Stable.c 13.5 KB
Newer Older
1 2
/* -----------------------------------------------------------------------------
 *
3
 * (c) The GHC Team, 1998-2002
4 5 6 7 8
 *
 * Stable names and stable pointers.
 *
 * ---------------------------------------------------------------------------*/

9 10 11
// Make static versions of inline functions in Stable.h:
#define RTS_STABLE_C

12
#include "PosixSource.h"
13 14 15
#include "Rts.h"
#include "Hash.h"
#include "RtsUtils.h"
16
#include "OSThreads.h"
17 18 19
#include "Storage.h"
#include "RtsAPI.h"
#include "RtsFlags.h"
20
#include "OSThreads.h"
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

/* Comment from ADR's implementation in old RTS:

  This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
  small change in @HpOverflow.lc@) consists of the changes in the
  runtime system required to implement "Stable Pointers". But we're
  getting a bit ahead of ourselves --- what is a stable pointer and what
  is it used for?

  When Haskell calls C, it normally just passes over primitive integers,
  floats, bools, strings, etc.  This doesn't cause any problems at all
  for garbage collection because the act of passing them makes a copy
  from the heap, stack or wherever they are onto the C-world stack.
  However, if we were to pass a heap object such as a (Haskell) @String@
  and a garbage collection occured before we finished using it, we'd run
  into problems since the heap object might have been moved or even
  deleted.

  So, if a C call is able to cause a garbage collection or we want to
  store a pointer to a heap object between C calls, we must be careful
  when passing heap objects. Our solution is to keep a table of all
  objects we've given to the C-world and to make sure that the garbage
  collector collects these objects --- updating the table as required to
  make sure we can still find the object.


  Of course, all this rather begs the question: why would we want to
  pass a boxed value?

  One very good reason is to preserve laziness across the language
  interface. Rather than evaluating an integer or a string because it
  {\em might\/} be required by the C function, we can wait until the C
  function actually wants the value and then force an evaluation.

  Another very good reason (the motivating reason!) is that the C code
  might want to execute an object of sort $IO ()$ for the side-effects
  it will produce. For example, this is used when interfacing to an X
  widgets library to allow a direct implementation of callbacks.


  The @makeStablePointer :: a -> IO (StablePtr a)@ function
  converts a value into a stable pointer.  It is part of the @PrimIO@
  monad, because we want to be sure we don't allocate one twice by
  accident, and then only free one of the copies.

  \begin{verbatim}
  makeStablePtr#  :: a -> State# RealWorld -> (# RealWorld, a #)
  freeStablePtr#  :: StablePtr# a -> State# RealWorld -> State# RealWorld
  deRefStablePtr# :: StablePtr# a -> State# RealWorld -> 
        (# State# RealWorld, a #)
  \end{verbatim}

  There may be additional functions on the C side to allow evaluation,
  application, etc of a stable pointer.

*/

78 79
snEntry *stable_ptr_table = NULL;
static snEntry *stable_ptr_free = NULL;
80

81
static unsigned int SPT_size = 0;
82

83
#ifdef THREADED_RTS
84
static Mutex stable_mutex;
85
#endif
86

87 88 89 90
/* This hash table maps Haskell objects to stable names, so that every
 * call to lookupStableName on a given object will return the same
 * stable name.
 *
91 92 93
 * OLD COMMENTS about reference counting follow.  The reference count
 * in a stable name entry is now just a counter.
 *
94 95 96 97 98 99 100 101 102 103
 * Reference counting
 * ------------------
 * A plain stable name entry has a zero reference count, which means
 * the entry will dissappear when the object it points to is
 * unreachable.  For stable pointers, we need an entry that sticks
 * around and keeps the object it points to alive, so each stable name
 * entry has an associated reference count.
 *
 * A stable pointer has a weighted reference count N attached to it
 * (actually in its upper 5 bits), which represents the weight
104
 * 2^(N-1).  The stable name entry keeps a 32-bit reference count, which
105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
 * represents any weight between 1 and 2^32 (represented as zero).
 * When the weight is 2^32, the stable name table owns "all" of the
 * stable pointers to this object, and the entry can be garbage
 * collected if the object isn't reachable.
 *
 * A new stable pointer is given the weight log2(W/2), where W is the
 * weight stored in the table entry.  The new weight in the table is W
 * - 2^log2(W/2).
 *
 * A stable pointer can be "split" into two stable pointers, by
 * dividing the weight by 2 and giving each pointer half.
 * When freeing a stable pointer, the weight of the pointer is added
 * to the weight stored in the table entry.
 * */

120
static HashTable *addrToStableHash = NULL;
121 122 123

#define INIT_SPT_SIZE 64

sof's avatar
sof committed
124
STATIC_INLINE void
125 126 127 128 129
initFreeList(snEntry *table, nat n, snEntry *free)
{
  snEntry *p;

  for (p = table + n - 1; p >= table; p--) {
sof's avatar
sof committed
130
    p->addr   = (P_)free;
131
    p->old    = NULL;
132
    p->ref    = 0;
sof's avatar
sof committed
133
    p->sn_obj = NULL;
134 135 136 137 138 139 140 141
    free = p;
  }
  stable_ptr_free = table;
}

void
initStablePtrTable(void)
{
142 143 144 145 146 147 148 149 150 151 152 153 154 155
	if (SPT_size > 0)
		return;

