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