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 if (to + size > ws->todo_lim) {
78 to = todo_block_full(size, ws);
80 ws->todo_free = to + size;
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 dbl_link_onto(bd, &ws->step->scavenged_large_objects);
299 ws->step->n_scavenged_large_blocks += bd->blocks;
301 bd->link = ws->todo_large_objects;
302 ws->todo_large_objects = bd;
305 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
308 /* ----------------------------------------------------------------------------
311 This is called (eventually) for every live object in the system.
313 The caller to evacuate specifies a desired generation in the
314 gct->evac_step thread-local variable. The following conditions apply to
315 evacuating an object which resides in generation M when we're
316 collecting up to generation N
318 if M >= gct->evac_step
320 else evac to step->to
322 if M < gct->evac_step evac to gct->evac_step, step 0
324 if the object is already evacuated, then we check which generation
327 if M >= gct->evac_step do nothing
328 if M < gct->evac_step set gct->failed_to_evac flag to indicate that we
329 didn't manage to evacuate this object into gct->evac_step.
334 evacuate() is the single most important function performance-wise
335 in the GC. Various things have been tried to speed it up, but as
336 far as I can tell the code generated by gcc 3.2 with -O2 is about
337 as good as it's going to get. We pass the argument to evacuate()
338 in a register using the 'regparm' attribute (see the prototype for
339 evacuate() near the top of this file).
341 Changing evacuate() to take an (StgClosure **) rather than
342 returning the new pointer seems attractive, because we can avoid
343 writing back the pointer when it hasn't changed (eg. for a static
344 object, or an object in a generation > N). However, I tried it and
345 it doesn't help. One reason is that the (StgClosure **) pointer
346 gets spilled to the stack inside evacuate(), resulting in far more
347 extra reads/writes than we save.
348 ------------------------------------------------------------------------- */
350 REGPARM1 GNUC_ATTR_HOT void
351 evacuate(StgClosure **p)
356 const StgInfoTable *info;
362 /* The tag and the pointer are split, to be merged after evacing */
363 tag = GET_CLOSURE_TAG(q);
364 q = UNTAG_CLOSURE(q);
366 ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
368 if (!HEAP_ALLOCED_GC(q)) {
370 if (!major_gc) return;
373 switch (info->type) {
376 if (info->srt_bitmap != 0) {
377 if (*THUNK_STATIC_LINK((StgClosure *)q) == NULL) {
379 *THUNK_STATIC_LINK((StgClosure *)q) = gct->static_objects;
380 gct->static_objects = (StgClosure *)q;
383 link = (StgPtr)cas((StgPtr)THUNK_STATIC_LINK((StgClosure *)q),
385 (StgWord)gct->static_objects);
387 gct->static_objects = (StgClosure *)q;
395 if (info->srt_bitmap != 0 &&
396 *FUN_STATIC_LINK((StgClosure *)q) == NULL) {
398 *FUN_STATIC_LINK((StgClosure *)q) = gct->static_objects;
399 gct->static_objects = (StgClosure *)q;
402 link = (StgPtr)cas((StgPtr)FUN_STATIC_LINK((StgClosure *)q),
404 (StgWord)gct->static_objects);
406 gct->static_objects = (StgClosure *)q;
413 /* If q->saved_info != NULL, then it's a revertible CAF - it'll be
414 * on the CAF list, so don't do anything with it here (we'll
415 * scavenge it later).
417 if (((StgIndStatic *)q)->saved_info == NULL) {
418 if (*IND_STATIC_LINK((StgClosure *)q) == NULL) {
420 *IND_STATIC_LINK((StgClosure *)q) = gct->static_objects;
421 gct->static_objects = (StgClosure *)q;
424 link = (StgPtr)cas((StgPtr)IND_STATIC_LINK((StgClosure *)q),
426 (StgWord)gct->static_objects);
428 gct->static_objects = (StgClosure *)q;
436 if (*STATIC_LINK(info,(StgClosure *)q) == NULL) {
438 *STATIC_LINK(info,(StgClosure *)q) = gct->static_objects;
439 gct->static_objects = (StgClosure *)q;
442 link = (StgPtr)cas((StgPtr)STATIC_LINK(info,(StgClosure *)q),
444 (StgWord)gct->static_objects);
446 gct->static_objects = (StgClosure *)q;
450 /* I am assuming that static_objects pointers are not
451 * written to other objects, and thus, no need to retag. */
454 case CONSTR_NOCAF_STATIC:
455 /* no need to put these on the static linked list, they don't need
461 barf("evacuate(static): strange closure type %d", (int)(info->type));
467 if ((bd->flags & (BF_LARGE | BF_MARKED | BF_EVACUATED)) != 0) {
469 // pointer into to-space: just return it. It might be a pointer
470 // into a generation that we aren't collecting (> N), or it
471 // might just be a pointer into to-space. The latter doesn't
472 // happen often, but allowing it makes certain things a bit
473 // easier; e.g. scavenging an object is idempotent, so it's OK to
474 // have an object on the mutable list multiple times.
