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);
146 for (i = 1; i < size; i++) { // unroll for small i
150 // if somebody else reads the forwarding pointer, we better make
151 // sure there's a closure at the end of it.
153 src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to);
155 // if (to+size+2 < bd->start + BLOCK_SIZE_W) {
156 // __builtin_prefetch(to + size + 2, 1);
160 // We store the size of the just evacuated object in the LDV word so that
161 // the profiler can guess the position of the next object later.
162 SET_EVACUAEE_FOR_LDV(from, size);
167 /* Special version of copy() for when we only want to copy the info
168 * pointer of an object, but reserve some padding after it. This is
169 * used to optimise evacuation of BLACKHOLEs.
172 copyPart(StgClosure **p, StgClosure *src, nat size_to_reserve,
173 nat size_to_copy, generation *gen)
179 #if defined(PARALLEL_GC)
181 info = xchg((StgPtr)&src->header.info, (W_)&stg_WHITEHOLE_info);
182 if (info == (W_)&stg_WHITEHOLE_info) {
188 if (IS_FORWARDING_PTR(info)) {
189 src->header.info = (const StgInfoTable *)info;
190 evacuate(p); // does the failed_to_evac stuff
194 info = (W_)src->header.info;
197 to = alloc_for_copy(size_to_reserve, gen);
198 *p = (StgClosure *)to;
202 for (i = 1; i < size_to_copy; i++) { // unroll for small i
206 #if defined(PARALLEL_GC)
209 src->header.info = (const StgInfoTable*)MK_FORWARDING_PTR(to);
212 // We store the size of the just evacuated object in the LDV word so that
213 // the profiler can guess the position of the next object later.
214 SET_EVACUAEE_FOR_LDV(from, size_to_reserve);
216 if (size_to_reserve - size_to_copy > 0)
217 LDV_FILL_SLOP(to + size_to_copy, (int)(size_to_reserve - size_to_copy));
224 /* Copy wrappers that don't tag the closure after copying */
225 STATIC_INLINE GNUC_ATTR_HOT void
226 copy(StgClosure **p, const StgInfoTable *info,
227 StgClosure *src, nat size, generation *gen)
229 copy_tag(p,info,src,size,gen,0);
232 /* -----------------------------------------------------------------------------
233 Evacuate a large object
235 This just consists of removing the object from the (doubly-linked)
236 gen->large_objects list, and linking it on to the (singly-linked)
237 gen->new_large_objects list, from where it will be scavenged later.
239 Convention: bd->flags has BF_EVACUATED set for a large object
240 that has been evacuated, or unset otherwise.
241 -------------------------------------------------------------------------- */
244 evacuate_large(StgPtr p)
247 generation *gen, *new_gen;
252 ACQUIRE_SPIN_LOCK(&gen->sync_large_objects);
254 // already evacuated?
255 if (bd->flags & BF_EVACUATED) {
256 /* Don't forget to set the gct->failed_to_evac flag if we didn't get
257 * the desired destination (see comments in evacuate()).
259 if (gen < gct->evac_gen) {
260 gct->failed_to_evac = rtsTrue;
261 TICK_GC_FAILED_PROMOTION();
263 RELEASE_SPIN_LOCK(&gen->sync_large_objects);
267 // remove from large_object list
269 bd->u.back->link = bd->link;
270 } else { // first object in the list
271 gen->large_objects = bd->link;
274 bd->link->u.back = bd->u.back;
277 /* link it on to the evacuated large object list of the destination gen
280 if (new_gen < gct->evac_gen) {
281 if (gct->eager_promotion) {
282 new_gen = gct->evac_gen;
284 gct->failed_to_evac = rtsTrue;
288 ws = &gct->gens[new_gen->no];
290 bd->flags |= BF_EVACUATED;
291 initBdescr(bd, new_gen, new_gen->to);
293 // If this is a block of pinned objects, we don't have to scan
294 // these objects, because they aren't allowed to contain any
295 // pointers. For these blocks, we skip the scavenge stage and put
296 // them straight on the scavenged_large_objects list.
