1 ;;;; the basics of the PCL wrapper cache mechanism
3 ;;;; This software is part of the SBCL system. See the README file for
6 ;;;; This software is derived from software originally released by Xerox
7 ;;;; Corporation. Copyright and release statements follow. Later modifications
8 ;;;; to the software are in the public domain and are provided with
9 ;;;; absolutely no warranty. See the COPYING and CREDITS files for more
12 ;;;; copyright information from original PCL sources:
14 ;;;; Copyright (c) 1985, 1986, 1987, 1988, 1989, 1990 Xerox Corporation.
15 ;;;; All rights reserved.
17 ;;;; Use and copying of this software and preparation of derivative works based
18 ;;;; upon this software are permitted. Any distribution of this software or
19 ;;;; derivative works must comply with all applicable United States export
22 ;;;; This software is made available AS IS, and Xerox Corporation makes no
23 ;;;; warranty about the software, its performance or its conformity to any
26 ;;;; Note: as of SBCL 1.0.6.3 it is questionable if cache.lisp can
27 ;;;; anymore be considered to be "derived from software originally
28 ;;;; released by Xerox Corporation", as at that time the whole cache
29 ;;;; implementation was essentially redone from scratch.
39 ;;;; emit-cache-lookup
41 ;;;; hash-table-to-cache
43 ;;;; This is a thread and interrupt safe reimplementation loosely
44 ;;;; based on the original PCL cache by Kickzales and Rodrigues,
45 ;;;; as described in "Efficient Method Dispatch in PCL".
47 ;;;; * Writes to cache are made atomic using compare-and-swap on
48 ;;;; wrappers. Wrappers are never moved or deleted after they have
49 ;;;; been written: to clean them out the cache need to be copied.
51 ;;;; * Copying or expanding the cache drops out incomplete and invalid
54 ;;;; * Since the cache is used for memoization only we don't need to
55 ;;;; worry about which of simultaneous replacements (when expanding
56 ;;;; the cache) takes place: the loosing one will have its work
57 ;;;; redone later. This also allows us to drop entries when the
58 ;;;; cache is about to grow insanely huge.
60 ;;;; The cache is essentially a specialized hash-table for layouts, used
61 ;;;; for memoization of effective methods, slot locations, and constant
64 ;;;; Subsequences of the cache vector are called cache lines.
66 ;;;; The cache vector uses the symbol SB-PCL::..EMPTY.. as a sentinel
67 ;;;; value, to allow storing NILs in the vector as well.
69 (defstruct (cache (:constructor %make-cache)
70 (:copier %copy-cache))
71 ;; Number of keys the cache uses.
72 (key-count 1 :type (integer 1 (#.call-arguments-limit)))
73 ;; True if we store values in the cache.
75 ;; Number of vector elements a single cache line uses in the vector.
76 ;; This is always a power of two, so that the vector length can be both
77 ;; an exact multiple of this and a power of two.
78 (line-size 1 :type (integer 1 #.most-positive-fixnum))
79 ;; Cache vector, its length is always both a multiple of line-size
80 ;; and a power of two. This is so that we can calculate
81 ;; (mod index (length vector))
83 (vector #() :type simple-vector)
84 ;; The bitmask used to calculate
85 ;; (mod (* line-size line-hash) (length vector))).
87 ;; Current probe-depth needed in the cache.
89 ;; Maximum allowed probe-depth before the cache needs to expand.
90 (limit 0 :type index))
92 (defun compute-cache-mask (vector-length line-size)
93 ;; Since both vector-length and line-size are powers of two, we
94 ;; can compute a bitmask such that
96 ;; (logand <mask> <combined-layout-hash>)
98 ;; is "morally equal" to
100 ;; (mod (* <line-size> <combined-layout-hash>) <vector-length>)
102 ;; This is it: (1- vector-length) is #b111... of the approriate size
103 ;; to get the MOD, and (- line-size) gives right the number of zero
104 ;; bits at the low end.
