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 * ---------------------------------------------------------------------------*/
23 #include "LdvProfile.h"
26 #if defined(PROF_SPIN) && defined(THREADED_RTS) && defined(PARALLEL_GC)
27 StgWord64 whitehole_spin = 0;
30 #if defined(THREADED_RTS) && !defined(PARALLEL_GC)
31 #define evacuate(p) evacuate1(p)
32 #define HEAP_ALLOCED_GC(p) HEAP_ALLOCED(p)
35 #if !defined(PARALLEL_GC)
36 #define copy_tag_nolock(p, info, src, size, stp, tag) \
37 copy_tag(p, info, src, size, stp, tag)
40 /* Used to avoid long recursion due to selector thunks
42 #define MAX_THUNK_SELECTOR_DEPTH 16
44 static void eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool);
45 STATIC_INLINE void evacuate_large(StgPtr p);
47 /* -----------------------------------------------------------------------------
48 Allocate some space in which to copy an object.
49 -------------------------------------------------------------------------- */
52 alloc_for_copy (nat size, step *stp)
57 /* Find out where we're going, using the handy "to" pointer in
58 * the step of the source object. If it turns out we need to
59 * evacuate to an older generation, adjust it here (see comment
62 if (stp < gct->evac_step) {
63 if (gct->eager_promotion) {
66 gct->failed_to_evac = rtsTrue;
70 ws = &gct->steps[stp->abs_no];
71 // this compiles to a single mem access to stp->abs_no only
73 /* chain a new block onto the to-space for the destination step if
77 ws->todo_free += size;
78 if (ws->todo_free > ws->todo_lim) {
79 to = todo_block_full(size, ws);
81 ASSERT(ws->todo_free >= ws->todo_bd->free && ws->todo_free <= ws->todo_lim);
86 /* -----------------------------------------------------------------------------
88 -------------------------------------------------------------------------- */
90 STATIC_INLINE GNUC_ATTR_HOT void
91 copy_tag(StgClosure **p, const StgInfoTable *info,
92 StgClosure *src, nat size, step *stp, StgWord tag)
97 to = alloc_for_copy(size,stp);
99 TICK_GC_WORDS_COPIED(size);
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, step *stp, StgWord tag)
141 to = alloc_for_copy(size,stp);
142 *p = TAG_CLOSURE(tag,(StgClosure*)to);
143 src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to);
145 TICK_GC_WORDS_COPIED(size);
149 for (i = 1; i < size; i++) { // unroll for small i
153 // if (to+size+2 < bd->start + BLOCK_SIZE_W) {
154 // __builtin_prefetch(to + size + 2, 1);
158 // We store the size of the just evacuated object in the LDV word so that
159 // the profiler can guess the position of the next object later.
160 SET_EVACUAEE_FOR_LDV(from, size);
165 /* Special version of copy() for when we only want to copy the info
166 * pointer of an object, but reserve some padding after it. This is
167 * used to optimise evacuation of BLACKHOLEs.
170 copyPart(StgClosure **p, StgClosure *src, nat size_to_reserve, nat size_to_copy, step *stp)
176 #if defined(PARALLEL_GC)
178 info = xchg((StgPtr)&src->header.info, (W_)&stg_WHITEHOLE_info);
179 if (info == (W_)&stg_WHITEHOLE_info) {
185 if (IS_FORWARDING_PTR(info)) {
186 src->header.info = (const StgInfoTable *)info;
187 evacuate(p); // does the failed_to_evac stuff
191 info = (W_)src->header.info;
194 to = alloc_for_copy(size_to_reserve, stp);
195 *p = (StgClosure *)to;
197 TICK_GC_WORDS_COPIED(size_to_copy);
201 for (i = 1; i < size_to_copy; i++) { // unroll for small i
205 #if defined(PARALLEL_GC)
208 src->header.info = (const StgInfoTable*)MK_FORWARDING_PTR(to);
211 // We store the size of the just evacuated object in the LDV word so that
212 // the profiler can guess the position of the next object later.
