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[ghc.git] / rts / Stable.c
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team, 1998-2002
4 *
5 * Stable names and stable pointers.
6 *
7 * ---------------------------------------------------------------------------*/
8
9 #include "PosixSource.h"
10 #include "Rts.h"
11 #include "RtsAPI.h"
12
13 #include "Hash.h"
14 #include "RtsUtils.h"
15 #include "Trace.h"
16 #include "Stable.h"
17
18 /* Comment from ADR's implementation in old RTS:
19
20 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
21 small change in @HpOverflow.lc@) consists of the changes in the
22 runtime system required to implement "Stable Pointers". But we're
23 getting a bit ahead of ourselves --- what is a stable pointer and what
24 is it used for?
25
26 When Haskell calls C, it normally just passes over primitive integers,
27 floats, bools, strings, etc. This doesn't cause any problems at all
28 for garbage collection because the act of passing them makes a copy
29 from the heap, stack or wherever they are onto the C-world stack.
30 However, if we were to pass a heap object such as a (Haskell) @String@
31 and a garbage collection occured before we finished using it, we'd run
32 into problems since the heap object might have been moved or even
33 deleted.
34
35 So, if a C call is able to cause a garbage collection or we want to
36 store a pointer to a heap object between C calls, we must be careful
37 when passing heap objects. Our solution is to keep a table of all
38 objects we've given to the C-world and to make sure that the garbage
39 collector collects these objects --- updating the table as required to
40 make sure we can still find the object.
41
42
43 Of course, all this rather begs the question: why would we want to
44 pass a boxed value?
45
46 One very good reason is to preserve laziness across the language
47 interface. Rather than evaluating an integer or a string because it
48 {\em might\/} be required by the C function, we can wait until the C
49 function actually wants the value and then force an evaluation.
50
51 Another very good reason (the motivating reason!) is that the C code
52 might want to execute an object of sort $IO ()$ for the side-effects
53 it will produce. For example, this is used when interfacing to an X
54 widgets library to allow a direct implementation of callbacks.
55
56
57 The @makeStablePointer :: a -> IO (StablePtr a)@ function
58 converts a value into a stable pointer. It is part of the @PrimIO@
59 monad, because we want to be sure we don't allocate one twice by
60 accident, and then only free one of the copies.
61
62 \begin{verbatim}
63 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
64 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
65 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
66 (# State# RealWorld, a #)
67 \end{verbatim}
68
69 There may be additional functions on the C side to allow evaluation,
70 application, etc of a stable pointer.
71
72 */
73
74 snEntry *stable_ptr_table = NULL;
75 static snEntry *stable_ptr_free = NULL;
76
77 static unsigned int SPT_size = 0;
78
79 #ifdef THREADED_RTS
80 Mutex stable_mutex;
81 #endif
82
83 static void enlargeStablePtrTable(void);
84
85 /* This hash table maps Haskell objects to stable names, so that every
86 * call to lookupStableName on a given object will return the same
87 * stable name.
88 *
89 * OLD COMMENTS about reference counting follow. The reference count
90 * in a stable name entry is now just a counter.
91 *
92 * Reference counting
93 * ------------------
94 * A plain stable name entry has a zero reference count, which means
95 * the entry will dissappear when the object it points to is
96 * unreachable. For stable pointers, we need an entry that sticks
97 * around and keeps the object it points to alive, so each stable name
98 * entry has an associated reference count.
99 *
100 * A stable pointer has a weighted reference count N attached to it
101 * (actually in its upper 5 bits), which represents the weight
102 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
103 * represents any weight between 1 and 2^32 (represented as zero).
104 * When the weight is 2^32, the stable name table owns "all" of the
105 * stable pointers to this object, and the entry can be garbage
106 * collected if the object isn't reachable.
107 *
108 * A new stable pointer is given the weight log2(W/2), where W is the
109 * weight stored in the table entry. The new weight in the table is W
110 * - 2^log2(W/2).
