;;; -*- Package: NEW-ASSEM -*-
;;;
;;; **********************************************************************
;;; This code was written as part of the CMU Common Lisp project at
;;; Carnegie Mellon University, and has been placed in the public domain.
;;;
(ext:file-comment

  "$Header: src/compiler/new-assem.lisp $")

;;;
;;; **********************************************************************
;;;
;;; Effecient retargetable scheduling assembler.
;;;
;;; Written by William Lott.
;;;
(in-package :new-assem)

(in-package :c)

(intl:textdomain "cmucl")

(import '(branch flushable) :new-assem)
(import '(sset-element sset make-sset do-elements
	  sset-adjoin sset-delete sset-empty)
	:new-assem)

(in-package :new-assem)

(export '(emit-byte emit-skip emit-back-patch emit-chooser emit-postit

	  define-emitter define-instruction define-instruction-macro

	  def-assembler-params branch flushable variable-length

	  segment make-segment segment-name segment-collect-dynamic-statistics

	  assemble align inst without-scheduling 

	  label label-p gen-label emit-label label-position
	  append-segment finalize-segment
	  segment-map-output release-segment))


;;;; Assembly control parameters.

(defstruct (assem-params
	    (:print-function %print-assem-params))
  (backend (ext:required-argument) :type c::backend)
  (scheduler-p nil :type (member t nil))

  (instructions (make-hash-table :test #'equal) :type hash-table)
  (max-locations 0 :type index))

;;;
(c::defprinter assem-params
  (backend :prin1 (c:backend-name backend)))

;;; DEF-ASSEMBLER-PARAMS -- Interface.
;;;
(defmacro def-assembler-params (&rest options)

  "Set up the assembler."

  `(eval-when (:compile-toplevel :load-toplevel :execute)

     (setf (c:backend-assembler-params c:*target-backend*)
	   (make-assem-params :backend c:*target-backend*
			      ,@options))))


;;;; Constants.

;;; ASSEMBLY-UNIT-BITS -- Number of bits in the minimum assembly unit,
;;; (also refered to as a ``byte'').  Hopefully, different instruction

;;; sets won't require changing this.

;;; 
(defconstant assembly-unit-bits 8)

(deftype assembly-unit ()
  `(unsigned-byte ,assembly-unit-bits))

;;; OUTPUT-BLOCK-SIZE -- The size (in bytes) to use per output block.  Each
;;; output block is a chunk of raw memory, pointed to by a sap.
;;;
(defconstant output-block-size (* 8 1024))

(deftype output-block-index ()
  `(integer 0 ,output-block-size))

;;; MAX-ALIGNMENT -- The maximum alignment we can guarentee given the object
;;; format.  If the loader only loads objects 8-byte aligned, we can't do

;;; any better than that ourselves.

;;;
(defconstant max-alignment 3)

(deftype alignment ()
  `(integer 0 ,max-alignment))

;;; MAX-INDEX -- The maximum an index will ever become.  Well, actually,
;;; just a bound on it so we can define a type.  There is no real hard
;;; limit on indexes, but we will run out of memory sometime.
;;; 
(defconstant max-index (1- most-positive-fixnum))

(deftype index ()
  `(integer 0 ,max-index))

;;; MAX-POSN -- Like MAX-INDEX, except for positions.
;;; 
(defconstant max-posn (1- most-positive-fixnum))

(deftype posn ()
  `(integer 0 ,max-posn))



;;;; The SEGMENT structure.

;;; SEGMENT -- This structure holds the state of the assembler.
;;; 
(defstruct (segment
	    (:print-function %print-segment)
	    (:constructor make-segment (&key name run-scheduler inst-hook)))
  ;;
  ;; The name of this segment.  Only using in trace files.
  (name "Unnamed" :type simple-base-string)
  ;;