105 (logand (1- vector-length) (- line-size)))
107 ;;; The smallest power of two that is equal to or greater then X.
108 (declaim (inline power-of-two-ceiling))
109 (defun power-of-two-ceiling (x)
110 (ash 1 (integer-length (1- x))))
112 (defun cache-statistics (cache)
113 (let* ((vector (cache-vector cache))
114 (size (length vector))
115 (line-size (cache-line-size cache))
116 (total-lines (/ size line-size))
117 (free-lines (loop for i from 0 by line-size below size
118 unless (eq (svref vector i) '..empty..)
120 (values (- total-lines free-lines) total-lines
121 (cache-depth cache) (cache-limit cache))))
123 ;;; Don't allocate insanely huge caches: this is 4096 lines for a
124 ;;; value cache with 8-15 keys -- probably "big enough for anyone",
125 ;;; and 16384 lines for a commonplace 2-key value cache.
126 (defconstant +cache-vector-max-length+ (expt 2 16))
128 ;;; Compute the maximum allowed probe depth as a function of cache size.
129 ;;; Cache size refers to number of cache lines, not the length of the
132 ;;; FIXME: It would be nice to take the generic function optimization
133 ;;; policy into account here (speed vs. space.)
134 (declaim (inline compute-limit))
135 (defun compute-limit (size)
136 (ceiling (sqrt (sqrt size))))
138 ;;; Returns VALUE if it is not ..EMPTY.., otherwise executes ELSE:
139 (defmacro non-empty-or (value else)
140 (with-unique-names (n-value)
141 `(let ((,n-value ,value))
142 (if (eq ,n-value '..empty..)
146 ;;; Fast way to check if a thing found at the position of a cache key is one:
147 ;;; it is always either a wrapper, or the ..EMPTY.. symbol.
148 (declaim (inline cache-key-p))
149 (defun cache-key-p (thing)
150 (not (symbolp thing)))
152 (eval-when (:compile-toplevel :load-toplevel :execute)
153 (sb-kernel:define-structure-slot-compare-and-swap compare-and-swap-cache-depth
157 ;;; Utility macro for atomic updates without locking... doesn't
158 ;;; do much right now, and it would be nice to make this more magical.
159 (defmacro compare-and-swap (place old new)
160 (unless (consp place)
161 (error "Don't know how to compare and swap ~S." place))
164 `(simple-vector-compare-and-swap ,@(cdr place) ,old ,new))
166 `(compare-and-swap-cache-depth ,@(cdr place) ,old ,new))))
168 ;;; Atomically update the current probe depth of a cache.
169 (defun note-cache-depth (cache depth)
170 (loop for old = (cache-depth cache)
171 while (and (< old depth)
172 (not (eq old (compare-and-swap (cache-depth cache)
175 ;;; Compute the starting index of the next cache line in the cache vector.
176 (declaim (inline next-cache-index))
177 (defun next-cache-index (mask index line-size)
178 (logand mask (+ index line-size)))
180 ;;; Returns the hash-value for layout, or executes ELSE if the layout
182 (defmacro hash-layout-or (layout else)
183 (with-unique-names (n-hash)
184 `(let ((,n-hash (layout-clos-hash ,layout)))
189 ;;; Compute cache index for the cache and a list of layouts.
190 (declaim (inline compute-cache-index))
191 (defun compute-cache-index (cache layouts)
192 (let ((index (hash-layout-or (car layouts)
193 (return-from compute-cache-index nil))))
194 (declare (fixnum index))
195 (dolist (layout (cdr layouts))
196 (mixf index (hash-layout-or layout (return-from compute-cache-index nil))))
197 ;; align with cache lines
198 (logand index (cache-mask cache))))
200 ;;; Emit code that does lookup in cache bound to CACHE-VAR using
201 ;;; layouts bound to LAYOUT-VARS. Go to MISS-TAG on event of a miss or
202 ;;; invalid layout. Otherwise, if VALUE-VAR is non-nil, set it to the
203 ;;; value found. (VALUE-VAR is non-nil only when CACHE-VALUE is true.)
