#| -*-Scheme-*-

$Id: lapopt.scm,v 1.6 2007/01/05 21:19:21 cph Exp $

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    1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007 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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USA.

|#

;;;; LAP Optimizer for MIPS.

(declare (usual-integrations))

(define (optimize-linear-lap instructions)
  ;; Find occurrences of LW/LBU/LWC1 followed by NOP, and delete the
  ;; NOP if the instruction following it has no reference to the
  ;; target register of the load.

  ;; **** This is pretty fragile. ****
  (letrec
      ((find-load
	(lambda (instructions)
	  (cond ((null? instructions) '())
		((and (pair? (car instructions))
		      (or (eq? 'LW (caar instructions))
			  (eq? 'LBU (caar instructions))
			  (eq? 'LWC1 (caar instructions))))
		 instructions)
		(else (find-load (cdr instructions))))))
       (get-next
	(lambda (instructions)
	  (let ((instructions (cdr instructions)))
	    (cond ((null? instructions) '())
		  ((or (not (pair? (car instructions)))
		       (eq? 'LABEL (caar instructions))
		       (eq? 'COMMENT (caar instructions)))
		   (get-next instructions))
		  (else instructions)))))
       (refers-to-register?
	(lambda (instruction register)
	  (let loop ((x instruction))
	    (if (pair? x)
		(or (loop (car x))
		    (loop (cdr x)))
		(eqv? register x))))))
    (let loop ((instructions instructions))
      (let ((first (find-load instructions)))
	(if (not (null? first))
	    (let ((second (get-next first)))
	      (if (not (null? second))
		  (let ((third (get-next second)))
		    (if (not (null? third))
			(if (and (equal? '(NOP) (car second))
				 ;; This is a crude way to test for a
				 ;; reference to the target register
				 ;; -- it will sometimes incorrectly
				 ;; say that there is a reference, but
				 ;; it will never incorrectly say that
				 ;; there is no reference.
				 (not (refers-to-register? (car third)
							   (cadar first)))
				 (or (not (and (eq? 'LWC1 (caar first))
					       (odd? (cadar first))))
				     (not (refers-to-register?
					   (car third)
					   (- (cadar first) 1)))))
			    (begin
			      (let loop ((this (cdr first)) (prev first))
				(if (eq? second this)
				    (set-cdr! prev (cdr this))
				    (loop (cdr this) this)))
			      (loop (if (equal? '(NOP) (car third))
					first
					third)))
			    (loop second))))))))))
  instructions)