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, step *stp)
59 /* Find out where we're going, using the handy "to" pointer in
60 * the step of the source object. If it turns out we need to
61 * evacuate to an older generation, adjust it here (see comment
64 if (stp < gct->evac_step) {
65 if (gct->eager_promotion) {
68 gct->failed_to_evac = rtsTrue;
72 ws = &gct->steps[stp->abs_no];
73 // this compiles to a single mem access to stp->abs_no only
75 /* chain a new block onto the to-space for the destination step 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, step *stp, StgWord tag)
99 to = alloc_for_copy(size,stp);
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);
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, nat size_to_copy, step *stp)
174 #if defined(PARALLEL_GC)
176 info = xchg((StgPtr)&src->header.info, (W_)&stg_WHITEHOLE_info);
177 if (info == (W_)&stg_WHITEHOLE_info) {
183 if (IS_FORWARDING_PTR(info)) {
184 src->header.info = (const StgInfoTable *)info;
185 evacuate(p); // does the failed_to_evac stuff
189 info = (W_)src->header.info;
192 to = alloc_for_copy(size_to_reserve, stp);
193 *p = (StgClosure *)to;
197 for (i = 1; i < size_to_copy; i++) { // unroll for small i
201 #if defined(PARALLEL_GC)
204 src->header.info = (const StgInfoTable*)MK_FORWARDING_PTR(to);
207 // We store the size of the just evacuated object in the LDV word so that
208 // the profiler can guess the position of the next object later.
209 SET_EVACUAEE_FOR_LDV(from, size_to_reserve);
211 if (size_to_reserve - size_to_copy > 0)
212 LDV_FILL_SLOP(to + size_to_copy, (int)(size_to_reserve - size_to_copy));
219 /* Copy wrappers that don't tag the closure after copying */
220 STATIC_INLINE GNUC_ATTR_HOT void
221 copy(StgClosure **p, const StgInfoTable *info,
222 StgClosure *src, nat size, step *stp)
224 copy_tag(p,info,src,size,stp,0);
227 /* -----------------------------------------------------------------------------
228 Evacuate a large object
230 This just consists of removing the object from the (doubly-linked)
231 step->large_objects list, and linking it on to the (singly-linked)
232 step->new_large_objects list, from where it will be scavenged later.
234 Convention: bd->flags has BF_EVACUATED set for a large object
235 that has been evacuated, or unset otherwise.
236 -------------------------------------------------------------------------- */
239 evacuate_large(StgPtr p)
241 bdescr *bd = Bdescr(p);
246 ACQUIRE_SPIN_LOCK(&stp->sync_large_objects);
248 // already evacuated?
249 if (bd->flags & BF_EVACUATED) {
250 /* Don't forget to set the gct->failed_to_evac flag if we didn't get
251 * the desired destination (see comments in evacuate()).
253 if (stp < gct->evac_step) {
254 gct->failed_to_evac = rtsTrue;
255 TICK_GC_FAILED_PROMOTION();
257 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
261 // remove from large_object list
263 bd->u.back->link = bd->link;
264 } else { // first object in the list
265 stp->large_objects = bd->link;
268 bd->link->u.back = bd->u.back;
271 /* link it on to the evacuated large object list of the destination step
274 if (new_stp < gct->evac_step) {
275 if (gct->eager_promotion) {
276 new_stp = gct->evac_step;
278 gct->failed_to_evac = rtsTrue;
282 ws = &gct->steps[new_stp->abs_no];
284 bd->flags |= BF_EVACUATED;
286 bd->gen_no = new_stp->gen_no;
288 // If this is a block of pinned objects, we don't have to scan
289 // these objects, because they aren't allowed to contain any
290 // pointers. For these blocks, we skip the scavenge stage and put
291 // them straight on the scavenged_large_objects list.