297 if (bd->flags & BF_PINNED) {
298 ASSERT(get_itbl((StgClosure *)p)->type == ARR_WORDS);
299 if (new_gen != gen) { ACQUIRE_SPIN_LOCK(&new_gen->sync_large_objects); }
300 dbl_link_onto(bd, &new_gen->scavenged_large_objects);
301 new_gen->n_scavenged_large_blocks += bd->blocks;
302 if (new_gen != gen) { RELEASE_SPIN_LOCK(&new_gen->sync_large_objects); }
304 bd->link = ws->todo_large_objects;
305 ws->todo_large_objects = bd;
308 RELEASE_SPIN_LOCK(&gen->sync_large_objects);
311 /* ----------------------------------------------------------------------------
314 This is called (eventually) for every live object in the system.
316 The caller to evacuate specifies a desired generation in the
317 gct->evac_gen thread-local variable. The following conditions apply to
318 evacuating an object which resides in generation M when we're
319 collecting up to generation N
321 if M >= gct->evac_gen
325 if M < gct->evac_gen evac to gct->evac_gen, step 0
327 if the object is already evacuated, then we check which generation
330 if M >= gct->evac_gen do nothing
331 if M < gct->evac_gen set gct->failed_to_evac flag to indicate that we
332 didn't manage to evacuate this object into gct->evac_gen.
337 evacuate() is the single most important function performance-wise
338 in the GC. Various things have been tried to speed it up, but as
339 far as I can tell the code generated by gcc 3.2 with -O2 is about
340 as good as it's going to get. We pass the argument to evacuate()
341 in a register using the 'regparm' attribute (see the prototype for
342 evacuate() near the top of this file).
344 Changing evacuate() to take an (StgClosure **) rather than
345 returning the new pointer seems attractive, because we can avoid
346 writing back the pointer when it hasn't changed (eg. for a static
347 object, or an object in a generation > N). However, I tried it and
348 it doesn't help. One reason is that the (StgClosure **) pointer
349 gets spilled to the stack inside evacuate(), resulting in far more
350 extra reads/writes than we save.
351 ------------------------------------------------------------------------- */
353 REGPARM1 GNUC_ATTR_HOT void
354 evacuate(StgClosure **p)
359 const StgInfoTable *info;
365 /* The tag and the pointer are split, to be merged after evacing */
366 tag = GET_CLOSURE_TAG(q);
367 q = UNTAG_CLOSURE(q);
369 ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
371 if (!HEAP_ALLOCED_GC(q)) {
373 if (!major_gc) return;
376 switch (info->type) {
379 if (info->srt_bitmap != 0) {
380 if (*THUNK_STATIC_LINK((StgClosure *)q) == NULL) {
382 *THUNK_STATIC_LINK((StgClosure *)q) = gct->static_objects;
383 gct->static_objects = (StgClosure *)q;
386 link = (StgPtr)cas((StgPtr)THUNK_STATIC_LINK((StgClosure *)q),
388 (StgWord)gct->static_objects);
390 gct->static_objects = (StgClosure *)q;
398 if (info->srt_bitmap != 0 &&
399 *FUN_STATIC_LINK((StgClosure *)q) == NULL) {
401 *FUN_STATIC_LINK((StgClosure *)q) = gct->static_objects;
402 gct->static_objects = (StgClosure *)q;
405 link = (StgPtr)cas((StgPtr)FUN_STATIC_LINK((StgClosure *)q),
407 (StgWord)gct->static_objects);
409 gct->static_objects = (StgClosure *)q;
416 /* If q->saved_info != NULL, then it's a revertible CAF - it'll be
417 * on the CAF list, so don't do anything with it here (we'll
418 * scavenge it later).
