#| -*-Scheme-*-
-$Id: assconv.scm,v 1.14 1996/02/08 17:10:59 adams Exp $
+$Id: assconv.scm,v 1.15 1996/02/09 04:38:25 adams Exp $
Copyright (c) 1994-1995 Massachusetts Institute of Technology
\f
;; Assignment conversion.
;;
-;; Each variable reference occurs in some dynamic extent. Each
-;; environment belongs to an extent. Some environments, like nested
-;; LET frames, belong to the same extent. Extents are represented by
-;; unique values (small integers). Later we partition bindings
-;; according to equivalence classes of the sets of extents in which
-;; references or assignments to them occur. The idea behind this is
-;; that mutable variables that travel around together will be placed
-;; in the same multi-cell but variables that are live in different
-;; extents are allocated to different cells, allowing better GC
-;; behaviour.
-
-;; Legal values: BY-EXTENT (as decribed above), INDIVIDUAL-CELLS (each
-;; mutable binding is allocated a separate cell), ONE-MULTICELL (all
-;; mutable bindings from a given frame are put in one multicell)
+;; Assignment conversion removes SET! from the language by introducing
+;; explicit locations (cells) and operations on those locations.
+;;
+;; It can be advantageous to collect locations together to make a larger
+;; `multicell' because only one pointer is required for the whole
+;; collection. Typically, a cell pointer will be stored on the stack
+;; or in the heap so one pointer is more efficient. The downside is
+;; that if too many (or the wrong) variables are allocated to the
+;; same multicell then it might keep live a pointer to a value that
+;; will never be used again because there are no procedures left
+;; which access that variable. This will prevent garbage collection
+;; from reclaiming the value. Using single cells reduces this false
+;; liveness problem at the cost of having more pointers. The scheme
+;; below is a middle course which has the benefits of both extremes.
+;;
+;; Each variable reference occurs in some dynamic extent. By `extent' we
+;; mean the duration or lifetime of a piece of program text. Each
+;; environment frame and the code within it has a separate extent,
+;; for example, escaping closures could have arbitrary unrelated
+;; lifetimes.
+;;
+;; Bindings are partitioned into equivalence classes according to the set
+;; of extents in which references or assignments to them occur. To
+;; make the partitioning easy we represent each extent by a unique
+;; small integer.
+;;
+;; Some frames happen to have the same or very similar extents.
+;; Discovering frames with `equivalent' extents will improve the
+;; partitioning (bigger multicells without risk of keeping data
+;; live). Currently we do only the trivial case of equating the
+;; extents of nested LET frames. It is probably not worth doing
+;; anything more elaborate.
+
+;; Legal values: BY-EXTENT (the scheme decribed above), INDIVIDUAL-CELLS
+;; (each mutable binding is allocated a separate cell), ONE-MULTICELL
+;; (all mutable bindings from a given frame are put in one multicell).
(define *assconv/partitioning* 'BY-EXTENT)
(NAMED-LAMBDA (,proc-name ENV FORM)
(ASSCONV/REMEMBER ,code form))))))))
-;;;; Variable manipulation forms
-
+\f
(define-assignment-converter LAMBDA (env lambda-list body)
(assconv/lambda* env lambda-list body (assconv/new-extent)))
name))
lambda-list))
,body*))))
-\f
+
(define-assignment-converter LET (env bindings body)
(call-with-values
(lambda ()
(cons result (assconv/binding/assignments binding)))
(assconv/binding/new-extent binding env)
result))))
-
-;;;; Trivial forms
-
+\f
(define-assignment-converter QUOTE (env object)
env ; ignored
`(QUOTE ,object))
(lambda/body rator)
(assconv/env/extent env)))
(finish (assconv/expr env rator))))
-\f
+
(define-assignment-converter BEGIN (env #!rest actions)
(let ((actions* (assconv/expr* env actions)))
(let loop ((actions actions*))
(define (assconv/remember new old)
(code-rewrite/remember new old)
new)
-
+\f
(define (assconv/new-name prefix)
(new-variable prefix))
(define (assconv/new-cell-name prefix)
(new-variable (string-append (symbol-name prefix) "-cell")))
-
(define *assconv/effect-only-forms*)
(define (assconv/form/set-effect-only! form)
(define (assconv/form/effect-only? form)
(hash-table/get *assconv/effect-only-forms* form #F))
-\f
+
;;;; Environments and extents.
(define *assconv/extent-counter*)
(define-structure (assconv/env
(conc-name assconv/env/)
(constructor assconv/env/make (bindings parent extent)))
- parent
- extent
- bindings)
+ (parent #F read-only true)
+ (extent #F read-only true)
+ (bindings #F read-only true))
(define-structure (assconv/binding
(conc-name assconv/binding/)
(constructor assconv/binding/make (name initial-extent)))
- (name false read-only true)
+ (name false read-only true)
(cell-name false read-only false)
(multicell-layout false read-only false)
- (references '() read-only false)
+ (references '() read-only false)
(assignments '() read-only false)
;; The extents associated with this binding. The INITIAL-EXTENT is the
- ;; extent in which the binding is introduced. The other extents
- ;; are for the references and assignments.
- (initial-extent #F read-only true)
- (extents '() read-only false))
-
+ ;; extent in which the binding is introduced (i.e. the extent of
+ ;; its frame). The other EXTENTS are those of the references and
+ ;; assignments, collected in a list.
+ (initial-extent #F read-only true)
+ (extents '() read-only false))
(define (assconv/binding/new-extent binding env)
(let ((extent (assconv/env/extent env)))
(car rib))
(else
(rib-loop (cdr rib))))))))
-
\f
(define (assconv/binding-body env next-extent names body)
;; (values shadowed-names body*)
\f
(define (assconv/cellify! binding)
(for-each (lambda (ref)
- (form/rewrite!
- ref
+ (form/rewrite! ref
(assconv/cell-reference binding)))
(assconv/binding/references binding))
(for-each (lambda (ass)
- (form/rewrite!
- ass
+ (form/rewrite! ass
(assconv/cell-assignment binding (set!/expr ass) ass)))
(assconv/binding/assignments binding))
(dbg-info/remember (assconv/binding/name binding)
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