#| -*-Scheme-*-

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    2017, 2018, 2019 Massachusetts Institute of Technology

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it under the terms of the GNU General Public License as published by
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WITHOUT ANY WARRANTY; without even the implied warranty of
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General Public License for more details.

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|#

;;;; RTL Generation: Return Statements

(declare (usual-integrations))

(define (generate/return return)
  (let loop ((returns (return/equivalence-class return)))
    (if (null? returns)
	(generate/return* (return/context return)
			  (return/operator return)
			  (application-continuation-push return)
			  (trivial-return-operand (return/operand return)))
	(let ((memoization (cfg-node-get (car returns) memoization-tag)))
	  (if (and memoization
		   (not (eq? memoization loop-memoization-marker)))
	      memoization
	      (loop (cdr returns)))))))

(define (generate/trivial-return context operator operand)
  (generate/return* context operator false (trivial-return-operand operand)))

(define (trivial-return-operand operand)
  (make-return-operand
   (lambda ()
     (make-null-cfg))
   (lambda (finish)
     (generate/rvalue operand scfg*scfg->scfg!
       (lambda (expression)
	 (finish (rtl:make-true-test expression)))))
   (lambda (finish)
     (generate/rvalue operand scfg*scfg->scfg! finish))
   (rvalue-known-value operand)))

(define-structure (return-operand (conc-name return-operand/))
  (effect-generator false read-only true)
  (predicate-generator false read-only true)
  (value-generator false read-only true)
  (known-value false read-only true))

(define-integrable (effect-prefix operand)
  ((return-operand/effect-generator operand)))

(define (generate/return* context operator not-on-stack? operand)
  (let ((continuation (rvalue-known-value operator)))
    (if (and continuation
	     (let ((procedure
		    (block-procedure
		     (continuation/closing-block continuation))))
	       (not (and (procedure? procedure)
			 (procedure/simplified? procedure)))))
	((method-table-lookup simple-methods (continuation/type continuation))
	 (if not-on-stack?
	     (return-operator/pop-frames context operator 0)
	     (scfg*scfg->scfg!
	      (return-operator/pop-frames
	       context
	       operator
	       (if (continuation/always-known-operator? continuation) 0 1))
	      (generate/continuation-entry/pop-extra continuation)))
	 operand
	 continuation)
	(scfg-append!
	 (if (and continuation (continuation/effect? continuation))
	     (effect-prefix operand)
	     ((return-operand/value-generator operand)
	      (lambda (expression)
		(rtl:make-assignment register:value expression))))
	 (return-operator/pop-frames context operator 0)
	 (rtl:make-pop-return)))))

(define-integrable (continuation/effect? continuation)
  (eq? continuation-type/effect (continuation/type continuation)))

(define simple-methods
  (make-method-table continuation-types false))

(define-method-table-entry 'EFFECT simple-methods
  (lambda (prefix operand continuation)
    (scfg-append!
     (effect-prefix operand)
     prefix
     (generate/node (continuation/entry-node continuation)))))

(define-method-table-entries '(REGISTER VALUE) simple-methods
  (lambda (prefix operand continuation)
    (scfg-append!
     (if (lvalue-integrated? (continuation/parameter continuation))
	 (effect-prefix operand)
	 ((return-operand/value-generator operand)
	  (lambda (expression)
	    (rtl:make-assignment (continuation/register continuation)
				 expression))))
     prefix
     (generate/node (continuation/entry-node continuation)))))

(define-method-table-entry 'PUSH simple-methods
  (lambda (prefix operand continuation)
    (scfg*scfg->scfg!
     (if (cfg-null? prefix)
	 ((return-operand/value-generator operand) rtl:make-push)
	 (use-temporary-register operand prefix rtl:make-push))
     (generate/node (continuation/entry-node continuation)))))

(define-method-table-entry 'PREDICATE simple-methods
  (lambda (prefix operand continuation)
    (let ((node (continuation/entry-node continuation))
	  (value (return-operand/known-value operand)))
      (if value
	  (scfg-append!
	   (effect-prefix operand)
	   prefix
	   (generate/node (if (and (rvalue/constant? value)
				   (false? (constant-value value)))
			      (pnode-alternative node)
			      (pnode-consequent node))))
	  (let ((finish
		 (lambda (pcfg)
		   (pcfg*scfg->scfg!
		    pcfg
		    (generate/node (pnode-consequent node))
		    (generate/node (pnode-alternative node))))))
	    (if (cfg-null? prefix)
		((return-operand/predicate-generator operand) finish)
		(use-temporary-register operand prefix
		  (lambda (expression)
		    (finish (rtl:make-true-test expression))))))))))

(define (use-temporary-register operand prefix finish)
  (let ((register (rtl:make-pseudo-register)))
    (let ((setup-register
	   ((return-operand/value-generator operand)
	    (lambda (expression)
	      (rtl:make-assignment register expression)))))
      (scfg-append!
       setup-register
       prefix
       (finish (rtl:make-fetch register))))))

(define (return-operator/pop-frames context operator extra)
  (let ((pop-extra
	 (lambda (extra)
	   (if (zero? extra)
	       (make-null-cfg)
	       (rtl:make-assignment register:stack-pointer
				    (rtl:make-address
				     (stack-locative-offset
				      (rtl:make-fetch register:stack-pointer)
				      extra)))))))
    (if (exact-integer? context)
	;; This kludge is used by open-coding of some primitives in
	;; reduction position.  In that case, there is no frame (and
	;; therefore no context) because adjustments prior to the
	;; open-coding have eliminated it.  So it is known that only
	;; the primitive's arguments are on the stack, and the return
	;; address appears directly above that.
	(pop-extra (+ context extra))
	(let ((block (reference-context/block context)))
	  (if (or (ic-block? block)
		  (return-operator/subproblem? operator))
	      (pop-extra extra)
	      (let ((popping-limit (block-popping-limit block)))
		(cond ((not popping-limit)
		       (scfg*scfg->scfg!
			(rtl:make-link->stack-pointer)
			(pop-extra extra)))
		      ((and (eq? popping-limit block)
			    (zero? (block-frame-size popping-limit))
			    (zero? (reference-context/offset context))
			    (zero? extra))
		       (make-null-cfg))
		      (else
		       (rtl:make-assignment
			register:stack-pointer
			(popping-limit/locative context
						popping-limit
						0
						extra))))))))))