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; -*- Mode: Scheme; Syntax: Scheme; Package: Scheme; -*-
; This is file stack.scm.
; *STACK-BEGIN* and *STACK-END* delimit the stack portion of memory.
; *STACK* points to the next unused cell on top of the stack.
; *STACK-LIMIT* is then end of the currently usable stack space.
; *BOTTOM-OF-STACK* is a continuation that lies a the base of the stack.
(define *stack-begin* (unassigned))
(define *stack-end* (unassigned))
(define *stack* (unassigned))
(define *stack-limit* (unassigned))
(define *bottom-of-stack* (unassigned))
(define *cont* (unassigned))
; For tracking the reason for copying closures and environments (used for
; debugging only).
(define-enumeration copy
(closure
overflow
preserve))
; At the bottom of the stack is a special continuation that is never removed.
; When it is invoked it copies the next continuation out of the heap (if there
; is any such) and invokes that instead.
(define (initialize-stack start size)
(set! *stack-begin* start)
(set! *stack-end* (+ start (cells->a-units size)))
(set! *stack-limit* *stack-begin*)
(set! *stack* (the-pointer-before *stack-end*))
(set! *cont* false)
(set! *env* quiescent)
(push-continuation-on-stack (make-template-containing-ops
(enum op get-cont-from-heap)
(enum op return))
(enter-fixnum 0)
0
universal-key)
(set! *bottom-of-stack* *cont*))
; The amount of heap space required to initialize the stack.
(define initial-stack-heap-space (op-template-size 2))
(define (reset-stack-pointer)
(set! *stack* (the-pointer-before
(the-pointer-before (address-after-header *bottom-of-stack*))))
(set-continuation-cont! *bottom-of-stack* (enter-boolean #f)))
(define (within-stack? p)
(and (addr>= p *stack-begin*)
(addr<= p *stack-end*)))
(define (stack-size)
(- *stack-end* *stack-begin*))
(define (available-on-stack? cells)
(addr> (addr- *stack* (cells->a-units cells)) *stack-limit*))
; The + 1 is to get room for one header that is made.
(define (current-stack-size)
(+ 1 (a-units->cells (addr- *stack-end* *stack*))))
; Value of *NARGS* indicating that the arguments overflowed the stack limit
(define arg-stack-overflow-nargs (+ maximum-stack-args 1))
; Room for MAXIMUM-STACK-ARGS plus one for the procedure argument to op/apply
(define maximum-stack-arg-count (+ maximum-stack-args 1))
; Add CELLS cells onto the stack.
; The stack grows towards negative memory.
(define (stack-add cells)
(set! *stack* (addr- *stack* (cells->a-units cells))))
(define (the-pointer-before x)
(addr- x (cells->a-units 1)))
(define (push x) ; check for overflow is done when continuations are pushed
(store! *stack* x)
(stack-add 1))
(define (pop)
(stack-add -1)
(fetch *stack*))
(define (stack-ref index)
(fetch (addr+ *stack* (cells->a-units (+ 1 index)))))
(define (stack-set! index value)
(store! (addr+ *stack* (cells->a-units (+ 1 index))) value))
(define (pointer-to-stack-arguments)
(addr+ *stack* (cells->a-units 1)))
(define (remove-stack-arguments count)
(stack-add (- 0 count)))
; Making sure that no one uses stack space without checking for overflow.
; Returns a key. Only the most recent key is valid for allocating storage.
; UNIVERSAL-KEY is always valid.
(define (ensure-stack-space space ensure-space)
(if (not (available-on-stack? space))
(copy-stack-into-heap (ensure-space (current-stack-size))))
(preallocate-stack-space space))
(define (preallocate-stack-space space)
(cond (check-stack-preallocation?
(set! *stack-key* (+ *stack-key* -1)) ;go down to distinguish from heap keys
(set! *okayed-stack-space* space)
*stack-key*)
(else 0)))
(define check-stack-preallocation? #f)
(define *stack-key* 0)
(define *okayed-stack-space* 0)
; Checks that KEY is the most recent key, and that the overflow check was
; made for at least CELLS space.
