duktape-1.8.0/src-separate/duk_heap_refcount.c

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/*
 *  Reference counting implementation.
 */
 
#include "duk_internal.h"
 
#ifdef DUK_USE_REFERENCE_COUNTING
 
#ifndef DUK_USE_DOUBLE_LINKED_HEAP
#error internal error, reference counting requires a double linked heap
#endif
 
/*
 *  Misc
 */
 
DUK_LOCAL void duk__queue_refzero(duk_heap *heap, duk_heaphdr *hdr) {
    /* tail insert: don't disturb head in case refzero is running */
 
    if (heap->refzero_list != NULL) {
        duk_heaphdr *hdr_prev;
 
        hdr_prev = heap->refzero_list_tail;
        DUK_ASSERT(hdr_prev != NULL);
        DUK_ASSERT(DUK_HEAPHDR_GET_NEXT(heap, hdr_prev) == NULL);
 
        DUK_HEAPHDR_SET_NEXT(heap, hdr, NULL);
        DUK_HEAPHDR_SET_PREV(heap, hdr, hdr_prev);
        DUK_HEAPHDR_SET_NEXT(heap, hdr_prev, hdr);
        DUK_ASSERT_HEAPHDR_LINKS(heap, hdr);
        DUK_ASSERT_HEAPHDR_LINKS(heap, hdr_prev);
        heap->refzero_list_tail = hdr;
    } else {
        DUK_ASSERT(heap->refzero_list_tail == NULL);
        DUK_HEAPHDR_SET_NEXT(heap, hdr, NULL);
        DUK_HEAPHDR_SET_PREV(heap, hdr, NULL);
        DUK_ASSERT_HEAPHDR_LINKS(heap, hdr);
        heap->refzero_list = hdr;
        heap->refzero_list_tail = hdr;
    }
}
 
/*
 *  Heap object refcount finalization.
 *
 *  When an object is about to be freed, all other objects it refers to must
 *  be decref'd.  Refcount finalization does NOT free the object or its inner
 *  allocations (mark-and-sweep shares these helpers), it just manipulates
 *  the refcounts.
 *
 *  Note that any of the decref's may cause a refcount to drop to zero, BUT
 *  it will not be processed inline; instead, because refzero is already
 *  running, the objects will just be queued to refzero list and processed
 *  later.  This eliminates C recursion.
 */
 
DUK_LOCAL void duk__refcount_finalize_hobject(duk_hthread *thr, duk_hobject *h) {
    duk_uint_fast32_t i;
 
    DUK_ASSERT(h);
    DUK_ASSERT(DUK_HEAPHDR_GET_TYPE((duk_heaphdr *) h) == DUK_HTYPE_OBJECT);
 
    /* XXX: better to get base and walk forwards? */
 
    for (i = 0; i < (duk_uint_fast32_t) DUK_HOBJECT_GET_ENEXT(h); i++) {
        duk_hstring *key = DUK_HOBJECT_E_GET_KEY(thr->heap, h, i);
        if (!key) {
            continue;
        }
        duk_heaphdr_decref(thr, (duk_heaphdr *) key);
        if (DUK_HOBJECT_E_SLOT_IS_ACCESSOR(thr->heap, h, i)) {
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) DUK_HOBJECT_E_GET_VALUE_GETTER(thr->heap, h, i));
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) DUK_HOBJECT_E_GET_VALUE_SETTER(thr->heap, h, i));
        } else {
            duk_tval_decref(thr, DUK_HOBJECT_E_GET_VALUE_TVAL_PTR(thr->heap, h, i));
        }
    }
 
    for (i = 0; i < (duk_uint_fast32_t) DUK_HOBJECT_GET_ASIZE(h); i++) {
        duk_tval_decref(thr, DUK_HOBJECT_A_GET_VALUE_PTR(thr->heap, h, i));
    }
 
