1 /* -----------------------------------------------------------------------------
2 * $Id: Storage.c,v 1.79 2003/03/26 18:59:34 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 )
322 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
323 cap->r.rNursery = allocNursery(NULL, RtsFlags.GcFlags.minAllocAreaSize);
324 cap->r.rCurrentNursery = cap->r.rNursery;
325 /* Set the back links to be equal to the Capability,
326 * so we can do slightly better informed locking.
328 for (bd = cap->r.rNursery; bd != NULL; bd = bd->link) {
329 bd->u.back = (bdescr *)cap;
333 g0s0->blocks = allocNursery(NULL, RtsFlags.GcFlags.minAllocAreaSize);
334 g0s0->n_blocks = RtsFlags.GcFlags.minAllocAreaSize;
335 g0s0->to_blocks = NULL;
336 g0s0->n_to_blocks = 0;
337 MainCapability.r.rNursery = g0s0->blocks;
338 MainCapability.r.rCurrentNursery = g0s0->blocks;
339 /* hp, hpLim, hp_bd, to_space etc. aren't used in G0S0 */
344 resetNurseries( void )
350 /* All tasks must be stopped */
351 ASSERT(n_free_capabilities == RtsFlags.ParFlags.nNodes);
353 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
354 for (bd = cap->r.rNursery; bd; bd = bd->link) {
355 bd->free = bd->start;
356 ASSERT(bd->gen_no == 0);
357 ASSERT(bd->step == g0s0);
358 IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
360 cap->r.rCurrentNursery = cap->r.rNursery;
363 for (bd = g0s0->blocks; bd; bd = bd->link) {
364 bd->free = bd->start;
365 ASSERT(bd->gen_no == 0);
366 ASSERT(bd->step == g0s0);
367 IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
369 MainCapability.r.rNursery = g0s0->blocks;
370 MainCapability.r.rCurrentNursery = g0s0->blocks;
375 allocNursery (bdescr *tail, nat blocks)
380 // Allocate a nursery: we allocate fresh blocks one at a time and
381 // cons them on to the front of the list, not forgetting to update
382 // the back pointer on the tail of the list to point to the new block.
383 for (i=0; i < blocks; i++) {
386 processNursery() in LdvProfile.c assumes that every block group in
387 the nursery contains only a single block. So, if a block group is
388 given multiple blocks, change processNursery() accordingly.
392 // double-link the nursery: we might need to insert blocks
399 bd->free = bd->start;
407 resizeNursery ( nat blocks )
413 barf("resizeNursery: can't resize in SMP mode");
416 nursery_blocks = g0s0->n_blocks;
417 if (nursery_blocks == blocks) {
421 else if (nursery_blocks < blocks) {
422 IF_DEBUG(gc, fprintf(stderr, "Increasing size of nursery to %d blocks\n",
424 g0s0->blocks = allocNursery(g0s0->blocks, blocks-nursery_blocks);
430 IF_DEBUG(gc, fprintf(stderr, "Decreasing size of nursery to %d blocks\n",
434 while (nursery_blocks > blocks) {
436 next_bd->u.back = NULL;
437 nursery_blocks -= bd->blocks; // might be a large block
442 // might have gone just under, by freeing a large block, so make
443 // up the difference.
444 if (nursery_blocks < blocks) {
445 g0s0->blocks = allocNursery(g0s0->blocks, blocks-nursery_blocks);
449 g0s0->n_blocks = blocks;
450 ASSERT(countBlocks(g0s0->blocks) == g0s0->n_blocks);
453 /* -----------------------------------------------------------------------------
454 The allocate() interface
456 allocate(n) always succeeds, and returns a chunk of memory n words
457 long. n can be larger than the size of a block if necessary, in
458 which case a contiguous block group will be allocated.