213 SET_EVACUAEE_FOR_LDV(from, size_to_reserve);
215 if (size_to_reserve - size_to_copy > 0)
216 LDV_FILL_SLOP(to + size_to_copy, (int)(size_to_reserve - size_to_copy));
223 /* Copy wrappers that don't tag the closure after copying */
224 STATIC_INLINE GNUC_ATTR_HOT void
225 copy(StgClosure **p, const StgInfoTable *info,
226 StgClosure *src, nat size, step *stp)
228 copy_tag(p,info,src,size,stp,0);
231 /* -----------------------------------------------------------------------------
232 Evacuate a large object
234 This just consists of removing the object from the (doubly-linked)
235 step->large_objects list, and linking it on to the (singly-linked)
236 step->new_large_objects list, from where it will be scavenged later.
238 Convention: bd->flags has BF_EVACUATED set for a large object
239 that has been evacuated, or unset otherwise.
240 -------------------------------------------------------------------------- */
243 evacuate_large(StgPtr p)
245 bdescr *bd = Bdescr(p);
250 ACQUIRE_SPIN_LOCK(&stp->sync_large_objects);
252 // already evacuated?
253 if (bd->flags & BF_EVACUATED) {
254 /* Don't forget to set the gct->failed_to_evac flag if we didn't get
255 * the desired destination (see comments in evacuate()).
257 if (stp < gct->evac_step) {
258 gct->failed_to_evac = rtsTrue;
259 TICK_GC_FAILED_PROMOTION();
261 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
265 // remove from large_object list
267 bd->u.back->link = bd->link;
268 } else { // first object in the list
269 stp->large_objects = bd->link;
272 bd->link->u.back = bd->u.back;
275 /* link it on to the evacuated large object list of the destination step
278 if (new_stp < gct->evac_step) {
279 if (gct->eager_promotion) {
280 new_stp = gct->evac_step;
282 gct->failed_to_evac = rtsTrue;
286 ws = &gct->steps[new_stp->abs_no];
288 bd->flags |= BF_EVACUATED;
290 bd->gen_no = new_stp->gen_no;
292 // If this is a block of pinned objects, we don't have to scan
293 // these objects, because they aren't allowed to contain any
294 // pointers. For these blocks, we skip the scavenge stage and put
295 // them straight on the scavenged_large_objects list.
296 if (bd->flags & BF_PINNED) {
297 ASSERT(get_itbl((StgClosure *)p)->type == ARR_WORDS);
298 if (new_stp != stp) { ACQUIRE_SPIN_LOCK(&new_stp->sync_large_objects); }
299 dbl_link_onto(bd, &new_stp->scavenged_large_objects);
300 new_stp->n_scavenged_large_blocks += bd->blocks;
301 if (new_stp != stp) { RELEASE_SPIN_LOCK(&new_stp->sync_large_objects); }
303 bd->link = ws->todo_large_objects;
304 ws->todo_large_objects = bd;
307 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
310 /* ----------------------------------------------------------------------------
313 This is called (eventually) for every live object in the system.
315 The caller to evacuate specifies a desired generation in the
316 gct->evac_step thread-local variable. The following conditions apply to
317 evacuating an object which resides in generation M when we're
318 collecting up to generation N
320 if M >= gct->evac_step
322 else evac to step->to
324 if M < gct->evac_step evac to gct->evac_step, step 0
326 if the object is already evacuated, then we check which generation
329 if M >= gct->evac_step do nothing
330 if M < gct->evac_step set gct->failed_to_evac flag to indicate that we
331 didn't manage to evacuate this object into gct->evac_step.
336 evacuate() is the single most important function performance-wise
337 in the GC. Various things have been tried to speed it up, but as
338 far as I can tell the code generated by gcc 3.2 with -O2 is about
339 as good as it's going to get. We pass the argument to evacuate()
340 in a register using the 'regparm' attribute (see the prototype for
341 evacuate() near the top of this file).
343 Changing evacuate() to take an (StgClosure **) rather than
344 returning the new pointer seems attractive, because we can avoid
345 writing back the pointer when it hasn't changed (eg. for a static
346 object, or an object in a generation > N). However, I tried it and
347 it doesn't help. One reason is that the (StgClosure **) pointer
348 gets spilled to the stack inside evacuate(), resulting in far more
349 extra reads/writes than we save.
