Simplify (and robustify) regular PACKing
[sbcl.git] / src / pcl / dlisp.lisp
index 46afb78..27f601c 100644 (file)
 ;;;; specification.
 
 (in-package "SB-PCL")
-\f
-;;; This file is (almost) functionally equivalent to dlap.lisp, but
-;;; easier to read.
 
-;;; Might generate faster code, too, depending on the compiler and
-;;; whether an implementation-specific lap assembler was used.
+
+;;;; some support stuff for getting a hold of symbols that we need when
+;;;; building the discriminator codes. It's OK for these to be interned
+;;;; symbols because we don't capture any user code in the scope in which
+;;;; these symbols are bound.
+
+(declaim (list *dfun-arg-symbols*))
+(defvar *dfun-arg-symbols* '(.ARG0. .ARG1. .ARG2. .ARG3.))
+
+(defun dfun-arg-symbol (arg-number)
+  (or (nth arg-number *dfun-arg-symbols*)
+      (format-symbol *pcl-package* ".ARG~A." arg-number)))
+
+(declaim (list *slot-vector-symbols*))
+(defvar *slot-vector-symbols* '(.SLOTS0. .SLOTS1. .SLOTS2. .SLOTS3.))
+
+(defun slot-vector-symbol (arg-number)
+  (or (nth arg-number *slot-vector-symbols*)
+      (format-symbol *pcl-package* ".SLOTS~A." arg-number)))
+
+(declaim (inline make-dfun-required-args))
+(defun make-dfun-required-args (count)
+  (declare (type index count))
+  (let (result)
+    (dotimes (i count (nreverse result))
+      (push (dfun-arg-symbol i) result))))
+
+(defun make-dfun-lambda-list (nargs applyp)
+  (let ((required (make-dfun-required-args nargs)))
+    (if applyp
+        (nconc required
+               ;; Use &MORE arguments to avoid consing up an &REST list
+               ;; that we might not need at all. See MAKE-EMF-CALL and
+               ;; INVOKE-EFFECTIVE-METHOD-FUNCTION for the other
+               ;; pieces.
+               '(&more .dfun-more-context. .dfun-more-count.))
+        required)))
+
+(defun make-dlap-lambda-list (nargs applyp)
+  (let* ((required (make-dfun-required-args nargs))
+         (lambda-list (if applyp
+                          (append required '(&more .more-context. .more-count.))
+                          required)))
+    ;; Return the full lambda list, the required arguments, a form
+    ;; that will generate a rest-list, and a list of the &MORE
+    ;; parameters used.
+    (values lambda-list
+            required
+            (when applyp
+              '((sb-c::%listify-rest-args
+                 .more-context.
+                 (the (and unsigned-byte fixnum)
+                   .more-count.))))
+            (when applyp
+              '(.more-context. .more-count.)))))
+
+(defun make-emf-call (nargs applyp fn-variable &optional emf-type)
+  (let ((required (make-dfun-required-args nargs)))
+    `(,(if (eq emf-type 'fast-method-call)
+           'invoke-effective-method-function-fast
+           'invoke-effective-method-function)
+       ,fn-variable
+       ,applyp
+       :required-args ,required
+       ;; INVOKE-EFFECTIVE-METHOD-FUNCTION will decide whether to use
+       ;; the :REST-ARG version or the :MORE-ARG version depending on
+       ;; the type of the EMF.
+       :rest-arg ,(if applyp
+                      ;; Creates a list from the &MORE arguments.
+                      '((sb-c::%listify-rest-args
+                         .dfun-more-context.
+                         (the (and unsigned-byte fixnum)
+                           .dfun-more-count.)))
+                      nil)
+       :more-arg ,(when applyp
+                    '(.dfun-more-context. .dfun-more-count.)))))
+
+(defun make-fast-method-call-lambda-list (nargs applyp)
+  (list* '.pv. '.next-method-call. (make-dfun-lambda-list nargs applyp)))
+\f
+;;; Emitting various accessors.
 