    /* hash part is a 'weak reference' and does not contribute */
 
    duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) DUK_HOBJECT_GET_PROTOTYPE(thr->heap, h));
 
    if (DUK_HOBJECT_IS_COMPILEDFUNCTION(h)) {
        duk_hcompiledfunction *f = (duk_hcompiledfunction *) h;
        duk_tval *tv, *tv_end;
        duk_hobject **funcs, **funcs_end;
 
        if (DUK_HCOMPILEDFUNCTION_GET_DATA(thr->heap, f) != NULL) {
            tv = DUK_HCOMPILEDFUNCTION_GET_CONSTS_BASE(thr->heap, f);
            tv_end = DUK_HCOMPILEDFUNCTION_GET_CONSTS_END(thr->heap, f);
            while (tv < tv_end) {
                duk_tval_decref(thr, tv);
                tv++;
            }
 
            funcs = DUK_HCOMPILEDFUNCTION_GET_FUNCS_BASE(thr->heap, f);
            funcs_end = DUK_HCOMPILEDFUNCTION_GET_FUNCS_END(thr->heap, f);
            while (funcs < funcs_end) {
                duk_heaphdr_decref(thr, (duk_heaphdr *) *funcs);
                funcs++;
            }
        } else {
            /* May happen in some out-of-memory corner cases. */
            DUK_D(DUK_DPRINT("duk_hcompiledfunction 'data' is NULL, skipping decref"));
        }
 
        duk_heaphdr_decref(thr, (duk_heaphdr *) DUK_HCOMPILEDFUNCTION_GET_DATA(thr->heap, f));
    } else if (DUK_HOBJECT_IS_NATIVEFUNCTION(h)) {
        duk_hnativefunction *f = (duk_hnativefunction *) h;
        DUK_UNREF(f);
        /* nothing to finalize */
    } else if (DUK_HOBJECT_IS_BUFFEROBJECT(h)) {
        duk_hbufferobject *b = (duk_hbufferobject *) h;
        if (b->buf) {
            duk_heaphdr_decref(thr, (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_tval_decref(thr, tv);
            tv++;
        }
 
        for (i = 0; i < (duk_uint_fast32_t) t->callstack_top; i++) {
            duk_activation *act = t->callstack + i;
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) DUK_ACT_GET_FUNC(act));
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) act->var_env);
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) act->lex_env);
#ifdef DUK_USE_NONSTD_FUNC_CALLER_PROPERTY
            duk_heaphdr_decref_allownull(thr, (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
 
        for (i = 0; i < DUK_NUM_BUILTINS; i++) {
            duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) t->builtins[i]);
        }
 
        duk_heaphdr_decref_allownull(thr, (duk_heaphdr *) t->resumer);
    }
}
 
DUK_INTERNAL void duk_heaphdr_refcount_finalize(duk_hthread *thr, duk_heaphdr *hdr) {
    DUK_ASSERT(hdr);
 
    switch ((int) DUK_HEAPHDR_GET_TYPE(hdr)) {
    case DUK_HTYPE_OBJECT:
        duk__refcount_finalize_hobject(thr, (duk_hobject *) hdr);
        break;
    case DUK_HTYPE_BUFFER:
        /* nothing to finalize */
        break;
    case DUK_HTYPE_STRING:
        /* cannot happen: strings are not put into refzero list (they don't even have the next/prev pointers) */
    default:
        DUK_UNREACHABLE();
    }
}
 
#if defined(DUK_USE_REFZERO_FINALIZER_TORTURE)
DUK_LOCAL duk_ret_t duk__refcount_fake_finalizer(duk_context *ctx) {
    DUK_UNREF(ctx);
    DUK_D(DUK_DPRINT("fake refcount torture finalizer executed"));
#if 0
    DUK_DD(DUK_DDPRINT("fake torture finalizer for: %!T", duk_get_tval(ctx, 0)));
#endif
    /* Require a lot of stack to force a value stack grow/shrink. */
    duk_require_stack(ctx, 100000);
 
    /* XXX: do something to force a callstack grow/shrink, perhaps
     * just a manual forced resize?
     */
    return 0;
}
 
DUK_LOCAL void duk__refcount_run_torture_finalizer(duk_hthread *thr, duk_hobject *obj) {
    duk_context *ctx;
    duk_int_t rc;
 
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(obj != NULL);
    ctx = (duk_context *) thr;
 
