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"
21 /* Comment from ADR's implementation in old RTS:
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
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
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.
46 Of course, all this rather begs the question: why would we want to
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.
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.
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.
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 #)
72 There may be additional functions on the C side to allow evaluation,
73 application, etc of a stable pointer.
77 snEntry *stable_ptr_table = NULL;
78 static snEntry *stable_ptr_free = NULL;
80 static unsigned int SPT_size = 0;
82 /* This hash table maps Haskell objects to stable names, so that every
83 * call to lookupStableName on a given object will return the same
86 * OLD COMMENTS about reference counting follow. The reference count
87 * in a stable name entry is now just a counter.
91 * A plain stable name entry has a zero reference count, which means
92 * the entry will dissappear when the object it points to is
93 * unreachable. For stable pointers, we need an entry that sticks
94 * around and keeps the object it points to alive, so each stable name
95 * entry has an associated reference count.
97 * A stable pointer has a weighted reference count N attached to it
98 * (actually in its upper 5 bits), which represents the weight
99 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
100 * represents any weight between 1 and 2^32 (represented as zero).
101 * When the weight is 2^32, the stable name table owns "all" of the
102 * stable pointers to this object, and the entry can be garbage
103 * collected if the object isn't reachable.
105 * A new stable pointer is given the weight log2(W/2), where W is the
106 * weight stored in the table entry. The new weight in the table is W
109 * A stable pointer can be "split" into two stable pointers, by
110 * dividing the weight by 2 and giving each pointer half.
111 * When freeing a stable pointer, the weight of the pointer is added
112 * to the weight stored in the table entry.
115 static HashTable *addrToStableHash = NULL;
117 #define INIT_SPT_SIZE 64
120 initFreeList(snEntry *table, nat n, snEntry *free)
124 for (p = table + n - 1; p >= table; p--) {
131 stable_ptr_free = table;
135 initStablePtrTable(void)
138 // the table will be allocated the first time makeStablePtr is
139 // called, and we want the table to persist through multiple inits.
141 // Also, getStablePtr is now called from __attribute__((constructor))
142 // functions, so initialising things here wouldn't work anyway.
146 * get at the real stuff...remove indirections.
148 * ToDo: move to a better home.
152 removeIndirections(StgClosure* p)
156 while (get_itbl(q)->type == IND ||
157 get_itbl(q)->type == IND_STATIC ||
158 get_itbl(q)->type == IND_OLDGEN ||
159 get_itbl(q)->type == IND_PERM ||
160 get_itbl(q)->type == IND_OLDGEN_PERM ) {
161 q = ((StgInd *)q)->indirectee;
167 lookupStableName(StgPtr p)
172 if (stable_ptr_free == NULL) {
173 enlargeStablePtrTable();
176 /* removing indirections increases the likelihood
177 * of finding a match in the stable name hash table.
179 p = (StgPtr)removeIndirections((StgClosure*)p);
181 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
182 sn = (StgWord)sn_tmp;
185 ASSERT(stable_ptr_table[sn].addr == p);
186 IF_DEBUG(stable,debugBelch("cached stable name %ld at %p\n",sn,p));
189 sn = stable_ptr_free - stable_ptr_table;
190 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
191 stable_ptr_table[sn].ref = 0;
192 stable_ptr_table[sn].addr = p;
193 stable_ptr_table[sn].sn_obj = NULL;
194 /* IF_DEBUG(stable,debugBelch("new stable name %d at %p\n",sn,p)); */
196 /* add the new stable name to the hash table */
197 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
204 freeStableName(snEntry *sn)
206 ASSERT(sn->sn_obj == NULL);
207 if (sn->addr != NULL) {
208 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
210 sn->addr = (P_)stable_ptr_free;
211 stable_ptr_free = sn;
215 getStablePtr(StgPtr p)
219 sn = lookupStableName(p);
220 stable_ptr_table[sn].ref++;
221 return (StgStablePtr)(sn);
225 freeStablePtr(StgStablePtr sp)
227 snEntry *sn = &stable_ptr_table[(StgWord)sp];
229 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
233 // If this entry has no StableName attached, then just free it
234 // immediately. This is important; it might be a while before the
235 // next major GC which actually collects the entry.
236 if (sn->sn_obj == NULL && sn->ref == 0) {
242 enlargeStablePtrTable(void)
244 nat old_SPT_size = SPT_size;
248 SPT_size = INIT_SPT_SIZE;
249 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
250 "enlargeStablePtrTable");
252 /* we don't use index 0 in the stable name table, because that
253 * would conflict with the hash table lookup operations which
254 * return NULL if an entry isn't found in the hash table.