    SPT_size = INIT_SPT_SIZE;
    stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
				      "initStablePtrTable");

    /* we don't use index 0 in the stable name table, because that
     * would conflict with the hash table lookup operations which
     * return NULL if an entry isn't found in the hash table.
     */
    initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
    addrToStableHash = allocHashTable();

156
#ifdef THREADED_RTS
157
    initMutex(&stable_mutex);
158
#endif
159 160
}

sof's avatar
sof committed
161 162 163 164 165 166 167 168 169 170 171
/*
 * get at the real stuff...remove indirections.
 *
 * ToDo: move to a better home.
 */
static
StgClosure*
removeIndirections(StgClosure* p)
{
  StgClosure* q = p;

172 173 174 175 176
  while (get_itbl(q)->type == IND ||
         get_itbl(q)->type == IND_STATIC ||
         get_itbl(q)->type == IND_OLDGEN ||
         get_itbl(q)->type == IND_PERM ||
         get_itbl(q)->type == IND_OLDGEN_PERM ) {
sof's avatar
sof committed
177 178 179 180 181
      q = ((StgInd *)q)->indirectee;
  }
  return q;
}

182 183
static StgWord
lookupStableName_(StgPtr p)
184 185
{
  StgWord sn;
186
  void* sn_tmp;
187 188 189 190

  if (stable_ptr_free == NULL) {
    enlargeStablePtrTable();
  }
sof's avatar
sof committed
191 192

  /* removing indirections increases the likelihood
193
   * of finding a match in the stable name hash table.
sof's avatar
sof committed
194 195 196
   */
  p = (StgPtr)removeIndirections((StgClosure*)p);

197 198
  sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
  sn = (StgWord)sn_tmp;
199 200 201
  
  if (sn != 0) {
    ASSERT(stable_ptr_table[sn].addr == p);
202
    IF_DEBUG(stable,debugBelch("cached stable name %ld at %p\n",sn,p));
203 204 205
    return sn;
  } else {
    sn = stable_ptr_free - stable_ptr_table;
206
    stable_ptr_free  = (snEntry*)(stable_ptr_free->addr);
207
    stable_ptr_table[sn].ref = 0;
208
    stable_ptr_table[sn].addr = p;
209
    stable_ptr_table[sn].sn_obj = NULL;
210
    /* IF_DEBUG(stable,debugBelch("new stable name %d at %p\n",sn,p)); */
211 212 213 214 215 216 217 218
    
    /* add the new stable name to the hash table */
    insertHashTable(addrToStableHash, (W_)p, (void *)sn);

    return sn;
  }
}

219 220 221 222
StgWord
lookupStableName(StgPtr p)
{
    StgWord res;
223 224

    initStablePtrTable();
225 226 227 228 229 230
    ACQUIRE_LOCK(&stable_mutex);
    res = lookupStableName_(p);
    RELEASE_LOCK(&stable_mutex);
    return res;
}

sof's avatar
sof committed
231
STATIC_INLINE void
232 233
freeStableName(snEntry *sn)
{
234
  ASSERT(sn->sn_obj == NULL);
235
  if (sn->addr != NULL) {
236
      removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
237
  }
238 239 240 241 242 243 244
  sn->addr = (P_)stable_ptr_free;
  stable_ptr_free = sn;
}

StgStablePtr
getStablePtr(StgPtr p)
{
245 246
  StgWord sn;

247
  initStablePtrTable();
248 249
  ACQUIRE_LOCK(&stable_mutex);
  sn = lookupStableName_(p);
250
  stable_ptr_table[sn].ref++;
251
  RELEASE_LOCK(&stable_mutex);
252
  return (StgStablePtr)(sn);
253 254
}

255 256 257
void
freeStablePtr(StgStablePtr sp)
{
258 259
    snEntry *sn;

260
	initStablePtrTable();
261 262 263
    ACQUIRE_LOCK(&stable_mutex);

    sn = &stable_ptr_table[(StgWord)sp];
264
    
265 266 267 268 269 270 271
    ASSERT((StgWord)sp < SPT_size  &&  sn->addr != NULL  &&  sn->ref > 0);

    sn->ref--;

    // If this entry has no StableName attached, then just free it
    // immediately.  This is important; it might be a while before the
    // next major GC which actually collects the entry.
272
    if (sn->sn_obj == NULL && sn->ref == 0) {
273 274
	freeStableName(sn);
    }
275 276

    RELEASE_LOCK(&stable_mutex);
277 278
}

279 280 281 282
void
enlargeStablePtrTable(void)
{
  nat old_SPT_size = SPT_size;
283

284
    // 2nd and subsequent times
285 286 287
  SPT_size *= 2;
  stable_ptr_table =
    stgReallocBytes(stable_ptr_table,
sof's avatar
sof committed
288
		      SPT_size * sizeof(snEntry),
289
		      "enlargeStablePtrTable");
sof's avatar
sof committed
290

