Pointer Tagging
[ghc.git] / rts / Stable.c
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team, 1998-2002
4 *
5 * Stable names and stable pointers.
6 *
7 * ---------------------------------------------------------------------------*/
8
9 // Make static versions of inline functions in Stable.h:
10 #define RTS_STABLE_C
11
12 #include "PosixSource.h"
13 #include "Rts.h"
14 #include "Hash.h"
15 #include "RtsUtils.h"
16 #include "OSThreads.h"
17 #include "Storage.h"
18 #include "RtsAPI.h"
19 #include "RtsFlags.h"
20 #include "OSThreads.h"
21 #include "Trace.h"
22 #include "Stable.h"
23
24 /* Comment from ADR's implementation in old RTS:
25
26 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
27 small change in @HpOverflow.lc@) consists of the changes in the
28 runtime system required to implement "Stable Pointers". But we're
29 getting a bit ahead of ourselves --- what is a stable pointer and what
30 is it used for?
31
32 When Haskell calls C, it normally just passes over primitive integers,
33 floats, bools, strings, etc. This doesn't cause any problems at all
34 for garbage collection because the act of passing them makes a copy
35 from the heap, stack or wherever they are onto the C-world stack.
36 However, if we were to pass a heap object such as a (Haskell) @String@
37 and a garbage collection occured before we finished using it, we'd run
38 into problems since the heap object might have been moved or even
39 deleted.
40
41 So, if a C call is able to cause a garbage collection or we want to
42 store a pointer to a heap object between C calls, we must be careful
43 when passing heap objects. Our solution is to keep a table of all
44 objects we've given to the C-world and to make sure that the garbage
45 collector collects these objects --- updating the table as required to
46 make sure we can still find the object.
47
48
49 Of course, all this rather begs the question: why would we want to
50 pass a boxed value?
51
52 One very good reason is to preserve laziness across the language
53 interface. Rather than evaluating an integer or a string because it
54 {\em might\/} be required by the C function, we can wait until the C
55 function actually wants the value and then force an evaluation.
56
57 Another very good reason (the motivating reason!) is that the C code
58 might want to execute an object of sort $IO ()$ for the side-effects
59 it will produce. For example, this is used when interfacing to an X
60 widgets library to allow a direct implementation of callbacks.
61
62
63 The @makeStablePointer :: a -> IO (StablePtr a)@ function
64 converts a value into a stable pointer. It is part of the @PrimIO@
65 monad, because we want to be sure we don't allocate one twice by
66 accident, and then only free one of the copies.
67
68 \begin{verbatim}
69 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
70 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
71 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
72 (# State# RealWorld, a #)
73 \end{verbatim}
74
75 There may be additional functions on the C side to allow evaluation,
76 application, etc of a stable pointer.
77
78 */
79
80 snEntry *stable_ptr_table = NULL;
81 static snEntry *stable_ptr_free = NULL;
82
83 static unsigned int SPT_size = 0;
84
85 #ifdef THREADED_RTS
86 static Mutex stable_mutex;
87 #endif
88
89 /* This hash table maps Haskell objects to stable names, so that every
90 * call to lookupStableName on a given object will return the same
91 * stable name.
92 *
93 * OLD COMMENTS about reference counting follow. The reference count
94 * in a stable name entry is now just a counter.
95 *
96 * Reference counting
97 * ------------------
98 * A plain stable name entry has a zero reference count, which means
99 * the entry will dissappear when the object it points to is
100 * unreachable. For stable pointers, we need an entry that sticks
101 * around and keeps the object it points to alive, so each stable name
102 * entry has an associated reference count.
103 *
104 * A stable pointer has a weighted reference count N attached to it
105 * (actually in its upper 5 bits), which represents the weight
106 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
107 * represents any weight between 1 and 2^32 (represented as zero).
108 * When the weight is 2^32, the stable name table owns "all" of the
109 * stable pointers to this object, and the entry can be garbage
110 * collected if the object isn't reachable.
111 *
112 * A new stable pointer is given the weight log2(W/2), where W is the
113 * weight stored in the table entry. The new weight in the table is W
114 * - 2^log2(W/2).
115 *
116 * A stable pointer can be "split" into two stable pointers, by
117 * dividing the weight by 2 and giving each pointer half.
118 * When freeing a stable pointer, the weight of the pointer is added
119 * to the weight stored in the table entry.
120 * */
121
122 static HashTable *addrToStableHash = NULL;
123
124 #define INIT_SPT_SIZE 64
125
126 STATIC_INLINE void
127 initFreeList(snEntry *table, nat n, snEntry *free)
128 {
129 snEntry *p;
130
131 for (p = table + n - 1; p >= table; p--) {
132 p->addr = (P_)free;
133 p->old = NULL;
134 p->ref = 0;
135 p->sn_obj = NULL;
136 free = p;
137 }
138 stable_ptr_free = table;
139 }
140
141 void
142 initStablePtrTable(void)
143 {
144 if (SPT_size > 0)
145 return;
146
147 SPT_size = INIT_SPT_SIZE;
148 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
149 "initStablePtrTable");
150
151 /* we don't use index 0 in the stable name table, because that
152 * would conflict with the hash table lookup operations which
153 * return NULL if an entry isn't found in the hash table.
