1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team, 1998-2002
5 * Stable names and stable pointers.
7 * ---------------------------------------------------------------------------*/
9 // Make static versions of inline functions in Stable.h:
12 #include "PosixSource.h"
16 #include "OSThreads.h"
20 #include "OSThreads.h"
22 /* Comment from ADR's implementation in old RTS:
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
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
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.
47 Of course, all this rather begs the question: why would we want to
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.
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.
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.
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 #)
73 There may be additional functions on the C side to allow evaluation,
74 application, etc of a stable pointer.
78 snEntry *stable_ptr_table = NULL;
79 static snEntry *stable_ptr_free = NULL;
81 static unsigned int SPT_size = 0;
84 static Mutex stable_mutex;
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
91 * OLD COMMENTS about reference counting follow. The reference count
92 * in a stable name entry is now just a counter.
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.
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.
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
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.
120 static HashTable *addrToStableHash = NULL;
122 #define INIT_SPT_SIZE 64
125 initFreeList(snEntry *table, nat n, snEntry *free)
129 for (p = table + n - 1; p >= table; p--) {
136 stable_ptr_free = table;
140 initStablePtrTable(void)
143 // the table will be allocated the first time makeStablePtr is
144 // called, and we want the table to persist through multiple inits.
146 // Also, getStablePtr is now called from __attribute__((constructor))
147 // functions, so initialising things here wouldn't work anyway.
149 initMutex(&stable_mutex);
154 * get at the real stuff...remove indirections.
156 * ToDo: move to a better home.
160 removeIndirections(StgClosure* p)
164 while (get_itbl(q)->type == IND ||
165 get_itbl(q)->type == IND_STATIC ||
166 get_itbl(q)->type == IND_OLDGEN ||
167 get_itbl(q)->type == IND_PERM ||
168 get_itbl(q)->type == IND_OLDGEN_PERM ) {
169 q = ((StgInd *)q)->indirectee;
175 lookupStableName_(StgPtr p)
180 if (stable_ptr_free == NULL) {
181 enlargeStablePtrTable();
184 /* removing indirections increases the likelihood
185 * of finding a match in the stable name hash table.
187 p = (StgPtr)removeIndirections((StgClosure*)p);
189 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
190 sn = (StgWord)sn_tmp;
193 ASSERT(stable_ptr_table[sn].addr == p);
194 IF_DEBUG(stable,debugBelch("cached stable name %ld at %p\n",sn,p));
195 RELEASE_LOCK(&stable_mutex);
198 sn = stable_ptr_free - stable_ptr_table;
199 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
200 stable_ptr_table[sn].ref = 0;
201 stable_ptr_table[sn].addr = p;
202 stable_ptr_table[sn].sn_obj = NULL;
203 /* IF_DEBUG(stable,debugBelch("new stable name %d at %p\n",sn,p)); */
205 /* add the new stable name to the hash table */
206 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
213 lookupStableName(StgPtr p)
216 ACQUIRE_LOCK(&stable_mutex);
217 res = lookupStableName_(p);
218 RELEASE_LOCK(&stable_mutex);
223 freeStableName(snEntry *sn)
225 ASSERT(sn->sn_obj == NULL);
226 if (sn->addr != NULL) {
227 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
229 sn->addr = (P_)stable_ptr_free;
230 stable_ptr_free = sn;
234 getStablePtr(StgPtr p)
238 ACQUIRE_LOCK(&stable_mutex);
239 sn = lookupStableName_(p);
240 stable_ptr_table[sn].ref++;
241 RELEASE_LOCK(&stable_mutex);
242 return (StgStablePtr)(sn);
246 freeStablePtr(StgStablePtr sp)
250 ACQUIRE_LOCK(&stable_mutex);
252 sn = &stable_ptr_table[(StgWord)sp];
254 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
258 // If this entry has no StableName attached, then just free it
259 // immediately. This is important; it might be a while before the
260 // next major GC which actually collects the entry.
261 if (sn->sn_obj == NULL && sn->ref == 0) {
265 RELEASE_LOCK(&stable_mutex);
269 enlargeStablePtrTable(void)
271 nat old_SPT_size = SPT_size;
275 SPT_size = INIT_SPT_SIZE;
276 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
277 "enlargeStablePtrTable");
279 /* we don't use index 0 in the stable name table, because that
280 * would conflict with the hash table lookup operations which
281 * return NULL if an entry isn't found in the hash table.