 (defun emit-one-class-reader (class-slot-p)
   (emit-reader/writer :reader 1 class-slot-p))
 
 ;;; FIXME: What do these variables mean?
 (defvar *precompiling-lap* nil)
-(defvar *emit-function-p* t)
-
-;;; FIXME: This variable is motivated by Gerd Moellman's observation,
-;;; in <867kga1wra.fsf@gerd.free-bsd.org> on cmucl-imp 2002-10-22,
-;;; that the functions returned from EMIT-xxx-FUNCTION can cause an
-;;; order-of-magnitude slowdown.  We include this variable for now,
-;;; but maybe its effect should rather be controlled by compilation
-;;; policy if there is a noticeable space difference between the
-;;; branches, or else maybe the EMIT-xxx-FUNCTION branches should be
-;;; deleted.  It's not clear to me how all of this works, though, so
-;;; until proper benchmarks are done it's probably safest simply to
-;;; have this pseudo-constant to hide code.  -- CSR, 2003-02-14
-(defvar *optimize-cache-functions-p* t)
 
 (defun emit-default-only (metatypes applyp)
-  (unless *optimize-cache-functions-p*
-    (when (and (null *precompiling-lap*) *emit-function-p*)
-      (return-from emit-default-only
-       (emit-default-only-function metatypes applyp))))
-  (let* ((dlap-lambda-list (make-dlap-lambda-list metatypes applyp))
-        (args (remove '&rest dlap-lambda-list))
-        (restl (when applyp '(.lap-rest-arg.))))
+  (multiple-value-bind (lambda-list args rest-arg more-arg)
+      (make-dlap-lambda-list (length metatypes) applyp)
     (generating-lisp '(emf)
-                    dlap-lambda-list
-                    `(invoke-effective-method-function emf
-                                                       ,applyp
-                                                       ,@args
-                                                       ,@restl))))
+                     lambda-list
+                     `(invoke-effective-method-function emf
+                                                        ,applyp
+                                                        :required-args ,args
+                                                        :more-arg ,more-arg
+                                                        :rest-arg ,rest-arg))))
 
 ;;; --------------------------------
 
 (defun generating-lisp (closure-variables args form)
-  (let* ((rest (memq '&rest args))
-        (ldiff (and rest (ldiff args rest)))
-        (args (if rest (append ldiff '(&rest .lap-rest-arg.)) args))
-        (lambda `(lambda ,closure-variables
-                   ,@(when (member 'miss-fn closure-variables)
-                       `((declare (type function miss-fn))))
-                   #'(sb-kernel:instance-lambda ,args
-                       (let ()
-                         (declare #.*optimize-speed*)
-                         ,form)))))
+  (let ((lambda `(lambda ,closure-variables
+                   ,@(when (member 'miss-fn closure-variables)
+                           `((declare (type function miss-fn))))
+                   #'(lambda ,args
+                       (let ()
+                         (declare #.*optimize-speed*)
+                         ,form)))))
     (values (if *precompiling-lap*
-               `#',lambda
-               (compile nil lambda))
-           nil)))
+                `#',lambda
+                (compile nil lambda))
+            nil)))
 