205 ;;; In other words, produces inlined code for COMPUTE-CACHE-INDEX when
206 ;;; number of keys and presence of values in the cache is known
208 (defun emit-cache-lookup (cache-var layout-vars miss-tag value-var)
209 (let ((line-size (power-of-two-ceiling (+ (length layout-vars)
210 (if value-var 1 0)))))
211 (with-unique-names (n-index n-vector n-depth n-pointer n-mask
212 MATCH-WRAPPERS EXIT-WITH-HIT)
213 `(let* ((,n-index (hash-layout-or ,(car layout-vars) (go ,miss-tag)))
214 (,n-vector (cache-vector ,cache-var))
215 (,n-mask (cache-mask ,cache-var)))
216 (declare (index ,n-index))
217 ,@(mapcar (lambda (layout-var)
218 `(mixf ,n-index (hash-layout-or ,layout-var (go ,miss-tag))))
220 ;; align with cache lines
221 (setf ,n-index (logand ,n-index ,n-mask))
222 (let ((,n-depth (cache-depth ,cache-var))
223 (,n-pointer ,n-index))
224 (declare (index ,n-depth ,n-pointer))
230 (eq ,layout-var (svref ,n-vector ,n-pointer))
234 `((setf ,value-var (non-empty-or (svref ,n-vector ,n-pointer)
240 (setf ,n-index (next-cache-index ,n-mask ,n-index ,line-size)
245 ;;; Probes CACHE for LAYOUTS.
247 ;;; Returns two values: a boolean indicating a hit or a miss, and a secondary
248 ;;; value that is the value that was stored in the cache if any.
249 (defun probe-cache (cache layouts)
250 (unless (consp layouts)
251 (setf layouts (list layouts)))
252 (let ((vector (cache-vector cache))
253 (key-count (cache-key-count cache))
254 (line-size (cache-line-size cache))
255 (mask (cache-mask cache)))
256 (flet ((probe-line (base)
258 (loop for offset from 0 below key-count
259 for layout in layouts do
260 (unless (eq layout (svref vector (+ base offset)))
263 ;; all layouts match!
264 (let ((value (when (cache-value cache)
265 (non-empty-or (svref vector (+ base key-count))
267 (return-from probe-cache (values t value)))
269 (return-from probe-line (next-cache-index mask base line-size)))))
270 (let ((index (compute-cache-index cache layouts)))
272 (loop repeat (1+ (cache-depth cache)) do
273 (setf index (probe-line index)))))))
276 ;;; Tries to write LAYOUTS and VALUE at the cache line starting at
277 ;;; the index BASE. Returns true on success, and false on failure.
278 (defun try-update-cache-line (cache base layouts value)
279 (declare (index base))
280 (let ((vector (cache-vector cache))
282 ;; If we unwind from here, we will be left with an incomplete
283 ;; cache line, but that is OK: next write using the same layouts
284 ;; will fill it, and reads will treat an incomplete line as a
285 ;; miss -- causing it to be filled.
286 (loop for old = (compare-and-swap (svref vector base) '..empty.. new) do
287 (when (and (cache-key-p old) (not (eq old new)))
288 ;; The place was already taken, and doesn't match our key.
289 (return-from try-update-cache-line nil))
291 ;; All keys match or succesfully saved, save our value --
292 ;; just smash it in. Until the first time it is written
293 ;; there is ..EMPTY.. here, which probes look for, so we
294 ;; don't get bogus hits. This is necessary because we want
295 ;; to be able store arbitrary values here for use with
296 ;; constant-value dispatch functions.
