1 /* -----------------------------------------------------------------------------
2 * $Id: Stable.c,v 1.26 2003/06/24 13:45:49 stolz 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.
78 snEntry *stable_ptr_table = NULL;
79 static snEntry *stable_ptr_free = NULL;
81 static unsigned int SPT_size = 0;
83 /* This hash table maps Haskell objects to stable names, so that every
84 * call to lookupStableName on a given object will return the same
87 * OLD COMMENTS about reference counting follow. The reference count
88 * in a stable name entry is now just a counter.
92 * A plain stable name entry has a zero reference count, which means
93 * the entry will dissappear when the object it points to is
94 * unreachable. For stable pointers, we need an entry that sticks
95 * around and keeps the object it points to alive, so each stable name
96 * entry has an associated reference count.
98 * A stable pointer has a weighted reference count N attached to it
99 * (actually in its upper 5 bits), which represents the weight
100 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
101 * represents any weight between 1 and 2^32 (represented as zero).
102 * When the weight is 2^32, the stable name table owns "all" of the
103 * stable pointers to this object, and the entry can be garbage
104 * collected if the object isn't reachable.
106 * A new stable pointer is given the weight log2(W/2), where W is the
107 * weight stored in the table entry. The new weight in the table is W
110 * A stable pointer can be "split" into two stable pointers, by
111 * dividing the weight by 2 and giving each pointer half.
112 * When freeing a stable pointer, the weight of the pointer is added
113 * to the weight stored in the table entry.
116 static HashTable *addrToStableHash = NULL;
118 #define INIT_SPT_SIZE 64
121 initFreeList(snEntry *table, nat n, snEntry *free)
125 for (p = table + n - 1; p >= table; p--) {
132 stable_ptr_free = table;
136 initStablePtrTable(void)
139 // the table will be allocated the first time makeStablePtr is
140 // called, and we want the table to persist through multiple inits.
144 * get at the real stuff...remove indirections.
146 * ToDo: move to a better home.
150 removeIndirections(StgClosure* p)
154 while (get_itbl(q)->type == IND ||
155 get_itbl(q)->type == IND_STATIC ||
156 get_itbl(q)->type == IND_OLDGEN ||
157 get_itbl(q)->type == IND_PERM ||
158 get_itbl(q)->type == IND_OLDGEN_PERM ) {
159 q = ((StgInd *)q)->indirectee;
165 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 (void *)sn = lookupHashTable(addrToStableHash,(W_)p);
181 ASSERT(stable_ptr_table[sn].addr == p);
182 IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
185 sn = stable_ptr_free - stable_ptr_table;
186 (P_)stable_ptr_free = stable_ptr_free->addr;
187 stable_ptr_table[sn].ref = 0;
188 stable_ptr_table[sn].addr = p;
189 stable_ptr_table[sn].sn_obj = NULL;
190 /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
193 /* add the new stable name to the hash table */
194 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
201 freeStableName(snEntry *sn)
203 ASSERT(sn->sn_obj == NULL);
204 if (sn->addr != NULL) {
205 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
207 sn->addr = (P_)stable_ptr_free;
208 stable_ptr_free = sn;
212 getStablePtr(StgPtr p)
216 sn = lookupStableName(p);
217 stable_ptr_table[sn].ref++;
218 return (StgStablePtr)(sn);
222 freeStablePtr(StgStablePtr sp)
224 snEntry *sn = &stable_ptr_table[(StgWord)sp];
226 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
230 // If this entry has no StableName attached, then just free it
231 // immediately. This is important; it might be a while before the
232 // next major GC which actually collects the entry.
233 if (sn->sn_obj == NULL && sn->ref == 0) {
239 enlargeStablePtrTable(void)
241 nat old_SPT_size = SPT_size;
245 SPT_size = INIT_SPT_SIZE;
246 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
247 "enlargeStablePtrTable");
249 /* we don't use index 0 in the stable name table, because that
250 * would conflict with the hash table lookup operations which
251 * return NULL if an entry isn't found in the hash table.
253 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
254 addrToStableHash = allocHashTable();
257 // 2nd and subsequent times
260 stgReallocBytes(stable_ptr_table,
261 SPT_size * sizeof(snEntry),
262 "enlargeStablePtrTable");
264 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
268 /* -----------------------------------------------------------------------------
269 * Treat stable pointers as roots for the garbage collector.