  ;; Whether or not run the scheduler.  Note: if the instruction defintions

  ;; were not compiled with the scheduler turned on, this has no effect.
  (run-scheduler nil)
  ;;
  ;; If a function, then it is funcalled for each inst emitted with the
  ;; segment, the VOP, the name of the inst (as a string), and the inst
  ;; arguments.
  (inst-hook nil :type (or function null))
  ;;
  ;; Where to deposit the next byte.
  (fill-pointer (system:int-sap 0) :type system:system-area-pointer)
  ;;
  ;; Where the current output block ends.  If fill-pointer is ever sap= to
  ;; this, don't deposit a byte.  Move the fill pointer into a new block.
  (block-end (system:int-sap 0) :type system:system-area-pointer)
  ;;
  ;; What position does this correspond to.  Initially, positions and indexes
  ;; are the same, but after we start collapsing choosers, positions can change
  ;; while indexes stay the same.
  (current-posn 0 :type posn)
  ;;
  ;; Were in the output blocks are we currently outputing.
  (current-index 0 :type index)
  ;;
  ;; A vector of the output blocks.
  (output-blocks (make-array 4 :initial-element nil) :type simple-vector)
  ;;
  ;; A list of all the annotations that have been output to this segment.
  (annotations nil :type list)
  ;;
  ;; A pointer to the last cons cell in the annotations list.  This is
  ;; so we can quickly add things to the end of the annotations list.
  (last-annotation nil :type list)
  ;;
  ;; The number of bits of alignment at the last time we synchronized.
  (alignment max-alignment :type alignment)
  ;;
  ;; The position the last time we synchronized.
  (sync-posn 0 :type posn)
  ;;
  ;; The posn and index everything ends at.  This is not maintained while the
  ;; data is being generated, but is filled in after.  Basically, we copy
  ;; current-posn and current-index so that we can trash them while processing
  ;; choosers and back-patches.
  (final-posn 0 :type posn)
  (final-index 0 :type index)
  ;;
  ;; *** State used by the scheduler during instruction queueing.
  ;;

  ;; List of postit's.  These are accumulated between instructions.
  (postits nil :type list)
  ;;

  ;; ``Number'' for last instruction queued.  Used only to supply insts
  ;; with unique sset-element-number's.
  (inst-number 0 :type index)
  ;;
  ;; Simple-Vectors mapping locations to the instruction that reads them and
  ;; instructions that write them.
  (readers (make-array (assem-params-max-locations
			(c:backend-assembler-params c:*backend*))
		       :initial-element nil)
	   :type simple-vector)
  (writers (make-array (assem-params-max-locations
			(c:backend-assembler-params c:*backend*))
		       :initial-element nil)
	   :type simple-vector)

  ;;
  ;; The number of additional cycles before the next control transfer, or NIL
  ;; if a control transfer hasn't been queued.  When a delayed branch is
  ;; queued, this slot is set to the delay count.
  (branch-countdown nil :type (or null (and fixnum unsigned-byte)))
  ;;
  ;; *** These two slots are used both by the queuing noise and the
  ;; scheduling noise.
  ;;

  ;; All the instructions that are pending and don't have any unresolved
  ;; dependents.  We don't list branches here even if they would otherwise
  ;; qualify.  They are listed above.

  ;;

  (emittable-insts-sset (make-sset) :type sset)

  ;;
  ;; List of queued branches.  We handle these specially, because they have to
  ;; be emitted at a specific place (e.g. one slot before the end of the
  ;; block).
  (queued-branches nil :type list)
  ;;
  ;; *** State used by the scheduler duing instruction scheduling.
  ;;
  ;; The instructions who would have had a read dependent removed if it were
  ;; not for a delay slot.  This is a list of lists.  Each element in the
  ;; top level list corresponds to yet another cycle of delay.  Each element
  ;; in the second level lists is a dotted pair, holding the dependency
  ;; instruction and the dependent to remove.

  (delayed nil :type list)
  ;;
  ;; The emittable insts again, except this time as a list sorted by depth.

  (emittable-insts-queue nil :type list)
  ;;
  ;; Whether or not to collect dynamic statistics.  This is just the same as
  ;; *collect-dynamic-statistics* but is faster to reference.
  (collect-dynamic-statistics nil))

(c::defprinter segment name)


;;;; Structures/types used by the scheduler.

(c:def-boolean-attribute instruction
  ;;
  ;; This attribute is set if the scheduler can freely flush this instruction
  ;; if it thinks it is not needed.  Examples are NOP and instructions that
  ;; have no side effect not described by the writes.
  flushable
  ;;
  ;; This attribute is set when an instruction can cause a control transfer.
  ;; For test instructions, the delay is used to determine how many
  ;; instructions follow the branch.
  branch

  ;;
  ;; This attribute indicates that this ``instruction'' can be variable length,
  ;; and therefore better never be used in a branch delay slot.
  variable-length

  )

(defstruct (instruction

	    (:include sset-element)

	    (:print-function %print-instruction)
	    (:conc-name inst-)

	    (:constructor make-instruction (number emitter attributes delay)))