(define (check-stack-cons cells key)
(cond ((and check-stack-preallocation?
(not (= key universal-key)))
(if (not (and (= key *stack-key*)
(>= *okayed-stack-space* cells)))
(error "invalid stack key" key cells))
(set! *okayed-stack-space* (- *okayed-stack-space* cells)))))
; Space for an exception continuation is reserved on the stack to
; ensure that pushing an exception continuation will not trigger a
; garbage collection. Exceptions occur at points where there are
; live values that will not be found by the GC.
(define (reserve-stack-space size)
(set! *stack-limit* (+ *stack-begin* (cells->a-units size))))
(define (enable-stack-reserve)
(set! *stack-limit* *stack-begin*))
(define *exception-space-used?* #t)
(define (exception-frame-space exception-frame-size)
(if (and *exception-space-used?*
(not (available-on-stack? exception-frame-size)))
(current-stack-size)
0))
(define (reserve-exception-space exception-frame-size key)
(cond (*exception-space-used?*
(if (not (available-on-stack? exception-frame-size))
(error "no space on stack to reserve exception space"))
(reserve-stack-space exception-frame-size)
(set! *exception-space-used?* #f))))
(define (allow-exception-consing exception-frame-size)
(cond ((not (available-on-stack? exception-frame-size))
(enable-stack-reserve)
(set! *exception-space-used?* #t)
(if (not (available-on-stack? exception-frame-size))
(error "insufficient space on stack for exception frame"))))
(preallocate-stack-space exception-frame-size))
(define (peek-at-current-continuation)
(if (addr= *cont* *bottom-of-stack*)
(continuation-cont *bottom-of-stack*)
*cont*))
; Skip to the continuation preceding the current one (used for multiple
; value returns).
(define (skip-current-continuation!)
(let ((next (continuation-cont *cont*)))
(if (addr= *cont* *bottom-of-stack*)
(set-continuation-cont! *cont* (continuation-cont next))
(set! *cont* next))))
; Called when replacing the current continuation with a new one.
(define (set-current-continuation! cont)
(set! *cont* (cond ((false? cont)
(reset-stack-pointer)
*bottom-of-stack*)
(else
(copy-continuation-from-heap cont)))))
; Called when returning off of the end of the stack.
(define (get-continuation-from-heap)
(continuation-cont *bottom-of-stack*))
; Copy CONT from heap onto stack just above *BOTTOM-OF-STACK*, linking it
; to *BOTTOM-OF-STACK* and *BOTTOM-OF-STACK* to CONT's continuation.
(define (copy-continuation-from-heap cont)
(assert (continuation? cont))
(let* ((top (addr- (address-at-header *bottom-of-stack*)
(addr1+ (cells->a-units (continuation-length cont)))))
(new-cont (address->stob-descriptor (addr1+ top))))
(add-copy-cont-from-heap-stats cont)
(set! *stack* (the-pointer-before top))
(copy-cells! (address-at-header cont) top (+ 1 (continuation-length cont)))
(set-continuation-cont! *bottom-of-stack* (continuation-cont new-cont))
(set-continuation-cont! new-cont *bottom-of-stack*)
new-cont))
; Pushing and popping continuations.
(define stack-continuation-size
(+ (+ continuation-cells 1) ; header
maximum-stack-arg-count)) ; pre-checking for pushed arguments
(define (push-continuation-on-stack template pc arg-count key)
(check-stack-cons stack-continuation-size key)
(add-continuation-stats arg-count)
(stack-add (+ 1 continuation-cells))
(store! (addr1+ *stack*) (make-continuation-header arg-count))
(let ((cont (address->stob-descriptor (addr1+ (addr1+ *stack*)))))
(set-continuation-pc! cont pc)
(set-continuation-template! cont template)
(set-continuation-env! cont *env*)
(set-continuation-cont! cont *cont*)
(set! *cont* cont)))
(define make-continuation-header
(let ((type (enum stob continuation)))
(lambda (arg-count)
(make-header-immutable
(make-header type (cells->bytes (+ arg-count continuation-cells)))))))
(define (pop-continuation-from-stack set-template!)
