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 (mod index (length vector))l
86 ;; Current probe-depth needed in the cache.
88 ;; Maximum allowed probe-depth before the cache needs to expand.
89 (limit 0 :type index))
91 ;;; The smallest power of two that is equal to or greater then X.
92 (declaim (inline power-of-two-ceiling))
93 (defun power-of-two-ceiling (x)
94 (ash 1 (integer-length (1- x))))
96 (defun cache-statistics (cache)
97 (let* ((vector (cache-vector cache))
98 (size (length vector))
99 (line-size (cache-line-size cache))
100 (total-lines (/ size line-size))
101 (free-lines (loop for i from 0 by line-size below size
102 unless (eq (svref vector i) '..empty..)
104 (values (- total-lines free-lines) total-lines
105 (cache-depth cache) (cache-limit cache))))
107 ;;; Don't allocate insanely huge caches.
108 (defconstant +cache-vector-max-length+ (expt 2 14))
110 ;;; Compute the maximum allowed probe depth as a function of cache size.
111 ;;; Cache size refers to number of cache lines, not the length of the
114 ;;; FIXME: It would be nice to take the generic function optimization
115 ;;; policy into account here (speed vs. space.)
116 (declaim (inline compute-limit))
117 (defun compute-limit (size)
118 (ceiling (sqrt size) 2))
120 ;;; Returns VALUE if it is not ..EMPTY.., otherwise executes ELSE:
121 (defmacro non-empty-or (value else)
122 (with-unique-names (n-value)
123 `(let ((,n-value ,value))
124 (if (eq ,n-value '..empty..)
128 ;;; Fast way to check if a thing found at the position of a cache key is one:
129 ;;; it is always either a wrapper, or the ..EMPTY.. symbol.
130 (declaim (inline cache-key-p))
131 (defun cache-key-p (thing)
132 (not (symbolp thing)))
134 (eval-when (:compile-toplevel :load-toplevel :execute)
135 (sb-kernel:define-structure-slot-compare-and-swap compare-and-swap-cache-depth
139 ;;; Utility macro for atomic updates without locking... doesn't
140 ;;; do much right now, and it would be nice to make this more magical.
141 (defmacro compare-and-swap (place old new)
142 (unless (consp place)
143 (error "Don't know how to compare and swap ~S." place))
146 `(simple-vector-compare-and-swap ,@(cdr place) ,old ,new))
148 `(compare-and-swap-cache-depth ,@(cdr place) ,old ,new))))
150 ;;; Atomically update the current probe depth of a cache.
151 (defun note-cache-depth (cache depth)
152 (loop for old = (cache-depth cache)
153 while (and (< old depth)
154 (not (eq old (compare-and-swap (cache-depth cache)
157 ;;; Compute the starting index of the next cache line in the cache vector.
158 (declaim (inline next-cache-index))
159 (defun next-cache-index (mask index line-size)
160 (logand mask (+ index line-size)))
162 ;;; Returns the hash-value for layout, or executes ELSE if the layout
164 (defmacro hash-layout-or (layout else)
165 (with-unique-names (n-hash)
166 `(let ((,n-hash (layout-clos-hash ,layout)))
171 ;;; Compute cache index for the cache and a list of layouts.
172 (declaim (inline compute-cache-index))
173 (defun compute-cache-index (cache layouts)
174 (let ((index (hash-layout-or (car layouts)
175 (return-from compute-cache-index nil))))
176 (dolist (layout (cdr layouts))
177 (mixf index (hash-layout-or layout (return-from compute-cache-index nil))))
178 ;; align with cache lines
179 (logand (* (cache-line-size cache) index) (cache-mask cache))))
181 ;;; Emit code that does lookup in cache bound to CACHE-VAR using
182 ;;; layouts bound to LAYOUT-VARS. Go to MISS-TAG on event of a miss or
183 ;;; invalid layout. Otherwise, if VALUE-VAR is non-nil, set it to the
184 ;;; value found. (VALUE-VAR is non-nil only when CACHE-VALUE is true.)