  ;;
  ;; The function to envoke to actually emit this instruction.  Gets called
  ;; with the segment as its one argument.
  (emitter (required-argument) :type (or null function))
  ;;
  ;; The attributes of this instruction.
  (attributes (instruction-attributes) :type c:attributes)
  ;;
  ;; Number of instructions or cycles of delay before additional instructions
  ;; can read our writes.
  (delay 0 :type (and fixnum unsigned-byte))
  ;;
  ;; The maximum number of instructions in the longest dependency chain from
  ;; this instruction to one of the independent instructions.  This is used
  ;; as a heuristic at to which instructions should be scheduled first.
  (depth nil :type (or null (and fixnum unsigned-byte)))
  ;;
  ;; ** When trying remember which of the next four is which, note that the
  ;; ``read'' or ``write'' always referes to the dependent (second)
  ;; instruction.
  ;;

  ;; Instructions whos writes this instruction tries to read.
  (read-dependencies (make-sset) :type sset)

  ;;

  ;; Instructions whos writes or reads are overwritten by this instruction.
  (write-dependencies (make-sset) :type sset)

  ;;

  ;; Instructions who write what we read or write.
  (write-dependents (make-sset) :type sset)

  ;;

  ;; Instructions who read what we write.
  (read-dependents (make-sset) :type sset))

;;;

#+debug (defvar *inst-ids* (make-hash-table :test #'eq))
#+debug (defvar *next-inst-id* 0)

(defun %print-instruction (inst stream depth)
  (declare (ignore depth))
  (print-unreadable-object (inst stream :type t :identity t)

    #+debug

    (princ (or (gethash inst *inst-ids*)
	       (setf (gethash inst *inst-ids*)
		     (incf *next-inst-id*)))
	   stream)

    (format stream #+debug " emitter=~S" #-debug "emitter=~S"

	    (let ((emitter (inst-emitter inst)))
	      (if emitter
		  (multiple-value-bind
		      (lambda lexenv-p name)
		      (function-lambda-expression emitter)
		    (declare (ignore lambda lexenv-p))
		    name)

		  "<flushed>")))

    (when (inst-depth inst)
      (format stream ", depth=~D" (inst-depth inst)))))

#+debug

(defun reset-inst-ids ()
  (clrhash *inst-ids*)
  (setf *next-inst-id* 0))


;;;; The scheduler itself.

;;; WITHOUT-SCHEDULING -- interface.
;;;

(defmacro without-scheduling ((&optional (segment '(%%current-segment%%)))

			      &body body)

  "Execute BODY (as a progn) without scheduling any of the instructions

   generated inside it.  DO NOT throw or return-from out of it."
  (let ((var (gensym))
	(seg (gensym)))
    `(let* ((,seg ,segment)
	    (,var (segment-run-scheduler ,seg)))
       (when ,var
	 (schedule-pending-instructions ,seg)
	 (setf (segment-run-scheduler ,seg) nil))
       ,@body
       (setf (segment-run-scheduler ,seg) ,var))))

(defmacro note-dependencies ((segment inst) &body body)
  (ext:once-only ((segment segment) (inst inst))
    `(macrolet ((reads (loc) `(note-read-dependency ,',segment ,',inst ,loc))
		(writes (loc &rest keys)
		  `(note-write-dependency ,',segment ,',inst ,loc ,@keys)))
       ,@body)))

(defun note-read-dependency (segment inst read)

  (multiple-value-bind (loc-num size)
      (c:location-number read)

    #+debug (format *trace-output* (intl:gettext "~&~S reads ~S[~D for ~D]~%")

	    inst read loc-num size)
    (when loc-num
      ;; Iterate over all the locations for this TN.
      (do ((index loc-num (1+ index))
	   (end-loc (+ loc-num (or size 1))))
	  ((>= index end-loc))
	(declare (type (mod 2048) index end-loc))
	(let ((writers (svref (segment-writers segment) index)))
	  (when writers
	    ;; The inst that wrote the value we want to read must have
	    ;; completed.
	    (let ((writer (car writers)))
	      (sset-adjoin writer (inst-read-dependencies inst))
	      (sset-adjoin inst (inst-read-dependents writer))
	      (sset-delete writer (segment-emittable-insts-sset segment))
	      ;; And it must have been completed *after* all other
	      ;; writes to that location.  Actually, that isn't quite
	      ;; true.  Each of the earlier writes could be done
	      ;; either before this last write, or after the read, but
	      ;; we have no way of representing that.
	      (dolist (other-writer (cdr writers))
		(sset-adjoin other-writer (inst-write-dependencies writer))
		(sset-adjoin writer (inst-write-dependents other-writer))
		(sset-delete other-writer
			     (segment-emittable-insts-sset segment))))
	    ;; And we don't need to remember about earlier writes any
	    ;; more.  Shortening the writers list means that we won't
	    ;; bother generating as many explicit arcs in the graph.
	    (setf (cdr writers) nil)))
	(push inst (svref (segment-readers segment) index)))))