154 */
155 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
156 addrToStableHash = allocHashTable();
157
158 #ifdef THREADED_RTS
159 initMutex(&stable_mutex);
160 #endif
161 }
162
163 void
164 exitStablePtrTable(void)
165 {
166 if (addrToStableHash)
167 freeHashTable(addrToStableHash, NULL);
168 addrToStableHash = NULL;
169 if (stable_ptr_table)
170 stgFree(stable_ptr_table);
171 stable_ptr_table = NULL;
172 SPT_size = 0;
173 #ifdef THREADED_RTS
174 closeMutex(&stable_mutex);
175 #endif
176 }
177
178 /*
179 * get at the real stuff...remove indirections.
180 * It untags pointers before dereferencing and
181 * retags the real stuff with its tag (if there
182 * is any) when returning.
183 *
184 * ToDo: move to a better home.
185 */
186 static
187 StgClosure*
188 removeIndirections(StgClosure* p)
189 {
190 StgWord tag = GET_CLOSURE_TAG(p);
191 StgClosure* q = UNTAG_CLOSURE(p);
192
193 while (get_itbl(q)->type == IND ||
194 get_itbl(q)->type == IND_STATIC ||
195 get_itbl(q)->type == IND_OLDGEN ||
196 get_itbl(q)->type == IND_PERM ||
197 get_itbl(q)->type == IND_OLDGEN_PERM ) {
198 tag = GET_CLOSURE_TAG(q);
199 q = UNTAG_CLOSURE(((StgInd *)q)->indirectee);
200 }
201 return TAG_CLOSURE(tag,q);
202 }
203
204 static StgWord
205 lookupStableName_(StgPtr p)
206 {
207 StgWord sn;
208 void* sn_tmp;
209
210 if (stable_ptr_free == NULL) {
211 enlargeStablePtrTable();
212 }
213
214 /* removing indirections increases the likelihood
215 * of finding a match in the stable name hash table.
216 */
217 p = (StgPtr)removeIndirections((StgClosure*)p);
218
219 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
220 sn = (StgWord)sn_tmp;
221
222 if (sn != 0) {
223 ASSERT(stable_ptr_table[sn].addr == p);
224 debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
225 return sn;
226 } else {
227 sn = stable_ptr_free - stable_ptr_table;
228 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
229 stable_ptr_table[sn].ref = 0;
230 stable_ptr_table[sn].addr = p;
231 stable_ptr_table[sn].sn_obj = NULL;
232 /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
233
234 /* add the new stable name to the hash table */
235 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
236
237 return sn;
238 }
239 }
240
241 StgWord
242 lookupStableName(StgPtr p)
243 {
244 StgWord res;
245
246 initStablePtrTable();
247 ACQUIRE_LOCK(&stable_mutex);
248 res = lookupStableName_(p);
249 RELEASE_LOCK(&stable_mutex);
250 return res;
251 }
252
253 STATIC_INLINE void
254 freeStableName(snEntry *sn)
255 {
256 ASSERT(sn->sn_obj == NULL);
257 if (sn->addr != NULL) {
258 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
259 }
260 sn->addr = (P_)stable_ptr_free;
261 stable_ptr_free = sn;
262 }
263
264 StgStablePtr
265 getStablePtr(StgPtr p)
266 {
267 StgWord sn;
268
269 initStablePtrTable();
270 ACQUIRE_LOCK(&stable_mutex);
271 sn = lookupStableName_(p);
272 stable_ptr_table[sn].ref++;
273 RELEASE_LOCK(&stable_mutex);
274 return (StgStablePtr)(sn);
275 }
276
277 void
278 freeStablePtr(StgStablePtr sp)
279 {
280 snEntry *sn;
281
282 initStablePtrTable();
283 ACQUIRE_LOCK(&stable_mutex);
284
285 sn = &stable_ptr_table[(StgWord)sp];
286
287 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
288
289 sn->ref--;
290
291 // If this entry has no StableName attached, then just free it
292 // immediately. This is important; it might be a while before the
293 // next major GC which actually collects the entry.
294 if (sn->sn_obj == NULL && sn->ref == 0) {
295 freeStableName(sn);
296 }
297
298 RELEASE_LOCK(&stable_mutex);
299 }
300
301 void
302 enlargeStablePtrTable(void)
303 {
304 nat old_SPT_size = SPT_size;
305
306 // 2nd and subsequent times
307 SPT_size *= 2;
308 stable_ptr_table =
309 stgReallocBytes(stable_ptr_table,
310 SPT_size * sizeof(snEntry),
311 "enlargeStablePtrTable");
312
313 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
314 }
315
316 /* -----------------------------------------------------------------------------
317 * Treat stable pointers as roots for the garbage collector.
318 *
319 * A stable pointer is any stable name entry with a ref > 0. We'll
320 * take the opportunity to zero the "keep" flags at the same time.
