1 /* -----------------------------------------------------------------------------
2 * $Id: Stable.c,v 1.18 2001/11/21 10:09:16 simonmar Exp $
4 * (c) The GHC Team, 1998-1999
6 * Stable names and stable pointers.
8 * ---------------------------------------------------------------------------*/
10 #include "PosixSource.h"
13 #include "StablePriv.h"
19 /* Comment from ADR's implementation in old RTS:
21 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
22 small change in @HpOverflow.lc@) consists of the changes in the
23 runtime system required to implement "Stable Pointers". But we're
24 getting a bit ahead of ourselves --- what is a stable pointer and what
27 When Haskell calls C, it normally just passes over primitive integers,
28 floats, bools, strings, etc. This doesn't cause any problems at all
29 for garbage collection because the act of passing them makes a copy
30 from the heap, stack or wherever they are onto the C-world stack.
31 However, if we were to pass a heap object such as a (Haskell) @String@
32 and a garbage collection occured before we finished using it, we'd run
33 into problems since the heap object might have been moved or even
36 So, if a C call is able to cause a garbage collection or we want to
37 store a pointer to a heap object between C calls, we must be careful
38 when passing heap objects. Our solution is to keep a table of all
39 objects we've given to the C-world and to make sure that the garbage
40 collector collects these objects --- updating the table as required to
41 make sure we can still find the object.
44 Of course, all this rather begs the question: why would we want to
47 One very good reason is to preserve laziness across the language
48 interface. Rather than evaluating an integer or a string because it
49 {\em might\/} be required by the C function, we can wait until the C
50 function actually wants the value and then force an evaluation.
52 Another very good reason (the motivating reason!) is that the C code
53 might want to execute an object of sort $IO ()$ for the side-effects
54 it will produce. For example, this is used when interfacing to an X
55 widgets library to allow a direct implementation of callbacks.
58 The @makeStablePointer :: a -> IO (StablePtr a)@ function
59 converts a value into a stable pointer. It is part of the @PrimIO@
60 monad, because we want to be sure we don't allocate one twice by
61 accident, and then only free one of the copies.
64 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
65 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
66 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
67 (# State# RealWorld, a #)
70 There may be additional functions on the C side to allow evaluation,
71 application, etc of a stable pointer.
73 When Haskell calls C, it normally just passes over primitive integers,
74 floats, bools, strings, etc. This doesn't cause any problems at all
75 for garbage collection because the act of passing them makes a copy
76 from the heap, stack or wherever they are onto the C-world stack.
77 However, if we were to pass a heap object such as a (Haskell) @String@
78 and a garbage collection occured before we finished using it, we'd run
79 into problems since the heap object might have been moved or even
82 So, if a C call is able to cause a garbage collection or we want to
83 store a pointer to a heap object between C calls, we must be careful
84 when passing heap objects. Our solution is to keep a table of all
85 objects we've given to the C-world and to make sure that the garbage
86 collector collects these objects --- updating the table as required to
87 make sure we can still find the object.
90 snEntry *stable_ptr_table;
91 snEntry *stable_ptr_free;
93 unsigned int SPT_size;
95 /* This hash table maps Haskell objects to stable names, so that every
96 * call to lookupStableName on a given object will return the same
99 * OLD COMMENTS about reference counting follow. The reference count
100 * in a stable name entry is now just a counter.
104 * A plain stable name entry has a zero reference count, which means
105 * the entry will dissappear when the object it points to is
106 * unreachable. For stable pointers, we need an entry that sticks
107 * around and keeps the object it points to alive, so each stable name
108 * entry has an associated reference count.
110 * A stable pointer has a weighted reference count N attached to it
111 * (actually in its upper 5 bits), which represents the weight
112 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
113 * represents any weight between 1 and 2^32 (represented as zero).
114 * When the weight is 2^32, the stable name table owns "all" of the
115 * stable pointers to this object, and the entry can be garbage
116 * collected if the object isn't reachable.
118 * A new stable pointer is given the weight log2(W/2), where W is the
119 * weight stored in the table entry. The new weight in the table is W
122 * A stable pointer can be "split" into two stable pointers, by
123 * dividing the weight by 2 and giving each pointer half.
124 * When freeing a stable pointer, the weight of the pointer is added
125 * to the weight stored in the table entry.
128 HashTable *addrToStableHash;
130 #define INIT_SPT_SIZE 64
133 initFreeList(snEntry *table, nat n, snEntry *free)
137 for (p = table + n - 1; p >= table; p--) {
144 stable_ptr_free = table;
148 initStablePtrTable(void)
150 /* the table will be allocated the first time makeStablePtr is
152 stable_ptr_table = NULL;
153 stable_ptr_free = NULL;
154 addrToStableHash = NULL;
159 * get at the real stuff...remove indirections.
161 * ToDo: move to a better home.
165 removeIndirections(StgClosure* p)
169 while (get_itbl(q)->type == IND ||
170 get_itbl(q)->type == IND_STATIC ||
171 get_itbl(q)->type == IND_OLDGEN ||
172 get_itbl(q)->type == IND_PERM ||
173 get_itbl(q)->type == IND_OLDGEN_PERM ) {
174 q = ((StgInd *)q)->indirectee;
180 lookupStableName(StgPtr p)
184 if (stable_ptr_free == NULL) {
185 enlargeStablePtrTable();
188 /* removing indirections increases the likelihood
189 * of finding a match in the stable name hash table.