350 ------------------------------------------------------------------------- */
352 REGPARM1 GNUC_ATTR_HOT void
353 evacuate(StgClosure **p)
358 const StgInfoTable *info;
364 /* The tag and the pointer are split, to be merged after evacing */
365 tag = GET_CLOSURE_TAG(q);
366 q = UNTAG_CLOSURE(q);
368 ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
370 if (!HEAP_ALLOCED_GC(q)) {
372 if (!major_gc) return;
375 switch (info->type) {
378 if (info->srt_bitmap != 0) {
379 if (*THUNK_STATIC_LINK((StgClosure *)q) == NULL) {
381 *THUNK_STATIC_LINK((StgClosure *)q) = gct->static_objects;
382 gct->static_objects = (StgClosure *)q;
385 link = (StgPtr)cas((StgPtr)THUNK_STATIC_LINK((StgClosure *)q),
387 (StgWord)gct->static_objects);
389 gct->static_objects = (StgClosure *)q;
397 if (info->srt_bitmap != 0 &&
398 *FUN_STATIC_LINK((StgClosure *)q) == NULL) {
400 *FUN_STATIC_LINK((StgClosure *)q) = gct->static_objects;
401 gct->static_objects = (StgClosure *)q;
404 link = (StgPtr)cas((StgPtr)FUN_STATIC_LINK((StgClosure *)q),
406 (StgWord)gct->static_objects);
408 gct->static_objects = (StgClosure *)q;
415 /* If q->saved_info != NULL, then it's a revertible CAF - it'll be
416 * on the CAF list, so don't do anything with it here (we'll
417 * scavenge it later).
419 if (((StgIndStatic *)q)->saved_info == NULL) {
420 if (*IND_STATIC_LINK((StgClosure *)q) == NULL) {
422 *IND_STATIC_LINK((StgClosure *)q) = gct->static_objects;
423 gct->static_objects = (StgClosure *)q;
426 link = (StgPtr)cas((StgPtr)IND_STATIC_LINK((StgClosure *)q),
428 (StgWord)gct->static_objects);
430 gct->static_objects = (StgClosure *)q;
438 if (*STATIC_LINK(info,(StgClosure *)q) == NULL) {
440 *STATIC_LINK(info,(StgClosure *)q) = gct->static_objects;
441 gct->static_objects = (StgClosure *)q;
444 link = (StgPtr)cas((StgPtr)STATIC_LINK(info,(StgClosure *)q),
446 (StgWord)gct->static_objects);
448 gct->static_objects = (StgClosure *)q;
452 /* I am assuming that static_objects pointers are not
453 * written to other objects, and thus, no need to retag. */
456 case CONSTR_NOCAF_STATIC:
457 /* no need to put these on the static linked list, they don't need
463 barf("evacuate(static): strange closure type %d", (int)(info->type));
469 if ((bd->flags & (BF_LARGE | BF_MARKED | BF_EVACUATED)) != 0) {
471 // pointer into to-space: just return it. It might be a pointer
472 // into a generation that we aren't collecting (> N), or it
473 // might just be a pointer into to-space. The latter doesn't
474 // happen often, but allowing it makes certain things a bit
475 // easier; e.g. scavenging an object is idempotent, so it's OK to
476 // have an object on the mutable list multiple times.
477 if (bd->flags & BF_EVACUATED) {
478 // We aren't copying this object, so we have to check
479 // whether it is already in the target generation. (this is
480 // the write barrier).
481 if (bd->step < gct->evac_step) {
482 gct->failed_to_evac = rtsTrue;
483 TICK_GC_FAILED_PROMOTION();
488 /* evacuate large objects by re-linking them onto a different list.
490 if (bd->flags & BF_LARGE) {
492 if (info->type == TSO &&
493 ((StgTSO *)q)->what_next == ThreadRelocated) {
494 q = (StgClosure *)((StgTSO *)q)->_link;
498 evacuate_large((P_)q);
502 /* If the object is in a step that we're compacting, then we
503 * need to use an alternative evacuate procedure.
505 if (!is_marked((P_)q,bd)) {
507 if (mark_stack_full()) {
508 debugTrace(DEBUG_gc,"mark stack overflowed");
509 mark_stack_overflowed = rtsTrue;
512 push_mark_stack((P_)q);
519 info = q->header.info;
520 if (IS_FORWARDING_PTR(info))
522 /* Already evacuated, just return the forwarding address.
523 * HOWEVER: if the requested destination generation (gct->evac_step) is
524 * older than the actual generation (because the object was
525 * already evacuated to a younger generation) then we have to
526 * set the gct->failed_to_evac flag to indicate that we couldn't
527 * manage to promote the object to the desired generation.