111 *
112 * A stable pointer can be "split" into two stable pointers, by
113 * dividing the weight by 2 and giving each pointer half.
114 * When freeing a stable pointer, the weight of the pointer is added
115 * to the weight stored in the table entry.
116 * */
117
118 static HashTable *addrToStableHash = NULL;
119
120 #define INIT_SPT_SIZE 64
121
122 STATIC_INLINE void
123 initFreeList(snEntry *table, nat n, snEntry *free)
124 {
125 snEntry *p;
126
127 for (p = table + n - 1; p >= table; p--) {
128 p->addr = (P_)free;
129 p->old = NULL;
130 p->ref = 0;
131 p->sn_obj = NULL;
132 free = p;
133 }
134 stable_ptr_free = table;
135 }
136
137 void
138 initStablePtrTable(void)
139 {
140 if (SPT_size > 0)
141 return;
142
143 SPT_size = INIT_SPT_SIZE;
144 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
145 "initStablePtrTable");
146
147 /* we don't use index 0 in the stable name table, because that
148 * would conflict with the hash table lookup operations which
149 * return NULL if an entry isn't found in the hash table.
150 */
151 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
152 addrToStableHash = allocHashTable();
153
154 #ifdef THREADED_RTS
155 initMutex(&stable_mutex);
156 #endif
157 }
158
159 void
160 exitStablePtrTable(void)
161 {
162 if (addrToStableHash)
163 freeHashTable(addrToStableHash, NULL);
164 addrToStableHash = NULL;
165 if (stable_ptr_table)
166 stgFree(stable_ptr_table);
167 stable_ptr_table = NULL;
168 SPT_size = 0;
169 #ifdef THREADED_RTS
170 closeMutex(&stable_mutex);
171 #endif
172 }
173
174 /*
175 * get at the real stuff...remove indirections.
176 * It untags pointers before dereferencing and
177 * retags the real stuff with its tag (if there
178 * is any) when returning.
179 *
180 * ToDo: move to a better home.
181 */
182 static
183 StgClosure*
184 removeIndirections(StgClosure* p)
185 {
186 StgWord tag = GET_CLOSURE_TAG(p);
187 StgClosure* q = UNTAG_CLOSURE(p);
188
189 while (get_itbl(q)->type == IND ||
190 get_itbl(q)->type == IND_STATIC ||
191 get_itbl(q)->type == IND_PERM) {
192 q = ((StgInd *)q)->indirectee;
193 tag = GET_CLOSURE_TAG(q);
194 q = UNTAG_CLOSURE(q);
195 }
196
197 return TAG_CLOSURE(tag,q);
198 }
199
200 static StgWord
201 lookupStableName_(StgPtr p)
202 {
203 StgWord sn;
204 void* sn_tmp;
205
206 if (stable_ptr_free == NULL) {
207 enlargeStablePtrTable();
208 }
209
210 /* removing indirections increases the likelihood
211 * of finding a match in the stable name hash table.
212 */
213 p = (StgPtr)removeIndirections((StgClosure*)p);
214
215 // register the untagged pointer. This just makes things simpler.
