update sources to ceph Nautilus 14.2.1

[ceph.git] / ceph / src / civetweb / src / third_party / duktape-1.5.2 / src-separate / duk_heap_markandsweep.c

/*
 *  Mark-and-sweep garbage collection.
 */

#include "duk_internal.h"

#ifdef DUK_USE_MARK_AND_SWEEP

DUK_LOCAL_DECL void duk__mark_heaphdr(duk_heap *heap, duk_heaphdr *h);
DUK_LOCAL_DECL void duk__mark_tval(duk_heap *heap, duk_tval *tv);

/*
 *  Misc
 */

/* Select a thread for mark-and-sweep use.
 *
 * XXX: This needs to change later.
 */
DUK_LOCAL duk_hthread *duk__get_temp_hthread(duk_heap *heap) {
        if (heap->curr_thread) {
                return heap->curr_thread;
        }
        return heap->heap_thread;  /* may be NULL, too */
}

/*
 *  Marking functions for heap types: mark children recursively
 */

DUK_LOCAL void duk__mark_hstring(duk_heap *heap, duk_hstring *h) {
        DUK_UNREF(heap);
        DUK_UNREF(h);

        DUK_DDD(DUK_DDDPRINT("duk__mark_hstring: %p", (void *) h));
        DUK_ASSERT(h);

        /* nothing to process */
}

DUK_LOCAL void duk__mark_hobject(duk_heap *heap, duk_hobject *h) {
        duk_uint_fast32_t i;

        DUK_DDD(DUK_DDDPRINT("duk__mark_hobject: %p", (void *) h));

        DUK_ASSERT(h);

        /* XXX: use advancing pointers instead of index macros -> faster and smaller? */

        for (i = 0; i < (duk_uint_fast32_t) DUK_HOBJECT_GET_ENEXT(h); i++) {
                duk_hstring *key = DUK_HOBJECT_E_GET_KEY(heap, h, i);
                if (!key) {
                        continue;
                }
                duk__mark_heaphdr(heap, (duk_heaphdr *) key);
                if (DUK_HOBJECT_E_SLOT_IS_ACCESSOR(heap, h, i)) {
                        duk__mark_heaphdr(heap, (duk_heaphdr *) DUK_HOBJECT_E_GET_VALUE_PTR(heap, h, i)->a.get);
                        duk__mark_heaphdr(heap, (duk_heaphdr *) DUK_HOBJECT_E_GET_VALUE_PTR(heap, h, i)->a.set);
                } else {
                        duk__mark_tval(heap, &DUK_HOBJECT_E_GET_VALUE_PTR(heap, h, i)->v);
                }
        }

        for (i = 0; i < (duk_uint_fast32_t) DUK_HOBJECT_GET_ASIZE(h); i++) {
                duk__mark_tval(heap, DUK_HOBJECT_A_GET_VALUE_PTR(heap, h, i));
        }

        /* hash part is a 'weak reference' and does not contribute */

        duk__mark_heaphdr(heap, (duk_heaphdr *) DUK_HOBJECT_GET_PROTOTYPE(heap, h));

        if (DUK_HOBJECT_IS_COMPILEDFUNCTION(h)) {
                duk_hcompiledfunction *f = (duk_hcompiledfunction *) h;
                duk_tval *tv, *tv_end;
                duk_hobject **fn, **fn_end;

                /* 'data' is reachable through every compiled function which
                 * contains a reference.
                 */

                duk__mark_heaphdr(heap, (duk_heaphdr *) DUK_HCOMPILEDFUNCTION_GET_DATA(heap, f));

                if (DUK_HCOMPILEDFUNCTION_GET_DATA(heap, f) != NULL) {
                        tv = DUK_HCOMPILEDFUNCTION_GET_CONSTS_BASE(heap, f);
                        tv_end = DUK_HCOMPILEDFUNCTION_GET_CONSTS_END(heap, f);
                        while (tv < tv_end) {
                                duk__mark_tval(heap, tv);
                                tv++;
                        }

                        fn = DUK_HCOMPILEDFUNCTION_GET_FUNCS_BASE(heap, f);
                        fn_end = DUK_HCOMPILEDFUNCTION_GET_FUNCS_END(heap, f);
                        while (fn < fn_end) {
                                duk__mark_heaphdr(heap, (duk_heaphdr *) *fn);
                                fn++;
                        }
                } else {
                        /* May happen in some out-of-memory corner cases. */
                        DUK_D(DUK_DPRINT("duk_hcompiledfunction 'data' is NULL, skipping marking"));
                }
        } else if (DUK_HOBJECT_IS_NATIVEFUNCTION(h)) {
                duk_hnativefunction *f = (duk_hnativefunction *) h;
                DUK_UNREF(f);
                /* nothing to mark */
        } else if (DUK_HOBJECT_IS_BUFFEROBJECT(h)) {
                duk_hbufferobject *b = (duk_hbufferobject *) h;
                duk__mark_heaphdr(heap, (duk_heaphdr *) b->buf);
        } else if (DUK_HOBJECT_IS_THREAD(h)) {
                duk_hthread *t = (duk_hthread *) h;
                duk_tval *tv;

                tv = t->valstack;
                while (tv < t->valstack_top) {
                        duk__mark_tval(heap, tv);
                        tv++;
                }

                for (i = 0; i < (duk_uint_fast32_t) t->callstack_top; i++) {
                        duk_activation *act = t->callstack + i;
                        duk__mark_heaphdr(heap, (duk_heaphdr *) DUK_ACT_GET_FUNC(act));
                        duk__mark_heaphdr(heap, (duk_heaphdr *) act->var_env);
                        duk__mark_heaphdr(heap, (duk_heaphdr *) act->lex_env);
#ifdef DUK_USE_NONSTD_FUNC_CALLER_PROPERTY
                        duk__mark_heaphdr(heap, (duk_heaphdr *) act->prev_caller);
#endif
                }

#if 0  /* nothing now */
                for (i = 0; i < (duk_uint_fast32_t) t->catchstack_top; i++) {
                        duk_catcher *cat = t->catchstack + i;
                }
#endif

                duk__mark_heaphdr(heap, (duk_heaphdr *) t->resumer);

                /* XXX: duk_small_uint_t would be enough for this loop */
                for (i = 0; i < DUK_NUM_BUILTINS; i++) {
                        duk__mark_heaphdr(heap, (duk_heaphdr *) t->builtins[i]);
                }
        }
}

