1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team, 1998-2002
5 * Stable names and stable pointers.
7 * ---------------------------------------------------------------------------*/
9 #include "PosixSource.h"
18 /* Comment from ADR's implementation in old RTS:
20 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
21 small change in @HpOverflow.lc@) consists of the changes in the
22 runtime system required to implement "Stable Pointers". But we're
23 getting a bit ahead of ourselves --- what is a stable pointer and what
26 When Haskell calls C, it normally just passes over primitive integers,
27 floats, bools, strings, etc. This doesn't cause any problems at all
28 for garbage collection because the act of passing them makes a copy
29 from the heap, stack or wherever they are onto the C-world stack.
30 However, if we were to pass a heap object such as a (Haskell) @String@
31 and a garbage collection occured before we finished using it, we'd run
32 into problems since the heap object might have been moved or even
35 So, if a C call is able to cause a garbage collection or we want to
36 store a pointer to a heap object between C calls, we must be careful
37 when passing heap objects. Our solution is to keep a table of all
38 objects we've given to the C-world and to make sure that the garbage
39 collector collects these objects --- updating the table as required to
40 make sure we can still find the object.
43 Of course, all this rather begs the question: why would we want to
46 One very good reason is to preserve laziness across the language
47 interface. Rather than evaluating an integer or a string because it
48 {\em might\/} be required by the C function, we can wait until the C
49 function actually wants the value and then force an evaluation.
51 Another very good reason (the motivating reason!) is that the C code
52 might want to execute an object of sort $IO ()$ for the side-effects
53 it will produce. For example, this is used when interfacing to an X
54 widgets library to allow a direct implementation of callbacks.
57 The @makeStablePointer :: a -> IO (StablePtr a)@ function
58 converts a value into a stable pointer. It is part of the @PrimIO@
59 monad, because we want to be sure we don't allocate one twice by
60 accident, and then only free one of the copies.
63 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
64 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
65 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
66 (# State# RealWorld, a #)
69 There may be additional functions on the C side to allow evaluation,
70 application, etc of a stable pointer.
74 snEntry *stable_ptr_table = NULL;
75 static snEntry *stable_ptr_free = NULL;
77 static unsigned int SPT_size = 0;
80 static Mutex stable_mutex;
83 static void enlargeStablePtrTable(void);
85 /* This hash table maps Haskell objects to stable names, so that every
86 * call to lookupStableName on a given object will return the same
89 * OLD COMMENTS about reference counting follow. The reference count
90 * in a stable name entry is now just a counter.
94 * A plain stable name entry has a zero reference count, which means
95 * the entry will dissappear when the object it points to is
96 * unreachable. For stable pointers, we need an entry that sticks
97 * around and keeps the object it points to alive, so each stable name
98 * entry has an associated reference count.
100 * A stable pointer has a weighted reference count N attached to it
101 * (actually in its upper 5 bits), which represents the weight
102 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
103 * represents any weight between 1 and 2^32 (represented as zero).
104 * When the weight is 2^32, the stable name table owns "all" of the
105 * stable pointers to this object, and the entry can be garbage
106 * collected if the object isn't reachable.
108 * A new stable pointer is given the weight log2(W/2), where W is the
109 * weight stored in the table entry. The new weight in the table is W
112 * A stable pointer can be "split" into two stable pointers, by
113 * dividing the weight by 2 and giving each pointer half.
114 * When freeing a stable pointer, the weight of the pointer is added
115 * to the weight stored in the table entry.
118 static HashTable *addrToStableHash = NULL;
120 #define INIT_SPT_SIZE 64
123 initFreeList(snEntry *table, nat n, snEntry *free)
127 for (p = table + n - 1; p >= table; p--) {
134 stable_ptr_free = table;
138 initStablePtrTable(void)
143 SPT_size = INIT_SPT_SIZE;
144 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
145 "initStablePtrTable");
147 /* we don't use index 0 in the stable name table, because that
148 * would conflict with the hash table lookup operations which
149 * return NULL if an entry isn't found in the hash table.
