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 (* line-size line-hash) (length vector))).
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 (defun compute-cache-mask (vector-length line-size
)
92 ;; Since both vector-length and line-size are powers of two, we
93 ;; can compute a bitmask such that
95 ;; (logand <mask> <combined-layout-hash>)
97 ;; is "morally equal" to
99 ;; (mod (* <line-size> <combined-layout-hash>) <vector-length>)
101 ;; This is it: (1- vector-length) is #b111... of the approriate size
102 ;; to get the MOD, and (- line-size) gives right the number of zero
103 ;; bits at the low end.
104 (logand (1- vector-length
) (- line-size
)))
106 ;;; The smallest power of two that is equal to or greater then X.
107 (declaim (inline power-of-two-ceiling
))
108 (defun power-of-two-ceiling (x)
109 (ash 1 (integer-length (1- x
))))
111 (defun cache-statistics (cache)
112 (let* ((vector (cache-vector cache
))
113 (size (length vector
))
114 (line-size (cache-line-size cache
))
115 (total-lines (/ size line-size
))
116 (free-lines (loop for i from
0 by line-size below size
117 unless
(eq (svref vector i
) '..empty..
)
119 (values (- total-lines free-lines
) total-lines
120 (cache-depth cache
) (cache-limit cache
))))
122 ;;; Don't allocate insanely huge caches: this is 4096 lines for a
123 ;;; value cache with 8-15 keys -- probably "big enough for anyone",
124 ;;; and 16384 lines for a commonplace 2-key value cache.
125 (defconstant +cache-vector-max-length
+ (expt 2 16))
127 ;;; Compute the maximum allowed probe depth as a function of cache size.
128 ;;; Cache size refers to number of cache lines, not the length of the
131 ;;; FIXME: It would be nice to take the generic function optimization
132 ;;; policy into account here (speed vs. space.)
133 (declaim (inline compute-limit
))
134 (defun compute-limit (size)
135 (ceiling (sqrt (sqrt size
))))
137 ;;; Returns VALUE if it is not ..EMPTY.., otherwise executes ELSE:
138 (defmacro non-empty-or
(value else
)
139 (with-unique-names (n-value)
140 `(let ((,n-value
,value
))
141 (if (eq ,n-value
'..empty..
)
145 ;;; Fast way to check if a thing found at the position of a cache key is one:
146 ;;; it is always either a wrapper, or the ..EMPTY.. symbol.
147 (declaim (inline cache-key-p
))
148 (defun cache-key-p (thing)
149 (not (symbolp thing
)))
151 (eval-when (:compile-toplevel
:load-toplevel
:execute
)
152 (sb-kernel:define-structure-slot-compare-and-swap compare-and-swap-cache-depth
156 ;;; Utility macro for atomic updates without locking... doesn't
157 ;;; do much right now, and it would be nice to make this more magical.
158 (defmacro compare-and-swap
(place old new
)
159 (unless (consp place
)
160 (error "Don't know how to compare and swap ~S." place
))
163 `(simple-vector-compare-and-swap ,@(cdr place
) ,old
,new
))
165 `(compare-and-swap-cache-depth ,@(cdr place
) ,old
,new
))))
167 ;;; Atomically update the current probe depth of a cache.
168 (defun note-cache-depth (cache depth
)
169 (loop for old
= (cache-depth cache
)
170 while
(and (< old depth
)
171 (not (eq old
(compare-and-swap (cache-depth cache
)
174 ;;; Compute the starting index of the next cache line in the cache vector.
175 (declaim (inline next-cache-index
))
176 (defun next-cache-index (mask index line-size
)
177 (logand mask
(+ index line-size
)))
179 ;;; Returns the hash-value for layout, or executes ELSE if the layout
181 (defmacro hash-layout-or
(layout else
)
182 (with-unique-names (n-hash)
183 `(let ((,n-hash
(layout-clos-hash ,layout
)))
188 ;;; Compute cache index for the cache and a list of layouts.
