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