151 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
152 addrToStableHash = allocHashTable();
155 initMutex(&stable_mutex);
160 exitStablePtrTable(void)
162 if (addrToStableHash)
163 freeHashTable(addrToStableHash, NULL);
164 addrToStableHash = NULL;
165 if (stable_ptr_table)
166 stgFree(stable_ptr_table);
167 stable_ptr_table = NULL;
170 closeMutex(&stable_mutex);
175 * get at the real stuff...remove indirections.
176 * It untags pointers before dereferencing and
177 * retags the real stuff with its tag (if there
178 * is any) when returning.
180 * ToDo: move to a better home.
184 removeIndirections(StgClosure* p)
186 StgWord tag = GET_CLOSURE_TAG(p);
187 StgClosure* q = UNTAG_CLOSURE(p);
189 while (get_itbl(q)->type == IND ||
190 get_itbl(q)->type == IND_STATIC ||
191 get_itbl(q)->type == IND_PERM) {
192 q = ((StgInd *)q)->indirectee;
193 tag = GET_CLOSURE_TAG(q);
194 q = UNTAG_CLOSURE(q);
197 return TAG_CLOSURE(tag,q);
201 lookupStableName_(StgPtr p)
206 if (stable_ptr_free == NULL) {
207 enlargeStablePtrTable();
210 /* removing indirections increases the likelihood
211 * of finding a match in the stable name hash table.
213 p = (StgPtr)removeIndirections((StgClosure*)p);
215 // register the untagged pointer. This just makes things simpler.
216 p = (StgPtr)UNTAG_CLOSURE((StgClosure*)p);
218 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
219 sn = (StgWord)sn_tmp;
222 ASSERT(stable_ptr_table[sn].addr == p);
223 debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
226 sn = stable_ptr_free - stable_ptr_table;
227 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
228 stable_ptr_table[sn].ref = 0;
229 stable_ptr_table[sn].addr = p;
230 stable_ptr_table[sn].sn_obj = NULL;
231 /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
233 /* add the new stable name to the hash table */
234 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
241 lookupStableName(StgPtr p)
245 initStablePtrTable();
246 ACQUIRE_LOCK(&stable_mutex);
247 res = lookupStableName_(p);
248 RELEASE_LOCK(&stable_mutex);
253 freeStableName(snEntry *sn)
255 ASSERT(sn->sn_obj == NULL);
256 if (sn->addr != NULL) {
257 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
259 sn->addr = (P_)stable_ptr_free;
260 stable_ptr_free = sn;
264 getStablePtr(StgPtr p)
268 initStablePtrTable();
269 ACQUIRE_LOCK(&stable_mutex);
270 sn = lookupStableName_(p);
271 stable_ptr_table[sn].ref++;
272 RELEASE_LOCK(&stable_mutex);
273 return (StgStablePtr)(sn);
277 freeStablePtr(StgStablePtr sp)
281 initStablePtrTable();
282 ACQUIRE_LOCK(&stable_mutex);
284 sn = &stable_ptr_table[(StgWord)sp];
286 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
290 // If this entry has no StableName attached, then just free it
291 // immediately. This is important; it might be a while before the
292 // next major GC which actually collects the entry.
293 if (sn->sn_obj == NULL && sn->ref == 0) {
297 RELEASE_LOCK(&stable_mutex);
301 enlargeStablePtrTable(void)
303 nat old_SPT_size = SPT_size;
305 // 2nd and subsequent times
308 stgReallocBytes(stable_ptr_table,
309 SPT_size * sizeof(snEntry),
310 "enlargeStablePtrTable");
312 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
315 /* -----------------------------------------------------------------------------
316 * We must lock the StablePtr table during GC, to prevent simultaneous
317 * calls to freeStablePtr().
318 * -------------------------------------------------------------------------- */
323 ACQUIRE_LOCK(&stable_mutex);
327 stablePtrPostGC(void)
329 RELEASE_LOCK(&stable_mutex);
332 /* -----------------------------------------------------------------------------
333 * Treat stable pointers as roots for the garbage collector.
335 * A stable pointer is any stable name entry with a ref > 0. We'll
336 * take the opportunity to zero the "keep" flags at the same time.
