1 /* -----------------------------------------------------------------------------
2 * $Id: Storage.c,v 1.78 2003/03/26 17:33:49 sof Exp $
4 * (c) The GHC Team, 1998-1999
6 * Storage manager front end
8 * ---------------------------------------------------------------------------*/
10 #include "PosixSource.h"
16 #include "BlockAlloc.h"
24 #include "OSThreads.h"
25 #include "StoragePriv.h"
27 #include "RetainerProfile.h" // for counting memory blocks (memInventory)
32 StgClosure *caf_list = NULL;
34 bdescr *small_alloc_list; /* allocate()d small objects */
35 bdescr *pinned_object_block; /* allocate pinned objects into this block */
36 nat alloc_blocks; /* number of allocate()d blocks since GC */
37 nat alloc_blocks_lim; /* approximate limit on alloc_blocks */
39 StgPtr alloc_Hp = NULL; /* next free byte in small_alloc_list */
40 StgPtr alloc_HpLim = NULL; /* end of block at small_alloc_list */
42 generation *generations = NULL; /* all the generations */
43 generation *g0 = NULL; /* generation 0, for convenience */
44 generation *oldest_gen = NULL; /* oldest generation, for convenience */
45 step *g0s0 = NULL; /* generation 0, step 0, for convenience */
47 lnat total_allocated = 0; /* total memory allocated during run */
50 * Storage manager mutex: protects all the above state from
51 * simultaneous access by two STG threads.
54 Mutex sm_mutex = INIT_MUTEX_VAR;
60 static void *stgAllocForGMP (size_t size_in_bytes);
61 static void *stgReallocForGMP (void *ptr, size_t old_size, size_t new_size);
62 static void stgDeallocForGMP (void *ptr, size_t size);
71 if (generations != NULL) {
72 // multi-init protection
76 /* Sanity check to make sure the LOOKS_LIKE_ macros appear to be
77 * doing something reasonable.
79 ASSERT(LOOKS_LIKE_INFO_PTR(&stg_BLACKHOLE_info));
80 ASSERT(LOOKS_LIKE_CLOSURE_PTR(&stg_dummy_ret_closure));
81 ASSERT(!HEAP_ALLOCED(&stg_dummy_ret_closure));
83 if (RtsFlags.GcFlags.maxHeapSize != 0 &&
84 RtsFlags.GcFlags.heapSizeSuggestion >
85 RtsFlags.GcFlags.maxHeapSize) {
86 RtsFlags.GcFlags.maxHeapSize = RtsFlags.GcFlags.heapSizeSuggestion;
89 if (RtsFlags.GcFlags.maxHeapSize != 0 &&
90 RtsFlags.GcFlags.minAllocAreaSize >
91 RtsFlags.GcFlags.maxHeapSize) {
92 prog_belch("maximum heap size (-M) is smaller than minimum alloc area size (-A)");
102 /* allocate generation info array */
103 generations = (generation *)stgMallocBytes(RtsFlags.GcFlags.generations
104 * sizeof(struct _generation),
105 "initStorage: gens");
107 /* Initialise all generations */
108 for(g = 0; g < RtsFlags.GcFlags.generations; g++) {
109 gen = &generations[g];
111 gen->mut_list = END_MUT_LIST;
112 gen->mut_once_list = END_MUT_LIST;
113 gen->collections = 0;
114 gen->failed_promotions = 0;
118 /* A couple of convenience pointers */
119 g0 = &generations[0];
120 oldest_gen = &generations[RtsFlags.GcFlags.generations-1];
122 /* Allocate step structures in each generation */
123 if (RtsFlags.GcFlags.generations > 1) {
124 /* Only for multiple-generations */
126 /* Oldest generation: one step */
127 oldest_gen->n_steps = 1;
129 stgMallocBytes(1 * sizeof(struct _step), "initStorage: last step");
131 /* set up all except the oldest generation with 2 steps */
132 for(g = 0; g < RtsFlags.GcFlags.generations-1; g++) {
133 generations[g].n_steps = RtsFlags.GcFlags.steps;
134 generations[g].steps =
135 stgMallocBytes (RtsFlags.GcFlags.steps * sizeof(struct _step),
136 "initStorage: steps");
140 /* single generation, i.e. a two-space collector */
142 g0->steps = stgMallocBytes (sizeof(struct _step), "initStorage: steps");
145 /* Initialise all steps */
146 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
