1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 1998-2008
5 * Generational garbage collector: evacuation functions
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
10 * http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
12 * ---------------------------------------------------------------------------*/
14 #include "PosixSource.h"
23 #include "MarkStack.h"
26 #include "LdvProfile.h"
28 #if defined(PROF_SPIN) && defined(THREADED_RTS) && defined(PARALLEL_GC)
29 StgWord64 whitehole_spin = 0;
32 #if defined(THREADED_RTS) && !defined(PARALLEL_GC)
33 #define evacuate(p) evacuate1(p)
34 #define HEAP_ALLOCED_GC(p) HEAP_ALLOCED(p)
37 #if !defined(PARALLEL_GC)
38 #define copy_tag_nolock(p, info, src, size, stp, tag) \
39 copy_tag(p, info, src, size, stp, tag)
42 /* Used to avoid long recursion due to selector thunks
44 #define MAX_THUNK_SELECTOR_DEPTH 16
46 static void eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool);
47 STATIC_INLINE void evacuate_large(StgPtr p);
49 /* -----------------------------------------------------------------------------
50 Allocate some space in which to copy an object.
51 -------------------------------------------------------------------------- */
54 alloc_for_copy (nat size, generation *gen)
59 /* Find out where we're going, using the handy "to" pointer in
60 * the gen of the source object. If it turns out we need to
61 * evacuate to an older generation, adjust it here (see comment
64 if (gen < gct->evac_gen) {
65 if (gct->eager_promotion) {
68 gct->failed_to_evac = rtsTrue;
72 ws = &gct->gens[gen->no];
73 // this compiles to a single mem access to gen->abs_no only
75 /* chain a new block onto the to-space for the destination gen if
79 ws->todo_free += size;
80 if (ws->todo_free > ws->todo_lim) {
81 to = todo_block_full(size, ws);
83 ASSERT(ws->todo_free >= ws->todo_bd->free && ws->todo_free <= ws->todo_lim);
88 /* -----------------------------------------------------------------------------
90 -------------------------------------------------------------------------- */
92 STATIC_INLINE GNUC_ATTR_HOT void
93 copy_tag(StgClosure **p, const StgInfoTable *info,
94 StgClosure *src, nat size, generation *gen, StgWord tag)
99 to = alloc_for_copy(size,gen);
103 for (i = 1; i < size; i++) { // unroll for small i
107 // if (to+size+2 < bd->start + BLOCK_SIZE_W) {
108 // __builtin_prefetch(to + size + 2, 1);
111 #if defined(PARALLEL_GC)
113 const StgInfoTable *new_info;
114 new_info = (const StgInfoTable *)cas((StgPtr)&src->header.info, (W_)info, MK_FORWARDING_PTR(to));
115 if (new_info != info) {
116 return evacuate(p); // does the failed_to_evac stuff
118 *p = TAG_CLOSURE(tag,(StgClosure*)to);
122 src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to);
123 *p = TAG_CLOSURE(tag,(StgClosure*)to);
127 // We store the size of the just evacuated object in the LDV word so that
128 // the profiler can guess the position of the next object later.
129 SET_EVACUAEE_FOR_LDV(from, size);
133 #if defined(PARALLEL_GC)
135 copy_tag_nolock(StgClosure **p, const StgInfoTable *info,
136 StgClosure *src, nat size, generation *gen, StgWord tag)
141 to = alloc_for_copy(size,gen);
142 *p = TAG_CLOSURE(tag,(StgClosure*)to);
143 src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to);
147 for (i = 1; i < size; i++) { // unroll for small i
151 // if (to+size+2 < bd->start + BLOCK_SIZE_W) {
152 // __builtin_prefetch(to + size + 2, 1);
156 // We store the size of the just evacuated object in the LDV word so that
157 // the profiler can guess the position of the next object later.