191 p = (StgPtr)removeIndirections((StgClosure*)p);
193 (void *)sn = lookupHashTable(addrToStableHash,(W_)p);
196 ASSERT(stable_ptr_table[sn].addr == p);
197 IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
200 sn = stable_ptr_free - stable_ptr_table;
201 (P_)stable_ptr_free = stable_ptr_free->addr;
202 stable_ptr_table[sn].ref = 0;
203 stable_ptr_table[sn].addr = p;
204 stable_ptr_table[sn].sn_obj = NULL;
205 /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
208 /* add the new stable name to the hash table */
209 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
216 freeStableName(snEntry *sn)
218 ASSERT(sn->sn_obj == NULL);
219 if (sn->addr != NULL) {
220 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
222 sn->addr = (P_)stable_ptr_free;
223 stable_ptr_free = sn;
227 getStablePtr(StgPtr p)
231 sn = lookupStableName(p);
232 stable_ptr_table[sn].ref++;
233 return (StgStablePtr)(sn);
237 enlargeStablePtrTable(void)
239 nat old_SPT_size = SPT_size;
243 SPT_size = INIT_SPT_SIZE;
244 stable_ptr_table = stgMallocWords(SPT_size * sizeof(snEntry),
245 "initStablePtrTable");
247 /* we don't use index 0 in the stable name table, because that
248 * would conflict with the hash table lookup operations which
249 * return NULL if an entry isn't found in the hash table.
251 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
252 addrToStableHash = allocHashTable();
255 // 2nd and subsequent times
258 stgReallocWords(stable_ptr_table, SPT_size * sizeof(snEntry),
259 "enlargeStablePtrTable");
261 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
265 /* -----------------------------------------------------------------------------
266 * Treat stable pointers as roots for the garbage collector.
268 * A stable pointer is any stable name entry with a ref > 0. We'll
269 * take the opportunity to zero the "keep" flags at the same time.
270 * -------------------------------------------------------------------------- */
273 markStablePtrTable(evac_fn evac)
275 snEntry *p, *end_stable_ptr_table;
278 end_stable_ptr_table = &stable_ptr_table[SPT_size];
280 // Mark all the stable *pointers* (not stable names).
281 // _starting_ at index 1; index 0 is unused.
282 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
285 // Internal pointers are free slots. If q == NULL, it's a
286 // stable name where the object has been GC'd, but the
287 // StableName object (sn_obj) is still alive.
288 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
290 // save the current addr away: we need to be able to tell
291 // whether the objects moved in order to be able to update
292 // the hash table later.
295 // if the ref is non-zero, treat addr as a root
297 evac((StgClosure **)&p->addr);
303 /* -----------------------------------------------------------------------------
304 * Thread the stable pointer table for compacting GC.
306 * Here we must call the supplied evac function for each pointer into
307 * the heap from the stable pointer table, because the compacting
308 * collector may move the object it points to.
309 * -------------------------------------------------------------------------- */
312 threadStablePtrTable( evac_fn evac )
314 snEntry *p, *end_stable_ptr_table;
317 end_stable_ptr_table = &stable_ptr_table[SPT_size];
319 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
321 if (p->sn_obj != NULL) {
322 evac((StgClosure **)&p->sn_obj);
326 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
327 evac((StgClosure **)&p->addr);
332 /* -----------------------------------------------------------------------------
333 * Garbage collect any dead entries in the stable pointer table.
337 * - a zero reference count
340 * Both of these conditions must be true in order to re-use the stable
341 * name table entry. We can re-use stable name table entries for live
342 * heap objects, as long as the program has no StableName objects that
343 * refer to the entry.
344 * -------------------------------------------------------------------------- */
347 gcStablePtrTable( void )
349 snEntry *p, *end_stable_ptr_table;
352 end_stable_ptr_table = &stable_ptr_table[SPT_size];
354 // NOTE: _starting_ at index 1; index 0 is unused.
355 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
357 // Update the pointer to the StableName object, if there is one
358 if (p->sn_obj != NULL) {
359 p->sn_obj = isAlive(p->sn_obj);
362 // Internal pointers are free slots. If q == NULL, it's a
363 // stable name where the object has been GC'd, but the
364 // StableName object (sn_obj) is still alive.
366 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
370 if (p->sn_obj == NULL) {
371 // StableName object is dead
373 IF_DEBUG(stable, fprintf(stderr,"GC'd Stable name %d\n",
374 p - stable_ptr_table));
378 (StgClosure *)p->addr = isAlive((StgClosure *)p->addr);
379 IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, ref %d\n", p - stable_ptr_table, p->addr, p->ref));
386 /* -----------------------------------------------------------------------------
387 * Update the StablePtr/StableName hash table
389 * The boolean argument 'full' indicates that a major collection is
390 * being done, so we might as well throw away the hash table and build
391 * a new one. For a minor collection, we just re-hash the elements
393 * -------------------------------------------------------------------------- */
396 updateStablePtrTable(rtsBool full)
398 snEntry *p, *end_stable_ptr_table;
400 if (full && addrToStableHash != NULL) {
401 freeHashTable(addrToStableHash,NULL);
402 addrToStableHash = allocHashTable();
405 end_stable_ptr_table = &stable_ptr_table[SPT_size];
407 // NOTE: _starting_ at index 1; index 0 is unused.
408 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
410 if (p->addr == NULL) {
411 if (p->old != NULL) {
412 // The target has been garbage collected. Remove its
413 // entry from the hash table.
414 removeHashTable(addrToStableHash, (W_)p->old, NULL);
418 else if (p->addr < (P_)stable_ptr_table
419 || p->addr >= (P_)end_stable_ptr_table) {
420 // Target still alive, Re-hash this stable name
422 insertHashTable(addrToStableHash, (W_)p->addr,
423 (void *)(p - stable_ptr_table));
424 } else if (p->addr != p->old) {
425 removeHashTable(addrToStableHash, (W_)p->old, NULL);
426 insertHashTable(addrToStableHash, (W_)p->addr,
427 (void *)(p - stable_ptr_table));