duktape-1.5.2/src-separate/duk_heap_markandsweep.c

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/*
 *  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 */