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 ;;; Atomically update the current probe depth of a cache.
153 (defun note-cache-depth (cache depth)
154 (loop for old = (cache-depth cache)
155 while (and (< old depth)
156 (not (eq old (compare-and-swap (cache-depth cache)
159 ;;; Compute the starting index of the next cache line in the cache vector.
160 (declaim (inline next-cache-index))
161 (defun next-cache-index (mask index line-size)
162 (declare (type (unsigned-byte #.sb-vm:n-word-bits) index line-size mask))
163 (logand mask (+ index line-size)))
165 ;;; Returns the hash-value for layout, or executes ELSE if the layout
167 (defmacro hash-layout-or (layout else)
168 (with-unique-names (n-hash)
169 `(let ((,n-hash (layout-clos-hash ,layout)))
174 ;;; Compute cache index for the cache and a list of layouts.
175 (declaim (inline compute-cache-index))
176 (defun compute-cache-index (cache layouts)
177 (let ((index (hash-layout-or (car layouts)
178 (return-from compute-cache-index nil))))
179 (declare (fixnum index))
180 (dolist (layout (cdr layouts))
181 (mixf index (hash-layout-or layout (return-from compute-cache-index nil))))
182 ;; align with cache lines
183 (logand index (cache-mask cache))))
185 ;;; Emit code that does lookup in cache bound to CACHE-VAR using
186 ;;; layouts bound to LAYOUT-VARS. Go to MISS-TAG on event of a miss or
187 ;;; invalid layout. Otherwise, if VALUE-VAR is non-nil, set it to the
188 ;;; value found. (VALUE-VAR is non-nil only when CACHE-VALUE is true.)
190 ;;; In other words, produces inlined code for COMPUTE-CACHE-INDEX when
191 ;;; number of keys and presence of values in the cache is known
193 (defun emit-cache-lookup (cache-var layout-vars miss-tag value-var)
194 (let ((line-size (power-of-two-ceiling (+ (length layout-vars)
195 (if value-var 1 0)))))
196 (with-unique-names (n-index n-vector n-depth n-pointer n-mask
197 MATCH-WRAPPERS EXIT-WITH-HIT)
198 `(let* ((,n-index (hash-layout-or ,(car layout-vars) (go ,miss-tag)))
199 (,n-vector (cache-vector ,cache-var))
200 (,n-mask (cache-mask ,cache-var)))
201 (declare (index ,n-index))
202 ,@(mapcar (lambda (layout-var)
203 `(mixf ,n-index (hash-layout-or ,layout-var (go ,miss-tag))))
205 ;; align with cache lines
206 (setf ,n-index (logand ,n-index ,n-mask))
207 (let ((,n-depth (cache-depth ,cache-var))
208 (,n-pointer ,n-index))
209 (declare (index ,n-depth ,n-pointer))
215 (eq ,layout-var (svref ,n-vector ,n-pointer))
219 `((setf ,value-var (non-empty-or (svref ,n-vector ,n-pointer)
225 (setf ,n-index (next-cache-index ,n-mask ,n-index ,line-size)
230 ;;; Probes CACHE for LAYOUTS.
232 ;;; Returns two values: a boolean indicating a hit or a miss, and a secondary
233 ;;; value that is the value that was stored in the cache if any.
234 (defun probe-cache (cache layouts)
235 (declare (optimize speed))
236 (unless (consp layouts)
237 (setf layouts (list layouts)))
238 (let ((vector (cache-vector cache))
239 (key-count (cache-key-count cache))
240 (line-size (cache-line-size cache))
241 (mask (cache-mask cache)))
242 (flet ((probe-line (base)
243 (declare (optimize (sb-c::type-check 0)))
245 (loop for offset of-type index from 0 below key-count
246 for layout in layouts do
247 (unless (eq layout (svref vector (+ base offset)))
250 ;; all layouts match!
251 (let ((value (when (cache-value cache)
252 (non-empty-or (svref vector (+ base key-count))
254 (return-from probe-cache (values t value)))
256 (return-from probe-line (next-cache-index mask base line-size)))))
257 (declare (ftype (function (index) (values index &optional)) probe-line))
258 (let ((index (compute-cache-index cache layouts)))
260 (loop repeat (1+ (cache-depth cache))
261 do (setf index (probe-line index)))))))
264 ;;; Tries to write LAYOUTS and VALUE at the cache line starting at
265 ;;; the index BASE. Returns true on success, and false on failure.
266 (defun try-update-cache-line (cache base layouts value)
267 (declare (index base))
268 (let ((vector (cache-vector cache))
270 ;; If we unwind from here, we will be left with an incomplete
271 ;; cache line, but that is OK: next write using the same layouts
272 ;; will fill it, and reads will treat an incomplete line as a
273 ;; miss -- causing it to be filled.
274 (loop for old = (compare-and-swap (svref vector base) '..empty.. new) do
275 (when (and (cache-key-p old) (not (eq old new)))
276 ;; The place was already taken, and doesn't match our key.