 ;;; note on implementation for CMU 17 and later (including SBCL):
 ;;; Since STD-INSTANCE-P is weakened, that branch may run on non-PCL
 ;;; FSC-INSTANCE-P returns true on funcallable structures as well as
 ;;; PCL fins.
 (defun emit-reader/writer (reader/writer 1-or-2-class class-slot-p)
-  (unless *optimize-cache-functions-p*
-    (when (and (null *precompiling-lap*) *emit-function-p*)
-      (return-from emit-reader/writer
-       (emit-reader/writer-function
-        reader/writer 1-or-2-class class-slot-p))))
   (let ((instance nil)
-       (arglist  ())
-       (closure-variables ())
-       (field +first-wrapper-cache-number-index+)
-       (read-form (emit-slot-read-form class-slot-p 'index 'slots)))
-    ;;we need some field to do the fast obsolete check
+        (arglist  ())
+        (closure-variables ())
+        (read-form (emit-slot-read-form class-slot-p 'index 'slots)))
     (ecase reader/writer
       ((:reader :boundp)
        (setq instance (dfun-arg-symbol 0)
-            arglist  (list instance)))
+             arglist  (list instance)))
       (:writer (setq instance (dfun-arg-symbol 1)
-                    arglist  (list (dfun-arg-symbol 0) instance))))
+                     arglist  (list (dfun-arg-symbol 0) instance))))
     (ecase 1-or-2-class
       (1 (setq closure-variables '(wrapper-0 index miss-fn)))
       (2 (setq closure-variables '(wrapper-0 wrapper-1 index miss-fn))))
      closure-variables
      arglist
      `(let* (,@(unless class-slot-p `((slots nil)))
-              (wrapper (cond ((std-instance-p ,instance)
-                              ,@(unless class-slot-p
-                                  `((setq slots
-                                          (std-instance-slots ,instance))))
-                              (std-instance-wrapper ,instance))
-                             ((fsc-instance-p ,instance)
-                              ,@(unless class-slot-p
-                                  `((setq slots
-                                          (fsc-instance-slots ,instance))))
-                              (fsc-instance-wrapper ,instance)))))
-       (block access
-         (when (and wrapper
-                    (/= (wrapper-cache-number-vector-ref wrapper ,field) 0)
-                    ,@(if (eql 1 1-or-2-class)
-                          `((eq wrapper wrapper-0))
-                          `((or (eq wrapper wrapper-0)
-                                (eq wrapper wrapper-1)))))
-           ,@(ecase reader/writer
-               (:reader
-                `((let ((value ,read-form))
-                    (unless (eq value +slot-unbound+)
-                      (return-from access value)))))
-               (:boundp
-                `((let ((value ,read-form))
-                      (return-from access (not (eq value +slot-unbound+))))))
-               (:writer
-                `((return-from access (setf ,read-form ,(car arglist)))))))
-         (funcall miss-fn ,@arglist))))))
+               (wrapper (cond ((std-instance-p ,instance)
+                               ,@(unless class-slot-p
+                                   `((setq slots
+                                           (std-instance-slots ,instance))))
+                               (std-instance-wrapper ,instance))
+                              ((fsc-instance-p ,instance)
+                               ,@(unless class-slot-p
+                                   `((setq slots
+                                           (fsc-instance-slots ,instance))))
+                               (fsc-instance-wrapper ,instance)))))
+        (block access
+          (when (and wrapper
+                     (not (zerop (layout-clos-hash wrapper)))
+                     ,@(if (eql 1 1-or-2-class)
+                           `((eq wrapper wrapper-0))
+                           `((or (eq wrapper wrapper-0)
+                                 (eq wrapper wrapper-1)))))
+            ,@(ecase reader/writer
+                (:reader
+                 `((let ((value ,read-form))
+                     (unless (eq value +slot-unbound+)
+                       (return-from access value)))))
+                (:boundp
+                 `((let ((value ,read-form))
+                     (return-from access (not (eq value +slot-unbound+))))))
+                (:writer
+                 `((return-from access (setf ,read-form ,(car arglist)))))))
+          (funcall miss-fn ,@arglist))))))
 
 (defun emit-slot-read-form (class-slot-p index slots)
   (if class-slot-p
 (defun emit-boundp-check (value-form miss-fn arglist)
   `(let ((value ,value-form))
      (if (eq value +slot-unbound+)
-        (funcall ,miss-fn ,@arglist)
-        value)))
+         (funcall ,miss-fn ,@arglist)
+         value)))
 
 (defun emit-slot-access (reader/writer class-slot-p slots
-                        index miss-fn arglist)
+                         index miss-fn arglist)
   (let ((read-form (emit-slot-read-form class-slot-p index slots)))
     (ecase reader/writer
       (:reader (emit-boundp-check read-form miss-fn arglist))
       (:writer `(setf ,read-form ,(car arglist))))))
 
 (defmacro emit-reader/writer-macro (reader/writer 1-or-2-class class-slot-p)
-  (let ((*emit-function-p* nil)
-       (*precompiling-lap* t))
+  (let ((*precompiling-lap* t))
     (values
      (emit-reader/writer reader/writer 1-or-2-class class-slot-p))))
 