216 p = (StgPtr)UNTAG_CLOSURE((StgClosure*)p);
217
218 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
219 sn = (StgWord)sn_tmp;
220
221 if (sn != 0) {
222 ASSERT(stable_ptr_table[sn].addr == p);
223 debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
224 return sn;
225 } else {
226 sn = stable_ptr_free - stable_ptr_table;
227 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
228 stable_ptr_table[sn].ref = 0;
229 stable_ptr_table[sn].addr = p;
230 stable_ptr_table[sn].sn_obj = NULL;
231 /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
232
233 /* add the new stable name to the hash table */
234 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
235
236 return sn;
237 }
238 }
239
240 StgWord
241 lookupStableName(StgPtr p)
242 {
243 StgWord res;
244
245 initStablePtrTable();
246 ACQUIRE_LOCK(&stable_mutex);
247 res = lookupStableName_(p);
248 RELEASE_LOCK(&stable_mutex);
249 return res;
250 }
251
252 STATIC_INLINE void
253 freeStableName(snEntry *sn)
254 {
255 ASSERT(sn->sn_obj == NULL);
256 if (sn->addr != NULL) {
257 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
258 }
259 sn->addr = (P_)stable_ptr_free;
260 stable_ptr_free = sn;
261 }
262
263 StgStablePtr
264 getStablePtr(StgPtr p)
265 {
266 StgWord sn;
267
268 initStablePtrTable();
269 ACQUIRE_LOCK(&stable_mutex);
270 sn = lookupStableName_(p);
271 stable_ptr_table[sn].ref++;
272 RELEASE_LOCK(&stable_mutex);
273 return (StgStablePtr)(sn);
274 }
275
276 void
277 freeStablePtr(StgStablePtr sp)
278 {
279 snEntry *sn;
280
281 initStablePtrTable();
282 ACQUIRE_LOCK(&stable_mutex);
283
284 sn = &stable_ptr_table[(StgWord)sp];
285
286 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
287
288 sn->ref--;
289
290 // If this entry has no StableName attached, then just free it
291 // immediately. This is important; it might be a while before the
292 // next major GC which actually collects the entry.
293 if (sn->sn_obj == NULL && sn->ref == 0) {
294 freeStableName(sn);
295 }
296
297 RELEASE_LOCK(&stable_mutex);
298 }
299
300 static void
301 enlargeStablePtrTable(void)
302 {
303 nat old_SPT_size = SPT_size;
304
305 // 2nd and subsequent times
306 SPT_size *= 2;
307 stable_ptr_table =
308 stgReallocBytes(stable_ptr_table,
309 SPT_size * sizeof(snEntry),
310 "enlargeStablePtrTable");
311
312 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
313 }
314
315 /* -----------------------------------------------------------------------------
316 * We must lock the StablePtr table during GC, to prevent simultaneous
317 * calls to freeStablePtr().
318 * -------------------------------------------------------------------------- */
319
320 void
321 stablePtrPreGC(void)
322 {
323 ACQUIRE_LOCK(&stable_mutex);
324 }
325
326 void
327 stablePtrPostGC(void)
328 {
329 RELEASE_LOCK(&stable_mutex);
330 }
331
332 /* -----------------------------------------------------------------------------
333 * Treat stable pointers as roots for the garbage collector.
334 *
335 * A stable pointer is any stable name entry with a ref > 0. We'll
336 * take the opportunity to zero the "keep" flags at the same time.
337 * -------------------------------------------------------------------------- */
338
339 void
340 markStablePtrTable(evac_fn evac, void *user)
341 {
342 snEntry *p, *end_stable_ptr_table;
343 StgPtr q;
344
345 end_stable_ptr_table = &stable_ptr_table[SPT_size];
346
347 // Mark all the stable *pointers* (not stable names).
348 // _starting_ at index 1; index 0 is unused.
349 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
350 q = p->addr;
351
352 // Internal pointers are free slots. If q == NULL, it's a
353 // stable name where the object has been GC'd, but the
354 // StableName object (sn_obj) is still alive.
355 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
356
357 // save the current addr away: we need to be able to tell
358 // whether the objects moved in order to be able to update
359 // the hash table later.
360 p->old = p->addr;
361
362 // if the ref is non-zero, treat addr as a root
363 if (p->ref != 0) {
364 evac(user, (StgClosure **)&p->addr);
365 }
366 }
367 }
368 }
369
370 /* -----------------------------------------------------------------------------
371 * Thread the stable pointer table for compacting GC.
372 *
373 * Here we must call the supplied evac function for each pointer into
374 * the heap from the stable pointer table, because the compacting
375 * collector may move the object it points to.