475 if (bd->flags & BF_EVACUATED) {
476 // We aren't copying this object, so we have to check
477 // whether it is already in the target generation. (this is
478 // the write barrier).
479 if (bd->step < gct->evac_step) {
480 gct->failed_to_evac = rtsTrue;
481 TICK_GC_FAILED_PROMOTION();
486 /* evacuate large objects by re-linking them onto a different list.
488 if (bd->flags & BF_LARGE) {
490 if (info->type == TSO &&
491 ((StgTSO *)q)->what_next == ThreadRelocated) {
492 q = (StgClosure *)((StgTSO *)q)->_link;
496 evacuate_large((P_)q);
500 /* If the object is in a step that we're compacting, then we
501 * need to use an alternative evacuate procedure.
503 if (!is_marked((P_)q,bd)) {
505 if (mark_stack_full()) {
506 debugTrace(DEBUG_gc,"mark stack overflowed");
507 mark_stack_overflowed = rtsTrue;
510 push_mark_stack((P_)q);
517 info = q->header.info;
518 if (IS_FORWARDING_PTR(info))
520 /* Already evacuated, just return the forwarding address.
521 * HOWEVER: if the requested destination generation (gct->evac_step) is
522 * older than the actual generation (because the object was
523 * already evacuated to a younger generation) then we have to
524 * set the gct->failed_to_evac flag to indicate that we couldn't
525 * manage to promote the object to the desired generation.
528 * Optimisation: the check is fairly expensive, but we can often
529 * shortcut it if either the required generation is 0, or the
530 * current object (the EVACUATED) is in a high enough generation.
531 * We know that an EVACUATED always points to an object in the
532 * same or an older generation. stp is the lowest step that the
533 * current object would be evacuated to, so we only do the full
534 * check if stp is too low.
536 StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
537 *p = TAG_CLOSURE(tag,e);
538 if (stp < gct->evac_step) { // optimisation
539 if (Bdescr((P_)e)->step < gct->evac_step) {
540 gct->failed_to_evac = rtsTrue;
541 TICK_GC_FAILED_PROMOTION();
547 switch (INFO_PTR_TO_STRUCT(info)->type) {
556 copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
561 StgWord w = (StgWord)q->payload[0];
562 if (info == Czh_con_info &&
563 // unsigned, so always true: (StgChar)w >= MIN_CHARLIKE &&
564 (StgChar)w <= MAX_CHARLIKE) {
565 *p = TAG_CLOSURE(tag,
566 (StgClosure *)CHARLIKE_CLOSURE((StgChar)w)
569 else if (info == Izh_con_info &&
570 (StgInt)w >= MIN_INTLIKE && (StgInt)w <= MAX_INTLIKE) {
571 *p = TAG_CLOSURE(tag,
572 (StgClosure *)INTLIKE_CLOSURE((StgInt)w)
576 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
584 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,stp,tag);
589 copy(p,info,q,sizeofW(StgThunk)+1,stp);
595 #ifdef NO_PROMOTE_THUNKS
596 if (bd->gen_no == 0 &&
598 bd->step->no == generations[bd->gen_no].n_steps-1) {
602 copy(p,info,q,sizeofW(StgThunk)+2,stp);
610 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
614 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,stp,tag);
618 copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp);
623 case IND_OLDGEN_PERM:
625 copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
630 copy_tag(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),stp,tag);
634 copy(p,info,q,bco_sizeW((StgBCO *)q),stp);
639 copyPart(p,q,BLACKHOLE_sizeW(),sizeofW(StgHeader),stp);
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),stp);
671 copy(p,info,q,ap_sizeW((StgAP*)q),stp);
675 copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),stp);
679 // just copy the block
680 copy(p,info,q,arr_words_sizeW((StgArrWords *)q),stp);
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),stp);
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 relocate the update frame
711 mine = copyPart(p,(StgClosure *)tso, tso_sizeW(tso),
712 sizeofW(StgTSO), stp);
714 new_tso = (StgTSO *)*p;
715 move_TSO(tso, new_tso);
716 for (r = tso->sp, s = new_tso->sp;
717 r < tso->stack+tso->stack_size;) {
726 copy(p,info,q,sizeofW(StgTRecHeader),stp);
729 case TVAR_WATCH_QUEUE:
730 copy(p,info,q,sizeofW(StgTVarWatchQueue),stp);
734 copy(p,info,q,sizeofW(StgTVar),stp);
738 copy(p,info,q,sizeofW(StgTRecChunk),stp);
741 case ATOMIC_INVARIANT:
742 copy(p,info,q,sizeofW(StgAtomicInvariant),stp);
745 case INVARIANT_CHECK_QUEUE:
746 copy(p,info,q,sizeofW(StgInvariantCheckQueue),stp);
750 barf("evacuate: strange closure type %d", (int)(INFO_PTR_TO_STRUCT(info)->type));
756 /* -----------------------------------------------------------------------------