/* recursion tracking happens here only */
DUK_LOCAL void duk__mark_heaphdr(duk_heap *heap, duk_heaphdr *h) {
        DUK_DDD(DUK_DDDPRINT("duk__mark_heaphdr %p, type %ld",
                             (void *) h,
                             (h != NULL ? (long) DUK_HEAPHDR_GET_TYPE(h) : (long) -1)));
        if (!h) {
                return;
        }
#if defined(DUK_USE_ROM_OBJECTS)
        if (DUK_HEAPHDR_HAS_READONLY(h)) {
                DUK_DDD(DUK_DDDPRINT("readonly object %p, skip", (void *) h));
                return;
        }
#endif
        if (DUK_HEAPHDR_HAS_REACHABLE(h)) {
                DUK_DDD(DUK_DDDPRINT("already marked reachable, skip"));
                return;
        }
        DUK_HEAPHDR_SET_REACHABLE(h);

        if (heap->mark_and_sweep_recursion_depth >= DUK_USE_MARK_AND_SWEEP_RECLIMIT) {
                /* log this with a normal debug level because this should be relatively rare */
                DUK_D(DUK_DPRINT("mark-and-sweep recursion limit reached, marking as temproot: %p", (void *) h));
                DUK_HEAP_SET_MARKANDSWEEP_RECLIMIT_REACHED(heap);
                DUK_HEAPHDR_SET_TEMPROOT(h);
                return;
        }

        heap->mark_and_sweep_recursion_depth++;

        switch ((int) DUK_HEAPHDR_GET_TYPE(h)) {
        case DUK_HTYPE_STRING:
                duk__mark_hstring(heap, (duk_hstring *) h);
                break;
        case DUK_HTYPE_OBJECT:
                duk__mark_hobject(heap, (duk_hobject *) h);
                break;
        case DUK_HTYPE_BUFFER:
                /* nothing to mark */
                break;
        default:
                DUK_D(DUK_DPRINT("attempt to mark heaphdr %p with invalid htype %ld", (void *) h, (long) DUK_HEAPHDR_GET_TYPE(h)));
                DUK_UNREACHABLE();
        }

        heap->mark_and_sweep_recursion_depth--;
}

DUK_LOCAL void duk__mark_tval(duk_heap *heap, duk_tval *tv) {
        DUK_DDD(DUK_DDDPRINT("duk__mark_tval %p", (void *) tv));
        if (!tv) {
                return;
        }
        if (DUK_TVAL_IS_HEAP_ALLOCATED(tv)) {
                duk__mark_heaphdr(heap, DUK_TVAL_GET_HEAPHDR(tv));
        }
}

/*
 *  Mark the heap.
 */

DUK_LOCAL void duk__mark_roots_heap(duk_heap *heap) {
        duk_small_uint_t i;

        DUK_DD(DUK_DDPRINT("duk__mark_roots_heap: %p", (void *) heap));

        duk__mark_heaphdr(heap, (duk_heaphdr *) heap->heap_thread);
        duk__mark_heaphdr(heap, (duk_heaphdr *) heap->heap_object);

        for (i = 0; i < DUK_HEAP_NUM_STRINGS; i++) {
                duk_hstring *h = DUK_HEAP_GET_STRING(heap, i);
                duk__mark_heaphdr(heap, (duk_heaphdr *) h);
        }

        duk__mark_tval(heap, &heap->lj.value1);
        duk__mark_tval(heap, &heap->lj.value2);

#if defined(DUK_USE_DEBUGGER_SUPPORT)
        for (i = 0; i < heap->dbg_breakpoint_count; i++) {
                duk__mark_heaphdr(heap, (duk_heaphdr *) heap->dbg_breakpoints[i].filename);
        }
#endif
}

/*
 *  Mark refzero_list objects.
 *
 *  Objects on the refzero_list have no inbound references.  They might have
 *  outbound references to objects that we might free, which would invalidate
 *  any references held by the refzero objects.  A refzero object might also
 *  be rescued by refcount finalization.  Refzero objects are treated as
 *  reachability roots to ensure they (or anything they point to) are not
 *  freed in mark-and-sweep.
 */

#ifdef DUK_USE_REFERENCE_COUNTING
DUK_LOCAL void duk__mark_refzero_list(duk_heap *heap) {
        duk_heaphdr *hdr;

        DUK_DD(DUK_DDPRINT("duk__mark_refzero_list: %p", (void *) heap));

        hdr = heap->refzero_list;
        while (hdr) {
                duk__mark_heaphdr(heap, hdr);
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
}
#endif

/*
 *  Mark unreachable, finalizable objects.
 *
 *  Such objects will be moved aside and their finalizers run later.  They have
 *  to be treated as reachability roots for their properties etc to remain
 *  allocated.  This marking is only done for unreachable values which would
 *  be swept later (refzero_list is thus excluded).
 *
 *  Objects are first marked FINALIZABLE and only then marked as reachability
 *  roots; otherwise circular references might be handled inconsistently.
 */

DUK_LOCAL void duk__mark_finalizable(duk_heap *heap) {
        duk_hthread *thr;
        duk_heaphdr *hdr;
        duk_size_t count_finalizable = 0;

        DUK_DD(DUK_DDPRINT("duk__mark_finalizable: %p", (void *) heap));

        thr = duk__get_temp_hthread(heap);
        DUK_ASSERT(thr != NULL);

        hdr = heap->heap_allocated;
        while (hdr) {
                /* A finalizer is looked up from the object and up its prototype chain
                 * (which allows inherited finalizers).  A prototype loop must not cause
                 * an error to be thrown here; duk_hobject_hasprop_raw() will ignore a
                 * prototype loop silently and indicate that the property doesn't exist.
                 */

                if (!DUK_HEAPHDR_HAS_REACHABLE(hdr) &&
                    DUK_HEAPHDR_GET_TYPE(hdr) == DUK_HTYPE_OBJECT &&
                    !DUK_HEAPHDR_HAS_FINALIZED(hdr) &&
                    duk_hobject_hasprop_raw(thr, (duk_hobject *) hdr, DUK_HTHREAD_STRING_INT_FINALIZER(thr))) {

                        /* heaphdr:
                         *  - is not reachable
                         *  - is an object
                         *  - is not a finalized object
                         *  - has a finalizer
                         */

                        DUK_DD(DUK_DDPRINT("unreachable heap object will be "
                                           "finalized -> mark as finalizable "
                                           "and treat as a reachability root: %p",
                                           (void *) hdr));
                        DUK_ASSERT(!DUK_HEAPHDR_HAS_READONLY(hdr));
                        DUK_HEAPHDR_SET_FINALIZABLE(hdr);
                        count_finalizable ++;
                }

                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }

        if (count_finalizable == 0) {
                return;
        }

        DUK_DD(DUK_DDPRINT("marked %ld heap objects as finalizable, now mark them reachable",
                           (long) count_finalizable));

        hdr = heap->heap_allocated;
        while (hdr) {
                if (DUK_HEAPHDR_HAS_FINALIZABLE(hdr)) {
                        duk__mark_heaphdr(heap, hdr);
                }

                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }

        /* Caller will finish the marking process if we hit a recursion limit. */
}

/*
 *  Mark objects on finalize_list.
 *
 */

DUK_LOCAL void duk__mark_finalize_list(duk_heap *heap) {
        duk_heaphdr *hdr;
#ifdef DUK_USE_DEBUG
        duk_size_t count_finalize_list = 0;
#endif

        DUK_DD(DUK_DDPRINT("duk__mark_finalize_list: %p", (void *) heap));

        hdr = heap->finalize_list;
        while (hdr) {
                duk__mark_heaphdr(heap, hdr);
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
#ifdef DUK_USE_DEBUG
                count_finalize_list++;
#endif
        }

#ifdef DUK_USE_DEBUG
        if (count_finalize_list > 0) {
                DUK_D(DUK_DPRINT("marked %ld objects on the finalize_list as reachable (previous finalizer run skipped)",
                                 (long) count_finalize_list));
        }
#endif
}