420 if (((StgIndStatic *)q)->saved_info == NULL) {
421 if (*IND_STATIC_LINK((StgClosure *)q) == NULL) {
423 *IND_STATIC_LINK((StgClosure *)q) = gct->static_objects;
424 gct->static_objects = (StgClosure *)q;
427 link = (StgPtr)cas((StgPtr)IND_STATIC_LINK((StgClosure *)q),
429 (StgWord)gct->static_objects);
431 gct->static_objects = (StgClosure *)q;
439 if (*STATIC_LINK(info,(StgClosure *)q) == NULL) {
441 *STATIC_LINK(info,(StgClosure *)q) = gct->static_objects;
442 gct->static_objects = (StgClosure *)q;
445 link = (StgPtr)cas((StgPtr)STATIC_LINK(info,(StgClosure *)q),
447 (StgWord)gct->static_objects);
449 gct->static_objects = (StgClosure *)q;
453 /* I am assuming that static_objects pointers are not
454 * written to other objects, and thus, no need to retag. */
457 case CONSTR_NOCAF_STATIC:
458 /* no need to put these on the static linked list, they don't need
464 barf("evacuate(static): strange closure type %d", (int)(info->type));
470 if ((bd->flags & (BF_LARGE | BF_MARKED | BF_EVACUATED)) != 0) {
472 // pointer into to-space: just return it. It might be a pointer
473 // into a generation that we aren't collecting (> N), or it
474 // might just be a pointer into to-space. The latter doesn't
475 // happen often, but allowing it makes certain things a bit
476 // easier; e.g. scavenging an object is idempotent, so it's OK to
477 // have an object on the mutable list multiple times.
478 if (bd->flags & BF_EVACUATED) {
479 // We aren't copying this object, so we have to check
480 // whether it is already in the target generation. (this is
481 // the write barrier).
482 if (bd->gen < gct->evac_gen) {
483 gct->failed_to_evac = rtsTrue;
484 TICK_GC_FAILED_PROMOTION();
489 /* evacuate large objects by re-linking them onto a different list.
491 if (bd->flags & BF_LARGE) {
493 if (info->type == TSO &&
494 ((StgTSO *)q)->what_next == ThreadRelocated) {
495 q = (StgClosure *)((StgTSO *)q)->_link;
499 evacuate_large((P_)q);
503 /* If the object is in a gen that we're compacting, then we
504 * need to use an alternative evacuate procedure.
506 if (!is_marked((P_)q,bd)) {
508 push_mark_stack((P_)q);
515 info = q->header.info;
516 if (IS_FORWARDING_PTR(info))
518 /* Already evacuated, just return the forwarding address.
519 * HOWEVER: if the requested destination generation (gct->evac_gen) is
520 * older than the actual generation (because the object was
521 * already evacuated to a younger generation) then we have to
522 * set the gct->failed_to_evac flag to indicate that we couldn't
523 * manage to promote the object to the desired generation.
526 * Optimisation: the check is fairly expensive, but we can often
527 * shortcut it if either the required generation is 0, or the
528 * current object (the EVACUATED) is in a high enough generation.
529 * We know that an EVACUATED always points to an object in the
530 * same or an older generation. gen is the lowest generation that the
531 * current object would be evacuated to, so we only do the full
532 * check if gen is too low.
534 StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
535 *p = TAG_CLOSURE(tag,e);
536 if (gen < gct->evac_gen) { // optimisation
537 if (Bdescr((P_)e)->gen < gct->evac_gen) {
538 gct->failed_to_evac = rtsTrue;
539 TICK_GC_FAILED_PROMOTION();
545 switch (INFO_PTR_TO_STRUCT(info)->type) {
554 copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
557 // For ints and chars of low value, save space by replacing references to
558 // these with closures with references to common, shared ones in the RTS.
560 // * Except when compiling into Windows DLLs which don't support cross-package
561 // data references very well.