292 if (bd->flags & BF_PINNED) {
293 ASSERT(get_itbl((StgClosure *)p)->type == ARR_WORDS);
294 if (new_stp != stp) { ACQUIRE_SPIN_LOCK(&new_stp->sync_large_objects); }
295 dbl_link_onto(bd, &new_stp->scavenged_large_objects);
296 new_stp->n_scavenged_large_blocks += bd->blocks;
297 if (new_stp != stp) { RELEASE_SPIN_LOCK(&new_stp->sync_large_objects); }
299 bd->link = ws->todo_large_objects;
300 ws->todo_large_objects = bd;
303 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
306 /* ----------------------------------------------------------------------------
309 This is called (eventually) for every live object in the system.
311 The caller to evacuate specifies a desired generation in the
312 gct->evac_step thread-local variable. The following conditions apply to
313 evacuating an object which resides in generation M when we're
314 collecting up to generation N
316 if M >= gct->evac_step
318 else evac to step->to
320 if M < gct->evac_step evac to gct->evac_step, step 0
322 if the object is already evacuated, then we check which generation
325 if M >= gct->evac_step do nothing
326 if M < gct->evac_step set gct->failed_to_evac flag to indicate that we
327 didn't manage to evacuate this object into gct->evac_step.
332 evacuate() is the single most important function performance-wise
333 in the GC. Various things have been tried to speed it up, but as
334 far as I can tell the code generated by gcc 3.2 with -O2 is about
335 as good as it's going to get. We pass the argument to evacuate()
336 in a register using the 'regparm' attribute (see the prototype for
337 evacuate() near the top of this file).
339 Changing evacuate() to take an (StgClosure **) rather than
340 returning the new pointer seems attractive, because we can avoid
341 writing back the pointer when it hasn't changed (eg. for a static
342 object, or an object in a generation > N). However, I tried it and
343 it doesn't help. One reason is that the (StgClosure **) pointer
344 gets spilled to the stack inside evacuate(), resulting in far more
345 extra reads/writes than we save.
346 ------------------------------------------------------------------------- */
348 REGPARM1 GNUC_ATTR_HOT void
349 evacuate(StgClosure **p)
354 const StgInfoTable *info;
360 /* The tag and the pointer are split, to be merged after evacing */
361 tag = GET_CLOSURE_TAG(q);
362 q = UNTAG_CLOSURE(q);
364 ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
366 if (!HEAP_ALLOCED_GC(q)) {
368 if (!major_gc) return;
371 switch (info->type) {
374 if (info->srt_bitmap != 0) {
375 if (*THUNK_STATIC_LINK((StgClosure *)q) == NULL) {
377 *THUNK_STATIC_LINK((StgClosure *)q) = gct->static_objects;
378 gct->static_objects = (StgClosure *)q;
381 link = (StgPtr)cas((StgPtr)THUNK_STATIC_LINK((StgClosure *)q),
383 (StgWord)gct->static_objects);
385 gct->static_objects = (StgClosure *)q;
393 if (info->srt_bitmap != 0 &&
394 *FUN_STATIC_LINK((StgClosure *)q) == NULL) {
396 *FUN_STATIC_LINK((StgClosure *)q) = gct->static_objects;
397 gct->static_objects = (StgClosure *)q;
400 link = (StgPtr)cas((StgPtr)FUN_STATIC_LINK((StgClosure *)q),
402 (StgWord)gct->static_objects);
404 gct->static_objects = (StgClosure *)q;
411 /* If q->saved_info != NULL, then it's a revertible CAF - it'll be
412 * on the CAF list, so don't do anything with it here (we'll
413 * scavenge it later).