/*
 *  Fallback marking handler if recursion limit is reached.
 *
 *  Iterates 'temproots' until recursion limit is no longer hit.  Note
 *  that temproots may reside either in heap allocated list or the
 *  refzero work list.  This is a slow scan, but guarantees that we
 *  finish with a bounded C stack.
 *
 *  Note that nodes may have been marked as temproots before this
 *  scan begun, OR they may have been marked during the scan (as
 *  we process nodes recursively also during the scan).  This is
 *  intended behavior.
 */

#ifdef DUK_USE_DEBUG
DUK_LOCAL void duk__handle_temproot(duk_heap *heap, duk_heaphdr *hdr, duk_size_t *count) {
#else
DUK_LOCAL void duk__handle_temproot(duk_heap *heap, duk_heaphdr *hdr) {
#endif
        if (!DUK_HEAPHDR_HAS_TEMPROOT(hdr)) {
                DUK_DDD(DUK_DDDPRINT("not a temp root: %p", (void *) hdr));
                return;
        }

        DUK_DDD(DUK_DDDPRINT("found a temp root: %p", (void *) hdr));
        DUK_HEAPHDR_CLEAR_TEMPROOT(hdr);
        DUK_HEAPHDR_CLEAR_REACHABLE(hdr);  /* done so that duk__mark_heaphdr() works correctly */
        duk__mark_heaphdr(heap, hdr);

#ifdef DUK_USE_DEBUG
        (*count)++;
#endif
}

DUK_LOCAL void duk__mark_temproots_by_heap_scan(duk_heap *heap) {
        duk_heaphdr *hdr;
#ifdef DUK_USE_DEBUG
        duk_size_t count;
#endif

        DUK_DD(DUK_DDPRINT("duk__mark_temproots_by_heap_scan: %p", (void *) heap));

        while (DUK_HEAP_HAS_MARKANDSWEEP_RECLIMIT_REACHED(heap)) {
                DUK_DD(DUK_DDPRINT("recursion limit reached, doing heap scan to continue from temproots"));

#ifdef DUK_USE_DEBUG
                count = 0;
#endif
                DUK_HEAP_CLEAR_MARKANDSWEEP_RECLIMIT_REACHED(heap);

                hdr = heap->heap_allocated;
                while (hdr) {
#ifdef DUK_USE_DEBUG
                        duk__handle_temproot(heap, hdr, &count);
#else
                        duk__handle_temproot(heap, hdr);
#endif
                        hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
                }

                /* must also check refzero_list */
#ifdef DUK_USE_REFERENCE_COUNTING
                hdr = heap->refzero_list;
                while (hdr) {
#ifdef DUK_USE_DEBUG
                        duk__handle_temproot(heap, hdr, &count);
#else
                        duk__handle_temproot(heap, hdr);
#endif
                        hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
                }
#endif  /* DUK_USE_REFERENCE_COUNTING */

#ifdef DUK_USE_DEBUG
                DUK_DD(DUK_DDPRINT("temproot mark heap scan processed %ld temp roots", (long) count));
#endif
        }
}

/*
 *  Finalize refcounts for heap elements just about to be freed.
 *  This must be done for all objects before freeing to avoid any
 *  stale pointer dereferences.
 *
 *  Note that this must deduce the set of objects to be freed
 *  identically to duk__sweep_heap().
 */

#ifdef DUK_USE_REFERENCE_COUNTING
DUK_LOCAL void duk__finalize_refcounts(duk_heap *heap) {
        duk_hthread *thr;
        duk_heaphdr *hdr;

        thr = duk__get_temp_hthread(heap);
        DUK_ASSERT(thr != NULL);

        DUK_DD(DUK_DDPRINT("duk__finalize_refcounts: heap=%p, hthread=%p",
                           (void *) heap, (void *) thr));

        hdr = heap->heap_allocated;
        while (hdr) {
                if (!DUK_HEAPHDR_HAS_REACHABLE(hdr)) {
                        /*
                         *  Unreachable object about to be swept.  Finalize target refcounts
                         *  (objects which the unreachable object points to) without doing
                         *  refzero processing.  Recursive decrefs are also prevented when
                         *  refzero processing is disabled.
                         *
                         *  Value cannot be a finalizable object, as they have been made
                         *  temporarily reachable for this round.
                         */

                        DUK_DDD(DUK_DDDPRINT("unreachable object, refcount finalize before sweeping: %p", (void *) hdr));
                        duk_heaphdr_refcount_finalize(thr, hdr);
                }

                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
}
#endif  /* DUK_USE_REFERENCE_COUNTING */

/*
 *  Clear (reachable) flags of refzero work list.
 */

#ifdef DUK_USE_REFERENCE_COUNTING
DUK_LOCAL void duk__clear_refzero_list_flags(duk_heap *heap) {
        duk_heaphdr *hdr;

        DUK_DD(DUK_DDPRINT("duk__clear_refzero_list_flags: %p", (void *) heap));

        hdr = heap->refzero_list;
        while (hdr) {
                DUK_HEAPHDR_CLEAR_REACHABLE(hdr);
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_TEMPROOT(hdr));
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
}
#endif  /* DUK_USE_REFERENCE_COUNTING */

/*
 *  Clear (reachable) flags of finalize_list
 *
 *  We could mostly do in the sweep phase when we move objects from the
 *  heap into the finalize_list.  However, if a finalizer run is skipped
 *  during a mark-and-sweep, the objects on the finalize_list will be marked
 *  reachable during the next mark-and-sweep.  Since they're already on the
 *  finalize_list, no-one will be clearing their REACHABLE flag so we do it
 *  here.  (This now overlaps with the sweep handling in a harmless way.)
 */

DUK_LOCAL void duk__clear_finalize_list_flags(duk_heap *heap) {
        duk_heaphdr *hdr;

        DUK_DD(DUK_DDPRINT("duk__clear_finalize_list_flags: %p", (void *) heap));

        hdr = heap->finalize_list;
        while (hdr) {
                DUK_HEAPHDR_CLEAR_REACHABLE(hdr);
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_TEMPROOT(hdr));
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
}

/*
 *  Sweep stringtable
 */

#if defined(DUK_USE_STRTAB_CHAIN)

/* XXX: skip count_free w/o debug? */
#if defined(DUK_USE_HEAPPTR16)
DUK_LOCAL void duk__sweep_string_chain16(duk_heap *heap, duk_uint16_t *slot, duk_size_t *count_keep, duk_size_t *count_free) {
        duk_uint16_t h16 = *slot;
        duk_hstring *h;
        duk_uint16_t null16 = heap->heapptr_null16;

        if (h16 == null16) {
                /* nop */
                return;
        }
        h = (duk_hstring *) DUK_USE_HEAPPTR_DEC16(heap->heap_udata, h16);
        DUK_ASSERT(h != NULL);

        if (DUK_HEAPHDR_HAS_REACHABLE((duk_heaphdr *) h)) {
                DUK_HEAPHDR_CLEAR_REACHABLE((duk_heaphdr *) h);
                (*count_keep)++;
        } else {
#if defined(DUK_USE_REFERENCE_COUNTING)
                DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT((duk_heaphdr *) h) == 0);
#endif
                /* deal with weak references first */
                duk_heap_strcache_string_remove(heap, (duk_hstring *) h);
                *slot = null16;