757 Evaluate a THUNK_SELECTOR if possible.
759 p points to a THUNK_SELECTOR that we want to evaluate. The
760 result of "evaluating" it will be evacuated and a pointer to the
761 to-space closure will be returned.
763 If the THUNK_SELECTOR could not be evaluated (its selectee is still
764 a THUNK, for example), then the THUNK_SELECTOR itself will be
766 -------------------------------------------------------------------------- */
768 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
776 ASSERT(p->header.info == &stg_WHITEHOLE_info);
778 ASSERT(p->header.info == &stg_BLACKHOLE_info);
780 // val must be in to-space. Not always: when we recursively
781 // invoke eval_thunk_selector(), the recursive calls will not
782 // evacuate the value (because we want to select on the value,
783 // not evacuate it), so in this case val is in from-space.
784 // ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
786 prev = (StgSelector*)((StgClosure *)p)->payload[0];
788 // Update the THUNK_SELECTOR with an indirection to the
789 // value. The value is still in from-space at this stage.
791 // (old note: Why not do upd_evacuee(q,p)? Because we have an
792 // invariant that an EVACUATED closure always points to an
793 // object in the same or an older generation (required by
794 // the short-cut test in the EVACUATED case, below).
795 if ((StgClosure *)p == val) {
796 // must be a loop; just leave a BLACKHOLE in place. This
797 // can happen when we have a chain of selectors that
798 // eventually loops back on itself. We can't leave an
799 // indirection pointing to itself, and we want the program
800 // to deadlock if it ever enters this closure, so
801 // BLACKHOLE is correct.
802 SET_INFO(p, &stg_BLACKHOLE_info);
804 ((StgInd *)p)->indirectee = val;
806 SET_INFO(p, &stg_IND_info);
809 // For the purposes of LDV profiling, we have created an
811 LDV_RECORD_CREATE(p);
818 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
819 // NB. for legacy reasons, p & q are swapped around :(
824 StgClosure *selectee;
825 StgSelector *prev_thunk_selector;
829 prev_thunk_selector = NULL;
830 // this is a chain of THUNK_SELECTORs that we are going to update
831 // to point to the value of the current THUNK_SELECTOR. Each
832 // closure on the chain is a BLACKHOLE, and points to the next in the
833 // chain with payload[0].
837 bd = Bdescr((StgPtr)p);
838 if (HEAP_ALLOCED_GC(p)) {
839 // If the THUNK_SELECTOR is in to-space or in a generation that we
840 // are not collecting, then bale out early. We won't be able to
841 // save any space in any case, and updating with an indirection is
842 // trickier in a non-collected gen: we would have to update the
844 if (bd->flags & BF_EVACUATED) {
845 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
846 *q = (StgClosure *)p;
847 // shortcut, behave as for: if (evac) evacuate(q);
848 if (evac && bd->step < gct->evac_step) {
849 gct->failed_to_evac = rtsTrue;
850 TICK_GC_FAILED_PROMOTION();
854 // we don't update THUNK_SELECTORS in the compacted
855 // generation, because compaction does not remove the INDs
856 // that result, this causes confusion later
857 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
858 // bit is very tricky to get right. If you make changes
859 // around here, test by compiling stage 3 with +RTS -c -RTS.
860 if (bd->flags & BF_MARKED) {
861 // must call evacuate() to mark this closure if evac==rtsTrue
862 *q = (StgClosure *)p;
863 if (evac) evacuate(q);
864 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
870 // BLACKHOLE the selector thunk, since it is now under evaluation.
871 // This is important to stop us going into an infinite loop if
872 // this selector thunk eventually refers to itself.
873 #if defined(THREADED_RTS)
874 // In threaded mode, we'll use WHITEHOLE to lock the selector
875 // thunk while we evaluate it.
878 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
879 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
881 // make sure someone else didn't get here first...
882 if (IS_FORWARDING_PTR(p) ||
883 INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
884 // v. tricky now. The THUNK_SELECTOR has been evacuated
885 // by another thread, and is now either a forwarding ptr or IND.