415 if (((StgIndStatic *)q)->saved_info == NULL) {
416 if (*IND_STATIC_LINK((StgClosure *)q) == NULL) {
418 *IND_STATIC_LINK((StgClosure *)q) = gct->static_objects;
419 gct->static_objects = (StgClosure *)q;
422 link = (StgPtr)cas((StgPtr)IND_STATIC_LINK((StgClosure *)q),
424 (StgWord)gct->static_objects);
426 gct->static_objects = (StgClosure *)q;
434 if (*STATIC_LINK(info,(StgClosure *)q) == NULL) {
436 *STATIC_LINK(info,(StgClosure *)q) = gct->static_objects;
437 gct->static_objects = (StgClosure *)q;
440 link = (StgPtr)cas((StgPtr)STATIC_LINK(info,(StgClosure *)q),
442 (StgWord)gct->static_objects);
444 gct->static_objects = (StgClosure *)q;
448 /* I am assuming that static_objects pointers are not
449 * written to other objects, and thus, no need to retag. */
452 case CONSTR_NOCAF_STATIC:
453 /* no need to put these on the static linked list, they don't need
459 barf("evacuate(static): strange closure type %d", (int)(info->type));
465 if ((bd->flags & (BF_LARGE | BF_MARKED | BF_EVACUATED)) != 0) {
467 // pointer into to-space: just return it. It might be a pointer
468 // into a generation that we aren't collecting (> N), or it
469 // might just be a pointer into to-space. The latter doesn't
470 // happen often, but allowing it makes certain things a bit
471 // easier; e.g. scavenging an object is idempotent, so it's OK to
472 // have an object on the mutable list multiple times.
473 if (bd->flags & BF_EVACUATED) {
474 // We aren't copying this object, so we have to check
475 // whether it is already in the target generation. (this is
476 // the write barrier).
477 if (bd->step < gct->evac_step) {
478 gct->failed_to_evac = rtsTrue;
479 TICK_GC_FAILED_PROMOTION();
484 /* evacuate large objects by re-linking them onto a different list.
486 if (bd->flags & BF_LARGE) {
488 if (info->type == TSO &&
489 ((StgTSO *)q)->what_next == ThreadRelocated) {
490 q = (StgClosure *)((StgTSO *)q)->_link;
494 evacuate_large((P_)q);
498 /* If the object is in a step that we're compacting, then we
499 * need to use an alternative evacuate procedure.
501 if (!is_marked((P_)q,bd)) {
503 push_mark_stack((P_)q);
510 info = q->header.info;
511 if (IS_FORWARDING_PTR(info))
513 /* Already evacuated, just return the forwarding address.
514 * HOWEVER: if the requested destination generation (gct->evac_step) is
515 * older than the actual generation (because the object was
516 * already evacuated to a younger generation) then we have to
517 * set the gct->failed_to_evac flag to indicate that we couldn't
518 * manage to promote the object to the desired generation.
521 * Optimisation: the check is fairly expensive, but we can often
522 * shortcut it if either the required generation is 0, or the
523 * current object (the EVACUATED) is in a high enough generation.
524 * We know that an EVACUATED always points to an object in the
525 * same or an older generation. stp is the lowest step that the
526 * current object would be evacuated to, so we only do the full
527 * check if stp is too low.