147 for (s = 0; s < generations[g].n_steps; s++) {
148 stp = &generations[g].steps[s];
151 stp->n_to_blocks = 0;
153 stp->gen = &generations[g];
160 stp->large_objects = NULL;
161 stp->n_large_blocks = 0;
162 stp->new_large_objects = NULL;
163 stp->scavenged_large_objects = NULL;
164 stp->n_scavenged_large_blocks = 0;
165 stp->is_compacted = 0;
170 /* Set up the destination pointers in each younger gen. step */
171 for (g = 0; g < RtsFlags.GcFlags.generations-1; g++) {
172 for (s = 0; s < generations[g].n_steps-1; s++) {
173 generations[g].steps[s].to = &generations[g].steps[s+1];
175 generations[g].steps[s].to = &generations[g+1].steps[0];
178 /* The oldest generation has one step and it is compacted. */
179 if (RtsFlags.GcFlags.compact) {
180 if (RtsFlags.GcFlags.generations == 1) {
181 belch("WARNING: compaction is incompatible with -G1; disabled");
183 oldest_gen->steps[0].is_compacted = 1;
186 oldest_gen->steps[0].to = &oldest_gen->steps[0];
188 /* generation 0 is special: that's the nursery */
189 generations[0].max_blocks = 0;
191 /* G0S0: the allocation area. Policy: keep the allocation area
192 * small to begin with, even if we have a large suggested heap
193 * size. Reason: we're going to do a major collection first, and we
194 * don't want it to be a big one. This vague idea is borne out by
195 * rigorous experimental evidence.
197 g0s0 = &generations[0].steps[0];
201 weak_ptr_list = NULL;
204 /* initialise the allocate() interface */
205 small_alloc_list = NULL;
207 alloc_blocks_lim = RtsFlags.GcFlags.minAllocAreaSize;
209 /* Tell GNU multi-precision pkg about our custom alloc functions */
210 mp_set_memory_functions(stgAllocForGMP, stgReallocForGMP, stgDeallocForGMP);
212 IF_DEBUG(gc, statDescribeGens());
218 stat_exit(calcAllocated());
221 /* -----------------------------------------------------------------------------
224 The entry code for every CAF does the following:
226 - builds a CAF_BLACKHOLE in the heap
227 - pushes an update frame pointing to the CAF_BLACKHOLE
228 - invokes UPD_CAF(), which:
229 - calls newCaf, below
230 - updates the CAF with a static indirection to the CAF_BLACKHOLE
232 Why do we build a BLACKHOLE in the heap rather than just updating
233 the thunk directly? It's so that we only need one kind of update
234 frame - otherwise we'd need a static version of the update frame too.
236 newCaf() does the following:
238 - it puts the CAF on the oldest generation's mut-once list.
239 This is so that we can treat the CAF as a root when collecting
242 For GHCI, we have additional requirements when dealing with CAFs:
244 - we must *retain* all dynamically-loaded CAFs ever entered,
245 just in case we need them again.
246 - we must be able to *revert* CAFs that have been evaluated, to
247 their pre-evaluated form.
249 To do this, we use an additional CAF list. When newCaf() is
250 called on a dynamically-loaded CAF, we add it to the CAF list
251 instead of the old-generation mutable list, and save away its
252 old info pointer (in caf->saved_info) for later reversion.
254 To revert all the CAFs, we traverse the CAF list and reset the
255 info pointer to caf->saved_info, then throw away the CAF list.
256 (see GC.c:revertCAFs()).
260 -------------------------------------------------------------------------- */
263 newCAF(StgClosure* caf)
265 /* Put this CAF on the mutable list for the old generation.
266 * This is a HACK - the IND_STATIC closure doesn't really have
267 * a mut_link field, but we pretend it has - in fact we re-use
268 * the STATIC_LINK field for the time being, because when we
269 * come to do a major GC we won't need the mut_link field
270 * any more and can use it as a STATIC_LINK.
