(in-package "SB!IMPL")

;;;; COMPARE-AND-SWAP
;;;;
;;;; SB-EXT:COMPARE-AND-SWAP is the public API for now.
;;;;
;;;; Internally our interface has CAS, GET-CAS-EXPANSION, DEFINE-CAS-EXPANDER,
;;;; DEFCAS, and #'(CAS ...) functions -- making things mostly isomorphic with
;;;; SETF.

;;; This is what it all comes down to.
(def!macro cas (place old new &environment env)
  #!+sb-doc
  "Synonym for COMPARE-AND-SWAP.

Additionally DEFUN, DEFGENERIC, DEFMETHOD, FLET, and LABELS can be also used to
define CAS-functions analogously to SETF-functions:

  (defvar *foo* nil)

  (defun (cas foo) (old new)
    (cas (symbol-value '*foo*) old new))

First argument of a CAS function is the expected old value, and the second
argument of is the new value. Note that the system provides no automatic
atomicity for CAS functions, nor can it verify that they are atomic: it is up
to the implementor of a CAS function to ensure its atomicity.

EXPERIMENTAL: Interface subject to change."
  (multiple-value-bind (temps place-args old-temp new-temp cas-form)
      (get-cas-expansion place env)
    `(let* (,@(mapcar #'list temps place-args)
            (,old-temp ,old)
            (,new-temp ,new))
       ,cas-form)))

(defun get-cas-expansion (place &optional environment)
  #!+sb-doc
  "Analogous to GET-SETF-EXPANSION. Returns the following six values:

 * list of temporary variables

 * list of value-forms whose results those variable must be bound

 * temporary variable for the old value of PLACE

 * temporary variable for the new value of PLACE

 * form using the aforementioned temporaries which performs the
   compare-and-swap operation on PLACE

 * form using the aforementioned temporaries with which to perform a volatile
   read of PLACE

Example:

  (get-cas-expansion '(car x))
  ; => (#:CONS871), (X), #:OLD872, #:NEW873,
  ;    (SB-KERNEL:%COMPARE-AND-SWAP-CAR #:CONS871 #:OLD872 :NEW873).
  ;    (CAR #:CONS871)

  (defmacro my-atomic-incf (place &optional (delta 1) &environment env)
    (multiple-value-bind (vars vals old new cas-form read-form)
        (get-cas-expansion place env)
     (let ((delta-value (gensym \"DELTA\")))
       `(let* (,@(mapcar 'list vars vals)
               (,old ,read-form)
               (,delta-value ,delta)
               (,new (+ ,old ,delta-value)))
          (loop until (eq ,old (setf ,old ,cas-form))
                do (setf ,new (+ ,old ,delta-value)))
          ,new))))

EXPERIMENTAL: Interface subject to change."
  ;; FIXME: this seems wrong on two points:
  ;; 1. if TRULY-THE had a CAS expander (which it doesn't) we'd want
  ;;    to use %MACROEXPAND[-1] so as not to lose the "truly-the"-ness
  ;; 2. if both a CAS expander and a macro exist, the CAS expander
  ;;    should be preferred before macroexpanding (just like SETF does)
    (let ((expanded (sb!xc:macroexpand place environment)))
      (flet ((invalid-place ()
           (error "Invalid place to CAS: ~S -> ~S" place expanded)))
      (unless (consp expanded)
        ;; FIXME: Allow (CAS *FOO* <OLD> <NEW>), maybe?
        (invalid-place))
      (let ((name (car expanded)))
        (unless (symbolp name)
          (invalid-place))
        (acond
            ((info :cas :expander name)
            ;; CAS expander.
             (funcall it expanded environment))

            ;; Structure accessor
            ((structure-instance-accessor-p name)
             (expand-structure-slot-cas it name expanded))

            ;; CAS function
            (t
             (with-unique-names (old new)
               (let ((vars nil)
                     (vals nil)
                     (args nil))
                 (dolist (x (reverse (cdr expanded)))
                   (cond ((constantp x environment)
                          (push x args))
                         (t
                          (let ((tmp (gensymify x)))
                            (push tmp args)
                            (push tmp vars)
                            (push x vals)))))
                 (values vars vals old new
                         `(funcall #'(cas ,name) ,old ,new ,@args)
                         `(,name ,@args))))))))))