                /* free inner references (these exist e.g. when external
                 * strings are enabled)
                 */
                duk_free_hstring_inner(heap, h);
                DUK_FREE(heap, h);
                (*count_free)++;
        }
}
#else  /* DUK_USE_HEAPPTR16 */
DUK_LOCAL void duk__sweep_string_chain(duk_heap *heap, duk_hstring **slot, duk_size_t *count_keep, duk_size_t *count_free) {
        duk_hstring *h = *slot;

        if (h == NULL) {
                /* nop */
                return;
        }

        if (DUK_HEAPHDR_HAS_REACHABLE((duk_heaphdr *) h)) {
                DUK_HEAPHDR_CLEAR_REACHABLE((duk_heaphdr *) h);
                (*count_keep)++;
        } else {
#if defined(DUK_USE_REFERENCE_COUNTING)
                DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT((duk_heaphdr *) h) == 0);
#endif
                /* deal with weak references first */
                duk_heap_strcache_string_remove(heap, (duk_hstring *) h);
                *slot = NULL;

                /* free inner references (these exist e.g. when external
                 * strings are enabled)
                 */
                duk_free_hstring_inner(heap, h);
                DUK_FREE(heap, h);
                (*count_free)++;
        }
}
#endif  /* DUK_USE_HEAPPTR16 */

DUK_LOCAL void duk__sweep_stringtable_chain(duk_heap *heap, duk_size_t *out_count_keep) {
        duk_strtab_entry *e;
        duk_uint_fast32_t i;
        duk_size_t count_free = 0;
        duk_size_t count_keep = 0;
        duk_size_t j, n;
#if defined(DUK_USE_HEAPPTR16)
        duk_uint16_t *lst;
#else
        duk_hstring **lst;
#endif

        DUK_DD(DUK_DDPRINT("duk__sweep_stringtable: %p", (void *) heap));

        /* Non-zero refcounts should not happen for unreachable strings,
         * because we refcount finalize all unreachable objects which
         * should have decreased unreachable string refcounts to zero
         * (even for cycles).
         */

        for (i = 0; i < DUK_STRTAB_CHAIN_SIZE; i++) {
                e = heap->strtable + i;
                if (e->listlen == 0) {
#if defined(DUK_USE_HEAPPTR16)
                        duk__sweep_string_chain16(heap, &e->u.str16, &count_keep, &count_free);
#else
                        duk__sweep_string_chain(heap, &e->u.str, &count_keep, &count_free);
#endif
                } else {
#if defined(DUK_USE_HEAPPTR16)
                        lst = (duk_uint16_t *) DUK_USE_HEAPPTR_DEC16(heap->heap_udata, e->u.strlist16);
#else
                        lst = e->u.strlist;
#endif
                        for (j = 0, n = e->listlen; j < n; j++) {
#if defined(DUK_USE_HEAPPTR16)
                                duk__sweep_string_chain16(heap, lst + j, &count_keep, &count_free);
#else
                                duk__sweep_string_chain(heap, lst + j, &count_keep, &count_free);
#endif
                        }
                }
        }

        DUK_D(DUK_DPRINT("mark-and-sweep sweep stringtable: %ld freed, %ld kept",
                         (long) count_free, (long) count_keep));
        *out_count_keep = count_keep;
}
#endif  /* DUK_USE_STRTAB_CHAIN */

#if defined(DUK_USE_STRTAB_PROBE)
DUK_LOCAL void duk__sweep_stringtable_probe(duk_heap *heap, duk_size_t *out_count_keep) {
        duk_hstring *h;
        duk_uint_fast32_t i;
#ifdef DUK_USE_DEBUG
        duk_size_t count_free = 0;
#endif
        duk_size_t count_keep = 0;

        DUK_DD(DUK_DDPRINT("duk__sweep_stringtable: %p", (void *) heap));

        for (i = 0; i < heap->st_size; i++) {
#if defined(DUK_USE_HEAPPTR16)
                h = (duk_hstring *) DUK_USE_HEAPPTR_DEC16(heap->heap_udata, heap->strtable16[i]);
#else
                h = heap->strtable[i];
#endif
                if (h == NULL || h == DUK_STRTAB_DELETED_MARKER(heap)) {
                        continue;
                } else if (DUK_HEAPHDR_HAS_REACHABLE((duk_heaphdr *) h)) {
                        DUK_HEAPHDR_CLEAR_REACHABLE((duk_heaphdr *) h);
                        count_keep++;
                        continue;
                }

#ifdef DUK_USE_DEBUG
                count_free++;
#endif

#if defined(DUK_USE_REFERENCE_COUNTING)
                /* Non-zero refcounts should not happen for unreachable strings,
                 * because we refcount finalize all unreachable objects which
                 * should have decreased unreachable string refcounts to zero
                 * (even for cycles).
                 */
                DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT((duk_heaphdr *) h) == 0);
#endif

                DUK_DDD(DUK_DDDPRINT("sweep string, not reachable: %p", (void *) h));

                /* deal with weak references first */
                duk_heap_strcache_string_remove(heap, (duk_hstring *) h);

                /* remove the string (mark DELETED), could also call
                 * duk_heap_string_remove() but that would be slow and
                 * pointless because we already know the slot.
                 */
#if defined(DUK_USE_HEAPPTR16)
                heap->strtable16[i] = heap->heapptr_deleted16;
#else
                heap->strtable[i] = DUK_STRTAB_DELETED_MARKER(heap);
#endif

                /* free inner references (these exist e.g. when external
                 * strings are enabled)
                 */
                duk_free_hstring_inner(heap, (duk_hstring *) h);

                /* finally free the struct itself */
                DUK_FREE(heap, h);
        }

#ifdef DUK_USE_DEBUG
        DUK_D(DUK_DPRINT("mark-and-sweep sweep stringtable: %ld freed, %ld kept",
                         (long) count_free, (long) count_keep));
#endif
        *out_count_keep = count_keep;
}
#endif  /* DUK_USE_STRTAB_PROBE */

/*
 *  Sweep heap
 */

DUK_LOCAL void duk__sweep_heap(duk_heap *heap, duk_int_t flags, duk_size_t *out_count_keep) {
        duk_heaphdr *prev;  /* last element that was left in the heap */
        duk_heaphdr *curr;
        duk_heaphdr *next;
#ifdef DUK_USE_DEBUG
        duk_size_t count_free = 0;
        duk_size_t count_finalize = 0;
        duk_size_t count_rescue = 0;
#endif
        duk_size_t count_keep = 0;

        DUK_UNREF(flags);
        DUK_DD(DUK_DDPRINT("duk__sweep_heap: %p", (void *) heap));

        prev = NULL;
        curr = heap->heap_allocated;
        heap->heap_allocated = NULL;
        while (curr) {
                /* Strings and ROM objects are never placed on the heap allocated list. */
                DUK_ASSERT(DUK_HEAPHDR_GET_TYPE(curr) != DUK_HTYPE_STRING);
                DUK_ASSERT(!DUK_HEAPHDR_HAS_READONLY(curr));

                next = DUK_HEAPHDR_GET_NEXT(heap, curr);

                if (DUK_HEAPHDR_HAS_REACHABLE(curr)) {
                        /*
                         *  Reachable object, keep
                         */

