src/code/early-setf.lisp

   1 ;;;; SETF and friends (except for stuff defined with COLLECT, which
   2 ;;;; comes later)
   3 ;;;;
   4 ;;;; Note: The expansions for SETF and friends sometimes create
   5 ;;;; needless LET-bindings of argument values. The compiler will
   6 ;;;; remove most of these spurious bindings, so SETF doesn't worry too
   7 ;;;; much about creating them.
   8
   9 ;;;; This software is part of the SBCL system. See the README file for
  10 ;;;; more information.
  11 ;;;;
  12 ;;;; This software is derived from the CMU CL system, which was
  13 ;;;; written at Carnegie Mellon University and released into the
  14 ;;;; public domain. The software is in the public domain and is
  15 ;;;; provided with absolutely no warranty. See the COPYING and CREDITS
  16 ;;;; files for more information.
  17
  18 (in-package "SB!IMPL")
  19
  20 ;;; The inverse for a generalized-variable reference function is stored in
  21 ;;; one of two ways:
  22 ;;;
  23 ;;; A SETF inverse property corresponds to the short form of DEFSETF. It is
  24 ;;; the name of a function takes the same args as the reference form, plus a
  25 ;;; new-value arg at the end.
  26 ;;;
  27 ;;; A SETF method expander is created by the long form of DEFSETF or
  28 ;;; by DEFINE-SETF-EXPANDER. It is a function that is called on the reference
  29 ;;; form and that produces five values: a list of temporary variables, a list
  30 ;;; of value forms, a list of the single store-value form, a storing function,
  31 ;;; and an accessing function.
  32 (declaim (ftype (function (t &optional (or null sb!c::lexenv))) sb!xc:get-setf-expansion))
  33 (defun sb!xc:get-setf-expansion (form &optional environment)
  34   #!+sb-doc
  35   "Return five values needed by the SETF machinery: a list of temporary
  36    variables, a list of values with which to fill them, a list of temporaries
  37    for the new values, the setting function, and the accessing function."
  38   (let (temp)
  39     (cond ((symbolp form)
  40            (multiple-value-bind (expansion expanded)
  41                (sb!xc:macroexpand-1 form environment)
  42              (if expanded
  43                  (sb!xc:get-setf-expansion expansion environment)
  44                  (let ((new-var (gensym)))
  45                    (values nil nil (list new-var)
  46                            `(setq ,form ,new-var) form)))))
  47           ;; Local functions inhibit global SETF methods.
  48           ((and environment
  49                 (let ((name (car form)))
  50                   (dolist (x (sb!c::lexenv-funs environment))
  51                     (when (and (eq (car x) name)
  52                                (not (sb!c::defined-fun-p (cdr x))))
  53                       (return t)))))
  54            (expand-or-get-setf-inverse form environment))
  55           ((setq temp (info :setf :inverse (car form)))
  56            (get-setf-method-inverse form `(,temp) nil))
  57           ((setq temp (info :setf :expander (car form)))
  58            ;; KLUDGE: It may seem as though this should go through
  59            ;; *MACROEXPAND-HOOK*, but the ANSI spec seems fairly explicit
  60            ;; that *MACROEXPAND-HOOK* is a hook for MACROEXPAND-1, not
  61            ;; for macroexpansion in general. -- WHN 19991128
  62            (funcall temp
  63                     form
  64                     ;; As near as I can tell from the ANSI spec,
  65                     ;; macroexpanders have a right to expect an actual
  66                     ;; lexical environment, not just a NIL which is to
  67                     ;; be interpreted as a null lexical environment.
  68                     ;; -- WHN 19991128
  69                     (coerce-to-lexenv environment)))
  70           (t
  71            (expand-or-get-setf-inverse form environment)))))
  72
  73 ;;; GET-SETF-METHOD existed in pre-ANSI Common Lisp, and various code inherited
  74 ;;; from CMU CL uses it repeatedly, so rather than rewrite a lot of code to not
  75 ;;; use it, we just define it in terms of ANSI's GET-SETF-EXPANSION (or
  76 ;;; actually, the cross-compiler version of that, i.e.