  (ext:undefined-value))

(defun note-write-dependency (segment inst write &key partially)

  (multiple-value-bind (loc-num size)
      (c:location-number write)

    #+debug (format *trace-output* (intl:gettext "~&~S writes ~S[~D for ~D]~%")

	    inst write loc-num size)
    (when loc-num
      ;; Iterate over all the locations for this TN.
      (do ((index loc-num (1+ index))
	   (end-loc (+ loc-num (or size 1))))
	  ((>= index end-loc))
	(declare (type (mod 2048) index end-loc))
	;; All previous reads of this location must have completed.
	(dolist (prev-inst (svref (segment-readers segment) index))
	  (unless (eq prev-inst inst)
	    (sset-adjoin prev-inst (inst-write-dependencies inst))
	    (sset-adjoin inst (inst-write-dependents prev-inst))
	    (sset-delete prev-inst (segment-emittable-insts-sset segment))))
	(when partially
	  ;; All previous writes to the location must have completed.
	  (dolist (prev-inst (svref (segment-writers segment) index))
	    (sset-adjoin prev-inst (inst-write-dependencies inst))
	    (sset-adjoin inst (inst-write-dependents prev-inst))
	    (sset-delete prev-inst (segment-emittable-insts-sset segment)))
	  ;; And we can forget about remembering them, because
	  ;; depending on us is as good as depending on them.
	  (setf (svref (segment-writers segment) index) nil))
	(push inst (svref (segment-writers segment) index)))))

  (ext:undefined-value))

;;; QUEUE-INST -- internal.
;;;
;;; This routine is called by due to uses of the INST macro when the scheduler

;;; is turned on.  The change to the dependency graph has already been
;;; computed, so we just have to check to see if the basic block is terminated.

;;; 

(defun queue-inst (segment inst)

  #+debug (format *trace-output* (intl:gettext "~&Queuing ~S~%") inst)

  #+debug

  (format *trace-output* (intl:gettext "  reads ~S~%  writes ~S~%")

	  (ext:collect ((reads))
	    (do-elements (read (inst-read-dependencies inst))
	      (reads read))
	    (reads))
	  (ext:collect ((writes))
	    (do-elements (write (inst-write-dependencies inst))
	      (writes write))
	    (writes)))

  (assert (segment-run-scheduler segment))

  (let ((countdown (segment-branch-countdown segment)))

    (when countdown

      (decf countdown)
      (assert (not (instruction-attributep (inst-attributes inst)
					   variable-length))))

    (cond ((instruction-attributep (inst-attributes inst) branch)
	   (unless countdown
	     (setf countdown (inst-delay inst)))
	   (push (cons countdown inst)
		 (segment-queued-branches segment)))
	  (t
	   (sset-adjoin inst (segment-emittable-insts-sset segment))))
    (when countdown

      (setf (segment-branch-countdown segment) countdown)
      (when (zerop countdown)
	(schedule-pending-instructions segment))))

  (ext:undefined-value))

;;; SCHEDULE-PENDING-INSTRUCTIONS -- internal.
;;;
;;; Emit all the pending instructions, and reset any state.  This is called
;;; whenever we hit a label (i.e. an entry point of some kind) and when the
;;; user turns the scheduler off (otherwise, the queued instructions would
;;; sit there until the scheduler was turned back on, and emitted in the
;;; wrong place).
;;; 
(defun schedule-pending-instructions (segment)
  (assert (segment-run-scheduler segment))
  ;;
  ;; Quick blow-out if nothing to do.

  (when (and (sset-empty (segment-emittable-insts-sset segment))
	     (null (segment-queued-branches segment)))

    (return-from schedule-pending-instructions
		 (ext:undefined-value)))
  ;;

  #+debug

  (format *trace-output* (intl:gettext "~&Scheduling pending instructions...~%"))

  ;;

  ;; Note that any values live at the end of the block have to be computed
  ;; last.

