1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 2001-2008
5 * Compacting garbage collector
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"
19 #include "BlockAlloc.h"
29 // Turn off inlining when debugging - it obfuscates things
32 # define STATIC_INLINE static
35 /* ----------------------------------------------------------------------------
36 Threading / unthreading pointers.
38 The basic idea here is to chain together all the fields pointing at
39 a particular object, with the root of the chain in the object's
40 info table field. The original contents of the info pointer goes
41 at the end of the chain.
43 Adding a new field to the chain is a matter of swapping the
44 contents of the field with the contents of the object's info table
47 To unthread the chain, we walk down it updating all the fields on
48 the chain with the new location of the object. We stop when we
49 reach the info pointer at the end.
51 The main difficulty here is that we need to be able to identify the
52 info pointer at the end of the chain. We can't use the low bits of
53 the pointer for this; they are already being used for
54 pointer-tagging. What's more, we need to retain the
55 pointer-tagging tag bits on each pointer during the
56 threading/unthreading process.
58 Our solution is as follows:
59 - an info pointer (chain length zero) is identified by having tag 0
60 - in a threaded chain of length > 0:
61 - the pointer-tagging tag bits are attached to the info pointer
62 - the first entry in the chain has tag 1
63 - second and subsequent entries in the chain have tag 2
65 This exploits the fact that the tag on each pointer to a given
66 closure is normally the same (if they are not the same, then
67 presumably the tag is not essential and it therefore doesn't matter
68 if we throw away some of the tags).
69 ------------------------------------------------------------------------- */
72 thread (StgClosure **p)
80 q = (StgPtr)UNTAG_CLOSURE(q0);
82 // It doesn't look like a closure at the moment, because the info
83 // ptr is possibly threaded:
84 // ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
86 if (HEAP_ALLOCED(q)) {
89 if (bd->flags & BF_MARKED)
92 switch (GET_CLOSURE_TAG((StgClosure *)iptr))
95 // this is the info pointer; we are creating a new chain.
96 // save the original tag at the end of the chain.
97 *p = (StgClosure *)((StgWord)iptr + GET_CLOSURE_TAG(q0));
102 // this is a chain of length 1 or more
103 *p = (StgClosure *)iptr;
112 thread_root (void *user STG_UNUSED, StgClosure **p)
117 // This version of thread() takes a (void *), used to circumvent
118 // warnings from gcc about pointer punning and strict aliasing.
119 STATIC_INLINE void thread_ (void *p) { thread((StgClosure **)p); }
122 unthread( StgPtr p, StgWord free )
129 switch (GET_CLOSURE_TAG((StgClosure *)q))
132 // nothing to do; the chain is length zero
136 r = *q0; // r is the info ptr, tagged with the pointer-tag
138 *p = (StgWord)UNTAG_CLOSURE((StgClosure *)r);
151 // Traverse a threaded chain and pull out the info pointer at the end.
152 // The info pointer is also tagged with the appropriate pointer tag
153 // for this closure, which should be attached to the pointer
154 // subsequently passed to unthread().
155 STATIC_INLINE StgWord
156 get_threaded_info( StgPtr p )
160 q = (W_)GET_INFO(UNTAG_CLOSURE((StgClosure *)p));
163 switch (GET_CLOSURE_TAG((StgClosure *)q))
166 ASSERT(LOOKS_LIKE_INFO_PTR(q));
170 StgWord r = *(StgPtr)(q-1);
171 ASSERT(LOOKS_LIKE_INFO_PTR((StgWord)UNTAG_CLOSURE((StgClosure *)r)));
178 barf("get_threaded_info");
182 // A word-aligned memmove will be faster for small objects than libc's or gcc's.
183 // Remember, the two regions *might* overlap, but: to <= from.