                        DUK_DDD(DUK_DDDPRINT("sweep, reachable: %p", (void *) curr));

                        if (DUK_HEAPHDR_HAS_FINALIZABLE(curr)) {
                                /*
                                 *  If object has been marked finalizable, move it to the
                                 *  "to be finalized" work list.  It will be collected on
                                 *  the next mark-and-sweep if it is still unreachable
                                 *  after running the finalizer.
                                 */

                                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(curr));
                                DUK_ASSERT(DUK_HEAPHDR_GET_TYPE(curr) == DUK_HTYPE_OBJECT);
                                DUK_DDD(DUK_DDDPRINT("object has finalizer, move to finalization work list: %p", (void *) curr));

#ifdef DUK_USE_DOUBLE_LINKED_HEAP
                                if (heap->finalize_list) {
                                        DUK_HEAPHDR_SET_PREV(heap, heap->finalize_list, curr);
                                }
                                DUK_HEAPHDR_SET_PREV(heap, curr, NULL);
#endif
                                DUK_HEAPHDR_SET_NEXT(heap, curr, heap->finalize_list);
                                DUK_ASSERT_HEAPHDR_LINKS(heap, curr);
                                heap->finalize_list = curr;
#ifdef DUK_USE_DEBUG
                                count_finalize++;
#endif
                        } else {
                                /*
                                 *  Object will be kept; queue object back to heap_allocated (to tail)
                                 */

                                if (DUK_HEAPHDR_HAS_FINALIZED(curr)) {
                                        /*
                                         *  Object's finalizer was executed on last round, and
                                         *  object has been happily rescued.
                                         */

                                        DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(curr));
                                        DUK_ASSERT(DUK_HEAPHDR_GET_TYPE(curr) == DUK_HTYPE_OBJECT);
                                        DUK_DD(DUK_DDPRINT("object rescued during mark-and-sweep finalization: %p", (void *) curr));
#ifdef DUK_USE_DEBUG
                                        count_rescue++;
#endif
                                } else {
                                        /*
                                         *  Plain, boring reachable object.
                                         */
                                        DUK_DD(DUK_DDPRINT("keep object: %!iO", curr));
                                        count_keep++;
                                }

                                if (!heap->heap_allocated) {
                                        heap->heap_allocated = curr;
                                }
                                if (prev) {
                                        DUK_HEAPHDR_SET_NEXT(heap, prev, curr);
                                }
#ifdef DUK_USE_DOUBLE_LINKED_HEAP
                                DUK_HEAPHDR_SET_PREV(heap, curr, prev);
#endif
                                DUK_ASSERT_HEAPHDR_LINKS(heap, prev);
                                DUK_ASSERT_HEAPHDR_LINKS(heap, curr);
                                prev = curr;
                        }

                        DUK_HEAPHDR_CLEAR_REACHABLE(curr);
                        DUK_HEAPHDR_CLEAR_FINALIZED(curr);
                        DUK_HEAPHDR_CLEAR_FINALIZABLE(curr);

                        DUK_ASSERT(!DUK_HEAPHDR_HAS_REACHABLE(curr));
                        DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(curr));
                        DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(curr));

                        curr = next;
                } else {
                        /*
                         *  Unreachable object, free
                         */

                        DUK_DDD(DUK_DDDPRINT("sweep, not reachable: %p", (void *) curr));

#if defined(DUK_USE_REFERENCE_COUNTING)
                        /* Non-zero refcounts should not happen because we refcount
                         * finalize all unreachable objects which should cancel out
                         * refcounts (even for cycles).
                         */
                        DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(curr) == 0);
#endif
                        DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(curr));

                        if (DUK_HEAPHDR_HAS_FINALIZED(curr)) {
                                DUK_DDD(DUK_DDDPRINT("finalized object not rescued: %p", (void *) curr));
                        }

                        /* Note: object cannot be a finalizable unreachable object, as
                         * they have been marked temporarily reachable for this round,
                         * and are handled above.
                         */

#ifdef DUK_USE_DEBUG
                        count_free++;
#endif

                        /* weak refs should be handled here, but no weak refs for
                         * any non-string objects exist right now.
                         */

                        /* free object and all auxiliary (non-heap) allocs */
                        duk_heap_free_heaphdr_raw(heap, curr);

                        curr = next;
                }
        }
        if (prev) {
                DUK_HEAPHDR_SET_NEXT(heap, prev, NULL);
        }
        DUK_ASSERT_HEAPHDR_LINKS(heap, prev);

#ifdef DUK_USE_DEBUG
        DUK_D(DUK_DPRINT("mark-and-sweep sweep objects (non-string): %ld freed, %ld kept, %ld rescued, %ld queued for finalization",
                         (long) count_free, (long) count_keep, (long) count_rescue, (long) count_finalize));
#endif
        *out_count_keep = count_keep;
}

/*
 *  Run (object) finalizers in the "to be finalized" work list.
 */

DUK_LOCAL void duk__run_object_finalizers(duk_heap *heap, duk_small_uint_t flags) {
        duk_heaphdr *curr;
        duk_heaphdr *next;
#ifdef DUK_USE_DEBUG
        duk_size_t count = 0;
#endif
        duk_hthread *thr;

        DUK_DD(DUK_DDPRINT("duk__run_object_finalizers: %p", (void *) heap));

        thr = duk__get_temp_hthread(heap);
        DUK_ASSERT(thr != NULL);

        curr = heap->finalize_list;
        while (curr) {
                DUK_DDD(DUK_DDDPRINT("mark-and-sweep finalize: %p", (void *) curr));

                DUK_ASSERT(DUK_HEAPHDR_GET_TYPE(curr) == DUK_HTYPE_OBJECT);  /* only objects have finalizers */
                DUK_ASSERT(!DUK_HEAPHDR_HAS_REACHABLE(curr));                /* flags have been already cleared */
                DUK_ASSERT(!DUK_HEAPHDR_HAS_TEMPROOT(curr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(curr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(curr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_READONLY(curr));  /* No finalizers for ROM objects */

                if (DUK_LIKELY((flags & DUK_MS_FLAG_SKIP_FINALIZERS) == 0)) {
                        /* Run the finalizer, duk_hobject_run_finalizer() sets FINALIZED.
                         * Next mark-and-sweep will collect the object unless it has
                         * become reachable (i.e. rescued).  FINALIZED prevents the
                         * finalizer from being executed again before that.
                         */
                        duk_hobject_run_finalizer(thr, (duk_hobject *) curr);  /* must never longjmp */
                        DUK_ASSERT(DUK_HEAPHDR_HAS_FINALIZED(curr));
                } else {
                        /* Used during heap destruction: don't actually run finalizers
                         * because we're heading into forced finalization.  Instead,
                         * queue finalizable objects back to the heap_allocated list.
                         */
                        DUK_D(DUK_DPRINT("skip finalizers flag set, queue object to heap_allocated without finalizing"));
                        DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(curr));
                }