  77 ;;; SB!XC:GET-SETF-EXPANSION).
  78 (declaim (ftype (function (t &optional (or null sb!c::lexenv))) get-setf-method))
  79 (defun get-setf-method (form &optional environment)
  80   #!+sb-doc
  81   "This is a specialized-for-one-value version of GET-SETF-EXPANSION (and
  82 a relic from pre-ANSI Common Lisp). Portable ANSI code should use
  83 GET-SETF-EXPANSION directly."
  84   (multiple-value-bind (temps value-forms store-vars store-form access-form)
  85       (sb!xc:get-setf-expansion form environment)
  86     (when (cdr store-vars)
  87       (error "GET-SETF-METHOD used for a form with multiple store ~
  88               variables:~%  ~S"
  89              form))
  90     (values temps value-forms store-vars store-form access-form)))
  91
  92 ;;; If a macro, expand one level and try again. If not, go for the
  93 ;;; SETF function.
  94 (declaim (ftype (function (t sb!c::lexenv)) expand-or-get-setf-inverse))
  95 (defun expand-or-get-setf-inverse (form environment)
  96   (multiple-value-bind (expansion expanded)
  97       (sb!xc:macroexpand-1 form environment)
  98     (if expanded
  99         (sb!xc:get-setf-expansion expansion environment)
 100         (get-setf-method-inverse form
 101                                  `(funcall #'(setf ,(car form)))
 102                                  t))))
 103
 104 (defun get-setf-method-inverse (form inverse setf-fun)
 105   (let ((new-var (gensym))
 106         (vars nil)
 107         (vals nil))
 108     (dolist (x (cdr form))
 109       (push (gensym) vars)
 110       (push x vals))
 111     (setq vals (nreverse vals))
 112     (values vars vals (list new-var)
 113             (if setf-fun
 114                 `(,@inverse ,new-var ,@vars)
 115                 `(,@inverse ,@vars ,new-var))
 116             `(,(car form) ,@vars))))
 117 \f
 118 ;;;; SETF itself
 119
 120 ;;; Except for atoms, we always call GET-SETF-EXPANSION, since it has
 121 ;;; some non-trivial semantics. But when there is a setf inverse, and
 122 ;;; G-S-E uses it, then we return a call to the inverse, rather than
 123 ;;; returning a hairy LET form. This is probably important mainly as a
 124 ;;; convenience in allowing the use of SETF inverses without the full
 125 ;;; interpreter.
 126 (defmacro-mundanely setf (&rest args &environment env)
 127   #!+sb-doc
 128   "Takes pairs of arguments like SETQ. The first is a place and the second
 129   is the value that is supposed to go into that place. Returns the last
 130   value. The place argument may be any of the access forms for which SETF
 131   knows a corresponding setting form."
 132   (let ((nargs (length args)))
 133     (cond
 134      ((= nargs 2)
 135       (let ((place (first args))
 136             (value-form (second args)))
 137         (if (atom place)
 138           `(setq ,place ,value-form)
 139           (multiple-value-bind (dummies vals newval setter getter)
 140               (sb!xc:get-setf-expansion place env)
 141             (declare (ignore getter))
 142             (let ((inverse (info :setf :inverse (car place))))
 143               (if (and inverse (eq inverse (car setter)))
 144                 `(,inverse ,@(cdr place) ,value-form)
 145                 `(let* (,@(mapcar #'list dummies vals))
 146                    (multiple-value-bind ,newval ,value-form
 147                      ,setter))))))))
 148      ((oddp nargs)
 149       (error "odd number of args to SETF"))
 150      (t
 151       (do ((a args (cddr a))
 152            (reversed-setfs nil))
 153           ((null a)
 154            `(progn ,@(nreverse reversed-setfs)))
 155         (push (list 'setf (car a) (cadr a)) reversed-setfs))))))
 156 \f
 157 ;;;; various SETF-related macros
 158
 159 (defmacro-mundanely shiftf (&whole form &rest args &environment env)
 160   #!+sb-doc
 161   "One or more SETF-style place expressions, followed by a single
 162    value expression. Evaluates all of the expressions in turn, then
 163    assigns the value of each expression to the place on its left,
 164    returning the value of the leftmost."