(defun expand-structure-slot-cas (info name place)
  (let* ((dd (car info))
         (structure (dd-name dd))
         (slotd (cdr info))
         (index (dsd-index slotd))
         (type (dsd-type slotd)))
    (unless (eq t (dsd-raw-type slotd))
      (error "Cannot use COMPARE-AND-SWAP with structure accessor ~
                for a typed slot: ~S"
             place))
    (when (dsd-read-only slotd)
      (error "Cannot use COMPARE-AND-SWAP with structure accessor ~
                for a read-only slot: ~S"
             place))
    (destructuring-bind (op arg) place
      (aver (eq op name))
      (with-unique-names (instance old new)
        (values (list instance)
                (list arg)
                old
                new
                `(truly-the (values ,type &optional)
                            (%compare-and-swap-instance-ref
                             (the ,structure ,instance)
                             ,index
                             (the ,type ,old)
                             (the ,type ,new)))
                `(,op ,instance))))))

(def!macro define-cas-expander (accessor lambda-list &body body)
  #!+sb-doc
  "Analogous to DEFINE-SETF-EXPANDER. Defines a CAS-expansion for ACCESSOR.
BODY must return six values as specified in GET-CAS-EXPANSION.

Note that the system provides no automatic atomicity for CAS expansion, nor
can it verify that they are atomic: it is up to the implementor of a CAS
expansion to ensure its atomicity.

EXPERIMENTAL: Interface subject to change."
  `(eval-when (:compile-toplevel :load-toplevel :execute)
     (setf (info :cas :expander ',accessor)
           ,(make-macro-lambda `(cas-expand ,accessor) lambda-list body
                               'define-cas-expander accessor))))

;; FIXME: this interface is bogus - short-form DEFSETF/CAS does not
;; want a lambda-list. You just blindly substitute
;;  (CAS (PLACE arg1 ... argN) old new) -> (F arg1 ... argN old new).
;; What role can this lambda-list have when there is no user-provided
;; code to read the variables?
;; And as mentioned no sbcl-devel, &REST is beyond bogus, it's broken.
;;
(def!macro defcas (accessor lambda-list function &optional docstring)
  #!+sb-doc
  "Analogous to short-form DEFSETF. Defines FUNCTION as responsible
for compare-and-swap on places accessed using ACCESSOR. LAMBDA-LIST
must correspond to the lambda-list of the accessor.

Note that the system provides no automatic atomicity for CAS expansions
resulting from DEFCAS, nor can it verify that they are atomic: it is up to the
user of DEFCAS to ensure that the function specified is atomic.

EXPERIMENTAL: Interface subject to change."
  (multiple-value-bind (llks reqs opts rest)
      (parse-lambda-list lambda-list
                         :accept (lambda-list-keyword-mask '(&optional &rest))
                         :context "a DEFCAS lambda-list")
    (declare (ignore llks))
    `(define-cas-expander ,accessor ,lambda-list
       ,@(when docstring (list docstring))
       ;; FIXME: if a &REST arg is present, this is really weird.
       (let ((temps (mapcar #'gensymify ',(append reqs opts rest)))
             (args (list ,@(append reqs opts rest)))
             (old (gensym "OLD"))
             (new (gensym "NEW")))
         (values temps
                 args
                 old
                 new
                 `(,',function ,@temps ,old ,new)
                 `(,',accessor ,@temps))))))

(def!macro compare-and-swap (place old new)
  #!+sb-doc
  "Atomically stores NEW in PLACE if OLD matches the current value of PLACE.
Two values are considered to match if they are EQ. Returns the previous value
of PLACE: if the returned value is EQ to OLD, the swap was carried out.