530 * Optimisation: the check is fairly expensive, but we can often
531 * shortcut it if either the required generation is 0, or the
532 * current object (the EVACUATED) is in a high enough generation.
533 * We know that an EVACUATED always points to an object in the
534 * same or an older generation. stp is the lowest step that the
535 * current object would be evacuated to, so we only do the full
536 * check if stp is too low.
538 StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
539 *p = TAG_CLOSURE(tag,e);
540 if (stp < gct->evac_step) { // optimisation
541 if (Bdescr((P_)e)->step < gct->evac_step) {
542 gct->failed_to_evac = rtsTrue;
543 TICK_GC_FAILED_PROMOTION();
549 switch (INFO_PTR_TO_STRUCT(info)->type) {
558 copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
563 StgWord w = (StgWord)q->payload[0];
564 if (info == Czh_con_info &&
565 // unsigned, so always true: (StgChar)w >= MIN_CHARLIKE &&
566 (StgChar)w <= MAX_CHARLIKE) {
567 *p = TAG_CLOSURE(tag,
568 (StgClosure *)CHARLIKE_CLOSURE((StgChar)w)
571 else if (info == Izh_con_info &&
572 (StgInt)w >= MIN_INTLIKE && (StgInt)w <= MAX_INTLIKE) {
573 *p = TAG_CLOSURE(tag,
574 (StgClosure *)INTLIKE_CLOSURE((StgInt)w)
578 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
586 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
591 copy(p,info,q,sizeofW(StgThunk)+1,stp);
597 #ifdef NO_PROMOTE_THUNKS
598 if (bd->gen_no == 0 &&
600 bd->step->no == generations[bd->gen_no].n_steps-1) {
604 copy(p,info,q,sizeofW(StgThunk)+2,stp);
612 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
616 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
620 copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
625 case IND_OLDGEN_PERM:
627 copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
632 copy_tag(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
636 copy(p,info,q,bco_sizeW((StgBCO *)q),stp);
641 copyPart(p,q,BLACKHOLE_sizeW(),sizeofW(StgHeader),stp);
645 eval_thunk_selector(p, (StgSelector *)q, rtsTrue);
650 // follow chains of indirections, don't evacuate them
651 q = ((StgInd*)q)->indirectee;
662 case CATCH_STM_FRAME:
663 case CATCH_RETRY_FRAME:
664 case ATOMICALLY_FRAME:
665 // shouldn't see these
666 barf("evacuate: stack frame at %p\n", q);
669 copy(p,info,q,pap_sizeW((StgPAP*)q),stp);
673 copy(p,info,q,ap_sizeW((StgAP*)q),stp);
677 copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),stp);
681 // just copy the block
682 copy(p,info,q,arr_words_sizeW((StgArrWords *)q),stp);
685 case MUT_ARR_PTRS_CLEAN:
686 case MUT_ARR_PTRS_DIRTY:
687 case MUT_ARR_PTRS_FROZEN:
688 case MUT_ARR_PTRS_FROZEN0:
689 // just copy the block
690 copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),stp);
695 StgTSO *tso = (StgTSO *)q;
697 /* Deal with redirected TSOs (a TSO that's had its stack enlarged).
699 if (tso->what_next == ThreadRelocated) {
700 q = (StgClosure *)tso->_link;
705 /* To evacuate a small TSO, we need to relocate the update frame
713 mine = copyPart(p,(StgClosure *)tso, tso_sizeW(tso),
714 sizeofW(StgTSO), stp);
716 new_tso = (StgTSO *)*p;
717 move_TSO(tso, new_tso);
718 for (r = tso->sp, s = new_tso->sp;
719 r < tso->stack+tso->stack_size;) {
728 copy(p,info,q,sizeofW(StgTRecHeader),stp);
731 case TVAR_WATCH_QUEUE:
732 copy(p,info,q,sizeofW(StgTVarWatchQueue),stp);
736 copy(p,info,q,sizeofW(StgTVar),stp);
740 copy(p,info,q,sizeofW(StgTRecChunk),stp);
743 case ATOMIC_INVARIANT:
744 copy(p,info,q,sizeofW(StgAtomicInvariant),stp);
747 case INVARIANT_CHECK_QUEUE:
748 copy(p,info,q,sizeofW(StgInvariantCheckQueue),stp);
752 barf("evacuate: strange closure type %d", (int)(INFO_PTR_TO_STRUCT(info)->type));
758 /* -----------------------------------------------------------------------------
759 Evaluate a THUNK_SELECTOR if possible.
761 p points to a THUNK_SELECTOR that we want to evaluate. The
762 result of "evaluating" it will be evacuated and a pointer to the
763 to-space closure will be returned.