                /* queue back to heap_allocated */
                next = DUK_HEAPHDR_GET_NEXT(heap, curr);
                DUK_HEAP_INSERT_INTO_HEAP_ALLOCATED(heap, curr);

                curr = next;
#ifdef DUK_USE_DEBUG
                count++;
#endif
        }

        /* finalize_list will always be processed completely */
        heap->finalize_list = NULL;

#ifdef DUK_USE_DEBUG
        DUK_D(DUK_DPRINT("mark-and-sweep finalize objects: %ld finalizers called", (long) count));
#endif
}

/*
 *  Object compaction.
 *
 *  Compaction is assumed to never throw an error.
 */

DUK_LOCAL int duk__protected_compact_object(duk_context *ctx) {
        /* XXX: for threads, compact value stack, call stack, catch stack? */

        duk_hobject *obj = duk_get_hobject(ctx, -1);
        DUK_ASSERT(obj != NULL);
        duk_hobject_compact_props((duk_hthread *) ctx, obj);
        return 0;
}

#ifdef DUK_USE_DEBUG
DUK_LOCAL void duk__compact_object_list(duk_heap *heap, duk_hthread *thr, duk_heaphdr *start, duk_size_t *p_count_check, duk_size_t *p_count_compact, duk_size_t *p_count_bytes_saved) {
#else
DUK_LOCAL void duk__compact_object_list(duk_heap *heap, duk_hthread *thr, duk_heaphdr *start) {
#endif
        duk_heaphdr *curr;
#ifdef DUK_USE_DEBUG
        duk_size_t old_size, new_size;
#endif
        duk_hobject *obj;

        DUK_UNREF(heap);

        curr = start;
        while (curr) {
                DUK_DDD(DUK_DDDPRINT("mark-and-sweep compact: %p", (void *) curr));

                if (DUK_HEAPHDR_GET_TYPE(curr) != DUK_HTYPE_OBJECT) {
                        goto next;
                }
                obj = (duk_hobject *) curr;

#ifdef DUK_USE_DEBUG
                old_size = DUK_HOBJECT_P_COMPUTE_SIZE(DUK_HOBJECT_GET_ESIZE(obj),
                                                      DUK_HOBJECT_GET_ASIZE(obj),
                                                      DUK_HOBJECT_GET_HSIZE(obj));
#endif

                DUK_DD(DUK_DDPRINT("compact object: %p", (void *) obj));
                duk_push_hobject((duk_context *) thr, obj);
                /* XXX: disable error handlers for duration of compaction? */
                duk_safe_call((duk_context *) thr, duk__protected_compact_object, 1, 0);

#ifdef DUK_USE_DEBUG
                new_size = DUK_HOBJECT_P_COMPUTE_SIZE(DUK_HOBJECT_GET_ESIZE(obj),
                                                      DUK_HOBJECT_GET_ASIZE(obj),
                                                      DUK_HOBJECT_GET_HSIZE(obj));
#endif

#ifdef DUK_USE_DEBUG
                (*p_count_compact)++;
                (*p_count_bytes_saved) += (duk_size_t) (old_size - new_size);
#endif

         next:
                curr = DUK_HEAPHDR_GET_NEXT(heap, curr);
#ifdef DUK_USE_DEBUG
                (*p_count_check)++;
#endif
        }
}

DUK_LOCAL void duk__compact_objects(duk_heap *heap) {
        /* XXX: which lists should participate?  to be finalized? */
#ifdef DUK_USE_DEBUG
        duk_size_t count_check = 0;
        duk_size_t count_compact = 0;
        duk_size_t count_bytes_saved = 0;
#endif
        duk_hthread *thr;

        DUK_DD(DUK_DDPRINT("duk__compact_objects: %p", (void *) heap));

        thr = duk__get_temp_hthread(heap);
        DUK_ASSERT(thr != NULL);

#ifdef DUK_USE_DEBUG
        duk__compact_object_list(heap, thr, heap->heap_allocated, &count_check, &count_compact, &count_bytes_saved);
        duk__compact_object_list(heap, thr, heap->finalize_list, &count_check, &count_compact, &count_bytes_saved);
#ifdef DUK_USE_REFERENCE_COUNTING
        duk__compact_object_list(heap, thr, heap->refzero_list, &count_check, &count_compact, &count_bytes_saved);
#endif
#else
        duk__compact_object_list(heap, thr, heap->heap_allocated);
        duk__compact_object_list(heap, thr, heap->finalize_list);
#ifdef DUK_USE_REFERENCE_COUNTING
        duk__compact_object_list(heap, thr, heap->refzero_list);
#endif
#endif

#ifdef DUK_USE_DEBUG
        DUK_D(DUK_DPRINT("mark-and-sweep compact objects: %ld checked, %ld compaction attempts, %ld bytes saved by compaction",
                         (long) count_check, (long) count_compact, (long) count_bytes_saved));
#endif
}

/*
 *  Assertion helpers.
 */

#ifdef DUK_USE_ASSERTIONS
DUK_LOCAL void duk__assert_heaphdr_flags(duk_heap *heap) {
        duk_heaphdr *hdr;

        hdr = heap->heap_allocated;
        while (hdr) {
                DUK_ASSERT(!DUK_HEAPHDR_HAS_REACHABLE(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_TEMPROOT(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(hdr));
                /* may have FINALIZED */
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }

#ifdef DUK_USE_REFERENCE_COUNTING
        hdr = heap->refzero_list;
        while (hdr) {
                DUK_ASSERT(!DUK_HEAPHDR_HAS_REACHABLE(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_TEMPROOT(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(hdr));
                DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZED(hdr));
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
#endif  /* DUK_USE_REFERENCE_COUNTING */
}

#ifdef DUK_USE_REFERENCE_COUNTING
DUK_LOCAL void duk__assert_valid_refcounts(duk_heap *heap) {
        duk_heaphdr *hdr = heap->heap_allocated;
        while (hdr) {
                if (DUK_HEAPHDR_GET_REFCOUNT(hdr) == 0 &&
                    DUK_HEAPHDR_HAS_FINALIZED(hdr)) {
                        /* An object may be in heap_allocated list with a zero
                         * refcount if it has just been finalized and is waiting
                         * to be collected by the next cycle.
                         */
                } else if (DUK_HEAPHDR_GET_REFCOUNT(hdr) == 0) {
                        /* An object may be in heap_allocated list with a zero
                         * refcount also if it is a temporary object created by
                         * a finalizer; because finalization now runs inside
                         * mark-and-sweep, such objects will not be queued to
                         * refzero_list and will thus appear here with refcount
                         * zero.
                         */
#if 0  /* this case can no longer occur because refcount is unsigned */
                } else if (DUK_HEAPHDR_GET_REFCOUNT(hdr) < 0) {
                        DUK_D(DUK_DPRINT("invalid refcount: %ld, %p -> %!O",
                                         (hdr != NULL ? (long) DUK_HEAPHDR_GET_REFCOUNT(hdr) : (long) 0),
                                         (void *) hdr, (duk_heaphdr *) hdr));
                        DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(hdr) > 0);
#endif
                }
                hdr = DUK_HEAPHDR_GET_NEXT(heap, hdr);
        }
}
#endif  /* DUK_USE_REFERENCE_COUNTING */
#endif  /* DUK_USE_ASSERTIONS */