565 #if defined(__PIC__) && defined(mingw32_HOST_OS)
566 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
568 StgWord w = (StgWord)q->payload[0];
569 if (info == Czh_con_info &&
570 // unsigned, so always true: (StgChar)w >= MIN_CHARLIKE &&
571 (StgChar)w <= MAX_CHARLIKE) {
572 *p = TAG_CLOSURE(tag,
573 (StgClosure *)CHARLIKE_CLOSURE((StgChar)w)
576 else if (info == Izh_con_info &&
577 (StgInt)w >= MIN_INTLIKE && (StgInt)w <= MAX_INTLIKE) {
578 *p = TAG_CLOSURE(tag,
579 (StgClosure *)INTLIKE_CLOSURE((StgInt)w)
583 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
592 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
597 copy(p,info,q,sizeofW(StgThunk)+1,gen);
603 #ifdef NO_PROMOTE_THUNKS
606 copy(p,info,q,sizeofW(StgThunk)+2,gen);
614 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
618 copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
622 copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
627 case IND_OLDGEN_PERM:
629 copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen,tag);
635 copy_tag(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen,tag);
639 copy(p,info,q,bco_sizeW((StgBCO *)q),gen);
644 copyPart(p,q,BLACKHOLE_sizeW(),sizeofW(StgHeader),gen);
648 eval_thunk_selector(p, (StgSelector *)q, rtsTrue);
653 // follow chains of indirections, don't evacuate them
654 q = ((StgInd*)q)->indirectee;
665 case CATCH_STM_FRAME:
666 case CATCH_RETRY_FRAME:
667 case ATOMICALLY_FRAME:
668 // shouldn't see these
669 barf("evacuate: stack frame at %p\n", q);
672 copy(p,info,q,pap_sizeW((StgPAP*)q),gen);
676 copy(p,info,q,ap_sizeW((StgAP*)q),gen);
680 copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),gen);
684 // just copy the block
685 copy(p,info,q,arr_words_sizeW((StgArrWords *)q),gen);
688 case MUT_ARR_PTRS_CLEAN:
689 case MUT_ARR_PTRS_DIRTY:
690 case MUT_ARR_PTRS_FROZEN:
691 case MUT_ARR_PTRS_FROZEN0:
692 // just copy the block
693 copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),gen);
698 StgTSO *tso = (StgTSO *)q;
700 /* Deal with redirected TSOs (a TSO that's had its stack enlarged).
702 if (tso->what_next == ThreadRelocated) {
703 q = (StgClosure *)tso->_link;
708 /* To evacuate a small TSO, we need to adjust the stack pointer
715 mine = copyPart(p,(StgClosure *)tso, tso_sizeW(tso),
716 sizeofW(StgTSO), gen);
718 new_tso = (StgTSO *)*p;
719 move_TSO(tso, new_tso);
720 for (r = tso->sp, s = new_tso->sp;
721 r < tso->stack+tso->stack_size;) {
730 copy(p,info,q,sizeofW(StgTRecChunk),gen);
734 barf("evacuate: strange closure type %d", (int)(INFO_PTR_TO_STRUCT(info)->type));
740 /* -----------------------------------------------------------------------------
741 Evaluate a THUNK_SELECTOR if possible.
743 p points to a THUNK_SELECTOR that we want to evaluate. The
744 result of "evaluating" it will be evacuated and a pointer to the
745 to-space closure will be returned.
747 If the THUNK_SELECTOR could not be evaluated (its selectee is still
748 a THUNK, for example), then the THUNK_SELECTOR itself will be
750 -------------------------------------------------------------------------- */
752 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
760 ASSERT(p->header.info == &stg_WHITEHOLE_info);
762 ASSERT(p->header.info == &stg_BLACKHOLE_info);
764 // val must be in to-space. Not always: when we recursively
765 // invoke eval_thunk_selector(), the recursive calls will not
766 // evacuate the value (because we want to select on the value,
767 // not evacuate it), so in this case val is in from-space.
