1 /* -----------------------------------------------------------------------------
2 * $Id: Stable.c,v 1.1 1999/01/27 10:11:27 simonm Exp $
4 * (c) The GHC Team 1998-1999
6 * Stable names and stable pointers.
8 * ---------------------------------------------------------------------------*/
12 #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 #)
69 There is also a C procedure @FreeStablePtr@ which frees a stable pointer.
71 There may be additional functions on the C side to allow evaluation,
72 application, etc of a stable pointer.
74 When Haskell calls C, it normally just passes over primitive integers,
75 floats, bools, strings, etc. This doesn't cause any problems at all
76 for garbage collection because the act of passing them makes a copy
77 from the heap, stack or wherever they are onto the C-world stack.
78 However, if we were to pass a heap object such as a (Haskell) @String@
79 and a garbage collection occured before we finished using it, we'd run
80 into problems since the heap object might have been moved or even
83 So, if a C call is able to cause a garbage collection or we want to
84 store a pointer to a heap object between C calls, we must be careful
85 when passing heap objects. Our solution is to keep a table of all
86 objects we've given to the C-world and to make sure that the garbage
87 collector collects these objects --- updating the table as required to
88 make sure we can still find the object.
91 snEntry *stable_ptr_table;
92 snEntry *stable_ptr_free;
94 unsigned int SPT_size;
96 /* This hash table maps Haskell objects to stable names, so that every
97 * call to lookupStableName on a given object will return the same
102 * A plain stable name entry has a zero reference count, which means
103 * the entry will dissappear when the object it points to is
104 * unreachable. For stable pointers, we need an entry that sticks
105 * around and keeps the object it points to alive, so each stable name
106 * entry has an associated reference count.
108 * A stable pointer has a weighted reference count N attached to it
109 * (actually in its upper 5 bits), which represents the weight
110 * 2^N. The stable name entry keeps a 32-bit reference count, which
111 * represents any weight between 1 and 2^32 (represented as zero).
112 * When the weight is 2^32, the stable name table owns "all" of the
113 * stable pointers to this object, and the entry can be garbage
114 * collected if the object isn't reachable.
116 * A new stable pointer is given the weight log2(W/2), where W is the
117 * weight stored in the table entry. The new weight in the table is W
120 * A stable pointer can be "split" into two stable pointers, by
121 * dividing the weight by 2 and giving each pointer half.
122 * When freeing a stable pointer, the weight of the pointer is added
123 * to the weight stored in the table entry.
126 HashTable *addrToStableHash;
128 #define INIT_SPT_SIZE 64
131 initFreeList(snEntry *table, nat n, snEntry *free)
135 for (p = table + n - 1; p >= table; p--) {
139 stable_ptr_free = table;
143 initStablePtrTable(void)
145 /* the table will be allocated the first time makeStablePtr is
147 stable_ptr_table = NULL;
148 stable_ptr_free = NULL;
149 addrToStableHash = NULL;
154 lookupStableName(StgPtr p)
158 if (stable_ptr_free == NULL) {
159 enlargeStablePtrTable();
162 (void *)sn = lookupHashTable(addrToStableHash,(W_)p);
165 ASSERT(stable_ptr_table[sn].addr == p);
166 IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
169 sn = stable_ptr_free - stable_ptr_table;
170 (P_)stable_ptr_free = stable_ptr_free->addr;
171 stable_ptr_table[sn].weight = 0;
172 stable_ptr_table[sn].addr = p;
173 /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
176 /* add the new stable name to the hash table */
177 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
184 freeStableName(snEntry *sn)
186 sn->addr = (P_)stable_ptr_free;
187 stable_ptr_free = sn;
191 getStablePtr(StgPtr p)
193 StgWord sn = lookupStableName(p);
194 StgWord weight, weight_2;
196 weight = stable_ptr_table[sn].weight;
198 weight = 1 << (BITS_IN(StgWord)-1);
199 stable_ptr_table[sn].weight = weight;
200 return (StgStablePtr)(sn + ((BITS_IN(StgWord)-1) << STABLEPTR_WEIGHT_SHIFT));
202 else if (weight == 1) {
203 barf("getStablePtr: too light");
207 /* find log2(weight) */
208 for (weight_2 = 1; weight != 1; weight_2++) {
211 stable_ptr_table[sn].weight -= 2^weight_2;
212 return (StgStablePtr)(sn + (weight_2 << STABLEPTR_WEIGHT_SHIFT));
217 enlargeStablePtrTable(void)
219 nat old_SPT_size = SPT_size;
223 SPT_size = INIT_SPT_SIZE;
224 stable_ptr_table = stgMallocWords(SPT_size * sizeof(snEntry),
225 "initStablePtrTable");
227 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
228 addrToStableHash = allocHashTable();
231 /* 2nd and subsequent times */
234 stgReallocWords(stable_ptr_table, SPT_size * sizeof(snEntry),
235 "enlargeStablePtrTable");
237 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
241 /* -----------------------------------------------------------------------------
242 * Treat stable pointers as roots for the garbage collector.
244 * A stable pointer is any stable name entry with a weight > 0. We'll
245 * take the opportunity to zero the "keep" flags at the same time.
246 * -------------------------------------------------------------------------- */
249 markStablePtrTable(rtsBool full)
251 snEntry *p, *end_stable_ptr_table;
259 freeHashTable(addrToStableHash,NULL);
260 addrToStableHash = allocHashTable();
263 end_stable_ptr_table = &stable_ptr_table[SPT_size];
265 /* Mark all the stable *pointers* (not stable names)
267 for (p = stable_ptr_table; p < end_stable_ptr_table; p++) {
269 /* internal pointers or NULL are free slots */
270 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
271 if (p->weight != 0) {
272 new = MarkRoot((StgClosure *)q);
273 /* Update the hash table */
275 insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
276 (StgClosure *)p->addr = new;
277 } else if ((P_)new != q) {
278 removeHashTable(addrToStableHash, (W_)q, NULL);
279 insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
280 (StgClosure *)p->addr = new;
282 /* IF_DEBUG(stable, fprintf(stderr,"Stable ptr %d still alive
283 at %p, weight %d\n", p - stable_ptr_table, new,
287 /* reset the keep flag */
294 /* -----------------------------------------------------------------------------
295 * Garbage collect any dead entries in the stable pointer table.
299 * - a weight of zero (i.e. 2^32)
300 * - a false keep flag
302 * The keep flag is set by the garbage collector whenever it
303 * encounters a StableName object on the heap.
305 * The boolean argument 'full' indicates that a major collection is
306 * being done, so we might as well throw away the hash table and build
307 * a new one. For a minor collection, we just re-hash the elements
309 * -------------------------------------------------------------------------- */
312 gcStablePtrTable(rtsBool full)
314 snEntry *p, *end_stable_ptr_table;
321 end_stable_ptr_table = &stable_ptr_table[SPT_size];
323 for (p = stable_ptr_table; p < end_stable_ptr_table; p++) {
326 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
328 /* We're only interested in Stable Names here. */
329 if (p->weight == 0) {
331 if (((StgClosure *)new = isAlive((StgClosure *)q))) {
332 IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, weight %d\n", p - stable_ptr_table, new, p->weight));
335 /* Re-hash this stable name */
337 insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
338 } else if (new != q) {
339 removeHashTable(addrToStableHash, (W_)q, NULL);
340 insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
345 /* If there are still StableName objects in the heap
346 * pointing to this entry (p->keep == rtsTrue), then
347 * don't free the entry just yet.