274 ((StgIndStatic *)caf)->saved_info = NULL;
275 ((StgMutClosure *)caf)->mut_link = oldest_gen->mut_once_list;
276 oldest_gen->mut_once_list = (StgMutClosure *)caf;
281 /* If we are PAR or DIST then we never forget a CAF */
283 //belch("<##> Globalising CAF %08x %s",caf,info_type(caf));
284 newGA=makeGlobal(caf,rtsTrue); /*given full weight*/
290 // An alternate version of newCaf which is used for dynamically loaded
291 // object code in GHCi. In this case we want to retain *all* CAFs in
292 // the object code, because they might be demanded at any time from an
293 // expression evaluated on the command line.
295 // The linker hackily arranges that references to newCaf from dynamic
296 // code end up pointing to newDynCAF.
298 newDynCAF(StgClosure *caf)
302 ((StgIndStatic *)caf)->saved_info = (StgInfoTable *)caf->header.info;
303 ((StgIndStatic *)caf)->static_link = caf_list;
309 /* -----------------------------------------------------------------------------
311 -------------------------------------------------------------------------- */
314 allocNurseries( void )
323 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
324 cap->r.rNursery = allocNursery(NULL, RtsFlags.GcFlags.minAllocAreaSize);
325 cap->r.rCurrentNursery = cap->r.rNursery;
326 for (bd = cap->r.rNursery; bd != NULL; bd = bd->link) {
327 bd->u.back = (bdescr *)cap;
330 /* Set the back links to be equal to the Capability,
331 * so we can do slightly better informed locking.
335 g0s0->blocks = allocNursery(NULL, RtsFlags.GcFlags.minAllocAreaSize);
336 g0s0->n_blocks = RtsFlags.GcFlags.minAllocAreaSize;
337 g0s0->to_blocks = NULL;
338 g0s0->n_to_blocks = 0;
339 MainCapability.r.rNursery = g0s0->blocks;
340 MainCapability.r.rCurrentNursery = g0s0->blocks;
341 /* hp, hpLim, hp_bd, to_space etc. aren't used in G0S0 */
346 resetNurseries( void )
352 /* All tasks must be stopped */
353 ASSERT(n_free_capabilities == RtsFlags.ParFlags.nNodes);
355 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
356 for (bd = cap->r.rNursery; bd; bd = bd->link) {
357 bd->free = bd->start;
358 ASSERT(bd->gen_no == 0);
359 ASSERT(bd->step == g0s0);
360 IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
362 cap->r.rCurrentNursery = cap->r.rNursery;
365 for (bd = g0s0->blocks; bd; bd = bd->link) {
366 bd->free = bd->start;
367 ASSERT(bd->gen_no == 0);
368 ASSERT(bd->step == g0s0);
369 IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
371 MainCapability.r.rNursery = g0s0->blocks;
372 MainCapability.r.rCurrentNursery = g0s0->blocks;
377 allocNursery (bdescr *tail, nat blocks)
382 // Allocate a nursery: we allocate fresh blocks one at a time and
383 // cons them on to the front of the list, not forgetting to update
384 // the back pointer on the tail of the list to point to the new block.
385 for (i=0; i < blocks; i++) {
388 processNursery() in LdvProfile.c assumes that every block group in
389 the nursery contains only a single block. So, if a block group is
390 given multiple blocks, change processNursery() accordingly.
394 // double-link the nursery: we might need to insert blocks
401 bd->free = bd->start;
409 resizeNursery ( nat blocks )
415 barf("resizeNursery: can't resize in SMP mode");
418 nursery_blocks = g0s0->n_blocks;
419 if (nursery_blocks == blocks) {
423 else if (nursery_blocks < blocks) {
424 IF_DEBUG(gc, fprintf(stderr, "Increasing size of nursery to %d blocks\n",
426 g0s0->blocks = allocNursery(g0s0->blocks, blocks-nursery_blocks);
432 IF_DEBUG(gc, fprintf(stderr, "Decreasing size of nursery to %d blocks\n",
436 while (nursery_blocks > blocks) {
438 next_bd->u.back = NULL;
439 nursery_blocks -= bd->blocks; // might be a large block
444 // might have gone just under, by freeing a large block, so make
445 // up the difference.