(let ((cont *cont*))
(set-template! (continuation-template cont)
(continuation-pc cont))
(set! *env* (continuation-env cont))
(set! *cont* (continuation-cont cont))
(set! *stack* (addr+ (address-at-header cont)
(cells->a-units continuation-cells)))))
; Making environments on the stack - the values are already there so this
; only needs to push space for a pointer and push a header.
; Copying the stack into the heap because there is no more room on the
; stack. This copies any arguments that are on the top of the stack into
; a vector, migrates and recovers the current, and then moves the arguments
; from the vector back to the stack.
;
; Why can't this move the arguments directly? The restored continuation
; cannot be larger than the original.
(define (copy-stack-into-heap key)
(let* ((arg-count (arguments-on-stack))
(vec (vm-make-vector arg-count key)))
(do ((i (+ -1 arg-count) (- i 1)))
((<= i -1))
(vm-vector-set! vec i (pop)))
(preserve-continuation key (enum copy overflow))
(do ((i 0 (+ i 1)))
((>= i arg-count))
(push (vm-vector-ref vec i)))
(unassigned)))
(define (arguments-on-stack)
(do ((p (addr1+ *stack*) (addr1+ p))
(i 0 (+ i 1)))
((header? (fetch p)) i)))
; Migrating the current continuation into the heap, saving the environment
; first.
(define current-continuation-size current-stack-size)
(define (current-continuation key)
(preserve-continuation key (enum copy preserve)))
(define (preserve-continuation key reason)
(preserve-current-env-with-reason key reason)
(let ((end (continuation-cont *bottom-of-stack*)))
(let loop ((cont *cont*) (previous *bottom-of-stack*))
(cond ((vm-eq? cont *bottom-of-stack*)
(set-continuation-cont! previous end))
(else
(if (within-stack? (continuation-env cont))
(save-env-in-heap (continuation-env cont) cont key reason)
0) ; for type inferencer (which could use some improvement)
(let ((new (copy-stob cont key)))
(add-preserve-cont-stats new reason)
(set-continuation-cont! previous new)
(loop (continuation-cont new) new)))))
(let ((cont (continuation-cont *bottom-of-stack*)))
(set! *cont* (if (false? cont)
*bottom-of-stack*
(copy-continuation-from-heap cont)))
cont)))
; Copy NARGS arguments from the top of the stack to just above CONT.
; Used by OP/MOVE-ARGS-AND-CALL to implement tail-recursive calls.
;
; The IF saves work in a rare case at the cost of a test in the common
; case; is it worth it?
(define (move-args-above-cont! nargs)
(let ((start-loc (the-pointer-before (address-at-header *cont*)))
(start-arg (addr+ *stack* (cells->a-units nargs))))
(if (not (addr<= start-loc start-arg))
(do ((loc start-loc (the-pointer-before loc))
(arg start-arg (the-pointer-before arg)))
((addr<= arg *stack*)
(set! *stack* loc))
(store! loc (fetch arg))))))
; Tracing the stack for garbage collection - first trace any arguments pushed
; above the current continuation, then loop down the continuations, tracing
; each one along with its environment (if the environment has not yet been
; done).
(define (trace-stack trace-locations)
(trace-locations (addr1+ *stack*) (address-at-header *cont*))
(let loop ((cont *cont*) (last-env 0))
(let ((env (continuation-env cont)))
(trace-locations (address-after-header cont) (address-after-stob cont))
(if (not (vm-eq? env last-env))
(trace-env env trace-locations))
(if (not (vm-eq? cont *bottom-of-stack*))
(loop (continuation-cont cont) env)))))
; I do not think that the recursive call is necessary as the superior
; env will be traced by TRACE-STACK as it goes down the list of
; continuations. For every superior env that is on the stack, there should
; be a continuation on the stack that points to it.