  (let ((emittable-insts (segment-emittable-insts-sset segment))
	(writers (segment-writers segment)))
    (dotimes (index (length writers))
      (let* ((writer (svref writers index))
	     (inst (car writer))
	     (overwritten (cdr writer)))
	(when writer
	  (when overwritten
	    (let ((write-dependencies (inst-write-dependencies inst)))
	      (dolist (other-inst overwritten)
		(sset-adjoin inst (inst-write-dependents other-inst))
		(sset-adjoin other-inst write-dependencies)
		(sset-delete other-inst emittable-insts))))
	  ;; If the value is live at the end of the block, we can't flush it.
	  (setf (instruction-attributep (inst-attributes inst) flushable)
		nil)))))

  ;;

  ;; Grovel through the entire graph in the forward direction finding all
  ;; the leaf instructions.
  (labels ((grovel-inst (inst)

	     (let ((max 0))
	       (do-elements (dep (inst-write-dependencies inst))

		 (let ((dep-depth (or (inst-depth dep) (grovel-inst dep))))
		   (when (> dep-depth max)
		     (setf max dep-depth))))

	       (do-elements (dep (inst-read-dependencies inst))

		 (let ((dep-depth
			(+ (or (inst-depth dep) (grovel-inst dep))
			   (inst-delay dep))))
		   (when (> dep-depth max)
		     (setf max dep-depth))))

	       (cond ((and (sset-empty (inst-read-dependents inst))

			   (instruction-attributep (inst-attributes inst)

						   flushable))

		      #+debug

		      (format *trace-output* (intl:gettext "Flushing ~S~%") inst)

		      (setf (inst-emitter inst) nil)
		      (setf (inst-depth inst) max))
		     (t

		      (setf (inst-depth inst) max))))))
    (let ((emittable-insts nil)

	  (delayed nil))

      (do-elements (inst (segment-emittable-insts-sset segment))

	(grovel-inst inst)
	(if (zerop (inst-delay inst))

	    (push inst emittable-insts)

	    (setf delayed
		  (add-to-nth-list delayed inst (1- (inst-delay inst))))))

      (setf (segment-emittable-insts-queue segment)
	    (sort emittable-insts #'> :key #'inst-depth))

      (setf (segment-delayed segment) delayed))
    (dolist (branch (segment-queued-branches segment))
      (grovel-inst (cdr branch))))

  #+debug

  (format *trace-output* (intl:gettext "Queued branches: ~S~%")

	  (segment-queued-branches segment))

  #+debug

  (format *trace-output* (intl:gettext "Initially emittable: ~S~%")

	  (segment-emittable-insts-queue segment))

  #+debug

  (format *trace-output* (intl:gettext "Initially delayed: ~S~%")

	  (segment-delayed segment))
  ;;
  ;; Accumulate the results in reverse order.  Well, actually, this list will
  ;; be in forward order, because we are generating the reverse order in
  ;; reverse.
  (let ((results nil))
    ;;
    ;; Schedule all the branches in their exact locations.

    (let ((insts-from-end (segment-branch-countdown segment)))

      (dolist (branch (segment-queued-branches segment))

	(let ((inst (cdr branch)))

	  (dotimes (i (- (car branch) insts-from-end))

	    ;; Each time through this loop we need to emit another instruction.
	    ;; First, we check to see if there is any instruction that must
	    ;; be emitted before (i.e. must come after) the branch inst.  If
	    ;; so, emit it.  Otherwise, just pick one of the emittable insts.
	    ;; If there is nothing to do, the emit a nop.
	    ;; ### Note: despite the fact that this is a loop, it really won't
	    ;; work for repetitions other then zero and one.  For example, if
	    ;; the branch has two dependents and one of them dpends on the
	    ;; other, then the stuff that grabs a dependent could easily
	    ;; grab the wrong one.  But I don't feel like fixing this because
	    ;; it doesn't matter for any of the architectures we are using
	    ;; or plan on using.

	    (flet ((maybe-schedule-dependent (dependents)

		     (do-elements (inst dependents)

		       ;; If do-elements enters the body, then there is a
		       ;; dependent.  Emit it.

		       (note-resolved-dependencies segment inst)

		       ;; Remove it from the emittable insts.

		       (setf (segment-emittable-insts-queue segment)
			     (delete inst
				     (segment-emittable-insts-queue segment)
				     :test #'eq))

		       ;; And if it was delayed, removed it from the delayed
		       ;; list.  This can happen if there is a load in a
		       ;; branch delay slot.
		       (block scan-delayed
			 (do ((delayed (segment-delayed segment)
				       (cdr delayed)))
			     ((null delayed))
			   (do ((prev nil cons)
				(cons (car delayed) (cdr cons)))
			       ((null cons))
			     (when (eq (car cons) inst)
			       (if prev
				   (setf (cdr prev) (cdr cons))
				   (setf (car delayed) (cdr cons)))
			       (return-from scan-delayed nil)))))
		       ;; And return it.

