1 /* -----------------------------------------------------------------------------
2 * $Id: Stable.c,v 1.29 2004/08/22 15:50:42 panne Exp $
4 * (c) The GHC Team, 1998-2002
6 * Stable names and stable pointers.
8 * ---------------------------------------------------------------------------*/
10 // Make static versions of inline functions in Stable.h:
13 #include "PosixSource.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.
143 * get at the real stuff...remove indirections.
145 * ToDo: move to a better home.
149 removeIndirections(StgClosure* p)
153 while (get_itbl(q)->type == IND ||
154 get_itbl(q)->type == IND_STATIC ||
155 get_itbl(q)->type == IND_OLDGEN ||
156 get_itbl(q)->type == IND_PERM ||
157 get_itbl(q)->type == IND_OLDGEN_PERM ) {
158 q = ((StgInd *)q)->indirectee;
164 lookupStableName(StgPtr p)
169 if (stable_ptr_free == NULL) {
170 enlargeStablePtrTable();
173 /* removing indirections increases the likelihood
174 * of finding a match in the stable name hash table.
176 p = (StgPtr)removeIndirections((StgClosure*)p);
178 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
179 sn = (StgWord)sn_tmp;
182 ASSERT(stable_ptr_table[sn].addr == p);
183 IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
186 sn = stable_ptr_free - stable_ptr_table;
187 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
188 stable_ptr_table[sn].ref = 0;
189 stable_ptr_table[sn].addr = p;
190 stable_ptr_table[sn].sn_obj = NULL;
191 /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
194 /* add the new stable name to the hash table */
195 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
202 freeStableName(snEntry *sn)
204 ASSERT(sn->sn_obj == NULL);
205 if (sn->addr != NULL) {
206 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
208 sn->addr = (P_)stable_ptr_free;
209 stable_ptr_free = sn;
213 getStablePtr(StgPtr p)
217 sn = lookupStableName(p);
218 stable_ptr_table[sn].ref++;
219 return (StgStablePtr)(sn);
223 freeStablePtr(StgStablePtr sp)
225 snEntry *sn = &stable_ptr_table[(StgWord)sp];
227 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
231 // If this entry has no StableName attached, then just free it
232 // immediately. This is important; it might be a while before the
233 // next major GC which actually collects the entry.
234 if (sn->sn_obj == NULL && sn->ref == 0) {
240 enlargeStablePtrTable(void)
242 nat old_SPT_size = SPT_size;
246 SPT_size = INIT_SPT_SIZE;
247 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
248 "enlargeStablePtrTable");
250 /* we don't use index 0 in the stable name table, because that
251 * would conflict with the hash table lookup operations which
252 * return NULL if an entry isn't found in the hash table.
254 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
255 addrToStableHash = allocHashTable();
258 // 2nd and subsequent times
261 stgReallocBytes(stable_ptr_table,
262 SPT_size * sizeof(snEntry),
263 "enlargeStablePtrTable");
265 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
269 /* -----------------------------------------------------------------------------
270 * Treat stable pointers as roots for the garbage collector.
272 * A stable pointer is any stable name entry with a ref > 0. We'll
273 * take the opportunity to zero the "keep" flags at the same time.
274 * -------------------------------------------------------------------------- */
277 markStablePtrTable(evac_fn evac)
279 snEntry *p, *end_stable_ptr_table;
282 end_stable_ptr_table = &stable_ptr_table[SPT_size];
284 // Mark all the stable *pointers* (not stable names).
285 // _starting_ at index 1; index 0 is unused.
286 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
289 // Internal pointers are free slots. If q == NULL, it's a
290 // stable name where the object has been GC'd, but the
291 // StableName object (sn_obj) is still alive.
292 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
294 // save the current addr away: we need to be able to tell
295 // whether the objects moved in order to be able to update
296 // the hash table later.
299 // if the ref is non-zero, treat addr as a root
301 evac((StgClosure **)&p->addr);
307 /* -----------------------------------------------------------------------------
308 * Thread the stable pointer table for compacting GC.
310 * Here we must call the supplied evac function for each pointer into
311 * the heap from the stable pointer table, because the compacting
312 * collector may move the object it points to.
313 * -------------------------------------------------------------------------- */
316 threadStablePtrTable( evac_fn evac )
318 snEntry *p, *end_stable_ptr_table;
321 end_stable_ptr_table = &stable_ptr_table[SPT_size];
323 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
325 if (p->sn_obj != NULL) {
326 evac((StgClosure **)&p->sn_obj);
330 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
331 evac((StgClosure **)&p->addr);
336 /* -----------------------------------------------------------------------------
337 * Garbage collect any dead entries in the stable pointer table.
341 * - a zero reference count
344 * Both of these conditions must be true in order to re-use the stable
345 * name table entry. We can re-use stable name table entries for live
346 * heap objects, as long as the program has no StableName objects that
347 * refer to the entry.
348 * -------------------------------------------------------------------------- */
351 gcStablePtrTable( void )
353 snEntry *p, *end_stable_ptr_table;
356 end_stable_ptr_table = &stable_ptr_table[SPT_size];
358 // NOTE: _starting_ at index 1; index 0 is unused.
359 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
361 // Update the pointer to the StableName object, if there is one
362 if (p->sn_obj != NULL) {
363 p->sn_obj = isAlive(p->sn_obj);
366 // Internal pointers are free slots. If q == NULL, it's a
367 // stable name where the object has been GC'd, but the
368 // StableName object (sn_obj) is still alive.
370 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
374 if (p->sn_obj == NULL) {
375 // StableName object is dead
377 IF_DEBUG(stable, fprintf(stderr,"GC'd Stable name %d\n",
378 p - stable_ptr_table));
382 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
383 IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, ref %d\n", p - stable_ptr_table, p->addr, p->ref));
390 /* -----------------------------------------------------------------------------
391 * Update the StablePtr/StableName hash table
393 * The boolean argument 'full' indicates that a major collection is
394 * being done, so we might as well throw away the hash table and build
395 * a new one. For a minor collection, we just re-hash the elements
397 * -------------------------------------------------------------------------- */
400 updateStablePtrTable(rtsBool full)
402 snEntry *p, *end_stable_ptr_table;
404 if (full && addrToStableHash != NULL) {
405 freeHashTable(addrToStableHash,NULL);
406 addrToStableHash = allocHashTable();
409 end_stable_ptr_table = &stable_ptr_table[SPT_size];
411 // NOTE: _starting_ at index 1; index 0 is unused.
412 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
414 if (p->addr == NULL) {
415 if (p->old != NULL) {
416 // The target has been garbage collected. Remove its
417 // entry from the hash table.
418 removeHashTable(addrToStableHash, (W_)p->old, NULL);
422 else if (p->addr < (P_)stable_ptr_table
423 || p->addr >= (P_)end_stable_ptr_table) {
424 // Target still alive, Re-hash this stable name
426 insertHashTable(addrToStableHash, (W_)p->addr,
427 (void *)(p - stable_ptr_table));
428 } else if (p->addr != p->old) {
429 removeHashTable(addrToStableHash, (W_)p->old, NULL);
430 insertHashTable(addrToStableHash, (W_)p->addr,
431 (void *)(p - stable_ptr_table));