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#lang sicp
(#%require (only racket/base print-as-expression print-mpair-curly-braces))
(print-as-expression #f)
(print-mpair-curly-braces #f)
;; Chapter 5
;; Computing with Register Machines
;; 5.3
;; Storage Allocation and Garbage Collection
;; Memory as Vectors
#| 5.20 |#
#| (define x (cons 1 2)) |#
#| (define y (list x x)) |#
#| | 0 | 1 | 2 | 3 | 4 | |#
#| ---------|----|----|----|----|----| |#
#| the-cars | n1 | p0 | p0 | |#
#| the-cdrs | n2 | e0 | p1 | |#
#| x = p0 |#
#| y = p2 |#
#| free = 3 |#
#| 5.21 |#
(define (count-leaves tree)
(cond
((null? tree) 0)
((not (pair? tree)) 1)
(else
(+
(count-leaves (car tree))
(count-leaves (cdr tree))))))
(define count-leaves-controller
'(controller
(assign continue (label count-leaves-done))
(assign tree (op read))
count-leaves
(test (op null?) (reg tree))
(branch (label if-null))
(assign t (op pair?) (reg tree))
(test (op not) (reg t))
(branch (label if-leaf))
(save continue)
(save tree)
(assign continue (label after-car-tree))
(assign tree (op car) (reg tree))
(goto (label count-leaves))
after-car-tree
(restore tree)
(save tree)
(save val)
(assign tree (op cdr) (reg tree))
(assign continue (label after-cdr-tree))
(goto (label count-leaves))
after-cdr-tree
(assign t (reg val))
(restore val)
(restore tree)
(restore continue)
(assign val (op +) (reg t) (reg val))
(goto (reg continue))
if-null
(assign val (const 0))
(goto (reg continue))
if-leaf
(assign val (const 1))
(goto (reg continue))
count-leaves-done))
(define (count-leaves- tree)
(define (count-iter tree n)
(cond
((null? tree) n)
((not (pair? tree)) (+ n 1))
(else
(count-iter
(cdr tree)
(count-iter (car tree) n)))))
(count-iter tree 0))
(define count-leaves-iter-controller
'(controller
(assign continue (label count-leaves-done))
(assign tree (op read))
(assign n (const 0))
count-iter
(test (op null?) (reg tree))
(branch (label if-null))
(assign t (op pair?) (reg tree))
(test (op not) (reg t))
(branch (label if-leaf))
(save tree)
(save continue)
(assign tree (car tree))
(assign continue (label after-car-tree))
(goto (label count-iter))
after-car-tree
(restore continue)
(restore tree)
(assign tree (cdr tree))
(goto (label count-iter))
if-null
(goto (reg continue))
if-leaf
(assign n (op +) (reg n) (const 1))
(goto (reg continue))
count-leaves-done))
#| 5.22 |#
(define (append x y)
(if (null? x)
y
(cons (car x) (append (cdr x) y))))
(define append-controller
'(controller
(assign x (op read))
(assign y (op read))
(assign continue (label append-done))
append
(test (op null?) (reg x))
(branch (label if-null))
(save x)
(save continue)
(assign x (op cdr) (reg x))
(assign continue (label after-append))
(goto (label append))
after-append
(restore continue)
(restore x)
(assign x (op car) (reg x))
(assign val (op cons) (reg x) (reg val))
(goto (reg continue))
if-null
(assign val (reg y))
(goto (reg continue))
append-done))
(define (last-pair x)
(if (null? (cdr x))
x
(last-pair (cdr x))))
(#%provide append!)
(define (append! x y)
(set-cdr! (last-pair x) y)
x)
(define append!-controller
'(controller
(assign x (op read))
(assign y (op read))
(assign head (reg x))
(assign rest (op cdr) (reg x))
last-pair
(test (op null?) (reg rest))
(branch (label found-last-pair))
(assign head (reg rest))
(assign rest (op cdr) (reg rest))
(goto (label last-pair))
found-last-pair
(perform (op set-cdr!) (reg head) (reg y))))
;; Maintaining the Illusion of Infinite Memory
(define gc-controller
'(begin-garbage-collection
(assign free (const 0))
(assign scan (const 0))
(assign old (reg root))
(assign relocate-continue (label reassign-root))
(goto (label relocate-old-result-in-new))
reassign-root
(assign root (reg new))
(goto (label gc-loop))
gc-loop
(test (op =) (reg scan) (reg free))
(branch (label gc-flip))
(assign old (op vector-ref) (reg new-cars) (reg scan))
(assign relocate-continue (label update-car))
(goto (label relocate-old-result-in-new))
update-car
(perform (op vector-set!) (reg new-cars) (reg scan) (reg new))
(assign old (op vector-ref) (reg new-cdrs) (reg scan))
(assign relocate-continue (label update-cdr))
(goto (label relocate-old-result-in-new))
update-cdr
(perform (op vector-set!) (reg new-cdrs) (reg scan) (reg new))
(assign scan (op +) (reg scan) (const 1))
(goto (label gc-loop))
relocate-old-result-in-new
(test (op pointer-to-pair?) (reg old))
(branch (label pair))
(assign new (reg old))
(goto (reg relocate-continue))
pair
(assign oldcr (op vector-ref) (reg the-cars) (reg old))
(test (op broken-heart?) (reg oldcr))
(branch (label already-moved))
(assign new (reg free))
(assign free (op +) (reg free) (const 1))
(perform (op vector-set!) (reg new-cars) (reg new) (reg olcr))
(assign oldcr (op vector-ref) (reg the-cdrs) (reg old))
(perform (op vector-set!) (reg new-cdrs) (reg new) (reg olcr))
(perform (op vector-set!) (reg the-cars) (reg old) (const broken-heart))
(perform (op vector-set!) (reg the-cdrs) (reg old) (reg new))
(goto (reg relocate-continue))
already-moved
(assign new (op vector-ref) (reg the-cdrs) (reg old))
(goto (reg relocate-continue))
gc-flip
(assign temp (reg the-cdrs))
(assign the-cdrs (reg new-cdrs))
(assign new-cdrs (reg temp))
(assign temp (reg the-cars))
(assign the-cars (reg new-cars))
(assign new-cars (reg temp))))
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