768 // ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
770 prev = (StgSelector*)((StgClosure *)p)->payload[0];
772 // Update the THUNK_SELECTOR with an indirection to the
773 // value. The value is still in from-space at this stage.
775 // (old note: Why not do upd_evacuee(q,p)? Because we have an
776 // invariant that an EVACUATED closure always points to an
777 // object in the same or an older generation (required by
778 // the short-cut test in the EVACUATED case, below).
779 if ((StgClosure *)p == val) {
780 // must be a loop; just leave a BLACKHOLE in place. This
781 // can happen when we have a chain of selectors that
782 // eventually loops back on itself. We can't leave an
783 // indirection pointing to itself, and we want the program
784 // to deadlock if it ever enters this closure, so
785 // BLACKHOLE is correct.
786 SET_INFO(p, &stg_BLACKHOLE_info);
788 ((StgInd *)p)->indirectee = val;
790 SET_INFO(p, &stg_IND_info);
793 // For the purposes of LDV profiling, we have created an
795 LDV_RECORD_CREATE(p);
802 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
803 // NB. for legacy reasons, p & q are swapped around :(
808 StgClosure *selectee;
809 StgSelector *prev_thunk_selector;
813 prev_thunk_selector = NULL;
814 // this is a chain of THUNK_SELECTORs that we are going to update
815 // to point to the value of the current THUNK_SELECTOR. Each
816 // closure on the chain is a BLACKHOLE, and points to the next in the
817 // chain with payload[0].
821 bd = Bdescr((StgPtr)p);
822 if (HEAP_ALLOCED_GC(p)) {
823 // If the THUNK_SELECTOR is in to-space or in a generation that we
824 // are not collecting, then bale out early. We won't be able to
825 // save any space in any case, and updating with an indirection is
826 // trickier in a non-collected gen: we would have to update the
828 if (bd->flags & BF_EVACUATED) {
829 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
830 *q = (StgClosure *)p;
831 // shortcut, behave as for: if (evac) evacuate(q);
832 if (evac && bd->gen < gct->evac_gen) {
833 gct->failed_to_evac = rtsTrue;
834 TICK_GC_FAILED_PROMOTION();
838 // we don't update THUNK_SELECTORS in the compacted
839 // generation, because compaction does not remove the INDs
840 // that result, this causes confusion later
841 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
842 // bit is very tricky to get right. If you make changes
843 // around here, test by compiling stage 3 with +RTS -c -RTS.
844 if (bd->flags & BF_MARKED) {
845 // must call evacuate() to mark this closure if evac==rtsTrue
846 *q = (StgClosure *)p;
847 if (evac) evacuate(q);
848 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
854 // BLACKHOLE the selector thunk, since it is now under evaluation.
855 // This is important to stop us going into an infinite loop if
856 // this selector thunk eventually refers to itself.
857 #if defined(THREADED_RTS)
858 // In threaded mode, we'll use WHITEHOLE to lock the selector
859 // thunk while we evaluate it.
862 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
863 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
865 // make sure someone else didn't get here first...
866 if (IS_FORWARDING_PTR(info_ptr) ||
867 INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
868 // v. tricky now. The THUNK_SELECTOR has been evacuated
869 // by another thread, and is now either a forwarding ptr or IND.
870 // We need to extract ourselves from the current situation
871 // as cleanly as possible.
872 // - unlock the closure
873 // - update *q, we may have done *some* evaluation
874 // - if evac, we need to call evacuate(), because we
875 // need the write-barrier stuff.
876 // - undo the chain we've built to point to p.
877 SET_INFO(p, (const StgInfoTable *)info_ptr);
878 *q = (StgClosure *)p;
879 if (evac) evacuate(q);
880 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
885 // Save the real info pointer (NOTE: not the same as get_itbl()).
886 info_ptr = (StgWord)p->header.info;
887 SET_INFO(p,&stg_BLACKHOLE_info);
890 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
892 // The selectee might be a constructor closure,
893 // so we untag the pointer.