    /* Avoid fake finalization for the duk__refcount_fake_finalizer function
     * itself, otherwise we're in infinite recursion.
     */
    if (DUK_HOBJECT_HAS_NATIVEFUNCTION(obj)) {
        if (((duk_hnativefunction *) obj)->func == duk__refcount_fake_finalizer) {
            DUK_DD(DUK_DDPRINT("avoid fake torture finalizer for duk__refcount_fake_finalizer itself"));
            return;
        }
    }
    /* Avoid fake finalization when callstack limit has been reached.
     * Otherwise a callstack limit error will be created, then refzero'ed,
     * and we're in an infinite loop.
     */
    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 torture finalizer"));
        return;
    }
 
    /* Run fake finalizer.  Avoid creating new refzero queue entries
     * so that we are not forced into a forever loop.
     */
    duk_push_c_function(ctx, duk__refcount_fake_finalizer, 1 /*nargs*/);
    duk_push_hobject(ctx, obj);
    rc = duk_pcall(ctx, 1);
    DUK_UNREF(rc);  /* ignored */
    duk_pop(ctx);
}
#endif  /* DUK_USE_REFZERO_FINALIZER_TORTURE */
 
/*
 *  Refcount memory freeing loop.
 *
 *  Frees objects in the refzero_pending list until the list becomes
 *  empty.  When an object is freed, its references get decref'd and
 *  may cause further objects to be queued for freeing.
 *
 *  This could be expanded to allow incremental freeing: just bail out
 *  early and resume at a future alloc/decref/refzero.
 */
 
DUK_LOCAL void duk__refzero_free_pending(duk_hthread *thr) {
    duk_heaphdr *h1, *h2;
    duk_heap *heap;
    duk_int_t count = 0;
 
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(thr->heap != NULL);
    heap = thr->heap;
    DUK_ASSERT(heap != NULL);
 
    /*
     *  Detect recursive invocation
     */
 
    if (DUK_HEAP_HAS_REFZERO_FREE_RUNNING(heap)) {
        DUK_DDD(DUK_DDDPRINT("refzero free running, skip run"));
        return;
    }
 
    /*
     *  Churn refzero_list until empty
     */
 
    DUK_HEAP_SET_REFZERO_FREE_RUNNING(heap);
    while (heap->refzero_list) {
        duk_hobject *obj;
        duk_bool_t rescued = 0;
 
        /*
         *  Pick an object from the head (don't remove yet).
         */
 
        h1 = heap->refzero_list;
        obj = (duk_hobject *) h1;
        DUK_DD(DUK_DDPRINT("refzero processing %p: %!O", (void *) h1, (duk_heaphdr *) h1));
        DUK_ASSERT(DUK_HEAPHDR_GET_PREV(heap, h1) == NULL);
        DUK_ASSERT(DUK_HEAPHDR_GET_TYPE(h1) == DUK_HTYPE_OBJECT);  /* currently, always the case */
 
#if defined(DUK_USE_REFZERO_FINALIZER_TORTURE)
        /* Torture option to shake out finalizer side effect issues:
         * make a bogus function call for every finalizable object,
         * essentially simulating the case where everything has a
         * finalizer.
         */
        DUK_DD(DUK_DDPRINT("refzero torture enabled, fake finalizer"));
        DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(h1) == 0);
        DUK_HEAPHDR_PREINC_REFCOUNT(h1);  /* bump refcount to prevent refzero during finalizer processing */
        duk__refcount_run_torture_finalizer(thr, obj);  /* must never longjmp */
        DUK_HEAPHDR_PREDEC_REFCOUNT(h1);  /* remove artificial bump */
        DUK_ASSERT_DISABLE(h1->h_refcount >= 0);  /* refcount is unsigned, so always true */
#endif
 
        /*
         *  Finalizer check.
         *
         *  Note: running a finalizer may have arbitrary side effects, e.g.
         *  queue more objects on refzero_list (tail), or even trigger a
         *  mark-and-sweep.
         *
         *  Note: quick reject check should match vast majority of
         *  objects and must be safe (not throw any errors, ever).
         */
 
        /* An object may have FINALIZED here if it was finalized by mark-and-sweep
         * on a previous run and refcount then decreased to zero.  We won't run the
         * finalizer again here.
         */
 
        /* A finalizer is looked up from the object and up its prototype chain
         * (which allows inherited finalizers).
         */
        if (duk_hobject_hasprop_raw(thr, obj, DUK_HTHREAD_STRING_INT_FINALIZER(thr))) {
            DUK_DDD(DUK_DDDPRINT("object has a finalizer, run it"));
 
            DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(h1) == 0);
            DUK_HEAPHDR_PREINC_REFCOUNT(h1);  /* bump refcount to prevent refzero during finalizer processing */
 
            duk_hobject_run_finalizer(thr, obj);  /* must never longjmp */
            DUK_ASSERT(DUK_HEAPHDR_HAS_FINALIZED(h1));  /* duk_hobject_run_finalizer() sets */
 
            DUK_HEAPHDR_PREDEC_REFCOUNT(h1);  /* remove artificial bump */
            DUK_ASSERT_DISABLE(h1->h_refcount >= 0);  /* refcount is unsigned, so always true */
 
            if (DUK_HEAPHDR_GET_REFCOUNT(h1) != 0) {
                DUK_DDD(DUK_DDDPRINT("-> object refcount after finalization non-zero, object will be rescued"));
                rescued = 1;
            } else {
                DUK_DDD(DUK_DDDPRINT("-> object refcount still zero after finalization, object will be freed"));
            }
        }
 
        /* Refzero head is still the same.  This is the case even if finalizer
         * inserted more refzero objects; they are inserted to the tail.
         */
        DUK_ASSERT(h1 == heap->refzero_list);
 
        /*
         *  Remove the object from the refzero list.  This cannot be done
         *  before a possible finalizer has been executed; the finalizer
         *  may trigger a mark-and-sweep, and mark-and-sweep must be able
         *  to traverse a complete refzero_list.
         */
 
        h2 = DUK_HEAPHDR_GET_NEXT(heap, h1);
        if (h2) {
            DUK_HEAPHDR_SET_PREV(heap, h2, NULL);  /* not strictly necessary */
            heap->refzero_list = h2;
        } else {
            heap->refzero_list = NULL;
            heap->refzero_list_tail = NULL;
        }
 
        /*
         *  Rescue or free.
         */
 
        if (rescued) {
            /* yes -> move back to heap allocated */
            DUK_DD(DUK_DDPRINT("object rescued during refcount finalization: %p", (void *) h1));
            DUK_ASSERT(!DUK_HEAPHDR_HAS_FINALIZABLE(h1));
            DUK_ASSERT(DUK_HEAPHDR_HAS_FINALIZED(h1));
            DUK_HEAPHDR_CLEAR_FINALIZED(h1);
            h2 = heap->heap_allocated;
            DUK_HEAPHDR_SET_PREV(heap, h1, NULL);
            if (h2) {
                DUK_HEAPHDR_SET_PREV(heap, h2, h1);
            }
            DUK_HEAPHDR_SET_NEXT(heap, h1, h2);
            DUK_ASSERT_HEAPHDR_LINKS(heap, h1);
            DUK_ASSERT_HEAPHDR_LINKS(heap, h2);
            heap->heap_allocated = h1;
        } else {
            /* no -> decref members, then free */
            duk__refcount_finalize_hobject(thr, obj);
            duk_heap_free_heaphdr_raw(heap, h1);
        }
 
        count++;
    }
    DUK_HEAP_CLEAR_REFZERO_FREE_RUNNING(heap);
 
    DUK_DDD(DUK_DDDPRINT("refzero processed %ld objects", (long) count));
 
    /*
     *  Once the whole refzero cascade has been freed, check for
     *  a voluntary mark-and-sweep.
     */
 
#if defined(DUK_USE_MARK_AND_SWEEP) && defined(DUK_USE_VOLUNTARY_GC)
    /* 'count' is more or less comparable to normal trigger counter update
     * which happens in memory block (re)allocation.
     */
    heap->mark_and_sweep_trigger_counter -= count;
    if (heap->mark_and_sweep_trigger_counter <= 0) {
        if (DUK_HEAP_HAS_MARKANDSWEEP_RUNNING(heap)) {
            DUK_D(DUK_DPRINT("mark-and-sweep in progress -> skip voluntary mark-and-sweep now"));
        } else {
            duk_bool_t rc;
            duk_small_uint_t flags = 0;  /* not emergency */
            DUK_D(DUK_DPRINT("refcount triggering mark-and-sweep"));
            rc = duk_heap_mark_and_sweep(heap, flags);
            DUK_UNREF(rc);
            DUK_D(DUK_DPRINT("refcount triggered mark-and-sweep => rc %ld", (long) rc));
        }
    }
#endif  /* DUK_USE_MARK_AND_SWEEP && DUK_USE_VOLUNTARY_GC */
}
 
