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));
197 sn = stable_ptr_free - stable_ptr_table;
198 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
199 stable_ptr_table[sn].ref = 0;
200 stable_ptr_table[sn].addr = p;
201 stable_ptr_table[sn].sn_obj = NULL;
202 /* IF_DEBUG(stable,debugBelch("new stable name %d at %p\n",sn,p)); */
204 /* add the new stable name to the hash table */
205 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
212 lookupStableName(StgPtr p)
215 ACQUIRE_LOCK(&stable_mutex);
216 res = lookupStableName_(p);
217 RELEASE_LOCK(&stable_mutex);
222 freeStableName(snEntry *sn)
224 ASSERT(sn->sn_obj == NULL);
225 if (sn->addr != NULL) {
226 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
228 sn->addr = (P_)stable_ptr_free;
229 stable_ptr_free = sn;
233 getStablePtr(StgPtr p)
237 ACQUIRE_LOCK(&stable_mutex);
238 sn = lookupStableName_(p);
239 stable_ptr_table[sn].ref++;
240 RELEASE_LOCK(&stable_mutex);
241 return (StgStablePtr)(sn);
245 freeStablePtr(StgStablePtr sp)
249 ACQUIRE_LOCK(&stable_mutex);
251 sn = &stable_ptr_table[(StgWord)sp];
253 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
257 // If this entry has no StableName attached, then just free it
258 // immediately. This is important; it might be a while before the
259 // next major GC which actually collects the entry.
260 if (sn->sn_obj == NULL && sn->ref == 0) {
264 RELEASE_LOCK(&stable_mutex);
268 enlargeStablePtrTable(void)
270 nat old_SPT_size = SPT_size;
274 SPT_size = INIT_SPT_SIZE;
275 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
276 "enlargeStablePtrTable");
278 /* we don't use index 0 in the stable name table, because that
279 * would conflict with the hash table lookup operations which
280 * return NULL if an entry isn't found in the hash table.
282 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
283 addrToStableHash = allocHashTable();
286 // 2nd and subsequent times
289 stgReallocBytes(stable_ptr_table,
290 SPT_size * sizeof(snEntry),
291 "enlargeStablePtrTable");
293 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
297 /* -----------------------------------------------------------------------------
298 * Treat stable pointers as roots for the garbage collector.
300 * A stable pointer is any stable name entry with a ref > 0. We'll
301 * take the opportunity to zero the "keep" flags at the same time.
302 * -------------------------------------------------------------------------- */
305 markStablePtrTable(evac_fn evac)
307 snEntry *p, *end_stable_ptr_table;
310 end_stable_ptr_table = &stable_ptr_table[SPT_size];
312 // Mark all the stable *pointers* (not stable names).
313 // _starting_ at index 1; index 0 is unused.
314 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
317 // Internal pointers are free slots. If q == NULL, it's a
318 // stable name where the object has been GC'd, but the
319 // StableName object (sn_obj) is still alive.
320 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
322 // save the current addr away: we need to be able to tell
323 // whether the objects moved in order to be able to update
324 // the hash table later.
327 // if the ref is non-zero, treat addr as a root
329 evac((StgClosure **)&p->addr);
335 /* -----------------------------------------------------------------------------
336 * Thread the stable pointer table for compacting GC.
338 * Here we must call the supplied evac function for each pointer into
339 * the heap from the stable pointer table, because the compacting
340 * collector may move the object it points to.
341 * -------------------------------------------------------------------------- */
344 threadStablePtrTable( evac_fn evac )
346 snEntry *p, *end_stable_ptr_table;
349 end_stable_ptr_table = &stable_ptr_table[SPT_size];
351 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
353 if (p->sn_obj != NULL) {
354 evac((StgClosure **)&p->sn_obj);
358 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
359 evac((StgClosure **)&p->addr);
364 /* -----------------------------------------------------------------------------
365 * Garbage collect any dead entries in the stable pointer table.
369 * - a zero reference count
372 * Both of these conditions must be true in order to re-use the stable
373 * name table entry. We can re-use stable name table entries for live
374 * heap objects, as long as the program has no StableName objects that
375 * refer to the entry.
376 * -------------------------------------------------------------------------- */
379 gcStablePtrTable( void )
381 snEntry *p, *end_stable_ptr_table;
384 end_stable_ptr_table = &stable_ptr_table[SPT_size];
386 // NOTE: _starting_ at index 1; index 0 is unused.
387 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
389 // Update the pointer to the StableName object, if there is one
390 if (p->sn_obj != NULL) {
391 p->sn_obj = isAlive(p->sn_obj);
394 // Internal pointers are free slots. If q == NULL, it's a
395 // stable name where the object has been GC'd, but the
396 // StableName object (sn_obj) is still alive.
398 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
402 if (p->sn_obj == NULL) {
403 // StableName object is dead
405 IF_DEBUG(stable, debugBelch("GC'd Stable name %ld\n",
406 p - stable_ptr_table));
410 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
411 IF_DEBUG(stable, debugBelch("Stable name %ld still alive at %p, ref %ld\n", p - stable_ptr_table, p->addr, p->ref));
418 /* -----------------------------------------------------------------------------
419 * Update the StablePtr/StableName hash table
421 * The boolean argument 'full' indicates that a major collection is
422 * being done, so we might as well throw away the hash table and build
423 * a new one. For a minor collection, we just re-hash the elements
425 * -------------------------------------------------------------------------- */
428 updateStablePtrTable(rtsBool full)
430 snEntry *p, *end_stable_ptr_table;
432 if (full && addrToStableHash != NULL) {
433 freeHashTable(addrToStableHash,NULL);
434 addrToStableHash = allocHashTable();
437 end_stable_ptr_table = &stable_ptr_table[SPT_size];
439 // NOTE: _starting_ at index 1; index 0 is unused.
440 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
442 if (p->addr == NULL) {
443 if (p->old != NULL) {
444 // The target has been garbage collected. Remove its
445 // entry from the hash table.
446 removeHashTable(addrToStableHash, (W_)p->old, NULL);
450 else if (p->addr < (P_)stable_ptr_table
451 || p->addr >= (P_)end_stable_ptr_table) {
452 // Target still alive, Re-hash this stable name
454 insertHashTable(addrToStableHash, (W_)p->addr,
455 (void *)(p - stable_ptr_table));
456 } else if (p->addr != p->old) {
457 removeHashTable(addrToStableHash, (W_)p->old, NULL);
458 insertHashTable(addrToStableHash, (W_)p->addr,
459 (void *)(p - stable_ptr_table));