446 if (nursery_blocks < blocks) {
447 g0s0->blocks = allocNursery(g0s0->blocks, blocks-nursery_blocks);
451 g0s0->n_blocks = blocks;
452 ASSERT(countBlocks(g0s0->blocks) == g0s0->n_blocks);
455 /* -----------------------------------------------------------------------------
456 The allocate() interface
458 allocate(n) always succeeds, and returns a chunk of memory n words
459 long. n can be larger than the size of a block if necessary, in
460 which case a contiguous block group will be allocated.
461 -------------------------------------------------------------------------- */
471 TICK_ALLOC_HEAP_NOCTR(n);
474 /* big allocation (>LARGE_OBJECT_THRESHOLD) */
475 /* ToDo: allocate directly into generation 1 */
476 if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
477 nat req_blocks = (lnat)BLOCK_ROUND_UP(n*sizeof(W_)) / BLOCK_SIZE;
478 bd = allocGroup(req_blocks);
479 dbl_link_onto(bd, &g0s0->large_objects);
482 bd->flags = BF_LARGE;
483 bd->free = bd->start + n;
484 /* don't add these blocks to alloc_blocks, since we're assuming
485 * that large objects are likely to remain live for quite a while
486 * (eg. running threads), so garbage collecting early won't make
489 alloc_blocks += req_blocks;
493 /* small allocation (<LARGE_OBJECT_THRESHOLD) */
494 } else if (small_alloc_list == NULL || alloc_Hp + n > alloc_HpLim) {
495 if (small_alloc_list) {
496 small_alloc_list->free = alloc_Hp;
499 bd->link = small_alloc_list;
500 small_alloc_list = bd;
504 alloc_Hp = bd->start;
505 alloc_HpLim = bd->start + BLOCK_SIZE_W;
516 allocated_bytes( void )
520 allocated = alloc_blocks * BLOCK_SIZE_W - (alloc_HpLim - alloc_Hp);
521 if (pinned_object_block != NULL) {
522 allocated -= (pinned_object_block->start + BLOCK_SIZE_W) -
523 pinned_object_block->free;
530 tidyAllocateLists (void)
532 if (small_alloc_list != NULL) {
533 ASSERT(alloc_Hp >= small_alloc_list->start &&
534 alloc_Hp <= small_alloc_list->start + BLOCK_SIZE);
535 small_alloc_list->free = alloc_Hp;
539 /* ---------------------------------------------------------------------------
540 Allocate a fixed/pinned object.
542 We allocate small pinned objects into a single block, allocating a
543 new block when the current one overflows. The block is chained
544 onto the large_object_list of generation 0 step 0.
546 NOTE: The GC can't in general handle pinned objects. This
547 interface is only safe to use for ByteArrays, which have no
548 pointers and don't require scavenging. It works because the
549 block's descriptor has the BF_LARGE flag set, so the block is
550 treated as a large object and chained onto various lists, rather
551 than the individual objects being copied. However, when it comes
552 to scavenge the block, the GC will only scavenge the first object.
553 The reason is that the GC can't linearly scan a block of pinned
554 objects at the moment (doing so would require using the
555 mostly-copying techniques). But since we're restricting ourselves
556 to pinned ByteArrays, not scavenging is ok.
558 This function is called by newPinnedByteArray# which immediately
559 fills the allocated memory with a MutableByteArray#.
560 ------------------------------------------------------------------------- */
563 allocatePinned( nat n )
566 bdescr *bd = pinned_object_block;
570 TICK_ALLOC_HEAP_NOCTR(n);
573 // If the request is for a large object, then allocate()
574 // will give us a pinned object anyway.
575 if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
580 // we always return 8-byte aligned memory. bd->free must be
581 // 8-byte aligned to begin with, so we just round up n to
582 // the nearest multiple of 8 bytes.
583 if (sizeof(StgWord) == 4) {
587 // If we don't have a block of pinned objects yet, or the current
588 // one isn't large enough to hold the new object, allocate a new one.