297 (when (cache-value cache)
298 (setf (svref vector (1+ base)) value))
299 (return-from try-update-cache-line t))
300 (setf new (pop layouts))
303 ;;; Tries to write LAYOUTS and VALUE somewhere in the cache. Returns
304 ;;; true on success and false on failure, meaning the cache is too
306 (defun try-update-cache (cache layouts value)
307 (let ((vector (cache-vector cache))
308 (index (or (compute-cache-index cache layouts)
309 ;; At least one of the layouts was invalid: just
310 ;; pretend we updated the cache, and let the next
311 ;; read pick up the mess.
312 (return-from try-update-cache t)))
313 (line-size (cache-line-size cache))
314 (mask (cache-mask cache)))
315 (declare (index index))
316 (loop for depth from 0 upto (cache-limit cache) do
317 (when (try-update-cache-line cache index layouts value)
318 (note-cache-depth cache depth)
319 (return-from try-update-cache t))
320 (setf index (next-cache-index mask index line-size)))))
322 ;;; Constructs a new cache.
323 (defun make-cache (&key (key-count (missing-arg)) (value (missing-arg))
325 (let* ((line-size (power-of-two-ceiling (+ key-count (if value 1 0))))
326 (adjusted-size (power-of-two-ceiling size))
327 (length (* adjusted-size line-size)))
328 (if (<= length +cache-vector-max-length+)
329 (%make-cache :key-count key-count
331 :vector (make-array length :initial-element '..empty..)
333 :mask (compute-cache-mask length line-size)
334 :limit (compute-limit adjusted-size))
335 ;; Make a smaller one, then
336 (make-cache :key-count key-count :value value :size (ceiling size 2)))))
338 ;;;; Copies and expands the cache, dropping any invalidated or
339 ;;;; incomplete lines.
340 (defun copy-and-expand-cache (cache)
341 (let ((copy (%copy-cache cache))
342 (length (length (cache-vector cache))))
343 (when (< length +cache-vector-max-length+)
344 (setf length (* 2 length)))
347 ;; Blow way the old vector first, so a GC potentially triggered by
348 ;; MAKE-ARRAY can collect it.
349 (setf (cache-vector copy) #()
350 (cache-vector copy) (make-array length :initial-element '..empty..)
352 (cache-mask copy) (compute-cache-mask length (cache-line-size cache))
353 (cache-limit copy) (compute-limit (/ length (cache-line-size cache))))
354 (map-cache (lambda (layouts value)
355 (unless (try-update-cache copy layouts value)
356 ;; If the cache would grow too much we drop the
357 ;; remaining the entries that don't fit. FIXME:
358 ;; It would be better to drop random entries to
359 ;; avoid getting into a rut here (best done by
360 ;; making MAP-CACHE map in a random order?), and
361 ;; possibly to downsize the cache more
362 ;; aggressively (on the assumption that most
363 ;; entries aren't getting used at the moment.)
364 (when (< length +cache-vector-max-length+)
365 (setf length (* 2 length))
370 (defun cache-has-invalid-entries-p (cache)
371 (let ((vector (cache-vector cache))
372 (line-size (cache-line-size cache))
373 (key-count (cache-key-count cache))
374 (mask (cache-mask cache))
377 ;; Check if the line is in use, and check validity of the keys.
378 (let ((key1 (svref vector index)))
379 (when (cache-key-p key1)
380 (if (zerop (layout-clos-hash key1))
381 ;; First key invalid.
382 (return-from cache-has-invalid-entries-p t)
383 ;; Line is in use and the first key is valid: check the rest.
384 (loop for offset from 1 below key-count
385 do (let ((thing (svref vector (+ index offset))))
386 (when (or (not (cache-key-p thing))
387 (zerop (layout-clos-hash thing)))
388 ;; Incomplete line or invalid layout.
389 (return-from cache-has-invalid-entries-p t)))))))
390 ;; Line empty of valid, onwards.