158 SET_EVACUAEE_FOR_LDV(from, size);
163 /* Special version of copy() for when we only want to copy the info
164 * pointer of an object, but reserve some padding after it. This is
165 * used to optimise evacuation of BLACKHOLEs.
168 copyPart(StgClosure **p, StgClosure *src, nat size_to_reserve,
169 nat size_to_copy, generation *gen)
175 #if defined(PARALLEL_GC)
177 info = xchg((StgPtr)&src->header.info, (W_)&stg_WHITEHOLE_info);
178 if (info == (W_)&stg_WHITEHOLE_info) {
184 if (IS_FORWARDING_PTR(info)) {
185 src->header.info = (const StgInfoTable *)info;
186 evacuate(p); // does the failed_to_evac stuff
190 info = (W_)src->header.info;
193 to = alloc_for_copy(size_to_reserve, gen);
194 *p = (StgClosure *)to;
198 for (i = 1; i < size_to_copy; i++) { // unroll for small i
202 #if defined(PARALLEL_GC)
205 src->header.info = (const StgInfoTable*)MK_FORWARDING_PTR(to);
208 // We store the size of the just evacuated object in the LDV word so that
209 // the profiler can guess the position of the next object later.
210 SET_EVACUAEE_FOR_LDV(from, size_to_reserve);
212 if (size_to_reserve - size_to_copy > 0)
213 LDV_FILL_SLOP(to + size_to_copy, (int)(size_to_reserve - size_to_copy));
220 /* Copy wrappers that don't tag the closure after copying */
221 STATIC_INLINE GNUC_ATTR_HOT void
222 copy(StgClosure **p, const StgInfoTable *info,
223 StgClosure *src, nat size, generation *gen)
225 copy_tag(p,info,src,size,gen,0);
228 /* -----------------------------------------------------------------------------
229 Evacuate a large object
231 This just consists of removing the object from the (doubly-linked)
232 gen->large_objects list, and linking it on to the (singly-linked)
233 gen->new_large_objects list, from where it will be scavenged later.
235 Convention: bd->flags has BF_EVACUATED set for a large object
236 that has been evacuated, or unset otherwise.
237 -------------------------------------------------------------------------- */
240 evacuate_large(StgPtr p)
243 generation *gen, *new_gen;
248 ACQUIRE_SPIN_LOCK(&gen->sync_large_objects);
250 // already evacuated?
251 if (bd->flags & BF_EVACUATED) {
252 /* Don't forget to set the gct->failed_to_evac flag if we didn't get
253 * the desired destination (see comments in evacuate()).
255 if (gen < gct->evac_gen) {
256 gct->failed_to_evac = rtsTrue;
257 TICK_GC_FAILED_PROMOTION();
259 RELEASE_SPIN_LOCK(&gen->sync_large_objects);
263 // remove from large_object list
265 bd->u.back->link = bd->link;
266 } else { // first object in the list
267 gen->large_objects = bd->link;
270 bd->link->u.back = bd->u.back;
273 /* link it on to the evacuated large object list of the destination gen
276 if (new_gen < gct->evac_gen) {
277 if (gct->eager_promotion) {
278 new_gen = gct->evac_gen;
280 gct->failed_to_evac = rtsTrue;
284 ws = &gct->gens[new_gen->no];
286 bd->flags |= BF_EVACUATED;
287 initBdescr(bd, new_gen, new_gen->to);
289 // If this is a block of pinned objects, we don't have to scan
290 // these objects, because they aren't allowed to contain any
291 // pointers. For these blocks, we skip the scavenge stage and put
292 // them straight on the scavenged_large_objects list.