459 -------------------------------------------------------------------------- */
469 TICK_ALLOC_HEAP_NOCTR(n);
472 /* big allocation (>LARGE_OBJECT_THRESHOLD) */
473 /* ToDo: allocate directly into generation 1 */
474 if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
475 nat req_blocks = (lnat)BLOCK_ROUND_UP(n*sizeof(W_)) / BLOCK_SIZE;
476 bd = allocGroup(req_blocks);
477 dbl_link_onto(bd, &g0s0->large_objects);
480 bd->flags = BF_LARGE;
481 bd->free = bd->start + n;
482 /* don't add these blocks to alloc_blocks, since we're assuming
483 * that large objects are likely to remain live for quite a while
484 * (eg. running threads), so garbage collecting early won't make
487 alloc_blocks += req_blocks;
491 /* small allocation (<LARGE_OBJECT_THRESHOLD) */
492 } else if (small_alloc_list == NULL || alloc_Hp + n > alloc_HpLim) {
493 if (small_alloc_list) {
494 small_alloc_list->free = alloc_Hp;
497 bd->link = small_alloc_list;
498 small_alloc_list = bd;
502 alloc_Hp = bd->start;
503 alloc_HpLim = bd->start + BLOCK_SIZE_W;
514 allocated_bytes( void )
518 allocated = alloc_blocks * BLOCK_SIZE_W - (alloc_HpLim - alloc_Hp);
519 if (pinned_object_block != NULL) {
520 allocated -= (pinned_object_block->start + BLOCK_SIZE_W) -
521 pinned_object_block->free;
528 tidyAllocateLists (void)
530 if (small_alloc_list != NULL) {
531 ASSERT(alloc_Hp >= small_alloc_list->start &&
532 alloc_Hp <= small_alloc_list->start + BLOCK_SIZE);
533 small_alloc_list->free = alloc_Hp;
537 /* ---------------------------------------------------------------------------
538 Allocate a fixed/pinned object.
540 We allocate small pinned objects into a single block, allocating a
541 new block when the current one overflows. The block is chained
542 onto the large_object_list of generation 0 step 0.
544 NOTE: The GC can't in general handle pinned objects. This
545 interface is only safe to use for ByteArrays, which have no
546 pointers and don't require scavenging. It works because the
547 block's descriptor has the BF_LARGE flag set, so the block is
548 treated as a large object and chained onto various lists, rather
549 than the individual objects being copied. However, when it comes
550 to scavenge the block, the GC will only scavenge the first object.
551 The reason is that the GC can't linearly scan a block of pinned
552 objects at the moment (doing so would require using the
553 mostly-copying techniques). But since we're restricting ourselves
554 to pinned ByteArrays, not scavenging is ok.
556 This function is called by newPinnedByteArray# which immediately
557 fills the allocated memory with a MutableByteArray#.
558 ------------------------------------------------------------------------- */
561 allocatePinned( nat n )
564 bdescr *bd = pinned_object_block;
566 // If the request is for a large object, then allocate()
567 // will give us a pinned object anyway.
568 if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
574 TICK_ALLOC_HEAP_NOCTR(n);
577 // we always return 8-byte aligned memory. bd->free must be
578 // 8-byte aligned to begin with, so we just round up n to
579 // the nearest multiple of 8 bytes.
580 if (sizeof(StgWord) == 4) {
584 // If we don't have a block of pinned objects yet, or the current
585 // one isn't large enough to hold the new object, allocate a new one.
586 if (bd == NULL || (bd->free + n) > (bd->start + BLOCK_SIZE_W)) {
587 pinned_object_block = bd = allocBlock();
588 dbl_link_onto(bd, &g0s0->large_objects);
591 bd->flags = BF_LARGE;
592 bd->free = bd->start;
602 /* -----------------------------------------------------------------------------
603 Allocation functions for GMP.
605 These all use the allocate() interface - we can't have any garbage
606 collection going on during a gmp operation, so we use allocate()
607 which always succeeds. The gmp operations which might need to
608 allocate will ask the storage manager (via doYouWantToGC()) whether
609 a garbage collection is required, in case we get into a loop doing
610 only allocate() style allocation.