765 If the THUNK_SELECTOR could not be evaluated (its selectee is still
766 a THUNK, for example), then the THUNK_SELECTOR itself will be
768 -------------------------------------------------------------------------- */
770 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
778 ASSERT(p->header.info == &stg_WHITEHOLE_info);
780 ASSERT(p->header.info == &stg_BLACKHOLE_info);
782 // val must be in to-space. Not always: when we recursively
783 // invoke eval_thunk_selector(), the recursive calls will not
784 // evacuate the value (because we want to select on the value,
785 // not evacuate it), so in this case val is in from-space.
786 // ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
788 prev = (StgSelector*)((StgClosure *)p)->payload[0];
790 // Update the THUNK_SELECTOR with an indirection to the
791 // value. The value is still in from-space at this stage.
793 // (old note: Why not do upd_evacuee(q,p)? Because we have an
794 // invariant that an EVACUATED closure always points to an
795 // object in the same or an older generation (required by
796 // the short-cut test in the EVACUATED case, below).
797 if ((StgClosure *)p == val) {
798 // must be a loop; just leave a BLACKHOLE in place. This
799 // can happen when we have a chain of selectors that
800 // eventually loops back on itself. We can't leave an
801 // indirection pointing to itself, and we want the program
802 // to deadlock if it ever enters this closure, so
803 // BLACKHOLE is correct.
804 SET_INFO(p, &stg_BLACKHOLE_info);
806 ((StgInd *)p)->indirectee = val;
808 SET_INFO(p, &stg_IND_info);
811 // For the purposes of LDV profiling, we have created an
813 LDV_RECORD_CREATE(p);
820 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
821 // NB. for legacy reasons, p & q are swapped around :(
826 StgClosure *selectee;
827 StgSelector *prev_thunk_selector;
831 prev_thunk_selector = NULL;
832 // this is a chain of THUNK_SELECTORs that we are going to update
833 // to point to the value of the current THUNK_SELECTOR. Each
834 // closure on the chain is a BLACKHOLE, and points to the next in the
835 // chain with payload[0].
839 bd = Bdescr((StgPtr)p);
840 if (HEAP_ALLOCED_GC(p)) {
841 // If the THUNK_SELECTOR is in to-space or in a generation that we
842 // are not collecting, then bale out early. We won't be able to
843 // save any space in any case, and updating with an indirection is
844 // trickier in a non-collected gen: we would have to update the
846 if (bd->flags & BF_EVACUATED) {
847 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
848 *q = (StgClosure *)p;
849 // shortcut, behave as for: if (evac) evacuate(q);
850 if (evac && bd->step < gct->evac_step) {
851 gct->failed_to_evac = rtsTrue;
852 TICK_GC_FAILED_PROMOTION();
856 // we don't update THUNK_SELECTORS in the compacted
857 // generation, because compaction does not remove the INDs
858 // that result, this causes confusion later
859 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
860 // bit is very tricky to get right. If you make changes
861 // around here, test by compiling stage 3 with +RTS -c -RTS.
862 if (bd->flags & BF_MARKED) {
863 // must call evacuate() to mark this closure if evac==rtsTrue
864 *q = (StgClosure *)p;
865 if (evac) evacuate(q);
866 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
872 // BLACKHOLE the selector thunk, since it is now under evaluation.
873 // This is important to stop us going into an infinite loop if
874 // this selector thunk eventually refers to itself.
875 #if defined(THREADED_RTS)
876 // In threaded mode, we'll use WHITEHOLE to lock the selector
877 // thunk while we evaluate it.
880 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
881 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
883 // make sure someone else didn't get here first...
884 if (IS_FORWARDING_PTR(p) ||
885 INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
886 // v. tricky now. The THUNK_SELECTOR has been evacuated
887 // by another thread, and is now either a forwarding ptr or IND.
888 // We need to extract ourselves from the current situation
889 // as cleanly as possible.
890 // - unlock the closure
891 // - update *q, we may have done *some* evaluation
892 // - if evac, we need to call evacuate(), because we
893 // need the write-barrier stuff.
