#| -*-Scheme-*-

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    2017, 2018, 2019 Massachusetts Institute of Technology

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|#

;;;; Garbage Collector Daemons
;;; package: (runtime gc-daemons)

(declare (usual-integrations))

(define (initialize-package!)
  (set! primitive-gc-daemons (make-serial-queue))
  (set! trigger-primitive-gc-daemons! (make-trigger primitive-gc-daemons))
  (set! add-primitive-gc-daemon! (make-adder primitive-gc-daemons))
  (set! add-primitive-gc-daemon!/unsafe
	(make-adder/unsafe primitive-gc-daemons))
  (set! gc-daemons (make-serial-queue))
  (set! trigger-gc-daemons! (make-trigger gc-daemons))
  (set! add-gc-daemon! (make-adder gc-daemons))
  (set! add-gc-daemon!/unsafe (make-adder/unsafe gc-daemons))
  (set! secondary-gc-daemons (make-serial-queue))
  (set! trigger-secondary-gc-daemons! (make-trigger secondary-gc-daemons))
  (set! add-secondary-gc-daemon! (make-adder secondary-gc-daemons))
  (set! add-secondary-gc-daemon!/unsafe
	(make-adder/unsafe secondary-gc-daemons))
  (let ((fixed-objects ((ucode-primitive get-fixed-objects-vector))))
    (vector-set! fixed-objects #x0B trigger-primitive-gc-daemons!)
    ((ucode-primitive set-fixed-objects-vector!) fixed-objects)))

;;; PRIMITIVE-GC-DAEMONS are executed during the GC.  They must not
;;; allocate any storage and they must be prepared to run at times
;;; when many data structures are not consistent.
(define primitive-gc-daemons)
(define trigger-primitive-gc-daemons!)
(define add-primitive-gc-daemon!)
(define add-primitive-gc-daemon!/unsafe)

;;; GC-DAEMONS are executed after each GC from an interrupt handler.
;;; This interrupt handler has lower priority than the GC interrupt,
;;; which guarantees that these daemons will not be run inside of
;;; critical sections.  As a result, the daemons may allocate storage
;;; and use most of the runtime facilities.
(define gc-daemons)
(define trigger-gc-daemons!)
(define add-gc-daemon!)
(define add-gc-daemon!/unsafe)
(define (add-gc-daemon!/no-restore daemon)
  (add-gc-daemon!
   (lambda ()
     (if (not (*within-restore-window?*))
	 (daemon)))))

;;; SECONDARY-GC-DAEMONS are executed rarely.  Their purpose is to
;;; reclaim storage that is either unlikely to be reclaimed or
;;; expensive to reclaim.
(define secondary-gc-daemons)
(define trigger-secondary-gc-daemons!)
(define add-secondary-gc-daemon!)
(define add-secondary-gc-daemon!/unsafe)

(define (make-trigger daemons)
  (lambda ()
    (for-each (lambda (thunk) (thunk))
	      (queue->list/unsafe daemons))))

(define (make-adder daemons)
  (lambda (daemon)
    (enqueue! daemons daemon)))

(define (make-adder/unsafe daemons)
  (lambda (daemon)
    (enqueue!/unsafe daemons daemon)))

(define (gc-clean #!optional threshold)
  (let ((threshold
	 (cond ((default-object? threshold) 100)
	       ((not (negative? threshold)) threshold)
	       (else (error "threshold must be non-negative" threshold)))))
    (let loop ((previous-free (gc-flip)))
      (trigger-secondary-gc-daemons!)
      (let ((this-free (gc-flip)))
	;; Don't bother to continue if the savings starts getting small.
	(if (<= (- this-free previous-free) threshold)
	    this-free
	    (loop this-free))))))