277 (return-from try-update-cache-line nil))
279 ;; All keys match or succesfully saved, save our value --
280 ;; just smash it in. Until the first time it is written
281 ;; there is ..EMPTY.. here, which probes look for, so we
282 ;; don't get bogus hits. This is necessary because we want
283 ;; to be able store arbitrary values here for use with
284 ;; constant-value dispatch functions.
285 (when (cache-value cache)
286 (setf (svref vector (1+ base)) value))
287 (return-from try-update-cache-line t))
288 (setf new (pop layouts))
291 ;;; Tries to write LAYOUTS and VALUE somewhere in the cache. Returns
292 ;;; true on success and false on failure, meaning the cache is too
294 (defun try-update-cache (cache layouts value)
295 (let ((index (or (compute-cache-index cache layouts)
296 ;; At least one of the layouts was invalid: just
297 ;; pretend we updated the cache, and let the next
298 ;; read pick up the mess.
299 (return-from try-update-cache t)))
300 (line-size (cache-line-size cache))
301 (mask (cache-mask cache)))
302 (declare (index index))
303 (loop for depth from 0 upto (cache-limit cache) do
304 (when (try-update-cache-line cache index layouts value)
305 (note-cache-depth cache depth)
306 (return-from try-update-cache t))
307 (setf index (next-cache-index mask index line-size)))))
309 ;;; Constructs a new cache.
310 (defun make-cache (&key (key-count (missing-arg)) (value (missing-arg))
312 (let* ((line-size (power-of-two-ceiling (+ key-count (if value 1 0))))
313 (adjusted-size (power-of-two-ceiling size))
314 (length (* adjusted-size line-size)))
315 (if (<= length +cache-vector-max-length+)
316 (%make-cache :key-count key-count
318 :vector (make-array length :initial-element '..empty..)
320 :mask (compute-cache-mask length line-size)
321 :limit (compute-limit adjusted-size))
322 ;; Make a smaller one, then
323 (make-cache :key-count key-count :value value :size (ceiling size 2)))))
325 (defconstant n-fixnum-bits #.(integer-length most-positive-fixnum))
327 ;;;; Copies and expands the cache, dropping any invalidated or
328 ;;;; incomplete lines.
329 (defun copy-and-expand-cache (cache layouts value)
330 (let ((copy (%copy-cache cache))
331 (length (length (cache-vector cache)))
332 (drop-random-entries nil))
333 (declare (index length))
334 (when (< length +cache-vector-max-length+)
335 (setf length (* 2 length)))
338 ;; Blow way the old vector first, so a GC potentially triggered by
339 ;; MAKE-ARRAY can collect it.
340 (setf (cache-vector copy) #()
341 (cache-vector copy) (make-array length :initial-element '..empty..)
343 (cache-mask copy) (compute-cache-mask length (cache-line-size cache))
344 (cache-limit copy) (compute-limit (/ length (cache-line-size cache))))
345 ;; First insert the new one -- if we don't do this first and
346 ;; the cache has reached it's maximum size we may end up
347 ;; looping in FILL-CACHE.
348 (unless (try-update-cache copy layouts value)
349 (bug "Could not insert ~S:~S to supposedly empty ~S." layouts value copy))
350 (map-cache (if drop-random-entries
351 ;; The cache is at maximum size, and all entries
352 ;; do not fit in. Drop a random ~50% of entries,
353 ;; to make space for new ones. This needs to be
354 ;; random, since otherwise we might get in a
355 ;; rut: add A causing B to drop, then add B
356 ;; causing A to drop... repeat ad nauseam,
357 ;; spending most of the time here instead of
358 ;; doing real work. 50% because if we drop to
359 ;; few we need to do this almost right away
360 ;; again, and if we drop to many, we need to
361 ;; recompute more then we'd like.
362 ;; _Experimentally_ 50% seems to perform the
363 ;; best, but it would be nice to have a proper
365 (flet ((random-fixnum ()
366 (random (1+ most-positive-fixnum))))
367 (let ((drops (random-fixnum))
368 (drop-pos n-fixnum-bits))
369 (declare (fixnum drops)
370 (type (integer 0 #.n-fixnum-bits) drop-pos))
371 (lambda (layouts value)
372 (when (logbitp (the unsigned-byte (decf drop-pos)) drops)
373 (try-update-cache copy layouts value))
374 (when (zerop drop-pos)
375 (setf drops (random-fixnum)
376 drop-pos n-fixnum-bits)))))
377 (lambda (layouts value)
378 (unless (try-update-cache copy layouts value)
379 ;; Didn't fit -- expand the cache, or drop
380 ;; a few unlucky ones.