291
  initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
292 293 294 295 296
}

/* -----------------------------------------------------------------------------
 * Treat stable pointers as roots for the garbage collector.
 *
297
 * A stable pointer is any stable name entry with a ref > 0.  We'll
298 299 300 301
 * take the opportunity to zero the "keep" flags at the same time.
 * -------------------------------------------------------------------------- */

void
302
markStablePtrTable(evac_fn evac)
303
{
304 305 306 307 308 309 310 311 312 313
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // Mark all the stable *pointers* (not stable names).
    // _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
	q = p->addr;

314 315 316
	// Internal pointers are free slots.  If q == NULL, it's a
	// stable name where the object has been GC'd, but the
	// StableName object (sn_obj) is still alive.
317 318 319 320 321 322 323
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {

	    // save the current addr away: we need to be able to tell
	    // whether the objects moved in order to be able to update
	    // the hash table later.
	    p->old = p->addr;

324 325
	    // if the ref is non-zero, treat addr as a root
	    if (p->ref != 0) {
326 327 328 329 330
		evac((StgClosure **)&p->addr);
	    }
	}
    }
}
331

332 333 334 335 336 337 338
/* -----------------------------------------------------------------------------
 * Thread the stable pointer table for compacting GC.
 * 
 * Here we must call the supplied evac function for each pointer into
 * the heap from the stable pointer table, because the compacting
 * collector may move the object it points to.
 * -------------------------------------------------------------------------- */
339

340 341 342 343 344 345 346 347 348 349
void
threadStablePtrTable( evac_fn evac )
{
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
	
350 351 352 353 354
	if (p->sn_obj != NULL) {
	    evac((StgClosure **)&p->sn_obj);
	}

	q = p->addr;
355
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
356
	    evac((StgClosure **)&p->addr);
357 358 359 360 361 362 363 364 365
	}
    }
}

/* -----------------------------------------------------------------------------
 * Garbage collect any dead entries in the stable pointer table.
 *
 * A dead entry has:
 *
366
 *          - a zero reference count
367
 *          - a dead sn_obj
368
 *
369 370 371 372
 * Both of these conditions must be true in order to re-use the stable
 * name table entry.  We can re-use stable name table entries for live
 * heap objects, as long as the program has no StableName objects that
 * refer to the entry.
373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390
 * -------------------------------------------------------------------------- */

void
gcStablePtrTable( void )
{
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // NOTE: _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
	
	// Update the pointer to the StableName object, if there is one
	if (p->sn_obj != NULL) {
	    p->sn_obj = isAlive(p->sn_obj);
	}
	
391 392 393
	// Internal pointers are free slots.  If q == NULL, it's a
	// stable name where the object has been GC'd, but the
	// StableName object (sn_obj) is still alive.
394 395 396 397
	q = p->addr;
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {

	    // StableNames only:
398
	    if (p->ref == 0) {
399 400 401
		if (p->sn_obj == NULL) {
		    // StableName object is dead
		    freeStableName(p);
402
		    IF_DEBUG(stable, debugBelch("GC'd Stable name %ld\n", 
403
						p - stable_ptr_table));
404 405 406
		    continue;
		    
		} else {
407
		  p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
408
		    IF_DEBUG(stable, debugBelch("Stable name %ld still alive at %p, ref %ld\n", p - stable_ptr_table, p->addr, p->ref));
409 410 411 412 413 414 415 416
		}
	    }
	}
    }
}

/* -----------------------------------------------------------------------------
 * Update the StablePtr/StableName hash table
417 418 419 420 421 422 423 424
 *
 * The boolean argument 'full' indicates that a major collection is
 * being done, so we might as well throw away the hash table and build
 * a new one.  For a minor collection, we just re-hash the elements
 * that changed.
 * -------------------------------------------------------------------------- */

void
425
updateStablePtrTable(rtsBool full)
426
{
427 428 429 430 431
    snEntry *p, *end_stable_ptr_table;
    
    if (full && addrToStableHash != NULL) {
	freeHashTable(addrToStableHash,NULL);
	addrToStableHash = allocHashTable();
432
    }
433 434 435 436 437
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // NOTE: _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
438
	
439 440 441 442 443 444 445 446 447 448 449
	if (p->addr == NULL) {
	    if (p->old != NULL) {
		// The target has been garbage collected.  Remove its
		// entry from the hash table.
		removeHashTable(addrToStableHash, (W_)p->old, NULL);
		p->old = NULL;
	    }
	}
	else if (p->addr < (P_)stable_ptr_table 
		 || p->addr >= (P_)end_stable_ptr_table) {
	    // Target still alive, Re-hash this stable name 
450
	    if (full) {
451 452 453 454 455 456
		insertHashTable(addrToStableHash, (W_)p->addr, 
				(void *)(p - stable_ptr_table));
	    } else if (p->addr != p->old) {
		removeHashTable(addrToStableHash, (W_)p->old, NULL);
		insertHashTable(addrToStableHash, (W_)p->addr, 
				(void *)(p - stable_ptr_table));
457
	    }
458 459 460
	}
    }
}