529 StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
530 *p = TAG_CLOSURE(tag,e);
531 if (stp < gct->evac_step) { // optimisation
532 if (Bdescr((P_)e)->step < gct->evac_step) {
533 gct->failed_to_evac = rtsTrue;
534 TICK_GC_FAILED_PROMOTION();
540 switch (INFO_PTR_TO_STRUCT(info)->type) {
549 copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
554 StgWord w = (StgWord)q->payload[0];
555 if (info == Czh_con_info &&
556 // unsigned, so always true: (StgChar)w >= MIN_CHARLIKE &&
557 (StgChar)w <= MAX_CHARLIKE) {
558 *p = TAG_CLOSURE(tag,
559 (StgClosure *)CHARLIKE_CLOSURE((StgChar)w)
562 else if (info == Izh_con_info &&
563 (StgInt)w >= MIN_INTLIKE && (StgInt)w <= MAX_INTLIKE) {
564 *p = TAG_CLOSURE(tag,
565 (StgClosure *)INTLIKE_CLOSURE((StgInt)w)
569 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
577 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
582 copy(p,info,q,sizeofW(StgThunk)+1,stp);
588 #ifdef NO_PROMOTE_THUNKS
589 if (bd->gen_no == 0 &&
591 bd->step->no == generations[bd->gen_no].n_steps-1) {
595 copy(p,info,q,sizeofW(StgThunk)+2,stp);
603 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
607 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
611 copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
616 case IND_OLDGEN_PERM:
618 copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
623 copy_tag(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
627 copy(p,info,q,bco_sizeW((StgBCO *)q),stp);
632 copyPart(p,q,BLACKHOLE_sizeW(),sizeofW(StgHeader),stp);
636 eval_thunk_selector(p, (StgSelector *)q, rtsTrue);
641 // follow chains of indirections, don't evacuate them
642 q = ((StgInd*)q)->indirectee;
653 case CATCH_STM_FRAME:
654 case CATCH_RETRY_FRAME:
655 case ATOMICALLY_FRAME:
656 // shouldn't see these
657 barf("evacuate: stack frame at %p\n", q);
660 copy(p,info,q,pap_sizeW((StgPAP*)q),stp);
664 copy(p,info,q,ap_sizeW((StgAP*)q),stp);
668 copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),stp);
672 // just copy the block
673 copy(p,info,q,arr_words_sizeW((StgArrWords *)q),stp);
676 case MUT_ARR_PTRS_CLEAN:
677 case MUT_ARR_PTRS_DIRTY:
678 case MUT_ARR_PTRS_FROZEN:
679 case MUT_ARR_PTRS_FROZEN0:
680 // just copy the block
681 copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),stp);
686 StgTSO *tso = (StgTSO *)q;
688 /* Deal with redirected TSOs (a TSO that's had its stack enlarged).
690 if (tso->what_next == ThreadRelocated) {
691 q = (StgClosure *)tso->_link;
696 /* To evacuate a small TSO, we need to adjust the stack pointer
703 mine = copyPart(p,(StgClosure *)tso, tso_sizeW(tso),
704 sizeofW(StgTSO), stp);
706 new_tso = (StgTSO *)*p;
707 move_TSO(tso, new_tso);
708 for (r = tso->sp, s = new_tso->sp;
709 r < tso->stack+tso->stack_size;) {
718 copy(p,info,q,sizeofW(StgTRecHeader),stp);
721 case TVAR_WATCH_QUEUE:
722 copy(p,info,q,sizeofW(StgTVarWatchQueue),stp);
726 copy(p,info,q,sizeofW(StgTVar),stp);
730 copy(p,info,q,sizeofW(StgTRecChunk),stp);
733 case ATOMIC_INVARIANT:
734 copy(p,info,q,sizeofW(StgAtomicInvariant),stp);
737 case INVARIANT_CHECK_QUEUE:
738 copy(p,info,q,sizeofW(StgInvariantCheckQueue),stp);
742 barf("evacuate: strange closure type %d", (int)(INFO_PTR_TO_STRUCT(info)->type));
748 /* -----------------------------------------------------------------------------
749 Evaluate a THUNK_SELECTOR if possible.
751 p points to a THUNK_SELECTOR that we want to evaluate. The
752 result of "evaluating" it will be evacuated and a pointer to the
753 to-space closure will be returned.
755 If the THUNK_SELECTOR could not be evaluated (its selectee is still
756 a THUNK, for example), then the THUNK_SELECTOR itself will be
758 -------------------------------------------------------------------------- */
760 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
768 ASSERT(p->header.info == &stg_WHITEHOLE_info);
770 ASSERT(p->header.info == &stg_BLACKHOLE_info);
772 // val must be in to-space. Not always: when we recursively
773 // invoke eval_thunk_selector(), the recursive calls will not
774 // evacuate the value (because we want to select on the value,
775 // not evacuate it), so in this case val is in from-space.
