
+(evals "(def c (->Circle 7))")
+(evals "(def sq (->Square 5))")
+
+;; analyze (area RECV), annotate it as a devirt call on `type`, and emit. RECV is a
+;; var name (c/sq) the emitted code resolves at eval time.
+(define (devirt-emit type recv)
+  (let* ((ir (analyze (make-analyze-ctx "user") (jolt-ce-read (string-append "(area " recv ")"))))
+         (dv (jolt-assoc ir (kw "devirt-type") type (kw "devirt-proto") "Shape"
+                         (kw "devirt-method") "area")))
+    (emit dv)))
+
+(define fails 0) (define total 0)
+(define (check label actual expected)
+  (set! total (+ total 1))
+  (unless (equal? actual expected)
+    (set! fails (+ fails 1))
+    (printf "  FAIL ~a: got ~s expected ~s\n" label actual expected)))
+(define (has-sub? s sub)
+  (let ((n (string-length s)) (m (string-length sub)))
+    (let loop ((i 0)) (cond ((> (+ i m) n) #f)
+                            ((string=? (substring s i (+ i m)) sub) #t)
+                            (else (loop (+ i 1)))))))
+;; eval an emitted Scheme string in the loaded runtime (var-deref resolves c/sq).
+(define (run-emit scm) (eval (read (open-input-string scm)) (interaction-environment)))
+
+(let ((e (devirt-emit "user.Circle" "c")))
+  (check "emit uses find-protocol-method" (has-sub? e "find-protocol-method") #t)
+  (check "devirt inline impl == dispatch" (run-emit e) (evals "(area c)")))   ; 7
+
+(let ((e (devirt-emit "user.Square" "sq")))
+  (check "devirt extend-type impl == dispatch" (run-emit e) (evals "(area sq)")))  ; 25
+
+;; a normal (no devirt) call still goes through dispatch and agrees — the path the
+;; megamorphic / unknown-receiver site keeps.
+(let ((e (emit (analyze (make-analyze-ctx "user") (jolt-ce-read "(area c)")))))
+  (check "non-devirt path no find-protocol-method" (has-sub? e "find-protocol-method") #f)
+  (check "non-devirt still dispatches" (run-emit e) 7))
+
+(if (= fails 0)
+    (begin (printf "devirt gate: ~a/~a passed\n" total total) (exit 0))
+    (begin (printf "devirt gate: ~a/~a passed (~a failed)\n" (- total fails) total fails) (exit 1)))