894 // - undo the chain we've built to point to p.
895 SET_INFO(p, (const StgInfoTable *)info_ptr);
896 *q = (StgClosure *)p;
897 if (evac) evacuate(q);
898 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
903 // Save the real info pointer (NOTE: not the same as get_itbl()).
904 info_ptr = (StgWord)p->header.info;
905 SET_INFO(p,&stg_BLACKHOLE_info);
908 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
910 // The selectee might be a constructor closure,
911 // so we untag the pointer.
912 selectee = UNTAG_CLOSURE(p->selectee);
915 // selectee now points to the closure that we're trying to select
916 // a field from. It may or may not be in to-space: we try not to
917 // end up in to-space, but it's impractical to avoid it in
918 // general. The compacting GC scatters to-space pointers in
919 // from-space during marking, for example. We rely on the property
920 // that evacuate() doesn't mind if it gets passed a to-space pointer.
922 info = (StgInfoTable*)selectee->header.info;
924 if (IS_FORWARDING_PTR(info)) {
925 // We don't follow pointers into to-space; the constructor
926 // has already been evacuated, so we won't save any space
927 // leaks by evaluating this selector thunk anyhow.
931 info = INFO_PTR_TO_STRUCT(info);
932 switch (info->type) {
934 goto bale_out; // about to be evacuated by another thread (or a loop).
943 case CONSTR_NOCAF_STATIC:
945 // check that the size is in range
946 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
947 info->layout.payload.nptrs));
949 // Select the right field from the constructor
950 val = selectee->payload[field];
953 // For the purposes of LDV profiling, we have destroyed
954 // the original selector thunk, p.
955 SET_INFO(p, (StgInfoTable *)info_ptr);
956 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
957 #if defined(THREADED_RTS)
958 SET_INFO(p, &stg_WHITEHOLE_info);
960 SET_INFO(p, &stg_BLACKHOLE_info);
964 // the closure in val is now the "value" of the
965 // THUNK_SELECTOR in p. However, val may itself be a
966 // THUNK_SELECTOR, in which case we want to continue
967 // evaluating until we find the real value, and then
968 // update the whole chain to point to the value.
970 info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
971 if (!IS_FORWARDING_PTR(info_ptr))
973 info = INFO_PTR_TO_STRUCT(info_ptr);
974 switch (info->type) {
978 case IND_OLDGEN_PERM:
980 val = ((StgInd *)val)->indirectee;
983 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
984 prev_thunk_selector = p;
985 p = (StgSelector*)val;
991 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
992 prev_thunk_selector = p;
996 // update the other selectors in the chain *before*
997 // evacuating the value. This is necessary in the case
998 // where the value turns out to be one of the selectors
999 // in the chain (i.e. we have a loop), and evacuating it
1000 // would corrupt the chain.
1001 unchain_thunk_selectors(prev_thunk_selector, val);
1003 // evacuate() cannot recurse through
1004 // eval_thunk_selector(), because we know val is not
1005 // a THUNK_SELECTOR.
1006 if (evac) evacuate(q);
1013 case IND_OLDGEN_PERM:
1015 // Again, we might need to untag a constructor.
1016 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
1019 case THUNK_SELECTOR:
1023 // recursively evaluate this selector. We don't want to
1024 // recurse indefinitely, so we impose a depth bound.
1025 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
1029 gct->thunk_selector_depth++;
1030 // rtsFalse says "don't evacuate the result". It will,
1031 // however, update any THUNK_SELECTORs that are evaluated
1033 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
1034 gct->thunk_selector_depth--;
1036 // did we actually manage to evaluate it?
1037 if (val == selectee) goto bale_out;
1039 // Of course this pointer might be tagged...
1040 selectee = UNTAG_CLOSURE(val);
1055 // not evaluated yet
1059 barf("eval_thunk_selector: strange selectee %d",
1064 // We didn't manage to evaluate this thunk; restore the old info
1065 // pointer. But don't forget: we still need to evacuate the thunk itself.
1066 SET_INFO(p, (const StgInfoTable *)info_ptr);
1067 // THREADED_RTS: we just unlocked the thunk, so another thread
1068 // might get in and update it. copy() will lock it again and
1069 // check whether it was updated in the meantime.
1070 *q = (StgClosure *)p;
1072 copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->step->to);
1074 unchain_thunk_selectors(prev_thunk_selector, *q);