186 ;;; In other words, produces inlined code for COMPUTE-CACHE-INDEX when
187 ;;; number of keys and presence of values in the cache is known
189 (defun emit-cache-lookup (cache-var layout-vars miss-tag value-var)
190 (let ((line-size (power-of-two-ceiling (+ (length layout-vars)
191 (if value-var 1 0)))))
192 (with-unique-names (n-index n-vector n-depth n-pointer n-mask
193 MATCH-WRAPPERS EXIT-WITH-HIT)
194 `(let* ((,n-index (hash-layout-or ,(car layout-vars) (go ,miss-tag)))
195 (,n-vector (cache-vector ,cache-var)))
196 (declare (index ,n-index))
197 ,@(mapcar (lambda (layout-var)
198 `(mixf ,n-index (hash-layout-or ,layout-var (go ,miss-tag))))
200 ;; align with cache lines
201 (setf ,n-index (logand (* ,line-size ,n-index) (cache-mask ,cache-var)))
202 (let ((,n-depth (cache-depth ,cache-var))
203 (,n-pointer ,n-index)
204 (,n-mask (cache-mask ,cache-var)))
205 (declare (index ,n-depth ,n-pointer))
211 (eq ,layout-var (svref ,n-vector ,n-pointer))
215 `((setf ,value-var (non-empty-or (svref ,n-vector ,n-pointer)
221 (setf ,n-index (next-cache-index ,n-mask ,n-index ,line-size)
226 ;;; Probes CACHE for LAYOUTS.
228 ;;; Returns two values: a boolean indicating a hit or a miss, and a secondary
229 ;;; value that is the value that was stored in the cache if any.
230 (defun probe-cache (cache layouts)
231 (unless (consp layouts)
232 (setf layouts (list layouts)))
233 (let ((vector (cache-vector cache))
234 (key-count (cache-key-count cache))
235 (line-size (cache-line-size cache))
236 (mask (cache-mask cache)))
237 (flet ((probe-line (base)
239 (loop for offset from 0 below key-count
240 for layout in layouts do
241 (unless (eq layout (svref vector (+ base offset)))
244 ;; all layouts match!
245 (let ((value (when (cache-value cache)
246 (non-empty-or (svref vector (+ base key-count))
248 (return-from probe-cache (values t value)))
250 (return-from probe-line (next-cache-index mask base line-size)))))
251 (let ((index (compute-cache-index cache layouts)))
253 (loop repeat (1+ (cache-depth cache)) do
254 (setf index (probe-line index)))))))
257 ;;; Tries to write LAYOUTS and VALUE at the cache line starting at
258 ;;; the index BASE. Returns true on success, and false on failure.
259 (defun try-update-cache-line (cache base layouts value)
260 (declare (index base))
261 (let ((vector (cache-vector cache))
263 ;; If we unwind from here, we will be left with an incomplete
264 ;; cache line, but that is OK: next write using the same layouts
265 ;; will fill it, and reads will treat an incomplete line as a
266 ;; miss -- causing it to be filled.
267 (loop for old = (compare-and-swap (svref vector base) '..empty.. new) do
268 (when (and (cache-key-p old) (not (eq old new)))
269 ;; The place was already taken, and doesn't match our key.
270 (return-from try-update-cache-line nil))
272 ;; All keys match or succesfully saved, save our value --
273 ;; just smash it in. Until the first time it is written
274 ;; there is ..EMPTY.. here, which probes look for, so we
275 ;; don't get bogus hits. This is necessary because we want
276 ;; to be able store arbitrary values here for use with
277 ;; constant-value dispatch functions.
278 (when (cache-value cache)
279 (setf (svref vector (1+ base)) value))
280 (return-from try-update-cache-line t))
281 (setf new (pop layouts))
284 ;;; Tries to write LAYOUTS and VALUE somewhere in the cache. Returns
285 ;;; true on success and false on failure, meaning the cache is too
287 (defun try-update-cache (cache layouts value)
288 (let ((vector (cache-vector cache))
289 (index (or (compute-cache-index cache layouts)
290 ;; At least one of the layouts was invalid: just
291 ;; pretend we updated the cache, and let the next
292 ;; read pick up the mess.
293 (return-from try-update-cache t)))
294 (line-size (cache-line-size cache))
295 (mask (cache-mask cache)))
296 (declare (index index))
297 (loop for depth from 0 upto (cache-limit cache) do
298 (when (try-update-cache-line cache index layouts value)
299 (note-cache-depth cache depth)
300 (return-from try-update-cache t))
301 (setf index (next-cache-index mask index line-size)))))
303 ;;; Constructs a new cache.
304 (defun make-cache (&key (key-count (missing-arg)) (value (missing-arg))
306 (let* ((line-size (power-of-two-ceiling (+ key-count (if value 1 0))))
307 (adjusted-size (power-of-two-ceiling size))
308 (length (* adjusted-size line-size)))
309 (if (<= length +cache-vector-max-length+)
310 (%make-cache :key-count key-count
312 :vector (make-array length :initial-element '..empty..)