886 // We need to extract ourselves from the current situation
887 // as cleanly as possible.
888 // - unlock the closure
889 // - update *q, we may have done *some* evaluation
890 // - if evac, we need to call evacuate(), because we
891 // need the write-barrier stuff.
892 // - undo the chain we've built to point to p.
893 SET_INFO(p, (const StgInfoTable *)info_ptr);
894 *q = (StgClosure *)p;
895 if (evac) evacuate(q);
896 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
901 // Save the real info pointer (NOTE: not the same as get_itbl()).
902 info_ptr = (StgWord)p->header.info;
903 SET_INFO(p,&stg_BLACKHOLE_info);
906 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
908 // The selectee might be a constructor closure,
909 // so we untag the pointer.
910 selectee = UNTAG_CLOSURE(p->selectee);
913 // selectee now points to the closure that we're trying to select
914 // a field from. It may or may not be in to-space: we try not to
915 // end up in to-space, but it's impractical to avoid it in
916 // general. The compacting GC scatters to-space pointers in
917 // from-space during marking, for example. We rely on the property
918 // that evacuate() doesn't mind if it gets passed a to-space pointer.
920 info = (StgInfoTable*)selectee->header.info;
922 if (IS_FORWARDING_PTR(info)) {
923 // We don't follow pointers into to-space; the constructor
924 // has already been evacuated, so we won't save any space
925 // leaks by evaluating this selector thunk anyhow.
929 info = INFO_PTR_TO_STRUCT(info);
930 switch (info->type) {
932 goto bale_out; // about to be evacuated by another thread (or a loop).
941 case CONSTR_NOCAF_STATIC:
943 // check that the size is in range
944 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
945 info->layout.payload.nptrs));
947 // Select the right field from the constructor
948 val = selectee->payload[field];
951 // For the purposes of LDV profiling, we have destroyed
952 // the original selector thunk, p.
953 SET_INFO(p, (StgInfoTable *)info_ptr);
954 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
955 #if defined(THREADED_RTS)
956 SET_INFO(p, &stg_WHITEHOLE_info);
958 SET_INFO(p, &stg_BLACKHOLE_info);
962 // the closure in val is now the "value" of the
963 // THUNK_SELECTOR in p. However, val may itself be a
964 // THUNK_SELECTOR, in which case we want to continue
965 // evaluating until we find the real value, and then
966 // update the whole chain to point to the value.
968 info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
969 if (!IS_FORWARDING_PTR(info_ptr))
971 info = INFO_PTR_TO_STRUCT(info_ptr);
972 switch (info->type) {
976 case IND_OLDGEN_PERM:
978 val = ((StgInd *)val)->indirectee;
981 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
982 prev_thunk_selector = p;
983 p = (StgSelector*)val;
989 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
990 prev_thunk_selector = p;
994 // update the other selectors in the chain *before*
995 // evacuating the value. This is necessary in the case
996 // where the value turns out to be one of the selectors
997 // in the chain (i.e. we have a loop), and evacuating it
998 // would corrupt the chain.
999 unchain_thunk_selectors(prev_thunk_selector, val);
1001 // evacuate() cannot recurse through
1002 // eval_thunk_selector(), because we know val is not
1003 // a THUNK_SELECTOR.
1004 if (evac) evacuate(q);
1011 case IND_OLDGEN_PERM:
1013 // Again, we might need to untag a constructor.
1014 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
1017 case THUNK_SELECTOR:
1021 // recursively evaluate this selector. We don't want to
1022 // recurse indefinitely, so we impose a depth bound.
1023 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
1027 gct->thunk_selector_depth++;
1028 // rtsFalse says "don't evacuate the result". It will,
1029 // however, update any THUNK_SELECTORs that are evaluated
1031 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
1032 gct->thunk_selector_depth--;
1034 // did we actually manage to evaluate it?
1035 if (val == selectee) goto bale_out;
1037 // Of course this pointer might be tagged...
1038 selectee = UNTAG_CLOSURE(val);
1053 // not evaluated yet
1057 barf("eval_thunk_selector: strange selectee %d",
1062 // We didn't manage to evaluate this thunk; restore the old info
1063 // pointer. But don't forget: we still need to evacuate the thunk itself.
1064 SET_INFO(p, (const StgInfoTable *)info_ptr);
1065 // THREADED_RTS: we just unlocked the thunk, so another thread
1066 // might get in and update it. copy() will lock it again and
1067 // check whether it was updated in the meantime.
1068 *q = (StgClosure *)p;
1070 copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->step->to);
1072 unchain_thunk_selectors(prev_thunk_selector, *q);