 (defun emit-one-or-n-index-reader/writer (reader/writer
-                                         cached-index-p
-                                         class-slot-p)
-  (unless *optimize-cache-functions-p*
-    (when (and (null *precompiling-lap*) *emit-function-p*)
-      (return-from emit-one-or-n-index-reader/writer
-       (emit-one-or-n-index-reader/writer-function
-        reader/writer cached-index-p class-slot-p))))
+                                          cached-index-p
+                                          class-slot-p)
   (multiple-value-bind (arglist metatypes)
       (ecase reader/writer
-       ((:reader :boundp)
-        (values (list (dfun-arg-symbol 0))
-                '(standard-instance)))
-       (:writer (values (list (dfun-arg-symbol 0) (dfun-arg-symbol 1))
-                        '(t standard-instance))))
+        ((:reader :boundp)
+         (values (list (dfun-arg-symbol 0))
+                 '(standard-instance)))
+        (:writer (values (list (dfun-arg-symbol 0) (dfun-arg-symbol 1))
+                         '(t standard-instance))))
     (generating-lisp
      `(cache ,@(unless cached-index-p '(index)) miss-fn)
      arglist
      `(let (,@(unless class-slot-p '(slots))
-           ,@(when cached-index-p '(index)))
-       ,(emit-dlap arglist metatypes
-                   (emit-slot-access reader/writer class-slot-p
-                                     'slots 'index 'miss-fn arglist)
-                   `(funcall miss-fn ,@arglist)
-                   (when cached-index-p 'index)
-                   (unless class-slot-p '(slots)))))))
+            ,@(when cached-index-p '(index)))
+        ,(emit-dlap 'cache arglist metatypes
+                    (emit-slot-access reader/writer class-slot-p
+                                      'slots 'index 'miss-fn arglist)
+                    `(funcall miss-fn ,@arglist)
+                    (when cached-index-p 'index)
+                    (unless class-slot-p '(slots)))))))
 
 (defmacro emit-one-or-n-index-reader/writer-macro
     (reader/writer cached-index-p class-slot-p)
-  (let ((*emit-function-p* nil)
-       (*precompiling-lap* t))
+  (let ((*precompiling-lap* t))
     (values
      (emit-one-or-n-index-reader/writer reader/writer
-                                       cached-index-p
-                                       class-slot-p))))
+                                        cached-index-p
+                                        class-slot-p))))
 
-(defun emit-miss (miss-fn args &optional applyp)
-  (let ((restl (when applyp '(.lap-rest-arg.))))
-    (if restl
-       `(apply ,miss-fn ,@args ,@restl)
-       `(funcall ,miss-fn ,@args ,@restl))))
+(defun emit-miss (miss-fn args applyp)
+  (if applyp
+      `(multiple-value-call ,miss-fn ,@args
+                            (sb-c::%more-arg-values .more-context.
+                                                    0
+                                                    .more-count.))
+      `(funcall ,miss-fn ,@args)))
 
 (defun emit-checking-or-caching (cached-emf-p return-value-p metatypes applyp)
-  (unless *optimize-cache-functions-p*
-    (when (and (null *precompiling-lap*) *emit-function-p*)
-      (return-from emit-checking-or-caching
-       (emit-checking-or-caching-function
-        cached-emf-p return-value-p metatypes applyp))))
-  (let* ((dlap-lambda-list (make-dlap-lambda-list metatypes applyp))
-        (args (remove '&rest dlap-lambda-list))
-        (restl (when applyp '(.lap-rest-arg.))))
+  (multiple-value-bind (lambda-list args rest-arg more-arg)
+      (make-dlap-lambda-list (length metatypes) applyp)
     (generating-lisp
      `(cache ,@(unless cached-emf-p '(emf)) miss-fn)
-     dlap-lambda-list
+     lambda-list
      `(let (,@(when cached-emf-p '(emf)))
-       ,(emit-dlap args
-                   metatypes
-                   (if return-value-p
-                       (if cached-emf-p 'emf t)
-                       `(invoke-effective-method-function
-                         emf ,applyp ,@args ,@restl))
-                   (emit-miss 'miss-fn args applyp)
-                   (when cached-emf-p 'emf))))))
+        ,(emit-dlap 'cache args metatypes
+                    (if return-value-p
+                        (if cached-emf-p 'emf t)
+                        `(invoke-effective-method-function
+                          emf ,applyp
+                          :required-args ,args
+                          :more-arg ,more-arg
+                          :rest-arg ,rest-arg))
+                    (emit-miss 'miss-fn args applyp)
+                    (when cached-emf-p 'emf))))))
 