376 * -------------------------------------------------------------------------- */
377
378 void
379 threadStablePtrTable( evac_fn evac, void *user )
380 {
381 snEntry *p, *end_stable_ptr_table;
382 StgPtr q;
383
384 end_stable_ptr_table = &stable_ptr_table[SPT_size];
385
386 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
387
388 if (p->sn_obj != NULL) {
389 evac(user, (StgClosure **)&p->sn_obj);
390 }
391
392 q = p->addr;
393 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
394 evac(user, (StgClosure **)&p->addr);
395 }
396 }
397 }
398
399 /* -----------------------------------------------------------------------------
400 * Garbage collect any dead entries in the stable pointer table.
401 *
402 * A dead entry has:
403 *
404 * - a zero reference count
405 * - a dead sn_obj
406 *
407 * Both of these conditions must be true in order to re-use the stable
408 * name table entry. We can re-use stable name table entries for live
409 * heap objects, as long as the program has no StableName objects that
410 * refer to the entry.
411 * -------------------------------------------------------------------------- */
412
413 void
414 gcStablePtrTable( void )
415 {
416 snEntry *p, *end_stable_ptr_table;
417 StgPtr q;
418
419 end_stable_ptr_table = &stable_ptr_table[SPT_size];
420
421 // NOTE: _starting_ at index 1; index 0 is unused.
422 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
423
424 // Update the pointer to the StableName object, if there is one
425 if (p->sn_obj != NULL) {
426 p->sn_obj = isAlive(p->sn_obj);
427 }
428
429 // Internal pointers are free slots. If q == NULL, it's a
430 // stable name where the object has been GC'd, but the
431 // StableName object (sn_obj) is still alive.
432 q = p->addr;
433 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
434
435 // StableNames only:
436 if (p->ref == 0) {
437 if (p->sn_obj == NULL) {
438 // StableName object is dead
439 freeStableName(p);
440 debugTrace(DEBUG_stable, "GC'd Stable name %ld",
441 (long)(p - stable_ptr_table));
442 continue;
443
444 } else {
445 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
446 debugTrace(DEBUG_stable,
447 "stable name %ld still alive at %p, ref %ld\n",
448 (long)(p - stable_ptr_table), p->addr, p->ref);
449 }
450 }
451 }
452 }
453 }
454
455 /* -----------------------------------------------------------------------------
456 * Update the StablePtr/StableName hash table
457 *
458 * The boolean argument 'full' indicates that a major collection is
459 * being done, so we might as well throw away the hash table and build
460 * a new one. For a minor collection, we just re-hash the elements
461 * that changed.
462 * -------------------------------------------------------------------------- */
463
464 void
465 updateStablePtrTable(rtsBool full)
466 {
467 snEntry *p, *end_stable_ptr_table;
468
469 if (full && addrToStableHash != NULL) {
470 freeHashTable(addrToStableHash,NULL);
471 addrToStableHash = allocHashTable();
472 }
473
474 end_stable_ptr_table = &stable_ptr_table[SPT_size];
475
476 // NOTE: _starting_ at index 1; index 0 is unused.
477 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
478
479 if (p->addr == NULL) {
480 if (p->old != NULL) {
481 // The target has been garbage collected. Remove its
482 // entry from the hash table.
483 removeHashTable(addrToStableHash, (W_)p->old, NULL);
484 p->old = NULL;
485 }
486 }
487 else if (p->addr < (P_)stable_ptr_table
488 || p->addr >= (P_)end_stable_ptr_table) {
489 // Target still alive, Re-hash this stable name
490 if (full) {
491 insertHashTable(addrToStableHash, (W_)p->addr,
492 (void *)(p - stable_ptr_table));
493 } else if (p->addr != p->old) {
494 removeHashTable(addrToStableHash, (W_)p->old, NULL);
495 insertHashTable(addrToStableHash, (W_)p->addr,
496 (void *)(p - stable_ptr_table));
497 }
498 }
499 }
500 }