		       (return inst))))

	      (let ((fill (or (maybe-schedule-dependent
			       (inst-read-dependents inst))
			      (maybe-schedule-dependent
			       (inst-write-dependents inst))
 			      (schedule-one-inst segment t)
			      :nop)))
		#+debug

		(format *trace-output* (intl:gettext "Filling branch delay slot with ~S~%")

			fill)
		(push fill results)))

	    (advance-one-inst segment)
	    (incf insts-from-end))

	  (note-resolved-dependencies segment inst)

	  (push inst results)
	  #+debug

	  (format *trace-output* (intl:gettext "Emitting ~S~%") inst)

	  (advance-one-inst segment))))

    ;;
    ;; Keep scheduling stuff until we run out.
    (loop

      (let ((inst (schedule-one-inst segment nil)))

	(unless inst
	  (return))
	(push inst results)
	(advance-one-inst segment)))
    ;;
    ;; Now call the emitters, but turn the scheduler off for the duration.
    (setf (segment-run-scheduler segment) nil)
    (dolist (inst results)
      (if (eq inst :nop)

	  (c:emit-nop segment)

	  (funcall (inst-emitter inst) segment)))
    (setf (segment-run-scheduler segment) t))
  ;;
  ;; Clear out any residue left over.

  (setf (segment-inst-number segment) 0)

  (setf (segment-queued-branches segment) nil)
  (setf (segment-branch-countdown segment) nil)

  (setf (segment-emittable-insts-sset segment) (make-sset))
  (fill (segment-readers segment) nil)
  (fill (segment-writers segment) nil)

  ;;
  ;; That's all folks.
  (ext:undefined-value))

;;; ADD-TO-NTH-LIST -- internal.
;;;
;;; Utility for maintaining the segment-delayed list.  We cdr down list
;;; n times (extending it if necessary) and then push thing on into the car
;;; of that cons cell.
;;; 
(defun add-to-nth-list (list thing n)
  (do ((cell (or list (setf list (list nil)))
	     (or (cdr cell) (setf (cdr cell) (list nil))))
       (i n (1- i)))
      ((zerop i)
       (push thing (car cell))
       list)))

;;; SCHEDULE-ONE-INST -- internal.
;;;
;;; Find the next instruction to schedule and return it after updating
;;; any dependency information.  If we can't do anything useful right
;;; now, but there is more work to be done, return :NOP to indicate that
;;; a nop must be emitted.  If we are all done, return NIL.
;;; 

(defun schedule-one-inst (segment delay-slot-p)
  (do ((prev nil remaining)
       (remaining (segment-emittable-insts-queue segment) (cdr remaining)))
      ((null remaining))
    (let ((inst (car remaining)))
      (unless (and delay-slot-p
		   (instruction-attributep (inst-attributes inst)
					   variable-length))
	;; We've got us a live one here.  Go for it.
	#+debug

	(format *Trace-output* (intl:gettext "Emitting ~S~%") inst)

	;; Delete it from the list of insts.
	(if prev
	    (setf (cdr prev) (cdr remaining))
	    (setf (segment-emittable-insts-queue segment)
		  (cdr remaining)))
	;; Note that this inst has been emitted.
	(note-resolved-dependencies segment inst)
	;; And return.
	(return-from schedule-one-inst
		     ;; Are we wanting to flush this instruction?
		     (if (inst-emitter inst)
			 ;; Nope, it's still a go.  So return it.
			 inst
			 ;; Yes, so pick a new one.  We have to start over,
			 ;; because note-resolved-dependencies might have
			 ;; changed the emittable-insts-queue.
			 (schedule-one-inst segment delay-slot-p))))))
  ;; Nothing to do, so make something up.
  (cond ((segment-delayed segment)
	 ;; No emittable instructions, but we have more work to do.  Emit
	 ;; a NOP to fill in a delay slot.