321 * -------------------------------------------------------------------------- */
322
323 void
324 markStablePtrTable(evac_fn evac)
325 {
326 snEntry *p, *end_stable_ptr_table;
327 StgPtr q;
328
329 end_stable_ptr_table = &stable_ptr_table[SPT_size];
330
331 // Mark all the stable *pointers* (not stable names).
332 // _starting_ at index 1; index 0 is unused.
333 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
334 q = p->addr;
335
336 // Internal pointers are free slots. If q == NULL, it's a
337 // stable name where the object has been GC'd, but the
338 // StableName object (sn_obj) is still alive.
339 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
340
341 // save the current addr away: we need to be able to tell
342 // whether the objects moved in order to be able to update
343 // the hash table later.
344 p->old = p->addr;
345
346 // if the ref is non-zero, treat addr as a root
347 if (p->ref != 0) {
348 evac((StgClosure **)&p->addr);
349 }
350 }
351 }
352 }
353
354 /* -----------------------------------------------------------------------------
355 * Thread the stable pointer table for compacting GC.
356 *
357 * Here we must call the supplied evac function for each pointer into
358 * the heap from the stable pointer table, because the compacting
359 * collector may move the object it points to.
360 * -------------------------------------------------------------------------- */
361
362 void
363 threadStablePtrTable( evac_fn evac )
364 {
365 snEntry *p, *end_stable_ptr_table;
366 StgPtr q;
367
368 end_stable_ptr_table = &stable_ptr_table[SPT_size];
369
370 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
371
372 if (p->sn_obj != NULL) {
373 evac((StgClosure **)&p->sn_obj);
374 }
375
376 q = p->addr;
377 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
378 evac((StgClosure **)&p->addr);
379 }
380 }
381 }
382
383 /* -----------------------------------------------------------------------------
384 * Garbage collect any dead entries in the stable pointer table.
385 *
386 * A dead entry has:
387 *
388 * - a zero reference count
389 * - a dead sn_obj
390 *
391 * Both of these conditions must be true in order to re-use the stable
392 * name table entry. We can re-use stable name table entries for live
393 * heap objects, as long as the program has no StableName objects that
394 * refer to the entry.
395 * -------------------------------------------------------------------------- */
396
397 void
398 gcStablePtrTable( void )
399 {
400 snEntry *p, *end_stable_ptr_table;
401 StgPtr q;
402
403 end_stable_ptr_table = &stable_ptr_table[SPT_size];
404
405 // NOTE: _starting_ at index 1; index 0 is unused.
406 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
407
408 // Update the pointer to the StableName object, if there is one
409 if (p->sn_obj != NULL) {
410 p->sn_obj = isAlive(p->sn_obj);
411 }
412
413 // Internal pointers are free slots. If q == NULL, it's a
414 // stable name where the object has been GC'd, but the
415 // StableName object (sn_obj) is still alive.
416 q = p->addr;
417 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
418
419 // StableNames only:
420 if (p->ref == 0) {
421 if (p->sn_obj == NULL) {
422 // StableName object is dead
423 freeStableName(p);
424 debugTrace(DEBUG_stable, "GC'd Stable name %ld",
425 (long)(p - stable_ptr_table));
426 continue;
427
428 } else {
429 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
430 debugTrace(DEBUG_stable,
431 "stable name %ld still alive at %p, ref %ld\n",
432 (long)(p - stable_ptr_table), p->addr, p->ref);
433 }
434 }
435 }
436 }
437 }
438
439 /* -----------------------------------------------------------------------------
440 * Update the StablePtr/StableName hash table
441 *
442 * The boolean argument 'full' indicates that a major collection is
443 * being done, so we might as well throw away the hash table and build
444 * a new one. For a minor collection, we just re-hash the elements
445 * that changed.
446 * -------------------------------------------------------------------------- */
447
448 void
449 updateStablePtrTable(rtsBool full)
450 {
451 snEntry *p, *end_stable_ptr_table;
452
453 if (full && addrToStableHash != NULL) {
454 freeHashTable(addrToStableHash,NULL);
455 addrToStableHash = allocHashTable();
456 }
457
458 end_stable_ptr_table = &stable_ptr_table[SPT_size];
459
460 // NOTE: _starting_ at index 1; index 0 is unused.
461 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
462
463 if (p->addr == NULL) {
464 if (p->old != NULL) {
465 // The target has been garbage collected. Remove its
466 // entry from the hash table.
467 removeHashTable(addrToStableHash, (W_)p->old, NULL);
468 p->old = NULL;
469 }
470 }
471 else if (p->addr < (P_)stable_ptr_table
472 || p->addr >= (P_)end_stable_ptr_table) {
473 // Target still alive, Re-hash this stable name
474 if (full) {
475 insertHashTable(addrToStableHash, (W_)p->addr,
476 (void *)(p - stable_ptr_table));
477 } else if (p->addr != p->old) {
478 removeHashTable(addrToStableHash, (W_)p->old, NULL);
479 insertHashTable(addrToStableHash, (W_)p->addr,
480 (void *)(p - stable_ptr_table));
481 }
482 }
483 }
484 }