1 /* -----------------------------------------------------------------------------
2 * $Id: Stable.c,v 1.23 2002/12/19 14:25:05 simonmar 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"
16 #include "StablePriv.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.
76 When Haskell calls C, it normally just passes over primitive integers,
77 floats, bools, strings, etc. This doesn't cause any problems at all
78 for garbage collection because the act of passing them makes a copy
79 from the heap, stack or wherever they are onto the C-world stack.
80 However, if we were to pass a heap object such as a (Haskell) @String@
81 and a garbage collection occured before we finished using it, we'd run
82 into problems since the heap object might have been moved or even
85 So, if a C call is able to cause a garbage collection or we want to
86 store a pointer to a heap object between C calls, we must be careful
87 when passing heap objects. Our solution is to keep a table of all
88 objects we've given to the C-world and to make sure that the garbage
89 collector collects these objects --- updating the table as required to
90 make sure we can still find the object.
93 snEntry *stable_ptr_table;
94 static snEntry *stable_ptr_free;
96 static unsigned int SPT_size;
98 /* This hash table maps Haskell objects to stable names, so that every
99 * call to lookupStableName on a given object will return the same
102 * OLD COMMENTS about reference counting follow. The reference count
103 * in a stable name entry is now just a counter.
107 * A plain stable name entry has a zero reference count, which means
108 * the entry will dissappear when the object it points to is
109 * unreachable. For stable pointers, we need an entry that sticks
110 * around and keeps the object it points to alive, so each stable name
111 * entry has an associated reference count.
113 * A stable pointer has a weighted reference count N attached to it
114 * (actually in its upper 5 bits), which represents the weight
115 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
116 * represents any weight between 1 and 2^32 (represented as zero).
117 * When the weight is 2^32, the stable name table owns "all" of the
118 * stable pointers to this object, and the entry can be garbage
119 * collected if the object isn't reachable.
121 * A new stable pointer is given the weight log2(W/2), where W is the
122 * weight stored in the table entry. The new weight in the table is W
125 * A stable pointer can be "split" into two stable pointers, by
126 * dividing the weight by 2 and giving each pointer half.
127 * When freeing a stable pointer, the weight of the pointer is added
128 * to the weight stored in the table entry.
131 static HashTable *addrToStableHash;
133 #define INIT_SPT_SIZE 64
136 initFreeList(snEntry *table, nat n, snEntry *free)
140 for (p = table + n - 1; p >= table; p--) {
147 stable_ptr_free = table;
151 initStablePtrTable(void)
153 /* the table will be allocated the first time makeStablePtr is
155 stable_ptr_table = NULL;
156 stable_ptr_free = NULL;
157 addrToStableHash = NULL;
162 * get at the real stuff...remove indirections.
164 * ToDo: move to a better home.
168 removeIndirections(StgClosure* p)
172 while (get_itbl(q)->type == IND ||
173 get_itbl(q)->type == IND_STATIC ||
174 get_itbl(q)->type == IND_OLDGEN ||
175 get_itbl(q)->type == IND_PERM ||
176 get_itbl(q)->type == IND_OLDGEN_PERM ) {
177 q = ((StgInd *)q)->indirectee;
183 lookupStableName(StgPtr p)
187 if (stable_ptr_free == NULL) {
188 enlargeStablePtrTable();
191 /* removing indirections increases the likelihood
192 * of finding a match in the stable name hash table.
194 p = (StgPtr)removeIndirections((StgClosure*)p);
196 (void *)sn = lookupHashTable(addrToStableHash,(W_)p);
199 ASSERT(stable_ptr_table[sn].addr == p);
200 IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
203 sn = stable_ptr_free - stable_ptr_table;
204 (P_)stable_ptr_free = stable_ptr_free->addr;
205 stable_ptr_table[sn].ref = 0;
206 stable_ptr_table[sn].addr = p;
207 stable_ptr_table[sn].sn_obj = NULL;
208 /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
211 /* add the new stable name to the hash table */
212 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
219 freeStableName(snEntry *sn)
221 ASSERT(sn->sn_obj == NULL);
222 if (sn->addr != NULL) {
223 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
225 sn->addr = (P_)stable_ptr_free;
226 stable_ptr_free = sn;
230 getStablePtr(StgPtr p)
234 sn = lookupStableName(p);
235 stable_ptr_table[sn].ref++;
236 return (StgStablePtr)(sn);
240 freeStablePtr(StgStablePtr sp)
242 snEntry *sn = &stable_ptr_table[(StgWord)sp];
244 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
248 // If this entry has no StableName attached, then just free it
249 // immediately. This is important; it might be a while before the
250 // next major GC which actually collects the entry.