189 (declaim (inline compute-cache-index
))
190 (defun compute-cache-index (cache layouts
)
191 (let ((index (hash-layout-or (car layouts
)
192 (return-from compute-cache-index nil
))))
193 (declare (fixnum index
))
194 (dolist (layout (cdr layouts
))
195 (mixf index
(hash-layout-or layout
(return-from compute-cache-index nil
))))
196 ;; align with cache lines
197 (logand index
(cache-mask cache
))))
199 ;;; Emit code that does lookup in cache bound to CACHE-VAR using
200 ;;; layouts bound to LAYOUT-VARS. Go to MISS-TAG on event of a miss or
201 ;;; invalid layout. Otherwise, if VALUE-VAR is non-nil, set it to the
202 ;;; value found. (VALUE-VAR is non-nil only when CACHE-VALUE is true.)
204 ;;; In other words, produces inlined code for COMPUTE-CACHE-INDEX when
205 ;;; number of keys and presence of values in the cache is known
207 (defun emit-cache-lookup (cache-var layout-vars miss-tag value-var
)
208 (let ((line-size (power-of-two-ceiling (+ (length layout-vars
)
209 (if value-var
1 0)))))
210 (with-unique-names (n-index n-vector n-depth n-pointer n-mask
211 MATCH-WRAPPERS EXIT-WITH-HIT
)
212 `(let* ((,n-index
(hash-layout-or ,(car layout-vars
) (go ,miss-tag
)))
213 (,n-vector
(cache-vector ,cache-var
))
214 (,n-mask
(cache-mask ,cache-var
)))
215 (declare (index ,n-index
))
216 ,@(mapcar (lambda (layout-var)
217 `(mixf ,n-index
(hash-layout-or ,layout-var
(go ,miss-tag
))))
219 ;; align with cache lines
220 (setf ,n-index
(logand ,n-index
,n-mask
))
221 (let ((,n-depth
(cache-depth ,cache-var
))
222 (,n-pointer
,n-index
))
223 (declare (index ,n-depth
,n-pointer
))
229 (eq ,layout-var
(svref ,n-vector
,n-pointer
))
233 `((setf ,value-var
(non-empty-or (svref ,n-vector
,n-pointer
)
239 (setf ,n-index
(next-cache-index ,n-mask
,n-index
,line-size
)
244 ;;; Probes CACHE for LAYOUTS.
246 ;;; Returns two values: a boolean indicating a hit or a miss, and a secondary
247 ;;; value that is the value that was stored in the cache if any.
248 (defun probe-cache (cache layouts
)
249 (unless (consp layouts
)
250 (setf layouts
(list layouts
)))
251 (let ((vector (cache-vector cache
))
252 (key-count (cache-key-count cache
))
253 (line-size (cache-line-size cache
))
254 (mask (cache-mask cache
)))
255 (flet ((probe-line (base)
257 (loop for offset from
0 below key-count
258 for layout in layouts do
259 (unless (eq layout
(svref vector
(+ base offset
)))
262 ;; all layouts match!
263 (let ((value (when (cache-value cache
)
264 (non-empty-or (svref vector
(+ base key-count
))
266 (return-from probe-cache
(values t value
)))
268 (return-from probe-line
(next-cache-index mask base line-size
)))))
269 (let ((index (compute-cache-index cache layouts
)))
271 (loop repeat
(1+ (cache-depth cache
)) do
272 (setf index
(probe-line index
)))))))
275 ;;; Tries to write LAYOUTS and VALUE at the cache line starting at
276 ;;; the index BASE. Returns true on success, and false on failure.
277 (defun try-update-cache-line (cache base layouts value
)
278 (declare (index base
))
279 (let ((vector (cache-vector cache
))
281 ;; If we unwind from here, we will be left with an incomplete
282 ;; cache line, but that is OK: next write using the same layouts
283 ;; will fill it, and reads will treat an incomplete line as a
284 ;; miss -- causing it to be filled.
285 (loop for old
= (compare-and-swap (svref vector base
) '..empty.. new
) do
286 (when (and (cache-key-p old
) (not (eq old new
)))
287 ;; The place was already taken, and doesn't match our key.
288 (return-from try-update-cache-line nil
))
290 ;; All keys match or succesfully saved, save our value --
291 ;; just smash it in. Until the first time it is written
292 ;; there is ..EMPTY.. here, which probes look for, so we
293 ;; don't get bogus hits. This is necessary because we want
294 ;; to be able store arbitrary values here for use with
295 ;; constant-value dispatch functions.