(define (trace-env env trace-locations)
(let loop ((env env))
(cond ((within-stack? env)
(trace-locations (address-after-header env) (address-after-stob env))
(loop (vm-vector-ref env 0))))))
; Collecting and printing statistics on stack usage
(define collect-statistics? #f)
(define *conts* 0)
(define *conts-slots* 0)
(define *conts-overflow* 0)
(define *conts-overflow-slots* 0)
(define *conts-preserved* 0)
(define *conts-preserved-slots* 0)
(define *conts-from-heap* 0)
(define *conts-from-heap-slots* 0)
(define *envs* 0)
(define *envs-slots* 0)
(define *envs-closed* 0)
(define *envs-closed-slots* 0)
(define *envs-overflow* 0)
(define *envs-overflow-slots* 0)
(define *envs-preserved* 0)
(define *envs-preserved-slots* 0)
(define (reset-stack-stats)
(cond (collect-statistics?
(set! *conts* 0)
(set! *conts-slots* 0)
(set! *conts-overflow* 0)
(set! *conts-overflow-slots* 0)
(set! *conts-preserved* 0)
(set! *conts-preserved-slots* 0)
(set! *conts-from-heap* 0)
(set! *conts-from-heap-slots* 0)
(set! *envs* 0)
(set! *envs-slots* 0)
(set! *envs-closed* 0)
(set! *envs-closed-slots* 0)
(set! *envs-overflow* 0)
(set! *envs-overflow-slots* 0)
(set! *envs-preserved* 0)
(set! *envs-preserved-slots* 0)
)
(else 0)))
(define (print-stack-stats port)
(if collect-statistics?
(really-print-stack-stats port)))
(define (really-print-stack-stats port)
(let ((one-record (lambda (name count slots port)
(newline port)
(write-string "(" port)
(write-string name port)
(write-string " " port)
(write-number count port)
(write-number slots port)
(write-string ")" port))))
(newline port)
(write-string "(continuations" port)
(one-record "made" *conts* *conts-slots* port)
(one-record "overflow" *conts-overflow* *conts-overflow-slots* port)
(one-record "preserved" *conts-preserved* *conts-preserved-slots* port)
(one-record "from-heap" *conts-from-heap* *conts-from-heap-slots* port)
(write-string ")" port)
(newline port)
(write-string "(environments" port)
(one-record "made" *envs* *envs-slots* port)
(one-record "closed" *envs-closed* *envs-closed-slots* port)
(one-record "overflow" *envs-overflow* *envs-overflow-slots* port)
(one-record "preserved" *envs-preserved* *envs-preserved-slots* port)
(write-string ")" port)
(newline port)
))
(define (add-continuation-stats arg-count)
(cond (collect-statistics?
(set! *conts* (+ *conts* 1))
(set! *conts-slots*
(+ *conts-slots* (+ arg-count continuation-cells))))))
(define (add-env-stats count)
(cond (collect-statistics?
(set! *envs* (+ *envs* 1))
(set! *envs-slots* (+ *envs-slots* (+ count 1))))))
(define (add-copy-env-stats env reason)
(cond ((not collect-statistics?)
(unassigned))
((= reason (enum copy closure))
(set! *envs-closed* (+ *envs-closed* 1))
(set! *envs-closed-slots*
(+ *envs-closed-slots* (vm-vector-length env))))
((= reason (enum copy overflow))
(set! *envs-overflow* (+ *envs-overflow* 1))
(set! *envs-overflow-slots*
(+ *envs-overflow-slots* (vm-vector-length env))))
((= reason (enum copy preserve))
(set! *envs-preserved* (+ *envs-preserved* 1))
(set! *envs-preserved-slots*
(+ *envs-preserved-slots* (vm-vector-length env))))))
(define (add-preserve-cont-stats new reason)
(cond ((not collect-statistics?)
(unassigned))
((= reason (enum copy overflow))
(set! *conts-overflow* (+ *conts-overflow* 1))
(set! *conts-overflow-slots*
(+ *conts-overflow-slots*
(continuation-length new))))
((= reason (enum copy preserve))
(set! *conts-preserved* (+ *conts-preserved* 1))
(set! *conts-preserved-slots*
(+ *conts-preserved-slots*
(continuation-length new))))))
(define (add-copy-cont-from-heap-stats cont)
(cond (collect-statistics?
(set! *conts-from-heap* (+ *conts-from-heap* 1))
(set! *conts-from-heap-slots* (+ *conts-from-heap-slots*
(continuation-length cont))))))
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