293 if (bd->flags & BF_PINNED) {
294 ASSERT(get_itbl((StgClosure *)p)->type == ARR_WORDS);
295 if (new_gen != gen) { ACQUIRE_SPIN_LOCK(&new_gen->sync_large_objects); }
296 dbl_link_onto(bd, &new_gen->scavenged_large_objects);
297 new_gen->n_scavenged_large_blocks += bd->blocks;
298 if (new_gen != gen) { RELEASE_SPIN_LOCK(&new_gen->sync_large_objects); }
300 bd->link = ws->todo_large_objects;
301 ws->todo_large_objects = bd;
304 RELEASE_SPIN_LOCK(&gen->sync_large_objects);
307 /* ----------------------------------------------------------------------------
310 This is called (eventually) for every live object in the system.
312 The caller to evacuate specifies a desired generation in the
313 gct->evac_gen thread-local variable. The following conditions apply to
314 evacuating an object which resides in generation M when we're
315 collecting up to generation N
317 if M >= gct->evac_gen
321 if M < gct->evac_gen evac to gct->evac_gen, step 0
323 if the object is already evacuated, then we check which generation
326 if M >= gct->evac_gen do nothing
327 if M < gct->evac_gen set gct->failed_to_evac flag to indicate that we
328 didn't manage to evacuate this object into gct->evac_gen.
333 evacuate() is the single most important function performance-wise
334 in the GC. Various things have been tried to speed it up, but as
335 far as I can tell the code generated by gcc 3.2 with -O2 is about
336 as good as it's going to get. We pass the argument to evacuate()
337 in a register using the 'regparm' attribute (see the prototype for
338 evacuate() near the top of this file).
340 Changing evacuate() to take an (StgClosure **) rather than
341 returning the new pointer seems attractive, because we can avoid
342 writing back the pointer when it hasn't changed (eg. for a static
343 object, or an object in a generation > N). However, I tried it and
344 it doesn't help. One reason is that the (StgClosure **) pointer
345 gets spilled to the stack inside evacuate(), resulting in far more
346 extra reads/writes than we save.
347 ------------------------------------------------------------------------- */
349 REGPARM1 GNUC_ATTR_HOT void
350 evacuate(StgClosure **p)
355 const StgInfoTable *info;
361 /* The tag and the pointer are split, to be merged after evacing */
362 tag = GET_CLOSURE_TAG(q);
363 q = UNTAG_CLOSURE(q);
365 ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
367 if (!HEAP_ALLOCED_GC(q)) {
369 if (!major_gc) return;
372 switch (info->type) {
375 if (info->srt_bitmap != 0) {
376 if (*THUNK_STATIC_LINK((StgClosure *)q) == NULL) {
378 *THUNK_STATIC_LINK((StgClosure *)q) = gct->static_objects;
379 gct->static_objects = (StgClosure *)q;
382 link = (StgPtr)cas((StgPtr)THUNK_STATIC_LINK((StgClosure *)q),
384 (StgWord)gct->static_objects);
386 gct->static_objects = (StgClosure *)q;
394 if (info->srt_bitmap != 0 &&
395 *FUN_STATIC_LINK((StgClosure *)q) == NULL) {
397 *FUN_STATIC_LINK((StgClosure *)q) = gct->static_objects;
398 gct->static_objects = (StgClosure *)q;
401 link = (StgPtr)cas((StgPtr)FUN_STATIC_LINK((StgClosure *)q),
403 (StgWord)gct->static_objects);
405 gct->static_objects = (StgClosure *)q;
412 /* If q->saved_info != NULL, then it's a revertible CAF - it'll be
413 * on the CAF list, so don't do anything with it here (we'll
414 * scavenge it later).
