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