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"
23 /* Comment from ADR's implementation in old RTS:
25 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
26 small change in @HpOverflow.lc@) consists of the changes in the
27 runtime system required to implement "Stable Pointers". But we're
28 getting a bit ahead of ourselves --- what is a stable pointer and what
31 When Haskell calls C, it normally just passes over primitive integers,
32 floats, bools, strings, etc. This doesn't cause any problems at all
33 for garbage collection because the act of passing them makes a copy
34 from the heap, stack or wherever they are onto the C-world stack.
35 However, if we were to pass a heap object such as a (Haskell) @String@
36 and a garbage collection occured before we finished using it, we'd run
37 into problems since the heap object might have been moved or even
40 So, if a C call is able to cause a garbage collection or we want to
41 store a pointer to a heap object between C calls, we must be careful
42 when passing heap objects. Our solution is to keep a table of all
43 objects we've given to the C-world and to make sure that the garbage
44 collector collects these objects --- updating the table as required to
45 make sure we can still find the object.
48 Of course, all this rather begs the question: why would we want to
51 One very good reason is to preserve laziness across the language
52 interface. Rather than evaluating an integer or a string because it
53 {\em might\/} be required by the C function, we can wait until the C
54 function actually wants the value and then force an evaluation.
56 Another very good reason (the motivating reason!) is that the C code
57 might want to execute an object of sort $IO ()$ for the side-effects
58 it will produce. For example, this is used when interfacing to an X
59 widgets library to allow a direct implementation of callbacks.
62 The @makeStablePointer :: a -> IO (StablePtr a)@ function
63 converts a value into a stable pointer. It is part of the @PrimIO@
64 monad, because we want to be sure we don't allocate one twice by
65 accident, and then only free one of the copies.
68 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
69 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
70 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
71 (# State# RealWorld, a #)
74 There may be additional functions on the C side to allow evaluation,
75 application, etc of a stable pointer.
79 snEntry *stable_ptr_table = NULL;
80 static snEntry *stable_ptr_free = NULL;
82 static unsigned int SPT_size = 0;
85 static Mutex stable_mutex;
88 /* This hash table maps Haskell objects to stable names, so that every
89 * call to lookupStableName on a given object will return the same
92 * OLD COMMENTS about reference counting follow. The reference count
93 * in a stable name entry is now just a counter.
97 * A plain stable name entry has a zero reference count, which means
98 * the entry will dissappear when the object it points to is
99 * unreachable. For stable pointers, we need an entry that sticks
100 * around and keeps the object it points to alive, so each stable name
101 * entry has an associated reference count.
103 * A stable pointer has a weighted reference count N attached to it
104 * (actually in its upper 5 bits), which represents the weight
105 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
106 * represents any weight between 1 and 2^32 (represented as zero).
107 * When the weight is 2^32, the stable name table owns "all" of the
108 * stable pointers to this object, and the entry can be garbage
109 * collected if the object isn't reachable.
111 * A new stable pointer is given the weight log2(W/2), where W is the
112 * weight stored in the table entry. The new weight in the table is W
115 * A stable pointer can be "split" into two stable pointers, by
116 * dividing the weight by 2 and giving each pointer half.
117 * When freeing a stable pointer, the weight of the pointer is added
118 * to the weight stored in the table entry.
121 static HashTable *addrToStableHash = NULL;
123 #define INIT_SPT_SIZE 64
126 initFreeList(snEntry *table, nat n, snEntry *free)
130 for (p = table + n - 1; p >= table; p--) {
137 stable_ptr_free = table;
141 initStablePtrTable(void)
146 SPT_size = INIT_SPT_SIZE;
147 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
148 "initStablePtrTable");
150 /* we don't use index 0 in the stable name table, because that
151 * would conflict with the hash table lookup operations which
152 * return NULL if an entry isn't found in the hash table.
154 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
155 addrToStableHash = allocHashTable();
158 initMutex(&stable_mutex);
163 * get at the real stuff...remove indirections.
165 * ToDo: move to a better home.
169 removeIndirections(StgClosure* p)
173 while (get_itbl(q)->type == IND ||
174 get_itbl(q)->type == IND_STATIC ||
175 get_itbl(q)->type == IND_OLDGEN ||
176 get_itbl(q)->type == IND_PERM ||
177 get_itbl(q)->type == IND_OLDGEN_PERM ) {
178 q = ((StgInd *)q)->indirectee;
184 lookupStableName_(StgPtr p)
189 if (stable_ptr_free == NULL) {
190 enlargeStablePtrTable();
193 /* removing indirections increases the likelihood
194 * of finding a match in the stable name hash table.