416 if (((StgIndStatic *)q)->saved_info == NULL) {
417 if (*IND_STATIC_LINK((StgClosure *)q) == NULL) {
419 *IND_STATIC_LINK((StgClosure *)q) = gct->static_objects;
420 gct->static_objects = (StgClosure *)q;
423 link = (StgPtr)cas((StgPtr)IND_STATIC_LINK((StgClosure *)q),
425 (StgWord)gct->static_objects);
427 gct->static_objects = (StgClosure *)q;
435 if (*STATIC_LINK(info,(StgClosure *)q) == NULL) {
437 *STATIC_LINK(info,(StgClosure *)q) = gct->static_objects;
438 gct->static_objects = (StgClosure *)q;
441 link = (StgPtr)cas((StgPtr)STATIC_LINK(info,(StgClosure *)q),
443 (StgWord)gct->static_objects);
445 gct->static_objects = (StgClosure *)q;
449 /* I am assuming that static_objects pointers are not
450 * written to other objects, and thus, no need to retag. */
453 case CONSTR_NOCAF_STATIC:
454 /* no need to put these on the static linked list, they don't need
460 barf("evacuate(static): strange closure type %d", (int)(info->type));
466 if ((bd->flags & (BF_LARGE | BF_MARKED | BF_EVACUATED)) != 0) {
468 // pointer into to-space: just return it. It might be a pointer
469 // into a generation that we aren't collecting (> N), or it
470 // might just be a pointer into to-space. The latter doesn't
471 // happen often, but allowing it makes certain things a bit
472 // easier; e.g. scavenging an object is idempotent, so it's OK to
473 // have an object on the mutable list multiple times.
474 if (bd->flags & BF_EVACUATED) {
475 // We aren't copying this object, so we have to check
476 // whether it is already in the target generation. (this is
477 // the write barrier).
478 if (bd->gen < gct->evac_gen) {
479 gct->failed_to_evac = rtsTrue;
480 TICK_GC_FAILED_PROMOTION();
485 /* evacuate large objects by re-linking them onto a different list.
487 if (bd->flags & BF_LARGE) {
489 if (info->type == TSO &&
490 ((StgTSO *)q)->what_next == ThreadRelocated) {
491 q = (StgClosure *)((StgTSO *)q)->_link;
495 evacuate_large((P_)q);
499 /* If the object is in a gen that we're compacting, then we
500 * need to use an alternative evacuate procedure.
502 if (!is_marked((P_)q,bd)) {
504 push_mark_stack((P_)q);
511 info = q->header.info;
512 if (IS_FORWARDING_PTR(info))
514 /* Already evacuated, just return the forwarding address.
515 * HOWEVER: if the requested destination generation (gct->evac_gen) is
516 * older than the actual generation (because the object was
517 * already evacuated to a younger generation) then we have to
518 * set the gct->failed_to_evac flag to indicate that we couldn't
519 * manage to promote the object to the desired generation.
522 * Optimisation: the check is fairly expensive, but we can often
523 * shortcut it if either the required generation is 0, or the
524 * current object (the EVACUATED) is in a high enough generation.
525 * We know that an EVACUATED always points to an object in the
526 * same or an older generation. gen is the lowest generation that the
527 * current object would be evacuated to, so we only do the full
528 * check if gen is too low.
530 StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
531 *p = TAG_CLOSURE(tag,e);
532 if (gen < gct->evac_gen) { // optimisation
533 if (Bdescr((P_)e)->gen < gct->evac_gen) {
534 gct->failed_to_evac = rtsTrue;
535 TICK_GC_FAILED_PROMOTION();
541 switch (INFO_PTR_TO_STRUCT(info)->type) {
550 copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
553 // For ints and chars of low value, save space by replacing references to
554 // these with closures with references to common, shared ones in the RTS.
556 // * Except when compiling into Windows DLLs which don't support cross-package
557 // data references very well.