	 #+debug (format *trace-output* (intl:gettext "Emitting a NOP.~%"))

	 :nop)
	(t
	 ;; All done.
	 nil)))

;;; NOTE-RESOLVED-DEPENDENCIES -- internal.
;;;
;;; This function is called whenever an instruction has been scheduled, and we
;;; want to know what possibilities that opens up.  So look at all the
;;; instructions that this one depends on, and remove this instruction from
;;; their dependents list.  If we were the last dependent, then that
;;; dependency can be emitted now.
;;;
(defun note-resolved-dependencies (segment inst)

  (assert (sset-empty (inst-read-dependents inst)))
  (assert (sset-empty (inst-write-dependents inst)))
  (do-elements (dep (inst-write-dependencies inst))

    ;; These are the instructions who have to be completed before our
    ;; write fires.  Doesn't matter how far before, just before.
    (let ((dependents (inst-write-dependents dep)))

      (sset-delete inst dependents)
      (when (and (sset-empty dependents)
		 (sset-empty (inst-read-dependents dep)))

	(insert-emittable-inst segment dep))))

  (do-elements (dep (inst-read-dependencies inst))

    ;; These are the instructions who write values we read.  If there
    ;; is no delay, then just remove us from the dependent list.
    ;; Otherwise, record the fact that in n cycles, we should be
    ;; removed.
    (if (zerop (inst-delay dep))
	(let ((dependents (inst-read-dependents dep)))

	  (sset-delete inst dependents)
	  (when (and (sset-empty dependents)
		     (sset-empty (inst-write-dependents dep)))

	    (insert-emittable-inst segment dep)))
	(setf (segment-delayed segment)
	      (add-to-nth-list (segment-delayed segment)
			       (cons dep inst)
			       (inst-delay dep)))))
  (ext:undefined-value))

;;; ADVANCE-ONE-INST -- internal.
;;;
;;; Process the next entry in segment-delayed.  This is called whenever anyone
;;; emits an instruction.
;;;
(defun advance-one-inst (segment)
  (let ((delayed-stuff (pop (segment-delayed segment))))
    (dolist (stuff delayed-stuff)
      (if (consp stuff)
	  (let* ((dependency (car stuff))
		 (dependent (cdr stuff))
		 (dependents (inst-read-dependents dependency)))

	    (sset-delete dependent dependents)
	    (when (and (sset-empty dependents)
		       (sset-empty (inst-write-dependents dependency)))

	      (insert-emittable-inst segment dependency)))
	  (insert-emittable-inst segment stuff)))))

;;; INSERT-EMITTABLE-INST -- internal.
;;;

;;; Note that inst is emittable by sticking it in the SEGMENT-EMITTABLE-INSTS-
;;; QUEUE list.  We keep the emittable-insts sorted with the largest ``depths''

;;; first.  Except that if INST is a branch, don't bother.  It will be handled
;;; correctly by the branch emitting code in SCHEDULE-PENDING-INSTRUCTIONS.

;;;
(defun insert-emittable-inst (segment inst)

  (unless (instruction-attributep (inst-attributes inst) branch)
    #+debug

    (format *Trace-output* (intl:gettext "Now emittable: ~S~%") inst)

    (do ((my-depth (inst-depth inst))

	 (remaining (segment-emittable-insts-queue segment) (cdr remaining))

	 (prev nil remaining))
	((or (null remaining) (> my-depth (inst-depth (car remaining))))
	 (if prev
	     (setf (cdr prev) (cons inst remaining))

	     (setf (segment-emittable-insts-queue segment)
		   (cons inst remaining))))))

  (ext:undefined-value))


;;;; Structure used during output emission.

;;; ANNOTATION -- Common supertype for all the different kinds of annotations.
;;; 
(defstruct (annotation
	    (:constructor nil))
  ;;
  ;; Where in the raw output stream was this annotation emitted.
  (index 0 :type index)
  ;;
  ;; What position does that correspond to.
  (posn nil :type (or index null)))

;;; LABEL -- Doesn't need any additional information beyond what is in the
;;; annotation structure.
;;; 
(defstruct (label
	    (:include annotation)
	    (:constructor gen-label ())
	    (:print-function %print-label))
  )
;;;
(defun %print-label (label stream depth)
  (declare (ignore depth))
  (if (or *print-escape* *print-readably*)
      (print-unreadable-object (label stream :type t)
	(prin1 (c:label-id label) stream))
      (format stream "L~D" (c:label-id label))))

;;; ALIGNMENT-NOTE -- A constraint on how the output stream must be aligned.
;;; 
(defstruct (alignment-note
	    (:include annotation)
	    (:conc-name alignment-)
	    (:predicate alignment-p)

	    (:constructor make-alignment (bits size fill-byte)))

  ;;
  ;; The minimum number of low-order bits that must be zero.
  (bits 0 :type alignment)
  ;;
  ;; The amount of filler we are assuming this alignment op will take.