185 move(StgPtr to, StgPtr from, nat size)
187 for(; size > 0; --size) {
193 thread_static( StgClosure* p )
195 const StgInfoTable *info;
197 // keep going until we've threaded all the objects on the linked
199 while (p != END_OF_STATIC_LIST) {
202 switch (info->type) {
205 thread(&((StgInd *)p)->indirectee);
206 p = *IND_STATIC_LINK(p);
210 p = *THUNK_STATIC_LINK(p);
213 p = *FUN_STATIC_LINK(p);
216 p = *STATIC_LINK(info,p);
220 barf("thread_static: strange closure %d", (int)(info->type));
227 thread_large_bitmap( StgPtr p, StgLargeBitmap *large_bitmap, nat size )
233 bitmap = large_bitmap->bitmap[b];
234 for (i = 0; i < size; ) {
235 if ((bitmap & 1) == 0) {
236 thread((StgClosure **)p);
240 if (i % BITS_IN(W_) == 0) {
242 bitmap = large_bitmap->bitmap[b];
244 bitmap = bitmap >> 1;
250 thread_arg_block (StgFunInfoTable *fun_info, StgClosure **args)
257 switch (fun_info->f.fun_type) {
259 bitmap = BITMAP_BITS(fun_info->f.b.bitmap);
260 size = BITMAP_SIZE(fun_info->f.b.bitmap);
263 size = GET_FUN_LARGE_BITMAP(fun_info)->size;
264 thread_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info), size);
268 bitmap = BITMAP_BITS(stg_arg_bitmaps[fun_info->f.fun_type]);
269 size = BITMAP_SIZE(stg_arg_bitmaps[fun_info->f.fun_type]);
272 if ((bitmap & 1) == 0) {
273 thread((StgClosure **)p);
276 bitmap = bitmap >> 1;
285 thread_stack(StgPtr p, StgPtr stack_end)
287 const StgRetInfoTable* info;
291 // highly similar to scavenge_stack, but we do pointer threading here.
293 while (p < stack_end) {
295 // *p must be the info pointer of an activation
296 // record. All activation records have 'bitmap' style layout
299 info = get_ret_itbl((StgClosure *)p);
301 switch (info->i.type) {
303 // Dynamic bitmap: the mask is stored on the stack
307 dyn = ((StgRetDyn *)p)->liveness;
309 // traverse the bitmap first
310 bitmap = RET_DYN_LIVENESS(dyn);
311 p = (P_)&((StgRetDyn *)p)->payload[0];
312 size = RET_DYN_BITMAP_SIZE;
314 if ((bitmap & 1) == 0) {
315 thread((StgClosure **)p);
318 bitmap = bitmap >> 1;
322 // skip over the non-ptr words
323 p += RET_DYN_NONPTRS(dyn) + RET_DYN_NONPTR_REGS_SIZE;
325 // follow the ptr words
326 for (size = RET_DYN_PTRS(dyn); size > 0; size--) {
327 thread((StgClosure **)p);
333 // small bitmap (<= 32 entries, or 64 on a 64-bit machine)
334 case CATCH_RETRY_FRAME:
335 case CATCH_STM_FRAME:
336 case ATOMICALLY_FRAME:
341 bitmap = BITMAP_BITS(info->i.layout.bitmap);
342 size = BITMAP_SIZE(info->i.layout.bitmap);
344 // NOTE: the payload starts immediately after the info-ptr, we
345 // don't have an StgHeader in the same sense as a heap closure.
347 if ((bitmap & 1) == 0) {
348 thread((StgClosure **)p);
351 bitmap = bitmap >> 1;
362 thread((StgClosure **)p);
364 size = BCO_BITMAP_SIZE(bco);
365 thread_large_bitmap(p, BCO_BITMAP(bco), size);
370 // large bitmap (> 32 entries, or 64 on a 64-bit machine)
373 size = GET_LARGE_BITMAP(&info->i)->size;
374 thread_large_bitmap(p, GET_LARGE_BITMAP(&info->i), size);
380 StgRetFun *ret_fun = (StgRetFun *)p;
381 StgFunInfoTable *fun_info;
383 fun_info = FUN_INFO_PTR_TO_STRUCT(UNTAG_CLOSURE((StgClosure *)
384 get_threaded_info((StgPtr)ret_fun->fun)));
385 // *before* threading it!