283 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
284 addrToStableHash = allocHashTable();
287 // 2nd and subsequent times
290 stgReallocBytes(stable_ptr_table,
291 SPT_size * sizeof(snEntry),
292 "enlargeStablePtrTable");
294 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
298 /* -----------------------------------------------------------------------------
299 * Treat stable pointers as roots for the garbage collector.
301 * A stable pointer is any stable name entry with a ref > 0. We'll
302 * take the opportunity to zero the "keep" flags at the same time.
303 * -------------------------------------------------------------------------- */
306 markStablePtrTable(evac_fn evac)
308 snEntry *p, *end_stable_ptr_table;
311 end_stable_ptr_table = &stable_ptr_table[SPT_size];
313 // Mark all the stable *pointers* (not stable names).
314 // _starting_ at index 1; index 0 is unused.
315 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
318 // Internal pointers are free slots. If q == NULL, it's a
319 // stable name where the object has been GC'd, but the
320 // StableName object (sn_obj) is still alive.
321 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
323 // save the current addr away: we need to be able to tell
324 // whether the objects moved in order to be able to update
325 // the hash table later.
328 // if the ref is non-zero, treat addr as a root
330 evac((StgClosure **)&p->addr);
336 /* -----------------------------------------------------------------------------
337 * Thread the stable pointer table for compacting GC.
339 * Here we must call the supplied evac function for each pointer into
340 * the heap from the stable pointer table, because the compacting
341 * collector may move the object it points to.
342 * -------------------------------------------------------------------------- */
345 threadStablePtrTable( evac_fn evac )
347 snEntry *p, *end_stable_ptr_table;
350 end_stable_ptr_table = &stable_ptr_table[SPT_size];
352 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
354 if (p->sn_obj != NULL) {
355 evac((StgClosure **)&p->sn_obj);
359 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
360 evac((StgClosure **)&p->addr);
365 /* -----------------------------------------------------------------------------
366 * Garbage collect any dead entries in the stable pointer table.
370 * - a zero reference count
373 * Both of these conditions must be true in order to re-use the stable
374 * name table entry. We can re-use stable name table entries for live
375 * heap objects, as long as the program has no StableName objects that
376 * refer to the entry.
377 * -------------------------------------------------------------------------- */
380 gcStablePtrTable( void )
382 snEntry *p, *end_stable_ptr_table;
385 end_stable_ptr_table = &stable_ptr_table[SPT_size];
387 // NOTE: _starting_ at index 1; index 0 is unused.
388 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
390 // Update the pointer to the StableName object, if there is one
391 if (p->sn_obj != NULL) {
392 p->sn_obj = isAlive(p->sn_obj);
395 // Internal pointers are free slots. If q == NULL, it's a
396 // stable name where the object has been GC'd, but the
397 // StableName object (sn_obj) is still alive.
399 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
403 if (p->sn_obj == NULL) {
404 // StableName object is dead
406 IF_DEBUG(stable, debugBelch("GC'd Stable name %ld\n",
407 p - stable_ptr_table));
411 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
412 IF_DEBUG(stable, debugBelch("Stable name %ld still alive at %p, ref %ld\n", p - stable_ptr_table, p->addr, p->ref));
419 /* -----------------------------------------------------------------------------
420 * Update the StablePtr/StableName hash table
422 * The boolean argument 'full' indicates that a major collection is
423 * being done, so we might as well throw away the hash table and build
424 * a new one. For a minor collection, we just re-hash the elements
426 * -------------------------------------------------------------------------- */
429 updateStablePtrTable(rtsBool full)
431 snEntry *p, *end_stable_ptr_table;
433 if (full && addrToStableHash != NULL) {
434 freeHashTable(addrToStableHash,NULL);
435 addrToStableHash = allocHashTable();
438 end_stable_ptr_table = &stable_ptr_table[SPT_size];
440 // NOTE: _starting_ at index 1; index 0 is unused.
441 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
443 if (p->addr == NULL) {
444 if (p->old != NULL) {
445 // The target has been garbage collected. Remove its
446 // entry from the hash table.
447 removeHashTable(addrToStableHash, (W_)p->old, NULL);
451 else if (p->addr < (P_)stable_ptr_table
452 || p->addr >= (P_)end_stable_ptr_table) {
453 // Target still alive, Re-hash this stable name
455 insertHashTable(addrToStableHash, (W_)p->addr,
456 (void *)(p - stable_ptr_table));
457 } else if (p->addr != p->old) {
458 removeHashTable(addrToStableHash, (W_)p->old, NULL);
459 insertHashTable(addrToStableHash, (W_)p->addr,
460 (void *)(p - stable_ptr_table));