776 // ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
778 prev = (StgSelector*)((StgClosure *)p)->payload[0];
780 // Update the THUNK_SELECTOR with an indirection to the
781 // value. The value is still in from-space at this stage.
783 // (old note: Why not do upd_evacuee(q,p)? Because we have an
784 // invariant that an EVACUATED closure always points to an
785 // object in the same or an older generation (required by
786 // the short-cut test in the EVACUATED case, below).
787 if ((StgClosure *)p == val) {
788 // must be a loop; just leave a BLACKHOLE in place. This
789 // can happen when we have a chain of selectors that
790 // eventually loops back on itself. We can't leave an
791 // indirection pointing to itself, and we want the program
792 // to deadlock if it ever enters this closure, so
793 // BLACKHOLE is correct.
794 SET_INFO(p, &stg_BLACKHOLE_info);
796 ((StgInd *)p)->indirectee = val;
798 SET_INFO(p, &stg_IND_info);
801 // For the purposes of LDV profiling, we have created an
803 LDV_RECORD_CREATE(p);
810 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
811 // NB. for legacy reasons, p & q are swapped around :(
816 StgClosure *selectee;
817 StgSelector *prev_thunk_selector;
821 prev_thunk_selector = NULL;
822 // this is a chain of THUNK_SELECTORs that we are going to update
823 // to point to the value of the current THUNK_SELECTOR. Each
824 // closure on the chain is a BLACKHOLE, and points to the next in the
825 // chain with payload[0].
829 bd = Bdescr((StgPtr)p);
830 if (HEAP_ALLOCED_GC(p)) {
831 // If the THUNK_SELECTOR is in to-space or in a generation that we
832 // are not collecting, then bale out early. We won't be able to
833 // save any space in any case, and updating with an indirection is
834 // trickier in a non-collected gen: we would have to update the
836 if (bd->flags & BF_EVACUATED) {
837 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
838 *q = (StgClosure *)p;
839 // shortcut, behave as for: if (evac) evacuate(q);
840 if (evac && bd->step < gct->evac_step) {
841 gct->failed_to_evac = rtsTrue;
842 TICK_GC_FAILED_PROMOTION();
846 // we don't update THUNK_SELECTORS in the compacted
847 // generation, because compaction does not remove the INDs
848 // that result, this causes confusion later
849 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
850 // bit is very tricky to get right. If you make changes
851 // around here, test by compiling stage 3 with +RTS -c -RTS.
852 if (bd->flags & BF_MARKED) {
853 // must call evacuate() to mark this closure if evac==rtsTrue
854 *q = (StgClosure *)p;
855 if (evac) evacuate(q);
856 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
862 // BLACKHOLE the selector thunk, since it is now under evaluation.
863 // This is important to stop us going into an infinite loop if
864 // this selector thunk eventually refers to itself.
865 #if defined(THREADED_RTS)
866 // In threaded mode, we'll use WHITEHOLE to lock the selector
867 // thunk while we evaluate it.
870 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
871 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
873 // make sure someone else didn't get here first...
874 if (IS_FORWARDING_PTR(p) ||
875 INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
876 // v. tricky now. The THUNK_SELECTOR has been evacuated
877 // by another thread, and is now either a forwarding ptr or IND.
878 // We need to extract ourselves from the current situation
879 // as cleanly as possible.
880 // - unlock the closure
881 // - update *q, we may have done *some* evaluation
882 // - if evac, we need to call evacuate(), because we
883 // need the write-barrier stuff.
884 // - undo the chain we've built to point to p.
885 SET_INFO(p, (const StgInfoTable *)info_ptr);
886 *q = (StgClosure *)p;
887 if (evac) evacuate(q);
888 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
893 // Save the real info pointer (NOTE: not the same as get_itbl()).
894 info_ptr = (StgWord)p->header.info;
895 SET_INFO(p,&stg_BLACKHOLE_info);
898 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
900 // The selectee might be a constructor closure,
901 // so we untag the pointer.