386 thread(&ret_fun->fun);
387 p = thread_arg_block(fun_info, ret_fun->payload);
392 barf("thread_stack: weird activation record found on stack: %d",
393 (int)(info->i.type));
399 thread_PAP_payload (StgClosure *fun, StgClosure **payload, StgWord size)
403 StgFunInfoTable *fun_info;
405 fun_info = FUN_INFO_PTR_TO_STRUCT(UNTAG_CLOSURE((StgClosure *)
406 get_threaded_info((StgPtr)fun)));
407 ASSERT(fun_info->i.type != PAP);
411 switch (fun_info->f.fun_type) {
413 bitmap = BITMAP_BITS(fun_info->f.b.bitmap);
416 thread_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info), size);
420 thread_large_bitmap((StgPtr)payload, BCO_BITMAP(fun), size);
424 bitmap = BITMAP_BITS(stg_arg_bitmaps[fun_info->f.fun_type]);
427 if ((bitmap & 1) == 0) {
428 thread((StgClosure **)p);
431 bitmap = bitmap >> 1;
441 thread_PAP (StgPAP *pap)
444 p = thread_PAP_payload(pap->fun, pap->payload, pap->n_args);
450 thread_AP (StgAP *ap)
453 p = thread_PAP_payload(ap->fun, ap->payload, ap->n_args);
459 thread_AP_STACK (StgAP_STACK *ap)
462 thread_stack((P_)ap->payload, (P_)ap->payload + ap->size);
463 return (P_)ap + sizeofW(StgAP_STACK) + ap->size;
467 thread_TSO (StgTSO *tso)
469 thread_(&tso->_link);
470 thread_(&tso->global_link);
472 if ( tso->why_blocked == BlockedOnMVar
473 || tso->why_blocked == BlockedOnBlackHole
474 || tso->why_blocked == BlockedOnMsgThrowTo
476 thread_(&tso->block_info.closure);
478 thread_(&tso->blocked_exceptions);
483 thread_stack(tso->sp, &(tso->stack[tso->stack_size]));
484 return (StgPtr)tso + tso_sizeW(tso);
489 update_fwd_large( bdescr *bd )
492 const StgInfoTable* info;
494 for (; bd != NULL; bd = bd->link) {
496 // nothing to do in a pinned block; it might not even have an object
498 if (bd->flags & BF_PINNED) continue;
501 info = get_itbl((StgClosure *)p);
503 switch (info->type) {
509 case MUT_ARR_PTRS_CLEAN:
510 case MUT_ARR_PTRS_DIRTY:
511 case MUT_ARR_PTRS_FROZEN:
512 case MUT_ARR_PTRS_FROZEN0:
517 a = (StgMutArrPtrs*)p;
518 for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
519 thread((StgClosure **)p);
525 thread_TSO((StgTSO *)p);
529 thread_AP_STACK((StgAP_STACK *)p);
533 thread_PAP((StgPAP *)p);
539 StgTRecChunk *tc = (StgTRecChunk *)p;
540 TRecEntry *e = &(tc -> entries[0]);
541 thread_(&tc->prev_chunk);
542 for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
544 thread(&e->expected_value);
545 thread(&e->new_value);
551 barf("update_fwd_large: unknown/strange object %d", (int)(info->type));
556 // ToDo: too big to inline
557 static /* STATIC_INLINE */ StgPtr
558 thread_obj (StgInfoTable *info, StgPtr p)
560 switch (info->type) {
562 return p + sizeofW(StgThunk) + 1;
566 return p + sizeofW(StgHeader) + 1;
570 thread(&((StgClosure *)p)->payload[0]);
571 return p + sizeofW(StgHeader) + 1;
574 thread(&((StgThunk *)p)->payload[0]);
575 return p + sizeofW(StgThunk) + 1;
578 return p + sizeofW(StgThunk) + 2;
582 return p + sizeofW(StgHeader) + 2;
585 thread(&((StgThunk *)p)->payload[0]);
586 return p + sizeofW(StgThunk) + 2;
590 thread(&((StgClosure *)p)->payload[0]);
591 return p + sizeofW(StgHeader) + 2;
594 thread(&((StgThunk *)p)->payload[0]);
595 thread(&((StgThunk *)p)->payload[1]);
596 return p + sizeofW(StgThunk) + 2;
600 thread(&((StgClosure *)p)->payload[0]);
601 thread(&((StgClosure *)p)->payload[1]);
602 return p + sizeofW(StgHeader) + 2;
605 StgBCO *bco = (StgBCO *)p;
606 thread_(&bco->instrs);