/*
 *  Finalizer torture.  Do one fake finalizer call which causes side effects
 *  similar to one or more finalizers on actual objects.
 */

#if defined(DUK_USE_MARKANDSWEEP_FINALIZER_TORTURE)
DUK_LOCAL duk_ret_t duk__markandsweep_fake_finalizer(duk_context *ctx) {
        DUK_D(DUK_DPRINT("fake mark-and-sweep torture finalizer executed"));

        /* Require a lot of stack to force a value stack grow/shrink.
         * Recursive mark-and-sweep is prevented by allocation macros
         * so this won't trigger another mark-and-sweep.
         */
        duk_require_stack(ctx, 100000);

        /* XXX: do something to force a callstack grow/shrink, perhaps
         * just a manual forced resize or a forced relocating realloc?
         */

        return 0;
}

DUK_LOCAL void duk__markandsweep_torture_finalizer(duk_hthread *thr) {
        duk_context *ctx;
        duk_int_t rc;

        DUK_ASSERT(thr != NULL);
        ctx = (duk_context *) thr;

        /* Avoid fake finalization when callstack limit has been reached.
         * Otherwise a callstack limit error will be created, then refzero'ed.
         */
        if (thr->heap->call_recursion_depth >= thr->heap->call_recursion_limit ||
            thr->callstack_size + 2 * DUK_CALLSTACK_GROW_STEP >= thr->callstack_max /*approximate*/) {
                DUK_D(DUK_DPRINT("call recursion depth reached, avoid fake mark-and-sweep torture finalizer"));
                return;
        }

        /* Run fake finalizer.  Avoid creating unnecessary garbage. */
        duk_push_c_function(ctx, duk__markandsweep_fake_finalizer, 0 /*nargs*/);
        rc = duk_pcall(ctx, 0 /*nargs*/);
        DUK_UNREF(rc);  /* ignored */
        duk_pop(ctx);
}
#endif  /* DUK_USE_MARKANDSWEEP_FINALIZER_TORTURE */

/*
 *  Main mark-and-sweep function.
 *
 *  'flags' represents the features requested by the caller.  The current
 *  heap->mark_and_sweep_base_flags is ORed automatically into the flags;
 *  the base flags mask typically prevents certain mark-and-sweep operations
 *  to avoid trouble.
 */

DUK_INTERNAL duk_bool_t duk_heap_mark_and_sweep(duk_heap *heap, duk_small_uint_t flags) {
        duk_hthread *thr;
        duk_size_t count_keep_obj;
        duk_size_t count_keep_str;
#ifdef DUK_USE_VOLUNTARY_GC
        duk_size_t tmp;
#endif

        /* XXX: thread selection for mark-and-sweep is currently a hack.
         * If we don't have a thread, the entire mark-and-sweep is now
         * skipped (although we could just skip finalizations).
         */

        /* If thr != NULL, the thr may still be in the middle of
         * initialization.
         * XXX: Improve the thread viability test.
         */
        thr = duk__get_temp_hthread(heap);
        if (thr == NULL) {
                DUK_D(DUK_DPRINT("gc skipped because we don't have a temp thread"));

                /* reset voluntary gc trigger count */
#ifdef DUK_USE_VOLUNTARY_GC
                heap->mark_and_sweep_trigger_counter = DUK_HEAP_MARK_AND_SWEEP_TRIGGER_SKIP;
#endif
                return 0;  /* OK */
        }

        /* If debugger is paused, garbage collection is disabled by default. */
        /* XXX: will need a force flag if garbage collection is triggered
         * explicitly during paused state.
         */
#if defined(DUK_USE_DEBUGGER_SUPPORT)
        if (DUK_HEAP_IS_PAUSED(heap)) {
                /* Checking this here rather that in memory alloc primitives
                 * reduces checking code there but means a failed allocation
                 * will go through a few retries before giving up.  That's
                 * fine because this only happens during debugging.
                 */
                DUK_D(DUK_DPRINT("gc skipped because debugger is paused"));
                return 0;
        }
#endif

        DUK_D(DUK_DPRINT("garbage collect (mark-and-sweep) starting, requested flags: 0x%08lx, effective flags: 0x%08lx",
                         (unsigned long) flags, (unsigned long) (flags | heap->mark_and_sweep_base_flags)));

        flags |= heap->mark_and_sweep_base_flags;

        /*
         *  Assertions before
         */

#ifdef DUK_USE_ASSERTIONS
        DUK_ASSERT(!DUK_HEAP_HAS_MARKANDSWEEP_RUNNING(heap));
        DUK_ASSERT(!DUK_HEAP_HAS_MARKANDSWEEP_RECLIMIT_REACHED(heap));
        DUK_ASSERT(heap->mark_and_sweep_recursion_depth == 0);
        duk__assert_heaphdr_flags(heap);
#ifdef DUK_USE_REFERENCE_COUNTING
        /* Note: DUK_HEAP_HAS_REFZERO_FREE_RUNNING(heap) may be true; a refcount
         * finalizer may trigger a mark-and-sweep.
         */
        duk__assert_valid_refcounts(heap);
#endif  /* DUK_USE_REFERENCE_COUNTING */
#endif  /* DUK_USE_ASSERTIONS */

        /*
         *  Begin
         */

        DUK_HEAP_SET_MARKANDSWEEP_RUNNING(heap);

        /*
         *  Mark roots, hoping that recursion limit is not normally hit.
         *  If recursion limit is hit, run additional reachability rounds
         *  starting from "temproots" until marking is complete.
         *
         *  Marking happens in two phases: first we mark actual reachability
         *  roots (and run "temproots" to complete the process).  Then we
         *  check which objects are unreachable and are finalizable; such
         *  objects are marked as FINALIZABLE and marked as reachability
         *  (and "temproots" is run again to complete the process).
         *
         *  The heap finalize_list must also be marked as a reachability root.
         *  There may be objects on the list from a previous round if the
         *  previous run had finalizer skip flag.
         */

        duk__mark_roots_heap(heap);               /* main reachability roots */
#ifdef DUK_USE_REFERENCE_COUNTING
        duk__mark_refzero_list(heap);             /* refzero_list treated as reachability roots */
#endif
        duk__mark_temproots_by_heap_scan(heap);   /* temproots */

        duk__mark_finalizable(heap);              /* mark finalizable as reachability roots */
        duk__mark_finalize_list(heap);            /* mark finalizer work list as reachability roots */
        duk__mark_temproots_by_heap_scan(heap);   /* temproots */

        /*
         *  Sweep garbage and remove marking flags, and move objects with
         *  finalizers to the finalizer work list.
         *
         *  Objects to be swept need to get their refcounts finalized before
         *  they are swept.  In other words, their target object refcounts
         *  need to be decreased.  This has to be done before freeing any
         *  objects to avoid decref'ing dangling pointers (which may happen
         *  even without bugs, e.g. with reference loops)
         *
         *  Because strings don't point to other heap objects, similar
         *  finalization is not necessary for strings.
         */