 165   (when (< (length args) 2)
 166     (error "~S called with too few arguments: ~S" 'shiftf form))
 167   (let ((resultvar (gensym)))
 168     (do ((arglist args (cdr arglist))
 169          (bindlist nil)
 170          (storelist nil)
 171          (lastvar resultvar))
 172         ((atom (cdr arglist))
 173          (push `(,lastvar ,(first arglist)) bindlist)
 174          `(let* ,(nreverse bindlist) ,@(nreverse storelist) ,resultvar))
 175       (multiple-value-bind (sm1 sm2 sm3 sm4 sm5)
 176           (get-setf-method (first arglist) env)
 177         (mapc (lambda (var val)
 178                 (push `(,var ,val) bindlist))
 179               sm1
 180               sm2)
 181         (push `(,lastvar ,sm5) bindlist)
 182         (push sm4 storelist)
 183         (setq lastvar (first sm3))))))
 184
 185 (defmacro-mundanely push (obj place &environment env)
 186   #!+sb-doc
 187   "Takes an object and a location holding a list. Conses the object onto
 188   the list, returning the modified list. OBJ is evaluated before PLACE."
 189   (if (symbolp place)
 190       `(setq ,place (cons ,obj ,place))
 191       (multiple-value-bind
 192           (dummies vals newval setter getter)
 193           (get-setf-method place env)
 194         (let ((g (gensym)))
 195           `(let* ((,g ,obj)
 196                   ,@(mapcar #'list dummies vals)
 197                   (,(car newval) (cons ,g ,getter)))
 198             ,setter)))))
 199
 200 (defmacro-mundanely pushnew (obj place &rest keys &environment env)
 201   #!+sb-doc
 202   "Takes an object and a location holding a list. If the object is already
 203   in the list, does nothing. Else, conses the object onto the list. Returns
 204   NIL. If there is a :TEST keyword, this is used for the comparison."
 205   (if (symbolp place)
 206       `(setq ,place (adjoin ,obj ,place ,@keys))
 207       (multiple-value-bind (dummies vals newval setter getter)
 208           (get-setf-method place env)
 209         (do* ((d dummies (cdr d))
 210               (v vals (cdr v))
 211               (let-list nil))
 212              ((null d)
 213               (push (list (car newval) `(adjoin ,obj ,getter ,@keys))
 214                     let-list)
 215               `(let* ,(nreverse let-list)
 216                  ,setter))
 217           (push (list (car d) (car v)) let-list)))))
 218
 219 (defmacro-mundanely pop (place &environment env)
 220   #!+sb-doc
 221   "The argument is a location holding a list. Pops one item off the front
 222   of the list and returns it."
 223   (if (symbolp place)
 224       `(prog1 (car ,place) (setq ,place (cdr ,place)))
 225       (multiple-value-bind (dummies vals newval setter getter)
 226           (get-setf-method place env)
 227         (do* ((d dummies (cdr d))
 228               (v vals (cdr v))
 229               (let-list nil))
 230              ((null d)
 231               (push (list (car newval) getter) let-list)
 232               `(let* ,(nreverse let-list)
 233                  (prog1 (car ,(car newval))
 234                         (setq ,(car newval) (cdr ,(car newval)))
 235                         ,setter)))
 236           (push (list (car d) (car v)) let-list)))))
 237
 238 (defmacro-mundanely remf (place indicator &environment env)
 239   #!+sb-doc
 240   "Place may be any place expression acceptable to SETF, and is expected
 241   to hold a property list or (). This list is destructively altered to
 242   remove the property specified by the indicator. Returns T if such a
 243   property was present, NIL if not."
