sicp-ex-5.34

  
  
  
 The difference is in (* n (factorial (- n 1))), before call (factorial (- n 1)), compiler need save continue, then restore it. But in iter, just (goto (reg val)), there is no need to callback. 

  
  
 (assign val (op make-compiled-procedure) (label entry24) (reg env)) 
 (goto (label after-lambda23)) 
  
 entry24 
 (assign env (op compiled-procedure-env) (reg proc)) 
 (assign env (op extend-environment) (const (n)) (reg argl) (reg env)) 
 ;;生成(define (iter product counter) ...) 
 (assign val (op make-compiled-procedure) (label entry29) (reg env)) 
 (goto (label after-lambda28)) 
  
 entry29 
 ;;设置环境啦 
 (assign env (op compiled-procedure-env) (reg proc)) 
 (assign env (op extend-environment) (const (product counter)) (reg argl) (reg env)) 
 ;;计算(> counter n) 
 (save continue) 
 (save env) 
 (assign proc (op lookup-variable-value) (const >) (reg env)) 
 (assign val (op lookup-variable-value) (const n) (reg env)) 
 (assign argl (op list) (reg val)) 
 (assign val (op lookup-variable-value) (const counter) (reg env)) 
 (assign argl (op cons) (reg val) (reg argl)) 
 (test (op primitive-procedure?) (reg proc)) 
 (branch (label primitive-branch44)) 
  
 compiled-branch43 
 (assign continue (label after-call42)) 
 (assign val (op compiled-procedure-entry) (reg proc)) 
 (goto (reg val)) 
  
 primitive-branch44 
 (assign val (op apply-primitive-procedure) (reg proc) (reg argl)) 
  
 after-call42 
 ;;虽然(> counter n)并未改变env。但过程应用的modified是all-regs，下面紧接着是另一个过程应用，need env，所以会产生这个步骤 
 (restore env) 
 (restore continue) 
 (test (op false?) (reg val)) 
 (branch (label false-branch31)) 
  
 true-branch32 
 (assign val (op lookup-variable-value) (const product) (reg env)) 
 (goto (reg continue)) 
  
 false-branch31 
 (assign proc (op lookup-variable-value) (const iter) (reg env)) 
 ;;计算(+ counter 1)，不会马上给回counter，不用担心product 
 (save continue) 
 (save proc) 
 (save env) 
 (assign proc (op lookup-variable-value) (const +) (reg env)) 
 (assign val (const 1)) 
 (assign argl (op list) (reg val)) 
 (assign val (op lookup-variable-value) (const counter) (reg env)) 
 (assign argl (op cons) (reg val) (reg argl)) 
 (test (op primitive-procedure?) (reg proc)) 
 (branch (label primitive-branch38)) 
  
 compiled-branch37 
 (assign continue (label after-call36)) 
 (assign val (op compiled-procedure-entry) (reg proc)) 
 (goto (reg val)) 
  
 primitive-branch38 
 (assign val (op apply-primitive-procedure) (reg proc) (reg argl)) 
  
 after-call36 
 (assign argl (op list) (reg val)) 
 ;;计算(* product counter) 
 (restore env) 
 (save argl) 
 (assign proc (op lookup-variable-value) (const *) (reg env)) 
 (assign val (op lookup-variable-value) (const counter) (reg env)) 
 (assign argl (op list) (reg val)) 
 (assign val (op lookup-variable-value) (const product) (reg env)) 
 (assign argl (op cons) (reg val) (reg argl)) 
 (test (op primitive-procedure?) (reg proc)) 
 (branch (label primitive-branch35)) 
  
 compiled-branch34 
 (assign continue (label after-call33)) 
 (assign val (op compiled-procedure-entry) (reg proc)) 
 (goto (reg val)) 
  
 primitive-branch35 
 (assign val (op apply-primitive-procedure) (reg proc) (reg argl)) 
  
 after-call33 
 (restore argl) 
 ;;给iter新的argl构建完毕 
 (assign argl (op cons) (reg val) (reg argl)) 
 (restore proc) 
 (restore continue) 
 ;;至此，栈又恢复到上次调用iter的情况 
 (test (op primitive-procedure?) (reg proc)) 
 (branch (label primitive-branch41)) 
  
 compiled-branch40 
 (assign val (op compiled-procedure-entry) (reg proc)) 
 (goto (reg val)) 
  
 primitive-branch41 
 (assign val (op apply-primitive-procedure) (reg proc) (reg argl)) 
 (goto (reg continue)) 
  
 ;;用不上了 
 after-call39 
 after-if30 
  
 ;;得到(define (iter ..) ...) 
 after-lambda28 
 (perform (op define-variable!) (const iter) (reg val) (reg env)) 
 (assign val (const ok)) 
 ;;计算(iter 1 1) 
 (assign proc (op lookup-variable-value) (const iter) (reg env)) 
 ;;构建argl 
 (assign val (const 1)) 
 (assign argl (op list) (reg val)) 
 (assign val (const 1)) 
 (assign argl (op cons) (reg val) (reg argl)) 
 (test (op primitive-procedure?) (reg proc)) 
 (branch (label primitive-branch27)) 
  
 ;;最后一条表达式，所以不用回到after-call25 
 compiled-branch26 
 (assign val (op compiled-procedure-entry) (reg proc)) 
 (goto (reg val)) 
  
 primitive-branch27 
 (assign val (op apply-primitive-procedure) (reg proc) (reg argl)) 
 (goto (reg continue)) 
  
 after-call25 
  
 after-lambda23 
 (perform (op define-variable!) (const factorial) (reg val) (reg env)) 
 (assign val (const ok))