561 #if defined(__PIC__) && defined(mingw32_HOST_OS)
562 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
564 StgWord w = (StgWord)q->payload[0];
565 if (info == Czh_con_info &&
566 // unsigned, so always true: (StgChar)w >= MIN_CHARLIKE &&
567 (StgChar)w <= MAX_CHARLIKE) {
568 *p = TAG_CLOSURE(tag,
569 (StgClosure *)CHARLIKE_CLOSURE((StgChar)w)
572 else if (info == Izh_con_info &&
573 (StgInt)w >= MIN_INTLIKE && (StgInt)w <= MAX_INTLIKE) {
574 *p = TAG_CLOSURE(tag,
575 (StgClosure *)INTLIKE_CLOSURE((StgInt)w)
579 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
588 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
593 copy(p,info,q,sizeofW(StgThunk)+1,gen);
599 #ifdef NO_PROMOTE_THUNKS
602 copy(p,info,q,sizeofW(StgThunk)+2,gen);
610 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
614 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
618 copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
623 case IND_OLDGEN_PERM:
625 copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen,tag);
630 copy_tag(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen,tag);
634 copy(p,info,q,bco_sizeW((StgBCO *)q),gen);
639 copyPart(p,q,BLACKHOLE_sizeW(),sizeofW(StgHeader),gen);
643 eval_thunk_selector(p, (StgSelector *)q, rtsTrue);
648 // follow chains of indirections, don't evacuate them
649 q = ((StgInd*)q)->indirectee;
660 case CATCH_STM_FRAME:
661 case CATCH_RETRY_FRAME:
662 case ATOMICALLY_FRAME:
663 // shouldn't see these
664 barf("evacuate: stack frame at %p\n", q);
667 copy(p,info,q,pap_sizeW((StgPAP*)q),gen);
671 copy(p,info,q,ap_sizeW((StgAP*)q),gen);
675 copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),gen);
679 // just copy the block
680 copy(p,info,q,arr_words_sizeW((StgArrWords *)q),gen);
683 case MUT_ARR_PTRS_CLEAN:
684 case MUT_ARR_PTRS_DIRTY:
685 case MUT_ARR_PTRS_FROZEN:
686 case MUT_ARR_PTRS_FROZEN0:
687 // just copy the block
688 copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),gen);
693 StgTSO *tso = (StgTSO *)q;
695 /* Deal with redirected TSOs (a TSO that's had its stack enlarged).
697 if (tso->what_next == ThreadRelocated) {
698 q = (StgClosure *)tso->_link;
703 /* To evacuate a small TSO, we need to adjust the stack pointer
710 mine = copyPart(p,(StgClosure *)tso, tso_sizeW(tso),
711 sizeofW(StgTSO), gen);
713 new_tso = (StgTSO *)*p;
714 move_TSO(tso, new_tso);
715 for (r = tso->sp, s = new_tso->sp;
716 r < tso->stack+tso->stack_size;) {
725 copy(p,info,q,sizeofW(StgTRecHeader),gen);
728 case TVAR_WATCH_QUEUE:
729 copy(p,info,q,sizeofW(StgTVarWatchQueue),gen);
733 copy(p,info,q,sizeofW(StgTVar),gen);
737 copy(p,info,q,sizeofW(StgTRecChunk),gen);
740 case ATOMIC_INVARIANT:
741 copy(p,info,q,sizeofW(StgAtomicInvariant),gen);
744 case INVARIANT_CHECK_QUEUE:
745 copy(p,info,q,sizeofW(StgInvariantCheckQueue),gen);
749 barf("evacuate: strange closure type %d", (int)(INFO_PTR_TO_STRUCT(info)->type));
755 /* -----------------------------------------------------------------------------
756 Evaluate a THUNK_SELECTOR if possible.
758 p points to a THUNK_SELECTOR that we want to evaluate. The
759 result of "evaluating" it will be evacuated and a pointer to the
760 to-space closure will be returned.
762 If the THUNK_SELECTOR could not be evaluated (its selectee is still
763 a THUNK, for example), then the THUNK_SELECTOR itself will be
765 -------------------------------------------------------------------------- */
767 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
775 ASSERT(p->header.info == &stg_WHITEHOLE_info);
777 ASSERT(p->header.info == &stg_BLACKHOLE_info);
779 // val must be in to-space. Not always: when we recursively
780 // invoke eval_thunk_selector(), the recursive calls will not
781 // evacuate the value (because we want to select on the value,
782 // not evacuate it), so in this case val is in from-space.