337 * -------------------------------------------------------------------------- */
340 markStablePtrTable(evac_fn evac, void *user)
342 snEntry *p, *end_stable_ptr_table;
345 end_stable_ptr_table = &stable_ptr_table[SPT_size];
347 // Mark all the stable *pointers* (not stable names).
348 // _starting_ at index 1; index 0 is unused.
349 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
352 // Internal pointers are free slots. If q == NULL, it's a
353 // stable name where the object has been GC'd, but the
354 // StableName object (sn_obj) is still alive.
355 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
357 // save the current addr away: we need to be able to tell
358 // whether the objects moved in order to be able to update
359 // the hash table later.
362 // if the ref is non-zero, treat addr as a root
364 evac(user, (StgClosure **)&p->addr);
370 /* -----------------------------------------------------------------------------
371 * Thread the stable pointer table for compacting GC.
373 * Here we must call the supplied evac function for each pointer into
374 * the heap from the stable pointer table, because the compacting
375 * collector may move the object it points to.
376 * -------------------------------------------------------------------------- */
379 threadStablePtrTable( evac_fn evac, void *user )
381 snEntry *p, *end_stable_ptr_table;
384 end_stable_ptr_table = &stable_ptr_table[SPT_size];
386 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
388 if (p->sn_obj != NULL) {
389 evac(user, (StgClosure **)&p->sn_obj);
393 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
394 evac(user, (StgClosure **)&p->addr);
399 /* -----------------------------------------------------------------------------
400 * Garbage collect any dead entries in the stable pointer table.
404 * - a zero reference count
407 * Both of these conditions must be true in order to re-use the stable
408 * name table entry. We can re-use stable name table entries for live
409 * heap objects, as long as the program has no StableName objects that
410 * refer to the entry.
411 * -------------------------------------------------------------------------- */
414 gcStablePtrTable( void )
416 snEntry *p, *end_stable_ptr_table;
419 end_stable_ptr_table = &stable_ptr_table[SPT_size];
421 // NOTE: _starting_ at index 1; index 0 is unused.
422 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
424 // Update the pointer to the StableName object, if there is one
425 if (p->sn_obj != NULL) {
426 p->sn_obj = isAlive(p->sn_obj);
429 // Internal pointers are free slots. If q == NULL, it's a
430 // stable name where the object has been GC'd, but the
431 // StableName object (sn_obj) is still alive.
433 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
437 if (p->sn_obj == NULL) {
438 // StableName object is dead
440 debugTrace(DEBUG_stable, "GC'd Stable name %ld",
441 (long)(p - stable_ptr_table));
445 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
446 debugTrace(DEBUG_stable,
447 "stable name %ld still alive at %p, ref %ld\n",
448 (long)(p - stable_ptr_table), p->addr, p->ref);
455 /* -----------------------------------------------------------------------------
456 * Update the StablePtr/StableName hash table
458 * The boolean argument 'full' indicates that a major collection is
459 * being done, so we might as well throw away the hash table and build
460 * a new one. For a minor collection, we just re-hash the elements
462 * -------------------------------------------------------------------------- */
465 updateStablePtrTable(rtsBool full)
467 snEntry *p, *end_stable_ptr_table;
469 if (full && addrToStableHash != NULL) {
470 freeHashTable(addrToStableHash,NULL);
471 addrToStableHash = allocHashTable();
474 end_stable_ptr_table = &stable_ptr_table[SPT_size];
476 // NOTE: _starting_ at index 1; index 0 is unused.
477 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
479 if (p->addr == NULL) {
480 if (p->old != NULL) {
481 // The target has been garbage collected. Remove its
482 // entry from the hash table.
483 removeHashTable(addrToStableHash, (W_)p->old, NULL);
487 else if (p->addr < (P_)stable_ptr_table
488 || p->addr >= (P_)end_stable_ptr_table) {
489 // Target still alive, Re-hash this stable name
491 insertHashTable(addrToStableHash, (W_)p->addr,
492 (void *)(p - stable_ptr_table));
493 } else if (p->addr != p->old) {
494 removeHashTable(addrToStableHash, (W_)p->old, NULL);
495 insertHashTable(addrToStableHash, (W_)p->addr,
496 (void *)(p - stable_ptr_table));