251 if (sn->sn_obj == NULL && sn->ref == 0) {
257 enlargeStablePtrTable(void)
259 nat old_SPT_size = SPT_size;
263 SPT_size = INIT_SPT_SIZE;
264 stable_ptr_table = stgMallocWords(SPT_size * sizeof(snEntry),
265 "initStablePtrTable");
267 /* we don't use index 0 in the stable name table, because that
268 * would conflict with the hash table lookup operations which
269 * return NULL if an entry isn't found in the hash table.
271 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
272 addrToStableHash = allocHashTable();
275 // 2nd and subsequent times
278 stgReallocWords(stable_ptr_table, SPT_size * sizeof(snEntry),
279 "enlargeStablePtrTable");
281 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
285 /* -----------------------------------------------------------------------------
286 * Treat stable pointers as roots for the garbage collector.
288 * A stable pointer is any stable name entry with a ref > 0. We'll
289 * take the opportunity to zero the "keep" flags at the same time.
290 * -------------------------------------------------------------------------- */
293 markStablePtrTable(evac_fn evac)
295 snEntry *p, *end_stable_ptr_table;
298 end_stable_ptr_table = &stable_ptr_table[SPT_size];
300 // Mark all the stable *pointers* (not stable names).
301 // _starting_ at index 1; index 0 is unused.
302 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
305 // Internal pointers are free slots. If q == NULL, it's a
306 // stable name where the object has been GC'd, but the
307 // StableName object (sn_obj) is still alive.
308 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
310 // save the current addr away: we need to be able to tell
311 // whether the objects moved in order to be able to update
312 // the hash table later.
315 // if the ref is non-zero, treat addr as a root
317 evac((StgClosure **)&p->addr);
323 /* -----------------------------------------------------------------------------
324 * Thread the stable pointer table for compacting GC.
326 * Here we must call the supplied evac function for each pointer into
327 * the heap from the stable pointer table, because the compacting
328 * collector may move the object it points to.
329 * -------------------------------------------------------------------------- */
332 threadStablePtrTable( evac_fn evac )
334 snEntry *p, *end_stable_ptr_table;
337 end_stable_ptr_table = &stable_ptr_table[SPT_size];
339 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
341 if (p->sn_obj != NULL) {
342 evac((StgClosure **)&p->sn_obj);
346 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
347 evac((StgClosure **)&p->addr);
352 /* -----------------------------------------------------------------------------
353 * Garbage collect any dead entries in the stable pointer table.
357 * - a zero reference count
360 * Both of these conditions must be true in order to re-use the stable
361 * name table entry. We can re-use stable name table entries for live
362 * heap objects, as long as the program has no StableName objects that
363 * refer to the entry.
364 * -------------------------------------------------------------------------- */
367 gcStablePtrTable( void )
369 snEntry *p, *end_stable_ptr_table;
372 end_stable_ptr_table = &stable_ptr_table[SPT_size];
374 // NOTE: _starting_ at index 1; index 0 is unused.
375 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
377 // Update the pointer to the StableName object, if there is one
378 if (p->sn_obj != NULL) {
379 p->sn_obj = isAlive(p->sn_obj);
382 // Internal pointers are free slots. If q == NULL, it's a
383 // stable name where the object has been GC'd, but the
384 // StableName object (sn_obj) is still alive.
386 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
390 if (p->sn_obj == NULL) {
391 // StableName object is dead
393 IF_DEBUG(stable, fprintf(stderr,"GC'd Stable name %d\n",
394 p - stable_ptr_table));
398 (StgClosure *)p->addr = isAlive((StgClosure *)p->addr);
399 IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, ref %d\n", p - stable_ptr_table, p->addr, p->ref));
406 /* -----------------------------------------------------------------------------
407 * Update the StablePtr/StableName hash table
409 * The boolean argument 'full' indicates that a major collection is
410 * being done, so we might as well throw away the hash table and build
411 * a new one. For a minor collection, we just re-hash the elements
413 * -------------------------------------------------------------------------- */
416 updateStablePtrTable(rtsBool full)
418 snEntry *p, *end_stable_ptr_table;
420 if (full && addrToStableHash != NULL) {
421 freeHashTable(addrToStableHash,NULL);
422 addrToStableHash = allocHashTable();
425 end_stable_ptr_table = &stable_ptr_table[SPT_size];
427 // NOTE: _starting_ at index 1; index 0 is unused.
428 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
430 if (p->addr == NULL) {
431 if (p->old != NULL) {
432 // The target has been garbage collected. Remove its
433 // entry from the hash table.
434 removeHashTable(addrToStableHash, (W_)p->old, NULL);
438 else if (p->addr < (P_)stable_ptr_table
439 || p->addr >= (P_)end_stable_ptr_table) {
440 // Target still alive, Re-hash this stable name
442 insertHashTable(addrToStableHash, (W_)p->addr,
443 (void *)(p - stable_ptr_table));
444 } else if (p->addr != p->old) {
445 removeHashTable(addrToStableHash, (W_)p->old, NULL);
446 insertHashTable(addrToStableHash, (W_)p->addr,
447 (void *)(p - stable_ptr_table));