381 (if (< length +cache-vector-max-length+)
382 (setf length (* 2 length))
383 (setf drop-random-entries t))
388 (defun cache-has-invalid-entries-p (cache)
389 (let ((vector (cache-vector cache))
390 (line-size (cache-line-size cache))
391 (key-count (cache-key-count cache))
392 (mask (cache-mask cache))
395 ;; Check if the line is in use, and check validity of the keys.
396 (let ((key1 (svref vector index)))
397 (when (cache-key-p key1)
398 (if (zerop (layout-clos-hash key1))
399 ;; First key invalid.
400 (return-from cache-has-invalid-entries-p t)
401 ;; Line is in use and the first key is valid: check the rest.
402 (loop for offset from 1 below key-count
403 do (let ((thing (svref vector (+ index offset))))
404 (when (or (not (cache-key-p thing))
405 (zerop (layout-clos-hash thing)))
406 ;; Incomplete line or invalid layout.
407 (return-from cache-has-invalid-entries-p t)))))))
408 ;; Line empty of valid, onwards.
409 (setf index (next-cache-index mask index line-size))
412 (return-from cache-has-invalid-entries-p nil)))))
414 (defun hash-table-to-cache (table &key value key-count)
415 (let ((cache (make-cache :key-count key-count :value value
416 :size (hash-table-count table))))
417 (maphash (lambda (class value)
418 (setq cache (fill-cache cache (class-wrapper class) value)))
422 ;;; Inserts VALUE to CACHE keyd by LAYOUTS. Expands the cache if
423 ;;; necessary, and returns the new cache.
424 (defun fill-cache (cache layouts value)
426 ((%fill-cache (cache layouts value expand)
427 (cond ((try-update-cache cache layouts value)
429 ((and (not expand) (cache-has-invalid-entries-p cache))
430 ;; Don't expand yet: maybe there will be enough space if
431 ;; we just drop the invalid entries.
432 (%fill-cache (copy-cache cache) layouts value t))
434 (copy-and-expand-cache cache layouts value)))))
436 (%fill-cache cache layouts value nil)
437 (%fill-cache cache (list layouts) value nil))))
439 ;;; Calls FUNCTION with all layouts and values in cache.
440 (defun map-cache (function cache)
441 (let* ((vector (cache-vector cache))
442 (key-count (cache-key-count cache))
443 (valuep (cache-value cache))
444 (line-size (cache-line-size cache))
445 (mask (cache-mask cache))
446 (fun (if (functionp function)
448 (fdefinition function)))
453 (loop for offset from 0 below key-count
454 collect (non-empty-or (svref vector (+ offset index))
456 (let ((value (when valuep
457 (non-empty-or (svref vector (+ index key-count))
459 ;; Let the callee worry about invalid layouts
460 (funcall fun layouts value)))
462 (setf index (next-cache-index mask index line-size))
463 (unless (zerop index)
467 ;;; Copying a cache without expanding it is very much like mapping it:
468 ;;; we need to be carefull because there may be updates while we are
469 ;;; copying it, and we don't want to copy incomplete entries or invalid
471 (defun copy-cache (cache)
472 (let* ((vector (cache-vector cache))
473 (copy (make-array (length vector) :initial-element '..empty..))
474 (line-size (cache-line-size cache))
475 (key-count (cache-key-count cache))
476 (valuep (cache-value cache))
477 (mask (cache-mask cache))
478 (size (/ (length vector) line-size))
483 (let ((layouts (loop for offset from 0 below key-count
484 collect (non-empty-or (svref vector (+ index offset))
486 ;; Check validity & compute primary index.
487 (let ((primary (or (compute-cache-index cache layouts)
489 ;; Check & copy value.
491 (setf (svref copy (+ index key-count))
492 (non-empty-or (svref vector (+ index key-count))
495 (loop for offset from 0 below key-count do
496 (setf (svref copy (+ index offset)) (pop layouts)))
497 ;; Update probe depth.
498 (let ((distance (/ (- index primary) line-size)))
499 (setf depth (max depth (if (minusp distance)
500 ;; account for wrap-around
504 (setf index (next-cache-index mask index line-size))
505 (unless (zerop index)
507 (%make-cache :vector copy
509 :key-count (cache-key-count cache)
513 :limit (cache-limit cache))))
515 ;;;; For debugging & collecting statistics.
517 (defun map-all-caches (function)
518 (dolist (p (list-all-packages))
520 (when (eq p (symbol-package s))
521 (dolist (name (list s
525 (slot-boundp-name s)))
527 (let ((fun (fdefinition name)))
528 (when (typep fun 'generic-function)
529 (let ((cache (gf-dfun-cache fun)))
531 (funcall function name cache)))))))))))
533 (defun check-cache-consistency (cache)
534 (let ((table (make-hash-table :test 'equal)))
535 (map-cache (lambda (layouts value)
536 (declare (ignore value))
537 (if (gethash layouts table)
538 (cerror "Check futher."
539 "Multiple appearances of ~S." layouts)
540 (setf (gethash layouts table) t)))