611 -------------------------------------------------------------------------- */
614 stgAllocForGMP (size_t size_in_bytes)
617 nat data_size_in_words, total_size_in_words;
619 /* round up to a whole number of words */
620 data_size_in_words = (size_in_bytes + sizeof(W_) + 1) / sizeof(W_);
621 total_size_in_words = sizeofW(StgArrWords) + data_size_in_words;
623 /* allocate and fill it in. */
624 arr = (StgArrWords *)allocate(total_size_in_words);
625 SET_ARR_HDR(arr, &stg_ARR_WORDS_info, CCCS, data_size_in_words);
627 /* and return a ptr to the goods inside the array */
628 return(BYTE_ARR_CTS(arr));
632 stgReallocForGMP (void *ptr, size_t old_size, size_t new_size)
634 void *new_stuff_ptr = stgAllocForGMP(new_size);
636 char *p = (char *) ptr;
637 char *q = (char *) new_stuff_ptr;
639 for (; i < old_size; i++, p++, q++) {
643 return(new_stuff_ptr);
647 stgDeallocForGMP (void *ptr STG_UNUSED,
648 size_t size STG_UNUSED)
650 /* easy for us: the garbage collector does the dealloc'n */
653 /* -----------------------------------------------------------------------------
655 * -------------------------------------------------------------------------- */
657 /* -----------------------------------------------------------------------------
660 * Approximate how much we've allocated: number of blocks in the
661 * nursery + blocks allocated via allocate() - unused nusery blocks.
662 * This leaves a little slop at the end of each block, and doesn't
663 * take into account large objects (ToDo).
664 * -------------------------------------------------------------------------- */
667 calcAllocated( void )
675 /* All tasks must be stopped. Can't assert that all the
676 capabilities are owned by the scheduler, though: one or more
677 tasks might have been stopped while they were running (non-main)
679 /* ASSERT(n_free_capabilities == RtsFlags.ParFlags.nNodes); */
682 n_free_capabilities * RtsFlags.GcFlags.minAllocAreaSize * BLOCK_SIZE_W
685 for (cap = free_capabilities; cap != NULL; cap = cap->link) {
686 for ( bd = cap->r.rCurrentNursery->link; bd != NULL; bd = bd->link ) {
687 allocated -= BLOCK_SIZE_W;
689 if (cap->r.rCurrentNursery->free < cap->r.rCurrentNursery->start
691 allocated -= (cap->r.rCurrentNursery->start + BLOCK_SIZE_W)
692 - cap->r.rCurrentNursery->free;
697 bdescr *current_nursery = MainCapability.r.rCurrentNursery;
699 allocated = (g0s0->n_blocks * BLOCK_SIZE_W) + allocated_bytes();
700 for ( bd = current_nursery->link; bd != NULL; bd = bd->link ) {
701 allocated -= BLOCK_SIZE_W;
703 if (current_nursery->free < current_nursery->start + BLOCK_SIZE_W) {
704 allocated -= (current_nursery->start + BLOCK_SIZE_W)
705 - current_nursery->free;
709 total_allocated += allocated;
713 /* Approximate the amount of live data in the heap. To be called just
714 * after garbage collection (see GarbageCollect()).
723 if (RtsFlags.GcFlags.generations == 1) {
724 live = (g0s0->n_to_blocks - 1) * BLOCK_SIZE_W +
725 ((lnat)g0s0->hp_bd->free - (lnat)g0s0->hp_bd->start) / sizeof(W_);
729 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
730 for (s = 0; s < generations[g].n_steps; s++) {
731 /* approximate amount of live data (doesn't take into account slop
732 * at end of each block).