783 // ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
785 prev = (StgSelector*)((StgClosure *)p)->payload[0];
787 // Update the THUNK_SELECTOR with an indirection to the
788 // value. The value is still in from-space at this stage.
790 // (old note: Why not do upd_evacuee(q,p)? Because we have an
791 // invariant that an EVACUATED closure always points to an
792 // object in the same or an older generation (required by
793 // the short-cut test in the EVACUATED case, below).
794 if ((StgClosure *)p == val) {
795 // must be a loop; just leave a BLACKHOLE in place. This
796 // can happen when we have a chain of selectors that
797 // eventually loops back on itself. We can't leave an
798 // indirection pointing to itself, and we want the program
799 // to deadlock if it ever enters this closure, so
800 // BLACKHOLE is correct.
801 SET_INFO(p, &stg_BLACKHOLE_info);
803 ((StgInd *)p)->indirectee = val;
805 SET_INFO(p, &stg_IND_info);
808 // For the purposes of LDV profiling, we have created an
810 LDV_RECORD_CREATE(p);
817 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
818 // NB. for legacy reasons, p & q are swapped around :(
823 StgClosure *selectee;
824 StgSelector *prev_thunk_selector;
828 prev_thunk_selector = NULL;
829 // this is a chain of THUNK_SELECTORs that we are going to update
830 // to point to the value of the current THUNK_SELECTOR. Each
831 // closure on the chain is a BLACKHOLE, and points to the next in the
832 // chain with payload[0].
836 bd = Bdescr((StgPtr)p);
837 if (HEAP_ALLOCED_GC(p)) {
838 // If the THUNK_SELECTOR is in to-space or in a generation that we
839 // are not collecting, then bale out early. We won't be able to
840 // save any space in any case, and updating with an indirection is
841 // trickier in a non-collected gen: we would have to update the
843 if (bd->flags & BF_EVACUATED) {
844 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
845 *q = (StgClosure *)p;
846 // shortcut, behave as for: if (evac) evacuate(q);
847 if (evac && bd->gen < gct->evac_gen) {
848 gct->failed_to_evac = rtsTrue;
849 TICK_GC_FAILED_PROMOTION();
853 // we don't update THUNK_SELECTORS in the compacted
854 // generation, because compaction does not remove the INDs
855 // that result, this causes confusion later
856 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
857 // bit is very tricky to get right. If you make changes
858 // around here, test by compiling stage 3 with +RTS -c -RTS.
859 if (bd->flags & BF_MARKED) {
860 // must call evacuate() to mark this closure if evac==rtsTrue
861 *q = (StgClosure *)p;
862 if (evac) evacuate(q);
863 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
869 // BLACKHOLE the selector thunk, since it is now under evaluation.
870 // This is important to stop us going into an infinite loop if
871 // this selector thunk eventually refers to itself.
872 #if defined(THREADED_RTS)
873 // In threaded mode, we'll use WHITEHOLE to lock the selector
874 // thunk while we evaluate it.
877 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
878 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
880 // make sure someone else didn't get here first...
881 if (IS_FORWARDING_PTR(p) ||
882 INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
883 // v. tricky now. The THUNK_SELECTOR has been evacuated
884 // by another thread, and is now either a forwarding ptr or IND.
885 // We need to extract ourselves from the current situation
886 // as cleanly as possible.
887 // - unlock the closure
888 // - update *q, we may have done *some* evaluation
889 // - if evac, we need to call evacuate(), because we
890 // need the write-barrier stuff.
891 // - undo the chain we've built to point to p.
892 SET_INFO(p, (const StgInfoTable *)info_ptr);
893 *q = (StgClosure *)p;
894 if (evac) evacuate(q);
895 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
900 // Save the real info pointer (NOTE: not the same as get_itbl()).
901 info_ptr = (StgWord)p->header.info;
902 SET_INFO(p,&stg_BLACKHOLE_info);
905 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
907 // The selectee might be a constructor closure,
908 // so we untag the pointer.