 (defmacro emit-checking-or-caching-macro (cached-emf-p
-                                         return-value-p
-                                         metatypes
-                                         applyp)
-  (let ((*emit-function-p* nil)
-       (*precompiling-lap* t))
+                                          return-value-p
+                                          metatypes
+                                          applyp)
+  (let ((*precompiling-lap* t))
     (values
      (emit-checking-or-caching cached-emf-p return-value-p metatypes applyp))))
 
-(defun emit-dlap (args metatypes hit miss value-reg &optional slot-regs)
+(defun emit-dlap (cache-var args metatypes hit-form miss-form value-var
+                  &optional slot-vars)
   (let* ((index -1)
-        (wrapper-bindings (mapcan (lambda (arg mt)
-                                    (unless (eq mt t)
-                                      (incf index)
-                                      `((,(intern (format nil
-                                                          "WRAPPER-~D"
-                                                          index)
-                                                  *pcl-package*)
-                                         ,(emit-fetch-wrapper
-                                           mt arg 'miss (pop slot-regs))))))
-                                  args metatypes))
-        (wrappers (mapcar #'car wrapper-bindings)))
+         (miss-tag (gensym "MISSED"))
+         (wrapper-bindings (mapcan (lambda (arg mt)
+                                     (unless (eq mt t)
+                                       (incf index)
+                                       `((,(format-symbol *pcl-package*
+                                                          "WRAPPER-~D"
+                                                          index)
+                                          ,(emit-fetch-wrapper
+                                            mt arg miss-tag (pop slot-vars))))))
+                                   args metatypes))
+         (wrapper-vars (mapcar #'car wrapper-bindings)))
     (declare (fixnum index))
-    (unless wrappers (error "Every metatype is T."))
-    `(block dfun
-       (tagbody
-         (let ((field (cache-field cache))
-               (cache-vector (cache-vector cache))
-               (mask (cache-mask cache))
-               (size (cache-size cache))
-               (overflow (cache-overflow cache))
-               ,@wrapper-bindings)
-           (declare (fixnum size field mask))
-           ,(cond ((cdr wrappers)
-                   (emit-greater-than-1-dlap wrappers 'miss value-reg))
-                  (value-reg
-                   (emit-1-t-dlap (car wrappers) 'miss value-reg))
-                  (t
-                   (emit-1-nil-dlap (car wrappers) 'miss)))
-           (return-from dfun ,hit))
-       miss
-         (return-from dfun ,miss)))))
-
-(defun emit-1-nil-dlap (wrapper miss-label)
-  `(let* ((primary ,(emit-1-wrapper-compute-primary-cache-location wrapper
-                                                                  miss-label))
-         (location primary))
-     (declare (fixnum primary location))
-     (block search
-       (loop (when (eq ,wrapper (cache-vector-ref cache-vector location))
-              (return-from search nil))
-            (setq location (the fixnum (+ location 1)))
-            (when (= location size)
-              (setq location 0))
-            (when (= location primary)
-              (dolist (entry overflow)
-                (when (eq (car entry) ,wrapper)
-                  (return-from search nil)))
-              (go ,miss-label))))))
-
-(defmacro get-cache-vector-lock-count (cache-vector)
-  `(let ((lock-count (cache-vector-lock-count ,cache-vector)))
-     (unless (typep lock-count 'fixnum)
-       (error "My cache got freed somehow."))
-     (the fixnum lock-count)))
-
-(defun emit-1-t-dlap (wrapper miss-label value)
-  `(let ((primary ,(emit-1-wrapper-compute-primary-cache-location wrapper
-                                                                 miss-label))
-        (initial-lock-count (get-cache-vector-lock-count cache-vector)))