        /* XXX: more emergency behavior, e.g. find smaller hash sizes etc */

#ifdef DUK_USE_REFERENCE_COUNTING
        duk__finalize_refcounts(heap);
#endif
        duk__sweep_heap(heap, flags, &count_keep_obj);
#if defined(DUK_USE_STRTAB_CHAIN)
        duk__sweep_stringtable_chain(heap, &count_keep_str);
#elif defined(DUK_USE_STRTAB_PROBE)
        duk__sweep_stringtable_probe(heap, &count_keep_str);
#else
#error internal error, invalid strtab options
#endif
#ifdef DUK_USE_REFERENCE_COUNTING
        duk__clear_refzero_list_flags(heap);
#endif
        duk__clear_finalize_list_flags(heap);

        /*
         *  Object compaction (emergency only).
         *
         *  Object compaction is a separate step after sweeping, as there is
         *  more free memory for it to work with.  Also, currently compaction
         *  may insert new objects into the heap allocated list and the string
         *  table which we don't want to do during a sweep (the reachability
         *  flags of such objects would be incorrect).  The objects inserted
         *  are currently:
         *
         *    - a temporary duk_hbuffer for a new properties allocation
         *    - if array part is abandoned, string keys are interned
         *
         *  The object insertions go to the front of the list, so they do not
         *  cause an infinite loop (they are not compacted).
         */

        if ((flags & DUK_MS_FLAG_EMERGENCY) &&
            !(flags & DUK_MS_FLAG_NO_OBJECT_COMPACTION)) {
                duk__compact_objects(heap);
        }

        /*
         *  String table resize check.
         *
         *  Note: this may silently (and safely) fail if GC is caused by an
         *  allocation call in stringtable resize_hash().  Resize_hash()
         *  will prevent a recursive call to itself by setting the
         *  DUK_MS_FLAG_NO_STRINGTABLE_RESIZE in heap->mark_and_sweep_base_flags.
         */

        /* XXX: stringtable emergency compaction? */

        /* XXX: remove this feature entirely? it would only matter for
         * emergency GC.  Disable for lowest memory builds.
         */
#if defined(DUK_USE_MS_STRINGTABLE_RESIZE)
        if (!(flags & DUK_MS_FLAG_NO_STRINGTABLE_RESIZE)) {
                DUK_DD(DUK_DDPRINT("resize stringtable: %p", (void *) heap));
                duk_heap_force_strtab_resize(heap);
        } else {
                DUK_D(DUK_DPRINT("stringtable resize skipped because DUK_MS_FLAG_NO_STRINGTABLE_RESIZE is set"));
        }
#endif

        /*
         *  Finalize objects in the finalization work list.  Finalized
         *  objects are queued back to heap_allocated with FINALIZED set.
         *
         *  Since finalizers may cause arbitrary side effects, they are
         *  prevented during string table and object property allocation
         *  resizing using the DUK_MS_FLAG_NO_FINALIZERS flag in
         *  heap->mark_and_sweep_base_flags.  In this case the objects
         *  remain in the finalization work list after mark-and-sweep
         *  exits and they may be finalized on the next pass.
         *
         *  Finalization currently happens inside "MARKANDSWEEP_RUNNING"
         *  protection (no mark-and-sweep may be triggered by the
         *  finalizers).  As a side effect:
         *
         *    1) an out-of-memory error inside a finalizer will not
         *       cause a mark-and-sweep and may cause the finalizer
         *       to fail unnecessarily
         *
         *    2) any temporary objects whose refcount decreases to zero
         *       during finalization will not be put into refzero_list;
         *       they can only be collected by another mark-and-sweep
         *
         *  This is not optimal, but since the sweep for this phase has
         *  already happened, this is probably good enough for now.
         */

#if defined(DUK_USE_MARKANDSWEEP_FINALIZER_TORTURE)
        /* Cannot simulate individual finalizers because finalize_list only
         * contains objects with actual finalizers.  But simulate side effects
         * from finalization by doing a bogus function call and resizing the
         * stacks.
         */
        if (flags & DUK_MS_FLAG_NO_FINALIZERS) {
                DUK_D(DUK_DPRINT("skip mark-and-sweep torture finalizer, DUK_MS_FLAG_NO_FINALIZERS is set"));
        } else if (!(thr->valstack != NULL && thr->callstack != NULL && thr->catchstack != NULL)) {
                DUK_D(DUK_DPRINT("skip mark-and-sweep torture finalizer, thread not yet viable"));
        } else {
                DUK_D(DUK_DPRINT("run mark-and-sweep torture finalizer"));
                duk__markandsweep_torture_finalizer(thr);
        }
#endif  /* DUK_USE_MARKANDSWEEP_FINALIZER_TORTURE */

        if (flags & DUK_MS_FLAG_NO_FINALIZERS) {
                DUK_D(DUK_DPRINT("finalizer run skipped because DUK_MS_FLAG_NO_FINALIZERS is set"));
        } else {
                duk__run_object_finalizers(heap, flags);
        }

        /*
         *  Finish
         */

        DUK_HEAP_CLEAR_MARKANDSWEEP_RUNNING(heap);

        /*
         *  Assertions after
         */

#ifdef DUK_USE_ASSERTIONS
        DUK_ASSERT(!DUK_HEAP_HAS_MARKANDSWEEP_RUNNING(heap));
        DUK_ASSERT(!DUK_HEAP_HAS_MARKANDSWEEP_RECLIMIT_REACHED(heap));
        DUK_ASSERT(heap->mark_and_sweep_recursion_depth == 0);
        duk__assert_heaphdr_flags(heap);
#ifdef DUK_USE_REFERENCE_COUNTING
        /* Note: DUK_HEAP_HAS_REFZERO_FREE_RUNNING(heap) may be true; a refcount
         * finalizer may trigger a mark-and-sweep.
         */
        duk__assert_valid_refcounts(heap);
#endif  /* DUK_USE_REFERENCE_COUNTING */
#endif  /* DUK_USE_ASSERTIONS */

        /*
         *  Reset trigger counter
         */

#ifdef DUK_USE_VOLUNTARY_GC
        tmp = (count_keep_obj + count_keep_str) / 256;
        heap->mark_and_sweep_trigger_counter = (duk_int_t) (
            (tmp * DUK_HEAP_MARK_AND_SWEEP_TRIGGER_MULT) +
            DUK_HEAP_MARK_AND_SWEEP_TRIGGER_ADD);
        DUK_D(DUK_DPRINT("garbage collect (mark-and-sweep) finished: %ld objects kept, %ld strings kept, trigger reset to %ld",
                         (long) count_keep_obj, (long) count_keep_str, (long) heap->mark_and_sweep_trigger_counter));
#else
        DUK_D(DUK_DPRINT("garbage collect (mark-and-sweep) finished: %ld objects kept, %ld strings kept, no voluntary trigger",
                         (long) count_keep_obj, (long) count_keep_str));
#endif

        return 0;  /* OK */
}

#else  /* DUK_USE_MARK_AND_SWEEP */

/* no mark-and-sweep gc */

#endif  /* DUK_USE_MARK_AND_SWEEP */