909 selectee = UNTAG_CLOSURE(p->selectee);
912 // selectee now points to the closure that we're trying to select
913 // a field from. It may or may not be in to-space: we try not to
914 // end up in to-space, but it's impractical to avoid it in
915 // general. The compacting GC scatters to-space pointers in
916 // from-space during marking, for example. We rely on the property
917 // that evacuate() doesn't mind if it gets passed a to-space pointer.
919 info = (StgInfoTable*)selectee->header.info;
921 if (IS_FORWARDING_PTR(info)) {
922 // We don't follow pointers into to-space; the constructor
923 // has already been evacuated, so we won't save any space
924 // leaks by evaluating this selector thunk anyhow.
928 info = INFO_PTR_TO_STRUCT(info);
929 switch (info->type) {
931 goto bale_out; // about to be evacuated by another thread (or a loop).
940 case CONSTR_NOCAF_STATIC:
942 // check that the size is in range
943 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
944 info->layout.payload.nptrs));
946 // Select the right field from the constructor
947 val = selectee->payload[field];
950 // For the purposes of LDV profiling, we have destroyed
951 // the original selector thunk, p.
952 SET_INFO(p, (StgInfoTable *)info_ptr);
953 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
954 #if defined(THREADED_RTS)
955 SET_INFO(p, &stg_WHITEHOLE_info);
957 SET_INFO(p, &stg_BLACKHOLE_info);
961 // the closure in val is now the "value" of the
962 // THUNK_SELECTOR in p. However, val may itself be a
963 // THUNK_SELECTOR, in which case we want to continue
964 // evaluating until we find the real value, and then
965 // update the whole chain to point to the value.
967 info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
968 if (!IS_FORWARDING_PTR(info_ptr))
970 info = INFO_PTR_TO_STRUCT(info_ptr);
971 switch (info->type) {
975 case IND_OLDGEN_PERM:
977 val = ((StgInd *)val)->indirectee;
980 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
981 prev_thunk_selector = p;
982 p = (StgSelector*)val;
988 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
989 prev_thunk_selector = p;
993 // update the other selectors in the chain *before*
994 // evacuating the value. This is necessary in the case
995 // where the value turns out to be one of the selectors
996 // in the chain (i.e. we have a loop), and evacuating it
997 // would corrupt the chain.
998 unchain_thunk_selectors(prev_thunk_selector, val);
1000 // evacuate() cannot recurse through
1001 // eval_thunk_selector(), because we know val is not
1002 // a THUNK_SELECTOR.
1003 if (evac) evacuate(q);
1010 case IND_OLDGEN_PERM:
1012 // Again, we might need to untag a constructor.
1013 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
1016 case THUNK_SELECTOR:
1020 // recursively evaluate this selector. We don't want to
1021 // recurse indefinitely, so we impose a depth bound.
1022 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
1026 gct->thunk_selector_depth++;
1027 // rtsFalse says "don't evacuate the result". It will,
1028 // however, update any THUNK_SELECTORs that are evaluated
1030 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
1031 gct->thunk_selector_depth--;
1033 // did we actually manage to evaluate it?
1034 if (val == selectee) goto bale_out;
1036 // Of course this pointer might be tagged...
1037 selectee = UNTAG_CLOSURE(val);
1052 // not evaluated yet
1056 barf("eval_thunk_selector: strange selectee %d",
1061 // We didn't manage to evaluate this thunk; restore the old info
1062 // pointer. But don't forget: we still need to evacuate the thunk itself.
1063 SET_INFO(p, (const StgInfoTable *)info_ptr);
1064 // THREADED_RTS: we just unlocked the thunk, so another thread
1065 // might get in and update it. copy() will lock it again and
1066 // check whether it was updated in the meantime.
1067 *q = (StgClosure *)p;
1069 copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->dest);
1071 unchain_thunk_selectors(prev_thunk_selector, *q);