607 thread_(&bco->literals);
609 return p + bco_sizeW(bco);
616 end = (P_)((StgThunk *)p)->payload +
617 info->layout.payload.ptrs;
618 for (p = (P_)((StgThunk *)p)->payload; p < end; p++) {
619 thread((StgClosure **)p);
621 return p + info->layout.payload.nptrs;
635 end = (P_)((StgClosure *)p)->payload +
636 info->layout.payload.ptrs;
637 for (p = (P_)((StgClosure *)p)->payload; p < end; p++) {
638 thread((StgClosure **)p);
640 return p + info->layout.payload.nptrs;
645 StgWeak *w = (StgWeak *)p;
646 thread(&w->cfinalizer);
649 thread(&w->finalizer);
650 if (w->link != NULL) {
653 return p + sizeofW(StgWeak);
659 StgMVar *mvar = (StgMVar *)p;
660 thread_(&mvar->head);
661 thread_(&mvar->tail);
662 thread(&mvar->value);
663 return p + sizeofW(StgMVar);
668 thread(&((StgInd *)p)->indirectee);
669 return p + sizeofW(StgInd);
673 StgSelector *s = (StgSelector *)p;
674 thread(&s->selectee);
675 return p + THUNK_SELECTOR_sizeW();
679 return thread_AP_STACK((StgAP_STACK *)p);
682 return thread_PAP((StgPAP *)p);
685 return thread_AP((StgAP *)p);
688 return p + arr_words_sizeW((StgArrWords *)p);
690 case MUT_ARR_PTRS_CLEAN:
691 case MUT_ARR_PTRS_DIRTY:
692 case MUT_ARR_PTRS_FROZEN:
693 case MUT_ARR_PTRS_FROZEN0:
698 a = (StgMutArrPtrs *)p;
699 for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
700 thread((StgClosure **)p);
703 return (StgPtr)a + mut_arr_ptrs_sizeW(a);
707 return thread_TSO((StgTSO *)p);
712 StgTRecChunk *tc = (StgTRecChunk *)p;
713 TRecEntry *e = &(tc -> entries[0]);
714 thread_(&tc->prev_chunk);
715 for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
717 thread(&e->expected_value);
718 thread(&e->new_value);
720 return p + sizeofW(StgTRecChunk);
724 barf("update_fwd: unknown/strange object %d", (int)(info->type));
730 update_fwd( bdescr *blocks )
738 // cycle through all the blocks in the step
739 for (; bd != NULL; bd = bd->link) {
742 // linearly scan the objects in this block
743 while (p < bd->free) {
744 ASSERT(LOOKS_LIKE_CLOSURE_PTR(p));
745 info = get_itbl((StgClosure *)p);
746 p = thread_obj(info, p);
752 update_fwd_compact( bdescr *blocks )
758 bdescr *bd, *free_bd;
765 free = free_bd->start;
767 // cycle through all the blocks in the step
768 for (; bd != NULL; bd = bd->link) {
771 while (p < bd->free ) {
773 while ( p < bd->free && !is_marked(p,bd) ) {
782 m = * ((StgPtr)bd->u.bitmap + ((p - bd->start) / (BITS_IN(StgWord))));
783 m >>= ((p - bd->start) & (BITS_IN(StgWord) - 1));
785 while ( p < bd->free ) {
790 if (((StgWord)p & (sizeof(W_) * BITS_IN(StgWord))) == 0) {
798 // Problem: we need to know the destination for this cell
799 // in order to unthread its info pointer. But we can't
800 // know the destination without the size, because we may
801 // spill into the next block. So we have to run down the
802 // threaded list and get the info ptr first.
804 // ToDo: one possible avenue of attack is to use the fact
805 // that if (p&BLOCK_MASK) >= (free&BLOCK_MASK), then we
806 // definitely have enough room. Also see bug #1147.
807 iptr = get_threaded_info(p);
808 info = INFO_PTR_TO_STRUCT(UNTAG_CLOSURE((StgClosure *)iptr));
812 p = thread_obj(info, p);
815 if (free + size > free_bd->start + BLOCK_SIZE_W) {
816 // set the next bit in the bitmap to indicate that
817 // this object needs to be pushed into the next
818 // block. This saves us having to run down the
819 // threaded info pointer list twice during the next pass.
