#| -*-Scheme-*-

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    2017, 2018, 2019, 2020 Massachusetts Institute of Technology

This file is part of MIT/GNU Scheme.

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it under the terms of the GNU General Public License as published by
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|#

;;;; Subproblem Free Variables
;;; package: (compiler fg-optimizer subproblem-free-variables)

(declare (usual-integrations))

(define (compute-subproblem-free-variables parallels)
  (with-analysis-state
   (lambda ()
     (for-each (lambda (parallel)
		 (for-each
		  (lambda (subproblem)
		    (set-subproblem-free-variables! subproblem 'UNKNOWN))
		  (parallel-subproblems parallel)))
	       parallels)
     (for-each (lambda (parallel)
		 (for-each walk-subproblem (parallel-subproblems parallel)))
	       parallels))))

(define (new-subproblem/compute-free-variables! subproblem)
  (with-analysis-state (lambda () (walk-subproblem subproblem))))

(define (walk-subproblem subproblem)
  (let ((free (subproblem-free-variables subproblem)))
    (case free
      ((UNKNOWN)
       (set-subproblem-free-variables! subproblem 'BEING-COMPUTED)
       (let ((free
	      (let ((free (walk-rvalue (subproblem-rvalue subproblem))))
		(if (subproblem-canonical? subproblem)
		    (eq-set-union
		     free
		     (walk-node (subproblem-entry-node subproblem)))
		    free))))
	 (set-subproblem-free-variables! subproblem free)
	 free))
      ((BEING-COMPUTED)
       (error "loop in subproblem free-variable analysis" subproblem))
      (else
       free))))

(define (walk-procedure proc)
  (define (default)
    (remove lvalue-integrated? (block-free-variables (procedure-block proc))))

  (define (closure)
    (eq-set-union
     (default)
     (block-bound-variables
      (block-shared-block (procedure-closing-block proc)))))

  (if (or (not (procedure/closure? proc))
	  (procedure/trivial-closure? proc))
      (default)
      (let ((how (procedure-closure-cons proc)))
	(case (car how)
	  ((NORMAL)
	   (closure))
	  ((DESCENDANT)
	   ;; This should automatically imply saving the ancestor
	   ;; for stack overwrites since that is how the free
	   ;; variables will be obtained.
	   (closure))
	  ((INDIRECTED)
	   ;; In reality, only the indirection variable or the default
	   ;; set is needed, depending on where the reference occurs.
	   ;; This is always safe, however.
	   (eq-set-adjoin (cdr how) (closure)))
	  (else
	   (error "walk-procedure: Unknown closure method" proc))))))

(define (walk-operator rvalue)
  (enumeration-case rvalue-type (tagged-vector/index rvalue)
    ((REFERENCE) (walk-lvalue (reference-lvalue rvalue) walk-operator))
    ((PROCEDURE)
     (if (procedure-continuation? rvalue)
	 (walk-next (continuation/entry-node rvalue) '())
	 (map-union walk-procedure
		    (eq-set-union (list rvalue)
				  (procedure-callees rvalue)))))
    (else '())))

(define (walk-rvalue rvalue)
  (enumeration-case rvalue-type (tagged-vector/index rvalue)
    ((REFERENCE) (walk-lvalue (reference-lvalue rvalue) walk-rvalue))
    ((PROCEDURE)
     (if (procedure-continuation? rvalue)
	 (walk-next (continuation/entry-node rvalue) '())
	 (walk-procedure rvalue)))
    (else '())))

(define (walk-lvalue lvalue walk-rvalue)
  (let ((value (lvalue-known-value lvalue)))
    (if value
	(if (lvalue-integrated? lvalue)
	    (walk-rvalue value)
	    (eq-set-adjoin lvalue (walk-rvalue value)))
	(if (and (variable? lvalue)
		 (variable-indirection lvalue))
	    (walk-lvalue (car (variable-indirection lvalue))
			 walk-rvalue)
	    (list lvalue)))))

(define *nodes*)

(define free-variables-tag
  "free-variables-tag")

(define (with-analysis-state thunk)
  (fluid-let ((*nodes* '()))
    (let ((value (with-new-node-marks thunk)))
      (for-each (lambda (node) (cfg-node-remove! node free-variables-tag))
		*nodes*)
      value)))

(define (walk-node node)
  (if (node-marked? node)
      (let ((free (cfg-node-get node free-variables-tag)))
	(if (eq? free 'BEING-COMPUTED)
	    (error "loop in node free-variable analysis" node))
	free)
      (begin
	(node-mark! node)
	(set! *nodes* (cons node *nodes*))
	(cfg-node-put! node free-variables-tag 'BEING-COMPUTED)
	(let ((free (walk-node-no-memoize node)))
	  (cfg-node-put! node free-variables-tag free)
	  free))))

(define (walk-node-no-memoize node)
  (cfg-node-case (tagged-vector/tag node)
    ((PARALLEL)
     (walk-next (snode-next node)
		(map-union walk-subproblem (parallel-subproblems node))))
    ((APPLICATION)
     (walk-next
      (snode-next node)
      (eq-set-union (walk-operator (application-operator node))
		    (map-union walk-rvalue (application-operands node)))))
    ((VIRTUAL-RETURN)
     (walk-next
      (snode-next node)
      (let ((operator (virtual-return-operator node))
	    (free (walk-rvalue (virtual-return-operand node))))
	(cond ((not (virtual-continuation? operator))
	       (eq-set-union (walk-rvalue operator) free))
	      ((virtual-continuation/reified? operator)
	       (eq-set-union
		(walk-rvalue (virtual-continuation/reification operator))
		free))
	      (else free)))))
    ((ASSIGNMENT)
     (walk-next
      (snode-next node)
      (eq-set-union (walk-lvalue (assignment-lvalue node) walk-rvalue)
		    (walk-rvalue (assignment-rvalue node)))))
    ((DEFINITION)
     (walk-next
      (snode-next node)
      (eq-set-union (walk-lvalue (definition-lvalue node) walk-rvalue)
		    (walk-rvalue (definition-rvalue node)))))
    ((TRUE-TEST)
     (walk-next (pnode-consequent node)
		(walk-next (pnode-alternative node)
			   (walk-rvalue (true-test-rvalue node)))))
    ((FG-NOOP)
     (walk-next (snode-next node) '()))))

(define (walk-next next free)
  (if next
      (eq-set-union (walk-node next) free)
      free))

(define (map-union procedure items)
  (if (null? items)
      '()
      (let loop ((items (cdr items)) (set (procedure (car items))))
	(if (null? items)
	    set
	    (loop (cdr items)
		  (eq-set-union (procedure (car items)) set))))))