902 selectee = UNTAG_CLOSURE(p->selectee);
905 // selectee now points to the closure that we're trying to select
906 // a field from. It may or may not be in to-space: we try not to
907 // end up in to-space, but it's impractical to avoid it in
908 // general. The compacting GC scatters to-space pointers in
909 // from-space during marking, for example. We rely on the property
910 // that evacuate() doesn't mind if it gets passed a to-space pointer.
912 info = (StgInfoTable*)selectee->header.info;
914 if (IS_FORWARDING_PTR(info)) {
915 // We don't follow pointers into to-space; the constructor
916 // has already been evacuated, so we won't save any space
917 // leaks by evaluating this selector thunk anyhow.
921 info = INFO_PTR_TO_STRUCT(info);
922 switch (info->type) {
924 goto bale_out; // about to be evacuated by another thread (or a loop).
933 case CONSTR_NOCAF_STATIC:
935 // check that the size is in range
936 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
937 info->layout.payload.nptrs));
939 // Select the right field from the constructor
940 val = selectee->payload[field];
943 // For the purposes of LDV profiling, we have destroyed
944 // the original selector thunk, p.
945 SET_INFO(p, (StgInfoTable *)info_ptr);
946 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
947 #if defined(THREADED_RTS)
948 SET_INFO(p, &stg_WHITEHOLE_info);
950 SET_INFO(p, &stg_BLACKHOLE_info);
954 // the closure in val is now the "value" of the
955 // THUNK_SELECTOR in p. However, val may itself be a
956 // THUNK_SELECTOR, in which case we want to continue
957 // evaluating until we find the real value, and then
958 // update the whole chain to point to the value.
960 info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
961 if (!IS_FORWARDING_PTR(info_ptr))
963 info = INFO_PTR_TO_STRUCT(info_ptr);
964 switch (info->type) {
968 case IND_OLDGEN_PERM:
970 val = ((StgInd *)val)->indirectee;
973 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
974 prev_thunk_selector = p;
975 p = (StgSelector*)val;
981 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
982 prev_thunk_selector = p;
986 // update the other selectors in the chain *before*
987 // evacuating the value. This is necessary in the case
988 // where the value turns out to be one of the selectors
989 // in the chain (i.e. we have a loop), and evacuating it
990 // would corrupt the chain.
991 unchain_thunk_selectors(prev_thunk_selector, val);
993 // evacuate() cannot recurse through
994 // eval_thunk_selector(), because we know val is not
996 if (evac) evacuate(q);
1003 case IND_OLDGEN_PERM:
1005 // Again, we might need to untag a constructor.
1006 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
1009 case THUNK_SELECTOR:
1013 // recursively evaluate this selector. We don't want to
1014 // recurse indefinitely, so we impose a depth bound.
1015 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
1019 gct->thunk_selector_depth++;
1020 // rtsFalse says "don't evacuate the result". It will,
1021 // however, update any THUNK_SELECTORs that are evaluated
1023 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
1024 gct->thunk_selector_depth--;
1026 // did we actually manage to evaluate it?
1027 if (val == selectee) goto bale_out;
1029 // Of course this pointer might be tagged...
1030 selectee = UNTAG_CLOSURE(val);
1045 // not evaluated yet
1049 barf("eval_thunk_selector: strange selectee %d",
1054 // We didn't manage to evaluate this thunk; restore the old info
1055 // pointer. But don't forget: we still need to evacuate the thunk itself.
1056 SET_INFO(p, (const StgInfoTable *)info_ptr);
1057 // THREADED_RTS: we just unlocked the thunk, so another thread
1058 // might get in and update it. copy() will lock it again and
1059 // check whether it was updated in the meantime.
1060 *q = (StgClosure *)p;
1062 copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->step->to);
1064 unchain_thunk_selectors(prev_thunk_selector, *q);