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