296 (when (cache-value cache
)
297 (setf (svref vector
(1+ base
)) value
))
298 (return-from try-update-cache-line t
))
299 (setf new
(pop layouts
))
302 ;;; Tries to write LAYOUTS and VALUE somewhere in the cache. Returns
303 ;;; true on success and false on failure, meaning the cache is too
305 (defun try-update-cache (cache layouts value
)
306 (let ((vector (cache-vector cache
))
307 (index (or (compute-cache-index cache layouts
)
308 ;; At least one of the layouts was invalid: just
309 ;; pretend we updated the cache, and let the next
310 ;; read pick up the mess.
311 (return-from try-update-cache t
)))
312 (line-size (cache-line-size cache
))
313 (mask (cache-mask cache
)))
314 (declare (index index
))
315 (loop for depth from
0 upto
(cache-limit cache
) do
316 (when (try-update-cache-line cache index layouts value
)
317 (note-cache-depth cache depth
)
318 (return-from try-update-cache t
))
319 (setf index
(next-cache-index mask index line-size
)))))
321 ;;; Constructs a new cache.
322 (defun make-cache (&key
(key-count (missing-arg)) (value (missing-arg))
324 (let* ((line-size (power-of-two-ceiling (+ key-count
(if value
1 0))))
325 (adjusted-size (power-of-two-ceiling size
))
326 (length (* adjusted-size line-size
)))
327 (if (<= length
+cache-vector-max-length
+)
328 (%make-cache
:key-count key-count
330 :vector
(make-array length
:initial-element
'..empty..
)
332 :mask
(compute-cache-mask length line-size
)
333 :limit
(compute-limit adjusted-size
))
334 ;; Make a smaller one, then
335 (make-cache :key-count key-count
:value value
:size
(ceiling size
2)))))
337 ;;;; Copies and expands the cache, dropping any invalidated or
338 ;;;; incomplete lines.
339 (defun copy-and-expand-cache (cache)
340 (let ((copy (%copy-cache cache
))
341 (length (length (cache-vector cache
))))
342 (when (< length
+cache-vector-max-length
+)
343 (setf length
(* 2 length
)))
346 (setf (cache-vector copy
) (make-array length
:initial-element
'..empty..
)
348 (cache-mask copy
) (compute-cache-mask length
(cache-line-size cache
))
349 (cache-limit copy
) (compute-limit (/ length
(cache-line-size cache
))))
350 (map-cache (lambda (layouts value
)
351 (unless (try-update-cache copy layouts value
)
352 ;; If the cache would grow too much we drop the
353 ;; remaining the entries that don't fit. FIXME:
354 ;; It would be better to drop random entries to
355 ;; avoid getting into a rut here (best done by
356 ;; making MAP-CACHE map in a random order?), and
357 ;; possibly to downsize the cache more
358 ;; aggressively (on the assumption that most
359 ;; entries aren't getting used at the moment.)
360 (when (< length
+cache-vector-max-length
+)
361 (setf length
(* 2 length
))
366 (defun cache-has-invalid-entries-p (cache)
367 (let ((vector (cache-vector cache
))
368 (line-size (cache-line-size cache
))
369 (key-count (cache-key-count cache
))
370 (mask (cache-mask cache
))
373 ;; Check if the line is in use, and check validity of the keys.
374 (let ((key1 (svref vector index
)))
375 (when (cache-key-p key1
)
376 (if (zerop (layout-clos-hash key1
))
377 ;; First key invalid.
378 (return-from cache-has-invalid-entries-p t
)
379 ;; Line is in use and the first key is valid: check the rest.
380 (loop for offset from
1 below key-count
381 do
(let ((thing (svref vector
(+ index offset
))))
382 (when (or (not (cache-key-p thing
))
383 (zerop (layout-clos-hash thing
)))
384 ;; Incomplete line or invalid layout.
385 (return-from cache-has-invalid-entries-p t
)))))))
386 ;; Line empty of valid, onwards.