589 if (bd == NULL || (bd->free + n) > (bd->start + BLOCK_SIZE_W)) {
590 pinned_object_block = bd = allocBlock();
591 dbl_link_onto(bd, &g0s0->large_objects);
594 bd->flags = BF_LARGE;
595 bd->free = bd->start;
605 /* -----------------------------------------------------------------------------
606 Allocation functions for GMP.
608 These all use the allocate() interface - we can't have any garbage
609 collection going on during a gmp operation, so we use allocate()
610 which always succeeds. The gmp operations which might need to
611 allocate will ask the storage manager (via doYouWantToGC()) whether
612 a garbage collection is required, in case we get into a loop doing
613 only allocate() style allocation.
614 -------------------------------------------------------------------------- */
617 stgAllocForGMP (size_t size_in_bytes)
620 nat data_size_in_words, total_size_in_words;
622 /* round up to a whole number of words */
623 data_size_in_words = (size_in_bytes + sizeof(W_) + 1) / sizeof(W_);
624 total_size_in_words = sizeofW(StgArrWords) + data_size_in_words;
626 /* allocate and fill it in. */
627 arr = (StgArrWords *)allocate(total_size_in_words);
628 SET_ARR_HDR(arr, &stg_ARR_WORDS_info, CCCS, data_size_in_words);
630 /* and return a ptr to the goods inside the array */
631 return(BYTE_ARR_CTS(arr));
635 stgReallocForGMP (void *ptr, size_t old_size, size_t new_size)
637 void *new_stuff_ptr = stgAllocForGMP(new_size);
639 char *p = (char *) ptr;
640 char *q = (char *) new_stuff_ptr;
642 for (; i < old_size; i++, p++, q++) {
646 return(new_stuff_ptr);
650 stgDeallocForGMP (void *ptr STG_UNUSED,
651 size_t size STG_UNUSED)
653 /* easy for us: the garbage collector does the dealloc'n */
656 /* -----------------------------------------------------------------------------
658 * -------------------------------------------------------------------------- */
660 /* -----------------------------------------------------------------------------
663 * Approximate how much we've allocated: number of blocks in the
664 * nursery + blocks allocated via allocate() - unused nusery blocks.
665 * This leaves a little slop at the end of each block, and doesn't
666 * take into account large objects (ToDo).
667 * -------------------------------------------------------------------------- */
670 calcAllocated( void )
678 /* All tasks must be stopped. Can't assert that all the
679 capabilities are owned by the scheduler, though: one or more
680 tasks might have been stopped while they were running (non-main)
682 /* ASSERT(n_free_capabilities == RtsFlags.ParFlags.nNodes); */
685 n_free_capabilities * RtsFlags.GcFlags.minAllocAreaSize * BLOCK_SIZE_W
688 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
689 for ( bd = cap->r.rCurrentNursery->link; bd != NULL; bd = bd->link ) {
690 allocated -= BLOCK_SIZE_W;
692 if (cap->r.rCurrentNursery->free < cap->r.rCurrentNursery->start
694 allocated -= (cap->r.rCurrentNursery->start + BLOCK_SIZE_W)
695 - cap->r.rCurrentNursery->free;
700 bdescr *current_nursery = MainCapability.r.rCurrentNursery;
702 allocated = (g0s0->n_blocks * BLOCK_SIZE_W) + allocated_bytes();
703 for ( bd = current_nursery->link; bd != NULL; bd = bd->link ) {
704 allocated -= BLOCK_SIZE_W;
706 if (current_nursery->free < current_nursery->start + BLOCK_SIZE_W) {
707 allocated -= (current_nursery->start + BLOCK_SIZE_W)
708 - current_nursery->free;
712 total_allocated += allocated;
716 /* Approximate the amount of live data in the heap. To be called just
717 * after garbage collection (see GarbageCollect()).
726 if (RtsFlags.GcFlags.generations == 1) {
727 live = (g0s0->n_to_blocks - 1) * BLOCK_SIZE_W +
728 ((lnat)g0s0->hp_bd->free - (lnat)g0s0->hp_bd->start) / sizeof(W_);
732 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
733 for (s = 0; s < generations[g].n_steps; s++) {
734 /* approximate amount of live data (doesn't take into account slop
735 * at end of each block).