391 (setf index (next-cache-index mask index line-size))
394 (return-from cache-has-invalid-entries-p nil)))))
396 (defun hash-table-to-cache (table &key value key-count)
397 (let ((cache (make-cache :key-count key-count :value value
398 :size (hash-table-count table))))
399 (maphash (lambda (class value)
400 (setq cache (fill-cache cache (class-wrapper class) value)))
404 ;;; Inserts VALUE to CACHE keyd by LAYOUTS. Expands the cache if
405 ;;; necessary, and returns the new cache.
406 (defun fill-cache (cache layouts value)
408 ((%fill-cache (cache layouts value)
409 (cond ((try-update-cache cache layouts value)
411 ((cache-has-invalid-entries-p cache)
412 ;; Don't expand yet: maybe there will be enough space if
413 ;; we just drop the invalid entries.
414 (%fill-cache (copy-cache cache) layouts value))
416 (%fill-cache (copy-and-expand-cache cache) layouts value)))))
418 (%fill-cache cache layouts value)
419 (%fill-cache cache (list layouts) value))))
421 ;;; Calls FUNCTION with all layouts and values in cache.
422 (defun map-cache (function cache)
423 (let* ((vector (cache-vector cache))
424 (key-count (cache-key-count cache))
425 (valuep (cache-value cache))
426 (line-size (cache-line-size cache))
427 (mask (cache-mask cache))
428 (fun (if (functionp function)
430 (fdefinition function)))
436 (loop for offset from 0 below key-count
437 collect (non-empty-or (svref vector (+ offset index))
439 (let ((value (when valuep
440 (non-empty-or (svref vector (+ index key-count))
442 ;; Let the callee worry about invalid layouts
443 (funcall fun layouts value)))
445 (setf index (next-cache-index mask index line-size))
446 (unless (zerop index)
450 ;;; Copying a cache without expanding it is very much like mapping it:
451 ;;; we need to be carefull because there may be updates while we are
452 ;;; copying it, and we don't want to copy incomplete entries or invalid
454 (defun copy-cache (cache)
455 (let* ((vector (cache-vector cache))
456 (copy (make-array (length vector) :initial-element '..empty..))
457 (line-size (cache-line-size cache))
458 (key-count (cache-key-count cache))
459 (valuep (cache-value cache))
460 (mask (cache-mask cache))
461 (size (/ (length vector) line-size))
467 (let ((layouts (loop for offset from 0 below key-count
468 collect (non-empty-or (svref vector (+ index offset))
470 ;; Check validity & compute primary index.
471 (let ((primary (or (compute-cache-index cache layouts)
473 ;; Check & copy value.
475 (setf (svref copy (+ index key-count))
476 (non-empty-or (svref vector (+ index key-count))
479 (loop for offset from 0 below key-count do
480 (setf (svref copy (+ index offset)) (pop layouts)))
481 ;; Update probe depth.
482 (let ((distance (/ (- index primary) line-size)))
483 (setf depth (max depth (if (minusp distance)
484 ;; account for wrap-around
488 (setf index (next-cache-index mask index line-size))
489 (unless (zerop index)
491 (%make-cache :vector copy
493 :key-count (cache-key-count cache)
497 :limit (cache-limit cache))))
499 ;;;; For debugging & collecting statistics.
501 (defun map-all-caches (function)
502 (dolist (p (list-all-packages))
504 (when (eq p (symbol-package s))
505 (dolist (name (list s
509 (slot-boundp-name s)))
511 (let ((fun (fdefinition name)))
512 (when (typep fun 'generic-function)
513 (let ((cache (gf-dfun-cache fun)))
515 (funcall function name cache)))))))))))
517 (defun check-cache-consistency (cache)
518 (let ((table (make-hash-table :test 'equal)))
519 (map-cache (lambda (layouts value)
520 (declare (ignore value))
521 (if (gethash layouts table)
522 (cerror "Check futher."
523 "Multiple appearances of ~S." layouts)
524 (setf (gethash layouts table) t)))