734 if (g == 0 && s == 0) {
737 stp = &generations[g].steps[s];
738 live += (stp->n_large_blocks + stp->n_blocks - 1) * BLOCK_SIZE_W;
739 if (stp->hp_bd != NULL) {
740 live += ((lnat)stp->hp_bd->free - (lnat)stp->hp_bd->start)
748 /* Approximate the number of blocks that will be needed at the next
749 * garbage collection.
751 * Assume: all data currently live will remain live. Steps that will
752 * be collected next time will therefore need twice as many blocks
753 * since all the data will be copied.
762 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
763 for (s = 0; s < generations[g].n_steps; s++) {
764 if (g == 0 && s == 0) { continue; }
765 stp = &generations[g].steps[s];
766 if (generations[g].steps[0].n_blocks +
767 generations[g].steps[0].n_large_blocks
768 > generations[g].max_blocks
769 && stp->is_compacted == 0) {
770 needed += 2 * stp->n_blocks;
772 needed += stp->n_blocks;
779 /* -----------------------------------------------------------------------------
782 memInventory() checks for memory leaks by counting up all the
783 blocks we know about and comparing that to the number of blocks
784 allegedly floating around in the system.
785 -------------------------------------------------------------------------- */
795 lnat total_blocks = 0, free_blocks = 0;
797 /* count the blocks we current have */
799 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
800 for (s = 0; s < generations[g].n_steps; s++) {
801 stp = &generations[g].steps[s];
802 total_blocks += stp->n_blocks;
803 if (RtsFlags.GcFlags.generations == 1) {
804 /* two-space collector has a to-space too :-) */
805 total_blocks += g0s0->n_to_blocks;
807 for (bd = stp->large_objects; bd; bd = bd->link) {
808 total_blocks += bd->blocks;
809 /* hack for megablock groups: they have an extra block or two in
810 the second and subsequent megablocks where the block
811 descriptors would normally go.
813 if (bd->blocks > BLOCKS_PER_MBLOCK) {
814 total_blocks -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
815 * (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
821 /* any blocks held by allocate() */
822 for (bd = small_alloc_list; bd; bd = bd->link) {
823 total_blocks += bd->blocks;
827 if (RtsFlags.ProfFlags.doHeapProfile == HEAP_BY_RETAINER) {
828 total_blocks += retainerStackBlocks();
832 // count the blocks allocated by the arena allocator
833 total_blocks += arenaBlocks();
835 /* count the blocks on the free list */
836 free_blocks = countFreeList();
838 if (total_blocks + free_blocks != mblocks_allocated *
840 fprintf(stderr, "Blocks: %ld live + %ld free = %ld total (%ld around)\n",
841 total_blocks, free_blocks, total_blocks + free_blocks,
842 mblocks_allocated * BLOCKS_PER_MBLOCK);
845 ASSERT(total_blocks + free_blocks == mblocks_allocated * BLOCKS_PER_MBLOCK);
850 countBlocks(bdescr *bd)
853 for (n=0; bd != NULL; bd=bd->link) {
859 /* Full heap sanity check. */
865 if (RtsFlags.GcFlags.generations == 1) {
866 checkHeap(g0s0->to_blocks);
867 checkChain(g0s0->large_objects);
870 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
871 for (s = 0; s < generations[g].n_steps; s++) {
872 ASSERT(countBlocks(generations[g].steps[s].blocks)
873 == generations[g].steps[s].n_blocks);
874 ASSERT(countBlocks(generations[g].steps[s].large_objects)
875 == generations[g].steps[s].n_large_blocks);
876 if (g == 0 && s == 0) { continue; }
877 checkHeap(generations[g].steps[s].blocks);
878 checkChain(generations[g].steps[s].large_objects);
880 checkMutableList(generations[g].mut_list, g);
881 checkMutOnceList(generations[g].mut_once_list, g);
885 checkFreeListSanity();
889 // handy function for use in gdb, because Bdescr() is inlined.
890 extern bdescr *_bdescr( StgPtr p );