#| -*-Scheme-*-

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    2017, 2018, 2019 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
the Free Software Foundation; either version 2 of the License, or (at
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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 Register Lifetime Analysis
;;;  Based on the GNU C Compiler

(declare (usual-integrations))

(define (lifetime-analysis rgraphs)
  (for-each walk-rgraph rgraphs))

(define (walk-rgraph rgraph)
  (let ((n-registers (rgraph-n-registers rgraph))
	(bblocks (rgraph-bblocks rgraph)))
    (set-rgraph-register-bblock! rgraph (make-vector n-registers false))
    (set-rgraph-register-n-refs! rgraph (make-vector n-registers 0))
    (set-rgraph-register-n-deaths! rgraph (make-vector n-registers 0))
    (set-rgraph-register-live-length! rgraph (make-vector n-registers 0))
    (set-rgraph-register-crosses-call?! rgraph
					(make-bit-string n-registers false))
    (for-each (lambda (bblock)
		(set-bblock-live-at-entry! bblock (make-regset n-registers))
		(set-bblock-live-at-exit! bblock (make-regset n-registers))
		(set-bblock-new-live-at-exit! bblock
					      (make-regset n-registers)))
	      bblocks)
    (fluid-let ((*current-rgraph* rgraph))
      (walk-bblocks bblocks))
    (for-each (lambda (bblock)
		(set-bblock-new-live-at-exit! bblock false))
	      (rgraph-bblocks rgraph))))

(define (walk-bblocks bblocks)
  (let ((changed? false))
    (define (loop first-pass?)
      (for-each (lambda (bblock)
		  (if (or first-pass?
			  (not (regset=? (bblock-live-at-exit bblock)
					 (bblock-new-live-at-exit bblock))))
		      (begin (set! changed? true)
			     (regset-copy! (bblock-live-at-exit bblock)
					   (bblock-new-live-at-exit bblock))
			     (regset-copy! (bblock-live-at-entry bblock)
					   (bblock-live-at-exit bblock))
			     (propagate-block bblock)
			     (for-each-previous-node bblock
			       (lambda (bblock*)
				 (regset-union!
				  (bblock-new-live-at-exit bblock*)
				  (bblock-live-at-entry bblock)))))))
		bblocks)
      (if changed?
	  (begin (set! changed? false)
		 (loop false))
	  (for-each (lambda (bblock)
		      (regset-copy! (bblock-live-at-entry bblock)
				    (bblock-live-at-exit bblock))
		      (propagate-block&delete! bblock))
		    bblocks)))
    (loop true)))

(define (propagate-block bblock)
  (propagation-loop bblock
    (lambda (dead live rinst)
      (update-live-registers! (bblock-live-at-entry bblock)
			      dead
			      live
			      (rinst-rtl rinst)
			      false false))))

(define (propagate-block&delete! bblock)
  (for-each-regset-member (bblock-live-at-entry bblock)
    (lambda (register)
      (set-register-bblock! register 'NON-LOCAL)))
  (propagation-loop bblock
    (lambda (dead live rinst)
      (let ((rtl (rinst-rtl rinst))
	    (old (bblock-live-at-entry bblock)))
	(if (rtl:invocation? rtl)
	    (for-each-regset-member old register-crosses-call!))
	(if (instruction-dead? rtl old)
	    (set-rinst-rtl! rinst false)
	    (begin
	      (update-live-registers! old dead live rtl bblock rinst)
	      (for-each-regset-member old increment-register-live-length!))))))
  (bblock-perform-deletions! bblock))

(define (propagation-loop bblock procedure)
  (let ((dead (regset-allocate (rgraph-n-registers *current-rgraph*)))
	(live (regset-allocate (rgraph-n-registers *current-rgraph*))))
    (bblock-walk-backward bblock
      (lambda (rinst)
	(regset-clear! dead)
	(regset-clear! live)
	(procedure dead live rinst)))))

(define (update-live-registers! old dead live rtl bblock rinst)
  (mark-set-registers! old dead rtl bblock)
  (mark-used-registers! old live rtl bblock rinst)
  (regset-difference! old dead)
  (regset-union! old live))

(define (mark-set-registers! needed dead rtl bblock)
  ;; **** This code safely ignores PRE-INCREMENT and POST-INCREMENT
  ;; modes, since they are only used on the stack pointer.
  needed
  (if (rtl:assign? rtl)
      (let ((address (rtl:assign-address rtl)))
	(if (interesting-register? address)
	    (let ((register (rtl:register-number address)))
	      (regset-adjoin! dead register)
	      (if bblock (record-register-reference register bblock)))))))

(define (mark-used-registers! needed live rtl bblock rinst)
  (define (loop expression)
    (if (interesting-register? expression)
	(let ((register (rtl:register-number expression)))
	  (regset-adjoin! live register)
	  (if bblock
	      (begin (record-register-reference register bblock)
		     (if (and (not (regset-member? needed register))
			      (not (rinst-dead-register? rinst register)))
			 (begin (set-rinst-dead-registers!
				 rinst
				 (cons register
				       (rinst-dead-registers rinst)))
				(increment-register-n-deaths! register))))))
	(rtl:for-each-subexpression expression loop)))
  (if (and (rtl:assign? rtl)
	   (rtl:register? (rtl:assign-address rtl)))
      (if (let ((register (rtl:register-number (rtl:assign-address rtl))))
	    (or (machine-register? register)
		(regset-member? needed register)))
	  (loop (rtl:assign-expression rtl)))
      (rtl:for-each-subexpression rtl loop)))

(define (record-register-reference register bblock)
  (let ((bblock* (register-bblock register)))
    (cond ((not bblock*)
	   (set-register-bblock! register bblock))
	  ((not (eq? bblock bblock*))
	   (set-register-bblock! register 'NON-LOCAL)))
    (increment-register-n-refs! register)))

(define (instruction-dead? rtl needed)
  (and (rtl:assign? rtl)
       (let ((address (rtl:assign-address rtl)))
	 (and (rtl:register? address)
	      (let ((register (rtl:register-number address)))
		(and (pseudo-register? register)
		     (not (regset-member? needed register))))))
       (not (rtl:expression-contains? (rtl:assign-expression rtl)
				      rtl:volatile-expression?))))

(define (interesting-register? expression)
  (and (rtl:register? expression)
       (pseudo-register? (rtl:register-number expression))))