 244   (multiple-value-bind (dummies vals newval setter getter)
 245       (get-setf-method place env)
 246     (do* ((d dummies (cdr d))
 247           (v vals (cdr v))
 248           (let-list nil)
 249           (ind-temp (gensym))
 250           (local1 (gensym))
 251           (local2 (gensym)))
 252          ((null d)
 253           (push (list (car newval) getter) let-list)
 254           (push (list ind-temp indicator) let-list)
 255           `(let* ,(nreverse let-list)
 256              (do ((,local1 ,(car newval) (cddr ,local1))
 257                   (,local2 nil ,local1))
 258                  ((atom ,local1) nil)
 259                (cond ((atom (cdr ,local1))
 260                       (error "Odd-length property list in REMF."))
 261                      ((eq (car ,local1) ,ind-temp)
 262                       (cond (,local2
 263                              (rplacd (cdr ,local2) (cddr ,local1))
 264                              (return t))
 265                             (t (setq ,(car newval) (cddr ,(car newval)))
 266                                ,setter
 267                                (return t))))))))
 268       (push (list (car d) (car v)) let-list))))
 269 \f
 270 ;;;; DEFINE-MODIFY-MACRO stuff
 271
 272 (def!macro sb!xc:define-modify-macro (name lambda-list function &optional doc-string)
 273   #!+sb-doc
 274   "Creates a new read-modify-write macro like PUSH or INCF."
 275   (let ((other-args nil)
 276         (rest-arg nil)
 277         (env (gensym))
 278         (reference (gensym)))
 279     ;; Parse out the variable names and &REST arg from the lambda list.
 280     (do ((ll lambda-list (cdr ll))
 281          (arg nil))
 282         ((null ll))
 283       (setq arg (car ll))
 284       (cond ((eq arg '&optional))
 285             ((eq arg '&rest)
 286              (if (symbolp (cadr ll))
 287                (setq rest-arg (cadr ll))
 288                (error "Non-symbol &REST argument in definition of ~S." name))
 289              (if (null (cddr ll))
 290                (return nil)
 291                (error "Illegal stuff after &REST argument.")))
 292             ((memq arg '(&key &allow-other-keys &aux))
 293              (error "~S not allowed in DEFINE-MODIFY-MACRO lambda list." arg))
 294             ((symbolp arg)
 295              (push arg other-args))
 296             ((and (listp arg) (symbolp (car arg)))
 297              (push (car arg) other-args))
 298             (t (error "Illegal stuff in lambda list."))))
 299     (setq other-args (nreverse other-args))
 300     `(#-sb-xc-host sb!xc:defmacro
 301       #+sb-xc-host defmacro-mundanely
 302          ,name (,reference ,@lambda-list &environment ,env)
 303        ,doc-string
 304        (multiple-value-bind (dummies vals newval setter getter)
 305            (get-setf-method ,reference ,env)
 306          (do ((d dummies (cdr d))
 307               (v vals (cdr v))
 308               (let-list nil (cons (list (car d) (car v)) let-list)))
 309              ((null d)
 310               (push (list (car newval)
 311                           ,(if rest-arg
 312                              `(list* ',function getter ,@other-args ,rest-arg)
 313                              `(list ',function getter ,@other-args)))
 314                     let-list)
 315               `(let* ,(nreverse let-list)
 316                  ,setter)))))))
 317
 318 (sb!xc:define-modify-macro incf (&optional (delta 1)) +
 319   #!+sb-doc
 320   "The first argument is some location holding a number. This number is
 321   incremented by the second argument, DELTA, which defaults to 1.")
 322
 323 (sb!xc:define-modify-macro decf (&optional (delta 1)) -
 324   #!+sb-doc
 325   "The first argument is some location holding a number. This number is
 326   decremented by the second argument, DELTA, which defaults to 1.")