-     (declare (fixnum primary initial-lock-count))
-     (let ((location primary))
-       (declare (fixnum location))
-       (block search
-        (loop (when (eq ,wrapper (cache-vector-ref cache-vector location))
-                (setq ,value (cache-vector-ref cache-vector (1+ location)))
-                (return-from search nil))
-              (setq location (the fixnum (+ location 2)))
-              (when (= location size)
-                (setq location 0))
-              (when (= location primary)
-                (dolist (entry overflow)
-                  (when (eq (car entry) ,wrapper)
-                    (setq ,value (cdr entry))
-                    (return-from search nil)))
-                (go ,miss-label))))
-       (unless (= initial-lock-count
-                 (get-cache-vector-lock-count cache-vector))
-        (go ,miss-label)))))
-
-(defun emit-greater-than-1-dlap (wrappers miss-label value)
-  (declare (type list wrappers))
-  (let ((cache-line-size (compute-line-size (+ (length wrappers)
-                                              (if value 1 0)))))
-    `(let ((primary 0)
-          (size-1 (the fixnum (- size 1))))
-       (declare (fixnum primary size-1))
-       ,(emit-n-wrapper-compute-primary-cache-location wrappers miss-label)
-       (let ((initial-lock-count (get-cache-vector-lock-count cache-vector)))
-        (declare (fixnum initial-lock-count))
-        (let ((location primary)
-              (next-location 0))
-          (declare (fixnum location next-location))
-          (block search
-            (loop (setq next-location
-                        (the fixnum (+ location ,cache-line-size)))
-                  (when (and ,@(mapcar
-                                (lambda (wrapper)
-                                  `(eq ,wrapper
-                                       (cache-vector-ref
-                                        cache-vector
-                                        (setq location
-                                              (the fixnum (+ location 1))))))
-                                wrappers))
-                    ,@(when value
-                        `((setq location (the fixnum (+ location 1)))
-                          (setq ,value (cache-vector-ref cache-vector
-                                                         location))))
-                    (return-from search nil))
-                  (setq location next-location)
-                  (when (= location size-1)
-                    (setq location 0))
-                  (when (= location primary)
-                    (dolist (entry overflow)
-                      (let ((entry-wrappers (car entry)))
-                        (when (and ,@(mapcar (lambda (wrapper)
-                                               `(eq ,wrapper
-                                                    (pop entry-wrappers)))
-                                             wrappers))
-                          ,@(when value
-                              `((setq ,value (cdr entry))))
-                          (return-from search nil))))
-                    (go ,miss-label))))
-          (unless (= initial-lock-count
-                     (get-cache-vector-lock-count cache-vector))
-            (go ,miss-label)))))))
-
-(defun emit-1-wrapper-compute-primary-cache-location (wrapper miss-label)
-  `(let ((wrapper-cache-no (wrapper-cache-number-vector-ref ,wrapper field)))
-     (declare (fixnum wrapper-cache-no))
-     (when (zerop wrapper-cache-no) (go ,miss-label))
-     ,(let ((form `(logand mask wrapper-cache-no)))
-       `(the fixnum ,form))))
-
-(defun emit-n-wrapper-compute-primary-cache-location (wrappers miss-label)
-  (declare (type list wrappers))
-  ;; This returns 1 less that the actual location.
-  `(progn
-     ,@(let ((adds 0) (len (length wrappers)))
-        (declare (fixnum adds len))
-        (mapcar (lambda (wrapper)
-                  `(let ((wrapper-cache-no (wrapper-cache-number-vector-ref
-                                            ,wrapper field)))
-                     (declare (fixnum wrapper-cache-no))
-                     (when (zerop wrapper-cache-no) (go ,miss-label))