256 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
257 addrToStableHash = allocHashTable();
260 // 2nd and subsequent times
263 stgReallocBytes(stable_ptr_table,
264 SPT_size * sizeof(snEntry),
265 "enlargeStablePtrTable");
267 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
271 /* -----------------------------------------------------------------------------
272 * Treat stable pointers as roots for the garbage collector.
274 * A stable pointer is any stable name entry with a ref > 0. We'll
275 * take the opportunity to zero the "keep" flags at the same time.
276 * -------------------------------------------------------------------------- */
279 markStablePtrTable(evac_fn evac)
281 snEntry *p, *end_stable_ptr_table;
284 end_stable_ptr_table = &stable_ptr_table[SPT_size];
286 // Mark all the stable *pointers* (not stable names).
287 // _starting_ at index 1; index 0 is unused.
288 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
291 // Internal pointers are free slots. If q == NULL, it's a
292 // stable name where the object has been GC'd, but the
293 // StableName object (sn_obj) is still alive.
294 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
296 // save the current addr away: we need to be able to tell
297 // whether the objects moved in order to be able to update
298 // the hash table later.
301 // if the ref is non-zero, treat addr as a root
303 evac((StgClosure **)&p->addr);
309 /* -----------------------------------------------------------------------------
310 * Thread the stable pointer table for compacting GC.
312 * Here we must call the supplied evac function for each pointer into
313 * the heap from the stable pointer table, because the compacting
314 * collector may move the object it points to.
315 * -------------------------------------------------------------------------- */
318 threadStablePtrTable( evac_fn evac )
320 snEntry *p, *end_stable_ptr_table;
323 end_stable_ptr_table = &stable_ptr_table[SPT_size];
325 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
327 if (p->sn_obj != NULL) {
328 evac((StgClosure **)&p->sn_obj);
332 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
333 evac((StgClosure **)&p->addr);
338 /* -----------------------------------------------------------------------------
339 * Garbage collect any dead entries in the stable pointer table.
343 * - a zero reference count
346 * Both of these conditions must be true in order to re-use the stable
347 * name table entry. We can re-use stable name table entries for live
348 * heap objects, as long as the program has no StableName objects that
349 * refer to the entry.
350 * -------------------------------------------------------------------------- */
353 gcStablePtrTable( void )
355 snEntry *p, *end_stable_ptr_table;
358 end_stable_ptr_table = &stable_ptr_table[SPT_size];
360 // NOTE: _starting_ at index 1; index 0 is unused.
361 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
363 // Update the pointer to the StableName object, if there is one
364 if (p->sn_obj != NULL) {
365 p->sn_obj = isAlive(p->sn_obj);
368 // Internal pointers are free slots. If q == NULL, it's a
369 // stable name where the object has been GC'd, but the
370 // StableName object (sn_obj) is still alive.
372 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
376 if (p->sn_obj == NULL) {
377 // StableName object is dead
379 IF_DEBUG(stable, debugBelch("GC'd Stable name %d\n",
380 p - stable_ptr_table));
384 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
385 IF_DEBUG(stable, debugBelch("Stable name %d still alive at %p, ref %ld\n", p - stable_ptr_table, p->addr, p->ref));
392 /* -----------------------------------------------------------------------------
393 * Update the StablePtr/StableName hash table
395 * The boolean argument 'full' indicates that a major collection is
396 * being done, so we might as well throw away the hash table and build
397 * a new one. For a minor collection, we just re-hash the elements
399 * -------------------------------------------------------------------------- */
402 updateStablePtrTable(rtsBool full)
404 snEntry *p, *end_stable_ptr_table;
406 if (full && addrToStableHash != NULL) {
407 freeHashTable(addrToStableHash,NULL);
408 addrToStableHash = allocHashTable();
411 end_stable_ptr_table = &stable_ptr_table[SPT_size];
413 // NOTE: _starting_ at index 1; index 0 is unused.
414 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
416 if (p->addr == NULL) {
417 if (p->old != NULL) {
418 // The target has been garbage collected. Remove its
419 // entry from the hash table.
420 removeHashTable(addrToStableHash, (W_)p->old, NULL);
424 else if (p->addr < (P_)stable_ptr_table
425 || p->addr >= (P_)end_stable_ptr_table) {
426 // Target still alive, Re-hash this stable name
428 insertHashTable(addrToStableHash, (W_)p->addr,
429 (void *)(p - stable_ptr_table));
430 } else if (p->addr != p->old) {
431 removeHashTable(addrToStableHash, (W_)p->old, NULL);
432 insertHashTable(addrToStableHash, (W_)p->addr,
433 (void *)(p - stable_ptr_table));