어휘 주소는 이름 검색을 두 인덱스로 바꾼다.
컴파일러가 변수를 프레임 깊이와 바인딩 오프셋으로 바꾸면 기계는 런타임 환경에서 값을 직접 가져올 수 있습니다.
컴파일할 때 환경에 관해 어떤 지식을 런타임 조회 밖으로 옮길 수 있을까요?
- Represent a lexical address as frame depth and binding offset
- Select a runtime frame without inspecting variable names
- Select one value slot inside that frame
- Connect compile-time name search to runtime direct lookup
The address (depth offset) counts outward through environment frames, then across one frame. lexical-ref follows only those two numbers. The runtime frames contain values rather than name-value pairs, so no assoc or symbol comparison occurs during the fetch.
find-address performs the name search against the compiler environment, whose frames contain variable names. Once it produces (1 0) for x, the runtime can use that address to fetch 42 from matching value frames. The example makes the compiler and machine agree on one frame layout.
(begin
(define (frame-at environment depth)
(if (= depth 0)
(car environment)
(frame-at (cdr environment) (- depth 1))))
(define (lexical-ref address environment)
(list-ref (frame-at environment (car address))
(cadr address)))
(define environment '((10 20) (30 40 50)))
(list (lexical-ref '(0 1) environment)
(lexical-ref '(1 0) environment)
(lexical-ref '(1 2) environment)))- 출력
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- 값
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- 진단
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The first program returns (20 30 50). The second resolves x to address (1 0), then returns ((1 0) 42).
In the first run, follow depth recursion separately from list-ref offset selection and confirm that no variable symbol is searched. In the second run, separate the compile-environment name search from the later value-frame lookup.
힌트를 보기 전에 프로그램을 바꿔 보세요.
Add w after z in the innermost compile-time frame and add 11 to the matching runtime frame. Predict the address and value of w.
힌트 하나 보기
The innermost frame has depth 0. Its second slot has offset 1.