PLACE must be an CAS-able place. Built-in CAS-able places are accessor forms
whose CAR is one of the following:

 CAR, CDR, FIRST, REST, SVREF, SYMBOL-PLIST, SYMBOL-VALUE, SVREF, SLOT-VALUE
 SB-MOP:STANDARD-INSTANCE-ACCESS, SB-MOP:FUNCALLABLE-STANDARD-INSTANCE-ACCESS,

or the name of a DEFSTRUCT created accessor for a slot whose declared type is
either FIXNUM or T. Results are unspecified if the slot has a declared type
other then FIXNUM or T.

In case of SLOT-VALUE, if the slot is unbound, SLOT-UNBOUND is called unless
OLD is EQ to SB-PCL:+SLOT-UNBOUND+ in which case SB-PCL:+SLOT-UNBOUND+ is
returned and NEW is assigned to the slot. Additionally, the results are
unspecified if there is an applicable method on either
SB-MOP:SLOT-VALUE-USING-CLASS, (SETF SB-MOP:SLOT-VALUE-USING-CLASS), or
SB-MOP:SLOT-BOUNDP-USING-CLASS.

Additionally, the PLACE can be a anything for which a CAS-expansion has been
specified using DEFCAS, DEFINE-CAS-EXPANDER, or for which a CAS-function has
been defined. (See SB-EXT:CAS for more information.)
"
  `(cas ,place ,old ,new))

;;; Out-of-line definitions for various primitive cas functions.
(macrolet ((def (name lambda-list ref &optional set)
             #!+compare-and-swap-vops
             (declare (ignore ref set))
             `(defun ,name (,@lambda-list old new)
                #!+compare-and-swap-vops
                (,name ,@lambda-list old new)
                #!-compare-and-swap-vops
                (progn
                  #!+sb-thread
                  ,(error "No COMPARE-AND-SWAP-VOPS on a threaded build?")
                  #!-sb-thread
                  (let ((current (,ref ,@lambda-list)))
                    ;; Shouldn't this be inside a WITHOUT-INTERRUPTS ?
                    (when (eq current old)
                      ,(if set
                           `(,set ,@lambda-list new)
                           `(setf (,ref ,@lambda-list) new)))
                    current)))))
  (def %compare-and-swap-car (cons) car)
  (def %compare-and-swap-cdr (cons) cdr)
  (def %compare-and-swap-instance-ref (instance index) %instance-ref %instance-set)
  (def %compare-and-swap-symbol-info (symbol) symbol-info)
  (def %compare-and-swap-symbol-value (symbol) symbol-value)
  (def %compare-and-swap-svref (vector index) svref))

;; Atomic increment/decrement ops on tagged storage cells (as contrasted with
;; specialized arrays and raw structure slots) are defined in terms of CAS.

;; This code would be more concise if workable versions
;; of +-MODFX, --MODFX were defined generically.
#-sb-xc-host
(macrolet ((modular (fun a b)
             #!+(or x86 x86-64)
             `(,(let ((*package* (find-package "SB!VM")))
                  (symbolicate fun "-MODFX"))
                ,a ,b)
             #!-(or x86 x86-64)
             ;; algorithm of https://graphics.stanford.edu/~seander/bithacks
             `(let ((res (logand (,fun ,a ,b)
                                 (ash sb!ext:most-positive-word
                                      (- sb!vm:n-fixnum-tag-bits))))
                    (m (ash 1 (1- sb!vm:n-fixnum-bits))))
                (- (logxor res m) m))))

  ;; Atomically frob the CAR or CDR of a cons, or a symbol-value.
  ;; The latter will be a global value because the ATOMIC-INCF/DECF
  ;; macros work on a symbol only if it is known global.
  (macrolet ((def-frob (name op type slot)
               `(defun ,name (place delta)
                  (declare (type ,type place) (type fixnum delta))
                  (loop (let ((old (the fixnum (,slot place))))
                          (when (eq (cas (,slot place) old
                                         (modular ,op old delta)) old)
                            (return old)))))))
    (def-frob %atomic-inc-symbol-global-value + symbol symbol-value)
    (def-frob %atomic-dec-symbol-global-value - symbol symbol-value)
    (def-frob %atomic-inc-car + cons car)
    (def-frob %atomic-dec-car - cons car)
    (def-frob %atomic-inc-cdr + cons cdr)
    (def-frob %atomic-dec-cdr - cons cdr)))