387 (setf index
(next-cache-index mask index line-size
))
390 (return-from cache-has-invalid-entries-p nil
)))))
392 (defun hash-table-to-cache (table &key value key-count
)
393 (let ((cache (make-cache :key-count key-count
:value value
394 :size
(hash-table-count table
))))
395 (maphash (lambda (class value
)
396 (setq cache
(fill-cache cache
(class-wrapper class
) value
)))
400 ;;; Inserts VALUE to CACHE keyd by LAYOUTS. Expands the cache if
401 ;;; necessary, and returns the new cache.
402 (defun fill-cache (cache layouts value
)
404 ((%fill-cache
(cache layouts value
)
405 (cond ((try-update-cache cache layouts value
)
407 ((cache-has-invalid-entries-p cache
)
408 ;; Don't expand yet: maybe there will be enough space if
409 ;; we just drop the invalid entries.
410 (%fill-cache
(copy-cache cache
) layouts value
))
412 (%fill-cache
(copy-and-expand-cache cache
) layouts value
)))))
414 (%fill-cache cache layouts value
)
415 (%fill-cache cache
(list layouts
) value
))))
417 ;;; Calls FUNCTION with all layouts and values in cache.
418 (defun map-cache (function cache
)
419 (let* ((vector (cache-vector cache
))
420 (key-count (cache-key-count cache
))
421 (valuep (cache-value cache
))
422 (line-size (cache-line-size cache
))
423 (mask (cache-mask cache
))
424 (fun (if (functionp function
)
426 (fdefinition function
)))
432 (loop for offset from
0 below key-count
433 collect
(non-empty-or (svref vector
(+ offset index
))
435 (let ((value (when valuep
436 (non-empty-or (svref vector
(+ index key-count
))
438 ;; Let the callee worry about invalid layouts
439 (funcall fun layouts value
)))
441 (setf index
(next-cache-index mask index line-size
))
442 (unless (zerop index
)
446 ;;; Copying a cache without expanding it is very much like mapping it:
447 ;;; we need to be carefull because there may be updates while we are
448 ;;; copying it, and we don't want to copy incomplete entries or invalid
450 (defun copy-cache (cache)
451 (let* ((vector (cache-vector cache
))
452 (copy (make-array (length vector
) :initial-element
'..empty..
))
453 (line-size (cache-line-size cache
))
454 (key-count (cache-key-count cache
))
455 (valuep (cache-value cache
))
456 (mask (cache-mask cache
))
457 (size (/ (length vector
) line-size
))
463 (let ((layouts (loop for offset from
0 below key-count
464 collect
(non-empty-or (svref vector
(+ index offset
))
466 ;; Check validity & compute primary index.
467 (let ((primary (or (compute-cache-index cache layouts
)
469 ;; Check & copy value.
471 (setf (svref copy
(+ index key-count
))
472 (non-empty-or (svref vector
(+ index key-count
))
475 (loop for offset from
0 below key-count do
476 (setf (svref copy
(+ index offset
)) (pop layouts
)))
477 ;; Update probe depth.
478 (let ((distance (/ (- index primary
) line-size
)))
479 (setf depth
(max depth
(if (minusp distance
)
480 ;; account for wrap-around
484 (setf index
(next-cache-index mask index line-size
))
485 (unless (zerop index
)
487 (%make-cache
:vector copy
489 :key-count
(cache-key-count cache
)
493 :limit
(cache-limit cache
))))
495 ;;;; For debugging & collecting statistics.
497 (defun map-all-caches (function)
498 (dolist (p (list-all-packages))
500 (when (eq p
(symbol-package s
))
501 (dolist (name (list s
505 (slot-boundp-name s
)))
507 (let ((fun (fdefinition name
)))
508 (when (typep fun
'generic-function
)
509 (let ((cache (gf-dfun-cache fun
)))
511 (funcall function name cache
)))))))))))
513 (defun check-cache-consistency (cache)
514 (let ((table (make-hash-table :test
'equal
)))
515 (map-cache (lambda (layouts value
)
516 (declare (ignore value
))
517 (if (gethash layouts table
)
518 (cerror "Check futher."
519 "Multiple appearances of ~S." layouts
)
520 (setf (gethash layouts table
) t
)))