196 p = (StgPtr)removeIndirections((StgClosure*)p);
198 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
199 sn = (StgWord)sn_tmp;
202 ASSERT(stable_ptr_table[sn].addr == p);
203 debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
206 sn = stable_ptr_free - stable_ptr_table;
207 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
208 stable_ptr_table[sn].ref = 0;
209 stable_ptr_table[sn].addr = p;
210 stable_ptr_table[sn].sn_obj = NULL;
211 /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
213 /* add the new stable name to the hash table */
214 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
221 lookupStableName(StgPtr p)
225 initStablePtrTable();
226 ACQUIRE_LOCK(&stable_mutex);
227 res = lookupStableName_(p);
228 RELEASE_LOCK(&stable_mutex);
233 freeStableName(snEntry *sn)
235 ASSERT(sn->sn_obj == NULL);
236 if (sn->addr != NULL) {
237 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
239 sn->addr = (P_)stable_ptr_free;
240 stable_ptr_free = sn;
244 getStablePtr(StgPtr p)
248 initStablePtrTable();
249 ACQUIRE_LOCK(&stable_mutex);
250 sn = lookupStableName_(p);
251 stable_ptr_table[sn].ref++;
252 RELEASE_LOCK(&stable_mutex);
253 return (StgStablePtr)(sn);
257 freeStablePtr(StgStablePtr sp)
261 initStablePtrTable();
262 ACQUIRE_LOCK(&stable_mutex);
264 sn = &stable_ptr_table[(StgWord)sp];
266 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
270 // If this entry has no StableName attached, then just free it
271 // immediately. This is important; it might be a while before the
272 // next major GC which actually collects the entry.
273 if (sn->sn_obj == NULL && sn->ref == 0) {
277 RELEASE_LOCK(&stable_mutex);
281 enlargeStablePtrTable(void)
283 nat old_SPT_size = SPT_size;
285 // 2nd and subsequent times
288 stgReallocBytes(stable_ptr_table,
289 SPT_size * sizeof(snEntry),
290 "enlargeStablePtrTable");
292 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
295 /* -----------------------------------------------------------------------------
296 * Treat stable pointers as roots for the garbage collector.
298 * A stable pointer is any stable name entry with a ref > 0. We'll
299 * take the opportunity to zero the "keep" flags at the same time.
300 * -------------------------------------------------------------------------- */
303 markStablePtrTable(evac_fn evac)
305 snEntry *p, *end_stable_ptr_table;
308 end_stable_ptr_table = &stable_ptr_table[SPT_size];
310 // Mark all the stable *pointers* (not stable names).
311 // _starting_ at index 1; index 0 is unused.
312 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
315 // Internal pointers are free slots. If q == NULL, it's a
316 // stable name where the object has been GC'd, but the
317 // StableName object (sn_obj) is still alive.
318 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
320 // save the current addr away: we need to be able to tell
321 // whether the objects moved in order to be able to update
322 // the hash table later.
325 // if the ref is non-zero, treat addr as a root
327 evac((StgClosure **)&p->addr);
333 /* -----------------------------------------------------------------------------
334 * Thread the stable pointer table for compacting GC.
336 * Here we must call the supplied evac function for each pointer into
337 * the heap from the stable pointer table, because the compacting
338 * collector may move the object it points to.
339 * -------------------------------------------------------------------------- */
342 threadStablePtrTable( evac_fn evac )
344 snEntry *p, *end_stable_ptr_table;
347 end_stable_ptr_table = &stable_ptr_table[SPT_size];
349 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
351 if (p->sn_obj != NULL) {
352 evac((StgClosure **)&p->sn_obj);
356 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
357 evac((StgClosure **)&p->addr);
362 /* -----------------------------------------------------------------------------
363 * Garbage collect any dead entries in the stable pointer table.
367 * - a zero reference count
370 * Both of these conditions must be true in order to re-use the stable
371 * name table entry. We can re-use stable name table entries for live
372 * heap objects, as long as the program has no StableName objects that
373 * refer to the entry.
374 * -------------------------------------------------------------------------- */
377 gcStablePtrTable( void )
379 snEntry *p, *end_stable_ptr_table;
382 end_stable_ptr_table = &stable_ptr_table[SPT_size];
384 // NOTE: _starting_ at index 1; index 0 is unused.
385 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
387 // Update the pointer to the StableName object, if there is one
388 if (p->sn_obj != NULL) {
389 p->sn_obj = isAlive(p->sn_obj);
392 // Internal pointers are free slots. If q == NULL, it's a
393 // stable name where the object has been GC'd, but the
394 // StableName object (sn_obj) is still alive.
396 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
400 if (p->sn_obj == NULL) {
401 // StableName object is dead
403 debugTrace(DEBUG_stable, "GC'd Stable name %ld",
404 p - stable_ptr_table);
408 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
409 debugTrace(DEBUG_stable,
410 "stable name %ld still alive at %p, ref %ld\n",
411 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));