 327 \f
 328 ;;;; DEFSETF
 329
 330 (eval-when (#-sb-xc :compile-toplevel :load-toplevel :execute)
 331   ;;; Assign SETF macro information for NAME, making all appropriate checks.
 332   (defun assign-setf-macro (name expander inverse doc)
 333     (cond ((gethash name sb!c:*setf-assumed-fboundp*)
 334            (warn
 335             "defining setf macro for ~S when ~S was previously ~
 336              treated as a function"
 337             name
 338             `(setf ,name)))
 339           ((not (fboundp `(setf ,name)))
 340            ;; All is well, we don't need any warnings.
 341            (values))
 342           ((not (eq (symbol-package name) (symbol-package 'aref)))
 343            (style-warn "defining setf macro for ~S when ~S is fbound"
 344                        name `(setf ,name))))
 345     (remhash name sb!c:*setf-assumed-fboundp*)
 346     ;; FIXME: It's probably possible to join these checks into one form which
 347     ;; is appropriate both on the cross-compilation host and on the target.
 348     (when (or inverse (info :setf :inverse name))
 349       (setf (info :setf :inverse name) inverse))
 350     (when (or expander (info :setf :expander name))
 351       (setf (info :setf :expander name) expander))
 352     (when doc
 353       (setf (fdocumentation name 'setf) doc))
 354     name))
 355
 356 (def!macro sb!xc:defsetf (access-fn &rest rest)
 357   #!+sb-doc
 358   "Associates a SETF update function or macro with the specified access
 359   function or macro. The format is complex. See the manual for details."
 360   (cond ((not (listp (car rest)))
 361          `(eval-when (:load-toplevel :compile-toplevel :execute)
 362             (assign-setf-macro ',access-fn
 363                                nil
 364                                ',(car rest)
 365                                 ,(when (and (car rest) (stringp (cadr rest)))
 366                                    `',(cadr rest)))))
 367         ((and (cdr rest) (listp (cadr rest)))
 368          (destructuring-bind
 369              (lambda-list (&rest store-variables) &body body)
 370              rest
 371            (let ((arglist-var (gensym "ARGS-"))
 372                  (access-form-var (gensym "ACCESS-FORM-"))
 373                  (env-var (gensym "ENVIRONMENT-")))
 374              (multiple-value-bind (body local-decs doc)
 375                  (parse-defmacro `(,lambda-list ,@store-variables)
 376                                  arglist-var body access-fn 'defsetf
 377                                  :anonymousp t)
 378                `(eval-when (:compile-toplevel :load-toplevel :execute)
 379                   (assign-setf-macro
 380                    ',access-fn
 381                    (lambda (,access-form-var ,env-var)
 382                      (declare (ignore ,env-var))
 383                      (%defsetf ,access-form-var ,(length store-variables)
 384                                (lambda (,arglist-var)
 385                                  ,@local-decs
 386                                  (block ,access-fn
 387                                    ,body))))
 388                    nil
 389                    ',doc))))))
 390         (t
 391          (error "ill-formed DEFSETF for ~S" access-fn))))
 392
 393 (defun %defsetf (orig-access-form num-store-vars expander)
 394   (let (subforms
 395         subform-vars
 396         subform-exprs
 397         store-vars)
 398     (dolist (subform (cdr orig-access-form))
 399       (if (constantp subform)
 400         (push subform subforms)
 401         (let ((var (gensym)))
 402           (push var subforms)
 403           (push var subform-vars)
 404           (push subform subform-exprs))))
 405     (dotimes (i num-store-vars)
 406       (push (gensym) store-vars))
 407     (let ((r-subforms (nreverse subforms))
 408           (r-subform-vars (nreverse subform-vars))
 409           (r-subform-exprs (nreverse subform-exprs))
 410           (r-store-vars (nreverse store-vars)))
 411       (values r-subform-vars
 412               r-subform-exprs
 413               r-store-vars
 414               (funcall expander (cons r-subforms r-store-vars))
 415               `(,(car orig-access-form) ,@r-subforms)))))
 416 \f
 417 ;;;; DEFMACRO DEFINE-SETF-EXPANDER and various DEFINE-SETF-EXPANDERs
 418
 419 ;;; DEFINE-SETF-EXPANDER is a lot like DEFMACRO.