737 if (g == 0 && s == 0) {
740 stp = &generations[g].steps[s];
741 live += (stp->n_large_blocks + stp->n_blocks - 1) * BLOCK_SIZE_W;
742 if (stp->hp_bd != NULL) {
743 live += ((lnat)stp->hp_bd->free - (lnat)stp->hp_bd->start)
751 /* Approximate the number of blocks that will be needed at the next
752 * garbage collection.
754 * Assume: all data currently live will remain live. Steps that will
755 * be collected next time will therefore need twice as many blocks
756 * since all the data will be copied.
765 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
766 for (s = 0; s < generations[g].n_steps; s++) {
767 if (g == 0 && s == 0) { continue; }
768 stp = &generations[g].steps[s];
769 if (generations[g].steps[0].n_blocks +
770 generations[g].steps[0].n_large_blocks
771 > generations[g].max_blocks
772 && stp->is_compacted == 0) {
773 needed += 2 * stp->n_blocks;
775 needed += stp->n_blocks;
782 /* -----------------------------------------------------------------------------
785 memInventory() checks for memory leaks by counting up all the
786 blocks we know about and comparing that to the number of blocks
787 allegedly floating around in the system.
788 -------------------------------------------------------------------------- */
798 lnat total_blocks = 0, free_blocks = 0;
800 /* count the blocks we current have */
802 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
803 for (s = 0; s < generations[g].n_steps; s++) {
804 stp = &generations[g].steps[s];
805 total_blocks += stp->n_blocks;
806 if (RtsFlags.GcFlags.generations == 1) {
807 /* two-space collector has a to-space too :-) */
808 total_blocks += g0s0->n_to_blocks;
810 for (bd = stp->large_objects; bd; bd = bd->link) {
811 total_blocks += bd->blocks;
812 /* hack for megablock groups: they have an extra block or two in
813 the second and subsequent megablocks where the block
814 descriptors would normally go.
816 if (bd->blocks > BLOCKS_PER_MBLOCK) {
817 total_blocks -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
818 * (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
824 /* any blocks held by allocate() */
825 for (bd = small_alloc_list; bd; bd = bd->link) {
826 total_blocks += bd->blocks;
830 if (RtsFlags.ProfFlags.doHeapProfile == HEAP_BY_RETAINER) {
831 total_blocks += retainerStackBlocks();
835 // count the blocks allocated by the arena allocator
836 total_blocks += arenaBlocks();
838 /* count the blocks on the free list */
839 free_blocks = countFreeList();
841 if (total_blocks + free_blocks != mblocks_allocated *
843 fprintf(stderr, "Blocks: %ld live + %ld free = %ld total (%ld around)\n",
844 total_blocks, free_blocks, total_blocks + free_blocks,
845 mblocks_allocated * BLOCKS_PER_MBLOCK);
848 ASSERT(total_blocks + free_blocks == mblocks_allocated * BLOCKS_PER_MBLOCK);
853 countBlocks(bdescr *bd)
856 for (n=0; bd != NULL; bd=bd->link) {
862 /* Full heap sanity check. */
868 if (RtsFlags.GcFlags.generations == 1) {
869 checkHeap(g0s0->to_blocks);
870 checkChain(g0s0->large_objects);
873 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
874 for (s = 0; s < generations[g].n_steps; s++) {
875 ASSERT(countBlocks(generations[g].steps[s].blocks)
876 == generations[g].steps[s].n_blocks);
877 ASSERT(countBlocks(generations[g].steps[s].large_objects)
878 == generations[g].steps[s].n_large_blocks);
879 if (g == 0 && s == 0) { continue; }
880 checkHeap(generations[g].steps[s].blocks);
881 checkChain(generations[g].steps[s].large_objects);
883 checkMutableList(generations[g].mut_list, g);
884 checkMutOnceList(generations[g].mut_once_list, g);
888 checkFreeListSanity();
892 // handy function for use in gdb, because Bdescr() is inlined.
893 extern bdescr *_bdescr( StgPtr p );