315 :limit (compute-limit adjusted-size))
316 ;; Make a smaller one, then
317 (make-cache :key-count key-count :value value :size (ceiling size 2)))))
319 ;;;; Copies and expands the cache, dropping any invalidated or
320 ;;;; incomplete lines.
321 (defun copy-and-expand-cache (cache)
322 (let ((copy (%copy-cache cache))
323 (length (length (cache-vector cache))))
324 (when (< length +cache-vector-max-length+)
325 (setf length (* 2 length)))
328 (setf (cache-vector copy) (make-array length :initial-element '..empty..)
330 (cache-mask copy) (1- length)
331 (cache-limit copy) (compute-limit (/ length (cache-line-size cache))))
332 (map-cache (lambda (layouts value)
333 (unless (try-update-cache copy layouts value)
334 ;; If the cache would grow too much we drop the
335 ;; remaining the entries that don't fit. FIXME:
336 ;; It would be better to drop random entries to
337 ;; avoid getting into a rut here (best done by
338 ;; making MAP-CACHE map in a random order?), and
339 ;; possibly to downsize the cache more
340 ;; aggressively (on the assumption that most
341 ;; entries aren't getting used at the moment.)
342 (when (< length +cache-vector-max-length+)
343 (setf length (* 2 length))
348 (defun cache-has-invalid-entries-p (cache)
349 (and (find-if (lambda (elt)
350 (and (typep elt 'layout)
351 (zerop (layout-clos-hash elt))))
352 (cache-vector cache))
355 (defun hash-table-to-cache (table &key value key-count)
356 (let ((cache (make-cache :key-count key-count :value value
357 :size (hash-table-count table))))
358 (maphash (lambda (class value)
359 (setq cache (fill-cache cache (class-wrapper class) value)))
363 ;;; Inserts VALUE to CACHE keyd by LAYOUTS. Expands the cache if
364 ;;; necessary, and returns the new cache.
365 (defun fill-cache (cache layouts value)
367 ((%fill-cache (cache layouts value)
368 (cond ((try-update-cache cache layouts value)
370 ((cache-has-invalid-entries-p cache)
371 ;; Don't expand yet: maybe there will be enough space if
372 ;; we just drop the invalid entries.
373 (%fill-cache (copy-cache cache) layouts value))
375 (%fill-cache (copy-and-expand-cache cache) layouts value)))))
377 (%fill-cache cache layouts value)
378 (%fill-cache cache (list layouts) value))))
380 ;;; Calls FUNCTION with all layouts and values in cache.
381 (defun map-cache (function cache)
382 (let* ((vector (cache-vector cache))
383 (key-count (cache-key-count cache))
384 (valuep (cache-value cache))
385 (line-size (cache-line-size cache))
386 (mask (cache-mask cache))
387 (fun (if (functionp function)
389 (fdefinition function)))
395 (loop for offset from 0 below key-count
396 collect (non-empty-or (svref vector (+ offset index))
398 (let ((value (when valuep
399 (non-empty-or (svref vector (+ index key-count))
401 ;; Let the callee worry about invalid layouts
402 (funcall fun layouts value)))
404 (setf index (next-cache-index mask index line-size))
405 (unless (zerop index)
409 ;;; Copying a cache without expanding it is very much like mapping it:
410 ;;; we need to be carefull because there may be updates while we are
411 ;;; copying it, and we don't want to copy incomplete entries or invalid
413 (defun copy-cache (cache)
414 (let* ((vector (cache-vector cache))
415 (copy (make-array (length vector) :initial-element '..empty..))
416 (line-size (cache-line-size cache))
417 (key-count (cache-key-count cache))
418 (valuep (cache-value cache))
419 (size (/ (length vector) line-size))
420 (mask (cache-mask cache))
426 (let ((layouts (loop for offset from 0 below key-count
427 collect (non-empty-or (svref vector (+ index offset))
429 ;; Check validity & compute primary index.
430 (let ((primary (or (compute-cache-index cache layouts)
432 ;; Check & copy value.
434 (setf (svref copy (+ index key-count))
435 (non-empty-or (svref vector (+ index key-count))
438 (loop for offset from 0 below key-count do
439 (setf (svref copy (+ index offset)) (pop layouts)))
440 ;; Update probe depth.
441 (let ((distance (/ (- index primary) line-size)))
442 (setf depth (max depth (if (minusp distance)
443 ;; account for wrap-around
447 (setf index (next-cache-index mask index line-size))
448 (unless (zerop index)
450 (%make-cache :vector copy
452 :key-count (cache-key-count cache)
455 :mask (cache-mask cache)
456 :limit (cache-limit cache))))