821 free_bd = free_bd->link;
822 free = free_bd->start;
824 ASSERT(!is_marked(q+1,bd));
827 unthread(q,(StgWord)free + GET_CLOSURE_TAG((StgClosure *)iptr));
837 update_bkwd_compact( generation *gen )
843 bdescr *bd, *free_bd;
845 nat size, free_blocks;
848 bd = free_bd = gen->old_blocks;
849 free = free_bd->start;
852 // cycle through all the blocks in the step
853 for (; bd != NULL; bd = bd->link) {
856 while (p < bd->free ) {
858 while ( p < bd->free && !is_marked(p,bd) ) {
867 m = * ((StgPtr)bd->u.bitmap + ((p - bd->start) / (BITS_IN(StgWord))));
868 m >>= ((p - bd->start) & (BITS_IN(StgWord) - 1));
870 while ( p < bd->free ) {
875 if (((StgWord)p & (sizeof(W_) * BITS_IN(StgWord))) == 0) {
883 if (is_marked(p+1,bd)) {
884 // don't forget to update the free ptr in the block desc.
885 free_bd->free = free;
886 free_bd = free_bd->link;
887 free = free_bd->start;
891 iptr = get_threaded_info(p);
892 unthread(p, (StgWord)free + GET_CLOSURE_TAG((StgClosure *)iptr));
893 ASSERT(LOOKS_LIKE_INFO_PTR((StgWord)((StgClosure *)p)->header.info));
894 info = get_itbl((StgClosure *)p);
895 size = closure_sizeW_((StgClosure *)p,info);
902 if (info->type == TSO) {
903 move_TSO((StgTSO *)p, (StgTSO *)free);
914 // free the remaining blocks and count what's left.
915 free_bd->free = free;
916 if (free_bd->link != NULL) {
917 freeChain(free_bd->link);
918 free_bd->link = NULL;
925 compact(StgClosure *static_objects)
930 // 1. thread the roots
931 markCapabilities((evac_fn)thread_root, NULL);
933 // the weak pointer lists...
934 if (weak_ptr_list != NULL) {
935 thread((void *)&weak_ptr_list);
937 if (old_weak_ptr_list != NULL) {
938 thread((void *)&old_weak_ptr_list); // tmp
942 for (g = 1; g < RtsFlags.GcFlags.generations; g++) {
946 for (bd = generations[g].mut_list; bd != NULL; bd = bd->link) {
947 for (p = bd->start; p < bd->free; p++) {
948 thread((StgClosure **)p);
951 for (n = 0; n < n_capabilities; n++) {
952 for (bd = capabilities[n].mut_lists[g];
953 bd != NULL; bd = bd->link) {
954 for (p = bd->start; p < bd->free; p++) {
955 thread((StgClosure **)p);
961 // the global thread list
962 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
963 thread((void *)&generations[g].threads);
966 // any threads resurrected during this GC
967 thread((void *)&resurrected_threads);
973 for (task = all_tasks; task != NULL; task = task->all_link) {
974 for (incall = task->incall; incall != NULL;
975 incall = incall->prev_stack) {
977 thread_(&incall->tso);
983 // the static objects
984 thread_static(static_objects /* ToDo: ok? */);
986 // the stable pointer table
987 threadStablePtrTable((evac_fn)thread_root, NULL);
989 // the CAF list (used by GHCi)
990 markCAFs((evac_fn)thread_root, NULL);
992 // 2. update forward ptrs
993 for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
994 gen = &generations[g];
995 debugTrace(DEBUG_gc, "update_fwd: %d", g);
997 update_fwd(gen->blocks);
998 update_fwd_large(gen->scavenged_large_objects);
999 if (g == RtsFlags.GcFlags.generations-1 && gen->old_blocks != NULL) {
1000 debugTrace(DEBUG_gc, "update_fwd: %d (compact)", g);
1001 update_fwd_compact(gen->old_blocks);
1005 // 3. update backward ptrs
1007 if (gen->old_blocks != NULL) {
1008 blocks = update_bkwd_compact(gen);
1009 debugTrace(DEBUG_gc,
1010 "update_bkwd: %d (compact, old: %d blocks, now %d blocks)",
1011 gen->no, gen->n_old_blocks, blocks);
1012 gen->n_old_blocks = blocks;