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Contification converges more quickly

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* module/language/cps/contification.scm (compute-contification):
  Converge more quickly by using the information we compute within a
  compute-contification pass.
This commit is contained in:
Andy Wingo 2013-11-25 20:07:27 +01:00
parent 7338a49fa1
commit 310da5e1ef
1 changed files with 47 additions and 10 deletions
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57
module/language/cps/contification.scm
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@ -41,6 +41,7 @@
(define (compute-contification fun)
(let* ((dfg (compute-dfg fun))
(cont-table (dfg-cont-table dfg))
(scope-table (make-hash-table))
(call-substs '())
(cont-substs '())
(fun-elisions '())
@ -52,9 +53,17 @@
(define (elide-function! k cont)
(set! fun-elisions (acons k cont fun-elisions)))
(define (splice-conts! scope conts)
(for-each (match-lambda
(($ $cont k) (hashq-set! scope-table k scope)))
conts)
(hashq-set! cont-splices scope
(append conts (hashq-ref cont-splices scope '()))))
(define (lookup-return-cont k)
(match (assq-ref cont-substs k)
(#f k)
(k (lookup-return-cont k))))
;; If K is a continuation that binds one variable, and it has only
;; one predecessor, return that variable.
(define (bound-symbol k)
@ -101,7 +110,9 @@
(match (find-call (lookup-cont use cont-table))
(($ $continue k src ($ $call proc* args))
(and (eq? proc proc*) (not (memq proc args)) (applicable? proc args)
k))
;; Converge more quickly by resolving already-contified
;; call targets.
(lookup-return-cont k)))
(_ #f)))
;; If this set of functions is always called with one
@ -132,23 +143,49 @@
;; defined, whose free variables are a superset of the free
;; variables of the functions.
;;
;; There is some slight trickiness here. Call-target already uses
;; the information we compute within this pass. Previous
;; contifications may cause functions to be contified not at their
;; point of definition but at their point of non-recursive use.
;; That will cause the scope nesting to change. (It may
;; effectively push a function deeper down the tree -- the second
;; case above, a call within the letrec body.) What if we contify
;; to the tail of a previously contified function? We have to
;; track what the new scope tree will be when asking whether K
;; will be bound in TERM-K's scope, not the scope tree that
;; existed when we started the pass.
;;
;; FIXME: Does this choose the right scope for contified let-bound
;; functions?
(define (find-contification-scope k)
(if (continuation-bound-in? k term-k dfg)
term-k
(let ((scope (lookup-block-scope k dfg)))
(match (lookup-cont scope cont-table)
;; The common continuation was the tail of some function
;; inside the letrec body. If that function has just
;; one clause, contify into that clause. Otherwise
;; bail.
(define (scope-contains? scope k)
(let ((k-scope (or (hashq-ref scope-table k)
(let ((k-scope (lookup-block-scope k dfg)))
(hashq-set! scope-table k k-scope)
k-scope))))
(or (eq? scope k-scope)
(and k-scope (scope-contains? scope k-scope)))))
;; Find the scope of K.
(define (continuation-scope k)
(or (hashq-ref scope-table k)
(let ((scope (lookup-block-scope k dfg)))
(hashq-set! scope-table k scope)
scope)))
(let ((k-scope (continuation-scope k)))
(if (scope-contains? k-scope term-k)
term-k
(match (lookup-cont k-scope cont-table)
(($ $kentry self tail clauses)
;; K is the tail of some function. If that function
;; has just one clause, return that clause. Otherwise
;; bail.
(match clauses
((($ $cont _ ($ $kclause arity ($ $cont kargs))))
kargs)
(_ #f)))
(_ scope)))))
(_ k-scope)))))
;; We are going to contify. Mark all SYMs for replacement in
;; calls, and mark the tail continuations for replacement by K.