  (size 0 :type (integer 0 #.(1- (ash 1 max-alignment))))
  ;;
  ;; The byte used as filling.
  (fill-byte 0 :type (or assembly-unit (signed-byte #.assembly-unit-bits))))

;;; BACK-PATCH -- a reference to someplace that needs to be back-patched when
;;; we actually know what label positions, etc. are.
;;; 
(defstruct (back-patch
	    (:include annotation)
	    (:constructor make-back-patch (size function)))
  ;;
  ;; The area effected by this back-patch.
  (size 0 :type index)
  ;;
  ;; The function to use to generate the real data
  (function nil :type function))

;;; CHOOSER -- Similar to a back-patch, but also an indication that the amount
;;; of stuff output depends on label-positions, etc.  Back-patches can't change
;;; their mind about how much stuff to emit, but choosers can.
;;; 
(defstruct (chooser
	    (:include annotation)
	    (:constructor make-chooser
			  (size alignment maybe-shrink worst-case-fun)))
  ;;
  ;; The worst case size for this chooser.  There is this much space allocated
  ;; in the output buffer.
  (size 0 :type index)
  ;;
  ;; The worst case alignment this chooser is guarenteed to preserve.
  (alignment 0 :type alignment)
  ;;
  ;; The function to call to determine of we can use a shorter sequence.  It
  ;; returns NIL if nothing shorter can be used, or emits that sequence and
  ;; returns T.
  (maybe-shrink nil :type function)
  ;;
  ;; The function to call to generate the worst case sequence.  This is used
  ;; when nothing else can be condensed.
  (worst-case-fun nil :type function))

;;; FILLER -- Used internally when we figure out a chooser or alignment doesn't
;;; really need as much space as we initially gave it.
;;;
(defstruct (filler
	    (:include annotation)
	    (:constructor make-filler (bytes)))
  ;;
  ;; The number of bytes of filler here.
  (bytes 0 :type index))



;;;; Output buffer utility functions.

;;; A list of all the output-blocks we have allocated but aren't using.
;;; We free-list them because allocation more is slow and the garbage collector
;;; doesn't know about them, so it can't be slowed down by use keep ahold of
;;; them.
;;; 
(defvar *available-output-blocks* nil)

;;; A list of all the output-blocks we have ever allocated.  We don't really
;;; need to keep tract of this if RELEASE-OUTPUT-BLOCK were always called,
;;; but...
;;;
(defvar *all-output-blocks* nil)

;;; NEW-OUTPUT-BLOCK -- internal.
;;;
;;; Return a new output block, allocating one if necessary.
;;;
(defun new-output-block ()
  (if *available-output-blocks*
      (pop *available-output-blocks*)
      (let ((block (system:allocate-system-memory output-block-size)))
	(push block *all-output-blocks*)
	block)))

;;; RELEASE-OUTPUT-BLOCK -- internal.
;;;
;;; Return block to the list of avaiable blocks.
;;;
(defun release-output-block (block)
  (push block *available-output-blocks*))

;;; FORGET-OUTPUT-BLOCKS -- internal.
;;;
;;; We call this whenever a core starts up, because system-memory isn't
;;; saves with the core.  If we didn't, we would find our hands full of
;;; bogus SAPs, which would make all sorts of things unhappy.
;;;
(defun forget-output-blocks ()

  (setf *all-output-blocks* nil)

  (setf *available-output-blocks* nil))
;;;
(pushnew 'forget-output-blocks ext:*after-save-initializations*)



;;;; Output functions.

;;; FIND-NEW-FILL-POINTER -- internal.
;;;
;;; Find us a new fill pointer for the current index in segment.  Allocate
;;; any additional storage as necessary.
;;; 
(defun find-new-fill-pointer (segment)
  (declare (type segment segment))
  (let* ((index (segment-current-index segment))
	 (blocks (segment-output-blocks segment))
	 (num-blocks (length blocks)))
    (multiple-value-bind
	(block-num offset)
	(truncate index output-block-size)
      (when (>= block-num num-blocks)

	(setf blocks
	      (adjust-array blocks (+ block-num 3) :initial-element nil))

	(setf (segment-output-blocks segment) blocks))
      (let ((block (or (aref blocks block-num)
		       (setf (aref blocks block-num) (new-output-block)))))
	(setf (segment-block-end segment)
	      (system:sap+ block output-block-size))
	(setf (segment-fill-pointer segment) (system:sap+ block offset))))))

;;; EMIT-BYTE -- interface.
;;;
;;; Emit the supplied BYTE to SEGMENT, growing it if necessary.
;;; 
(declaim (inline emit-byte))
(defun emit-byte (segment byte)