 420 (def!macro sb!xc:define-setf-expander (access-fn lambda-list &body body)
 421   #!+sb-doc
 422   "Syntax like DEFMACRO, but creates a Setf-Method generator. The body
 423   must be a form that returns the five magical values."
 424   (unless (symbolp access-fn)
 425     (error "DEFINE-SETF-EXPANDER access-function name ~S is not a symbol."
 426            access-fn))
 427   (let ((whole (gensym "WHOLE-"))
 428         (environment (gensym "ENV-")))
 429     (multiple-value-bind (body local-decs doc)
 430         (parse-defmacro lambda-list whole body access-fn
 431                         'sb!xc:define-setf-expander
 432                         :environment environment)
 433       `(eval-when (:compile-toplevel :load-toplevel :execute)
 434          (assign-setf-macro ',access-fn
 435                             (lambda (,whole ,environment)
 436                               ,@local-decs
 437                               (block ,access-fn ,body))
 438                             nil
 439                             ',doc)))))
 440
 441 (sb!xc:define-setf-expander getf (place prop
 442                                   &optional default
 443                                   &environment env)
 444   (declare (type sb!c::lexenv env))
 445   (multiple-value-bind (temps values stores set get)
 446       (get-setf-method place env)
 447     (let ((newval (gensym))
 448           (ptemp (gensym))
 449           (def-temp (if default (gensym))))
 450       (values `(,@temps ,ptemp ,@(if default `(,def-temp)))
 451               `(,@values ,prop ,@(if default `(,default)))
 452               `(,newval)
 453               `(let ((,(car stores) (%putf ,get ,ptemp ,newval)))
 454                  ,set
 455                  ,newval)
 456               `(getf ,get ,ptemp ,@(if default `(,def-temp)))))))
 457
 458 (sb!xc:define-setf-expander get (symbol prop &optional default)
 459   (let ((symbol-temp (gensym))
 460         (prop-temp (gensym))
 461         (def-temp (gensym))
 462         (newval (gensym)))
 463     (values `(,symbol-temp ,prop-temp ,@(if default `(,def-temp)))
 464             `(,symbol ,prop ,@(if default `(,default)))
 465             (list newval)
 466             `(%put ,symbol-temp ,prop-temp ,newval)
 467             `(get ,symbol-temp ,prop-temp ,@(if default `(,def-temp))))))
 468
 469 (sb!xc:define-setf-expander gethash (key hashtable &optional default)
 470   (let ((key-temp (gensym))
 471         (hashtable-temp (gensym))
 472         (default-temp (gensym))
 473         (new-value-temp (gensym)))
 474     (values
 475      `(,key-temp ,hashtable-temp ,@(if default `(,default-temp)))
 476      `(,key ,hashtable ,@(if default `(,default)))
 477      `(,new-value-temp)
 478      `(%puthash ,key-temp ,hashtable-temp ,new-value-temp)
 479      `(gethash ,key-temp ,hashtable-temp ,@(if default `(,default-temp))))))
 480
 481 (sb!xc:define-setf-expander logbitp (index int &environment env)
 482   (declare (type sb!c::lexenv env))
 483   (multiple-value-bind (temps vals stores store-form access-form)
 484       (get-setf-method int env)
 485     (let ((ind (gensym))
 486           (store (gensym))
 487           (stemp (first stores)))
 488       (values `(,ind ,@temps)
 489               `(,index
 490                 ,@vals)
 491               (list store)
 492               `(let ((,stemp
 493                       (dpb (if ,store 1 0) (byte 1 ,ind) ,access-form)))
 494                  ,store-form
 495                  ,store)
 496               `(logbitp ,ind ,access-form)))))
 497
 498 ;;; CMU CL had a comment here that:
 499 ;;;   Evil hack invented by the gnomes of Vassar Street (though not as evil as
 500 ;;;   it used to be.)  The function arg must be constant, and is converted to
 501 ;;;   an APPLY of the SETF function, which ought to exist.