-                     (setq primary (the fixnum (+ primary wrapper-cache-no)))
-                     ,@(progn
-                         (incf adds)
-                         (when (or (zerop (mod adds
-                                               wrapper-cache-number-adds-ok))
-                                   (eql adds len))
-                           `((setq primary
-                                   ,(let ((form `(logand primary mask)))
-                                      `(the fixnum ,form))))))))
-                wrappers))))
-
-;;; CMU17 (and SBCL) note: Since STD-INSTANCE-P is weakened in the
-;;; CMU/SBCL approach of using funcallable instances, that branch may
-;;; run on non-pcl instances (structures). The result will be the
-;;; non-wrapper layout for the structure, which will cause a miss. The
-;;; "slots" will be whatever the first slot is, but will be ignored.
-;;; Similarly, FSC-INSTANCE-P returns true on funcallable structures
-;;; as well as PCL fins.
-(defun emit-fetch-wrapper (metatype argument miss-label &optional slot)
+    (unless wrapper-vars
+      (error "Every metatype is T."))
+    `(prog ()
+        (return
+          (let ,wrapper-bindings
+            ,(emit-cache-lookup cache-var wrapper-vars miss-tag value-var)
+            ,hit-form))
+      ,miss-tag
+        (return ,miss-form))))
+
+(defun emit-fetch-wrapper (metatype argument miss-tag &optional slot)
   (ecase metatype
-    ((standard-instance) 
-     `(cond ((std-instance-p ,argument)
-            ,@(when slot `((setq ,slot (std-instance-slots ,argument))))
-            (std-instance-wrapper ,argument))
-           ((fsc-instance-p ,argument)
-            ,@(when slot `((setq ,slot (fsc-instance-slots ,argument))))
-            (fsc-instance-wrapper ,argument))
-           (t
-            (go ,miss-label))))
-    (class
-     (when slot (error "can't do a slot reg for this metatype"))
-     `(wrapper-of-macro ,argument))
-    ((built-in-instance structure-instance)
-     (when slot (error "can't do a slot reg for this metatype"))
-     `(built-in-or-structure-wrapper
-       ,argument))))
-
+    ((standard-instance)
+     ;; This branch may run on non-pcl instances (structures). The
+     ;; result will be the non-wrapper layout for the structure, which
+     ;; will cause a miss. Since refencing the structure is rather iffy
+     ;; if it should have no slots, or only raw slots, we use FOR-STD-CLASS-P
+     ;; to ensure that we have a wrapper.
+     ;;
+     ;; FIXME: If we unify layouts and wrappers we can use
+     ;; instance-slots-layout instead of for-std-class-p, as if there
+     ;; are no layouts there are no slots to worry about.
+     (with-unique-names (wrapper)
+       `(cond
+          ((std-instance-p ,argument)
+           (let ((,wrapper (std-instance-wrapper ,argument)))
+             ,@(when slot
+                     `((when (layout-for-std-class-p ,wrapper)
+                         (setq ,slot (std-instance-slots ,argument)))))
+             ,wrapper))
+          ((fsc-instance-p ,argument)
+           (let ((,wrapper (fsc-instance-wrapper ,argument)))
+             ,@(when slot
+                     `((when (layout-for-std-class-p ,wrapper)
+                         (setq ,slot (fsc-instance-slots ,argument)))))
+             ,wrapper))
+          (t (go ,miss-tag)))))
+    ;; Sep92 PCL used to distinguish between some of these cases (and
+    ;; spuriously exclude others).  Since in SBCL
+    ;; WRAPPER-OF/LAYOUT-OF/BUILT-IN-OR-STRUCTURE-WRAPPER are all
+    ;; equivalent and inlined to each other, we can collapse some
+    ;; spurious differences.
+    ((class built-in-instance structure-instance condition-instance)
+     (when slot
+       (bug "SLOT requested for metatype ~S, but it isnt' going to happen."
+            metatype))
+     `(wrapper-of ,argument))
+    ;; a metatype of NIL should never be seen here, as NIL is only in
+    ;; the metatypes before a generic function is fully initialized.
+    ;; T should never be seen because we never need to get a wrapper
+    ;; to do dispatch if all methods have T as the respective
+    ;; specializer.
+    ((t nil)
+     (bug "~@<metatype ~S seen in ~S.~@:>" metatype 'emit-fetch-wrapper))))