894 selectee = UNTAG_CLOSURE(p->selectee);
897 // selectee now points to the closure that we're trying to select
898 // a field from. It may or may not be in to-space: we try not to
899 // end up in to-space, but it's impractical to avoid it in
900 // general. The compacting GC scatters to-space pointers in
901 // from-space during marking, for example. We rely on the property
902 // that evacuate() doesn't mind if it gets passed a to-space pointer.
904 info = (StgInfoTable*)selectee->header.info;
906 if (IS_FORWARDING_PTR(info)) {
907 // We don't follow pointers into to-space; the constructor
908 // has already been evacuated, so we won't save any space
909 // leaks by evaluating this selector thunk anyhow.
913 info = INFO_PTR_TO_STRUCT(info);
914 switch (info->type) {
916 goto bale_out; // about to be evacuated by another thread (or a loop).
925 case CONSTR_NOCAF_STATIC:
927 // check that the size is in range
928 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
929 info->layout.payload.nptrs));
931 // Select the right field from the constructor
932 val = selectee->payload[field];
935 // For the purposes of LDV profiling, we have destroyed
936 // the original selector thunk, p.
937 SET_INFO(p, (StgInfoTable *)info_ptr);
938 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
939 #if defined(THREADED_RTS)
940 SET_INFO(p, &stg_WHITEHOLE_info);
942 SET_INFO(p, &stg_BLACKHOLE_info);
946 // the closure in val is now the "value" of the
947 // THUNK_SELECTOR in p. However, val may itself be a
948 // THUNK_SELECTOR, in which case we want to continue
949 // evaluating until we find the real value, and then
950 // update the whole chain to point to the value.
952 info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
953 if (!IS_FORWARDING_PTR(info_ptr))
955 info = INFO_PTR_TO_STRUCT(info_ptr);
956 switch (info->type) {
960 case IND_OLDGEN_PERM:
962 val = ((StgInd *)val)->indirectee;
965 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
966 prev_thunk_selector = p;
967 p = (StgSelector*)val;
973 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
974 prev_thunk_selector = p;
978 // update the other selectors in the chain *before*
979 // evacuating the value. This is necessary in the case
980 // where the value turns out to be one of the selectors
981 // in the chain (i.e. we have a loop), and evacuating it
982 // would corrupt the chain.
983 unchain_thunk_selectors(prev_thunk_selector, val);
985 // evacuate() cannot recurse through
986 // eval_thunk_selector(), because we know val is not
988 if (evac) evacuate(q);
995 case IND_OLDGEN_PERM:
997 // Again, we might need to untag a constructor.
998 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
1001 case THUNK_SELECTOR:
1005 // recursively evaluate this selector. We don't want to
1006 // recurse indefinitely, so we impose a depth bound.
1007 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
1011 gct->thunk_selector_depth++;
1012 // rtsFalse says "don't evacuate the result". It will,
1013 // however, update any THUNK_SELECTORs that are evaluated
1015 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
1016 gct->thunk_selector_depth--;
1018 // did we actually manage to evaluate it?
1019 if (val == selectee) goto bale_out;
1021 // Of course this pointer might be tagged...
1022 selectee = UNTAG_CLOSURE(val);
1037 // not evaluated yet
1041 barf("eval_thunk_selector: strange selectee %d",
1046 // We didn't manage to evaluate this thunk; restore the old info
1047 // pointer. But don't forget: we still need to evacuate the thunk itself.
1048 SET_INFO(p, (const StgInfoTable *)info_ptr);
1049 // THREADED_RTS: we just unlocked the thunk, so another thread
1050 // might get in and update it. copy() will lock it again and
1051 // check whether it was updated in the meantime.
1052 *q = (StgClosure *)p;
1054 copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->dest);
1056 unchain_thunk_selectors(prev_thunk_selector, *q);