 502 ;;;
 503 ;;; It may not be clear (wasn't to me..) that this is a standard thing, but See
 504 ;;; "5.1.2.5 APPLY Forms as Places" in the ANSI spec. I haven't actually
 505 ;;; verified that this code has any correspondence to that code, but at least
 506 ;;; ANSI has some place for SETF APPLY. -- WHN 19990604
 507 (sb!xc:define-setf-expander apply (functionoid &rest args)
 508   (unless (and (listp functionoid)
 509                (= (length functionoid) 2)
 510                (eq (first functionoid) 'function)
 511                (symbolp (second functionoid)))
 512     (error "SETF of APPLY is only defined for function args like #'SYMBOL."))
 513   (let ((function (second functionoid))
 514         (new-var (gensym))
 515         (vars (make-gensym-list (length args))))
 516     (values vars args (list new-var)
 517             `(apply #'(setf ,function) ,new-var ,@vars)
 518             `(apply #',function ,@vars))))
 519
 520 ;;; Special-case a BYTE bytespec so that the compiler can recognize it.
 521 (sb!xc:define-setf-expander ldb (bytespec place &environment env)
 522   #!+sb-doc
 523   "The first argument is a byte specifier. The second is any place form
 524   acceptable to SETF. Replace the specified byte of the number in this
 525   place with bits from the low-order end of the new value."
 526   (declare (type sb!c::lexenv env))
 527   (multiple-value-bind (dummies vals newval setter getter)
 528       (get-setf-method place env)
 529     (if (and (consp bytespec) (eq (car bytespec) 'byte))
 530         (let ((n-size (gensym))
 531               (n-pos (gensym))
 532               (n-new (gensym)))
 533           (values (list* n-size n-pos dummies)
 534                   (list* (second bytespec) (third bytespec) vals)
 535                   (list n-new)
 536                   `(let ((,(car newval) (dpb ,n-new (byte ,n-size ,n-pos)
 537                                              ,getter)))
 538                      ,setter
 539                      ,n-new)
 540                   `(ldb (byte ,n-size ,n-pos) ,getter)))
 541         (let ((btemp (gensym))
 542               (gnuval (gensym)))
 543           (values (cons btemp dummies)
 544                   (cons bytespec vals)
 545                   (list gnuval)
 546                   `(let ((,(car newval) (dpb ,gnuval ,btemp ,getter)))
 547                      ,setter
 548                      ,gnuval)
 549                   `(ldb ,btemp ,getter))))))
 550
 551 (sb!xc:define-setf-expander mask-field (bytespec place &environment env)
 552   #!+sb-doc
 553   "The first argument is a byte specifier. The second is any place form
 554   acceptable to SETF. Replaces the specified byte of the number in this place
 555   with bits from the corresponding position in the new value."
 556   (declare (type sb!c::lexenv env))
 557   (multiple-value-bind (dummies vals newval setter getter)
 558       (get-setf-method place env)
 559     (let ((btemp (gensym))
 560           (gnuval (gensym)))
 561       (values (cons btemp dummies)
 562               (cons bytespec vals)
 563               (list gnuval)
 564               `(let ((,(car newval) (deposit-field ,gnuval ,btemp ,getter)))
 565                  ,setter
 566                  ,gnuval)
 567               `(mask-field ,btemp ,getter)))))
 568
 569 (sb!xc:define-setf-expander the (type place &environment env)
 570   (declare (type sb!c::lexenv env))
 571   (multiple-value-bind (dummies vals newval setter getter)
 572       (get-setf-method place env)
 573     (values dummies
 574               vals
 575               newval
 576               (subst `(the ,type ,(car newval)) (car newval) setter)
 577               `(the ,type ,getter))))