271 * A stable pointer is any stable name entry with a ref > 0. We'll
272 * take the opportunity to zero the "keep" flags at the same time.
273 * -------------------------------------------------------------------------- */
276 markStablePtrTable(evac_fn evac)
278 snEntry *p, *end_stable_ptr_table;
281 end_stable_ptr_table = &stable_ptr_table[SPT_size];
283 // Mark all the stable *pointers* (not stable names).
284 // _starting_ at index 1; index 0 is unused.
285 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
288 // Internal pointers are free slots. If q == NULL, it's a
289 // stable name where the object has been GC'd, but the
290 // StableName object (sn_obj) is still alive.
291 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
293 // save the current addr away: we need to be able to tell
294 // whether the objects moved in order to be able to update
295 // the hash table later.
298 // if the ref is non-zero, treat addr as a root
300 evac((StgClosure **)&p->addr);
306 /* -----------------------------------------------------------------------------
307 * Thread the stable pointer table for compacting GC.
309 * Here we must call the supplied evac function for each pointer into
310 * the heap from the stable pointer table, because the compacting
311 * collector may move the object it points to.
312 * -------------------------------------------------------------------------- */
315 threadStablePtrTable( evac_fn evac )
317 snEntry *p, *end_stable_ptr_table;
320 end_stable_ptr_table = &stable_ptr_table[SPT_size];
322 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
324 if (p->sn_obj != NULL) {
325 evac((StgClosure **)&p->sn_obj);
329 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
330 evac((StgClosure **)&p->addr);
335 /* -----------------------------------------------------------------------------
336 * Garbage collect any dead entries in the stable pointer table.
340 * - a zero reference count
343 * Both of these conditions must be true in order to re-use the stable
344 * name table entry. We can re-use stable name table entries for live
345 * heap objects, as long as the program has no StableName objects that
346 * refer to the entry.
347 * -------------------------------------------------------------------------- */
350 gcStablePtrTable( void )
352 snEntry *p, *end_stable_ptr_table;
355 end_stable_ptr_table = &stable_ptr_table[SPT_size];
357 // NOTE: _starting_ at index 1; index 0 is unused.
358 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
360 // Update the pointer to the StableName object, if there is one
361 if (p->sn_obj != NULL) {
362 p->sn_obj = isAlive(p->sn_obj);
365 // Internal pointers are free slots. If q == NULL, it's a
366 // stable name where the object has been GC'd, but the
367 // StableName object (sn_obj) is still alive.
369 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
373 if (p->sn_obj == NULL) {
374 // StableName object is dead
376 IF_DEBUG(stable, fprintf(stderr,"GC'd Stable name %d\n",
377 p - stable_ptr_table));
381 (StgClosure *)p->addr = isAlive((StgClosure *)p->addr);
382 IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, ref %d\n", p - stable_ptr_table, p->addr, p->ref));
389 /* -----------------------------------------------------------------------------
390 * Update the StablePtr/StableName hash table
392 * The boolean argument 'full' indicates that a major collection is
393 * being done, so we might as well throw away the hash table and build
394 * a new one. For a minor collection, we just re-hash the elements
396 * -------------------------------------------------------------------------- */
399 updateStablePtrTable(rtsBool full)
401 snEntry *p, *end_stable_ptr_table;
403 if (full && addrToStableHash != NULL) {
404 freeHashTable(addrToStableHash,NULL);
405 addrToStableHash = allocHashTable();
408 end_stable_ptr_table = &stable_ptr_table[SPT_size];
410 // NOTE: _starting_ at index 1; index 0 is unused.
411 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
413 if (p->addr == NULL) {
414 if (p->old != NULL) {
415 // The target has been garbage collected. Remove its
416 // entry from the hash table.
417 removeHashTable(addrToStableHash, (W_)p->old, NULL);
421 else if (p->addr < (P_)stable_ptr_table
422 || p->addr >= (P_)end_stable_ptr_table) {
423 // Target still alive, Re-hash this stable name
425 insertHashTable(addrToStableHash, (W_)p->addr,
426 (void *)(p - stable_ptr_table));
427 } else if (p->addr != p->old) {
428 removeHashTable(addrToStableHash, (W_)p->old, NULL);
429 insertHashTable(addrToStableHash, (W_)p->addr,
430 (void *)(p - stable_ptr_table));