/*
 *  Incref and decref functions.
 *
 *  Decref may trigger immediate refzero handling, which may free and finalize
 *  an arbitrary number of objects.
 *
 */
 
DUK_INTERNAL void duk_heaphdr_refzero(duk_hthread *thr, duk_heaphdr *h) {
    duk_heap *heap;
 
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(h != NULL);
 
    heap = thr->heap;
    DUK_DDD(DUK_DDDPRINT("refzero %p: %!O", (void *) h, (duk_heaphdr *) h));
 
    /*
     *  Refzero handling is skipped entirely if (1) mark-and-sweep is
     *  running or (2) execution is paused in the debugger.  The objects
     *  are left in the heap, and will be freed by mark-and-sweep or
     *  eventual heap destruction.
     *
     *  This is necessary during mark-and-sweep because refcounts are also
     *  updated during the sweep phase (otherwise objects referenced by a
     *  swept object would have incorrect refcounts) which then calls here.
     *  This could be avoided by using separate decref macros in
     *  mark-and-sweep; however, mark-and-sweep also calls finalizers which
     *  would use the ordinary decref macros anyway and still call this
     *  function.
     *
     *  This check must be enabled also when mark-and-sweep support has been
     *  disabled: the flag is also used in heap destruction when running
     *  finalizers for remaining objects, and the flag prevents objects from
     *  being moved around in heap linked lists.
     */
 
    /* XXX: ideally this would be just one flag (maybe a derived one) so
     * that a single bit test is sufficient to check the condition.
     */
#if defined(DUK_USE_DEBUGGER_SUPPORT)
    if (DUK_UNLIKELY(DUK_HEAP_HAS_MARKANDSWEEP_RUNNING(heap) || DUK_HEAP_IS_PAUSED(heap))) {
#else
    if (DUK_UNLIKELY(DUK_HEAP_HAS_MARKANDSWEEP_RUNNING(heap))) {
#endif
        DUK_DDD(DUK_DDDPRINT("refzero handling suppressed when mark-and-sweep running, object: %p", (void *) h));
        return;
    }
 
    switch ((duk_small_int_t) DUK_HEAPHDR_GET_TYPE(h)) {
    case DUK_HTYPE_STRING:
        /*
         *  Strings have no internal references but do have "weak"
         *  references in the string cache.  Also note that strings
         *  are not on the heap_allocated list like other heap
         *  elements.
         */
 
        duk_heap_strcache_string_remove(heap, (duk_hstring *) h);
        duk_heap_string_remove(heap, (duk_hstring *) h);
        duk_heap_free_heaphdr_raw(heap, h);
        break;
 
    case DUK_HTYPE_OBJECT:
        /*
         *  Objects have internal references.  Must finalize through
         *  the "refzero" work list.
         */
 
        duk_heap_remove_any_from_heap_allocated(heap, h);
        duk__queue_refzero(heap, h);
        duk__refzero_free_pending(thr);
        break;
 
    case DUK_HTYPE_BUFFER:
        /*
         *  Buffers have no internal references.  However, a dynamic
         *  buffer has a separate allocation for the buffer.  This is
         *  freed by duk_heap_free_heaphdr_raw().
         */
 
        duk_heap_remove_any_from_heap_allocated(heap, h);
        duk_heap_free_heaphdr_raw(heap, h);
        break;
 
    default:
        DUK_D(DUK_DPRINT("invalid heap type in decref: %ld", (long) DUK_HEAPHDR_GET_TYPE(h)));
        DUK_UNREACHABLE();
    }
}
 
#if !defined(DUK_USE_FAST_REFCOUNT_DEFAULT)
DUK_INTERNAL void duk_tval_incref(duk_tval *tv) {
    DUK_ASSERT(tv != NULL);
 
    if (DUK_TVAL_NEEDS_REFCOUNT_UPDATE(tv)) {
        duk_heaphdr *h = DUK_TVAL_GET_HEAPHDR(tv);
        DUK_ASSERT(h != NULL);
        DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
        DUK_ASSERT_DISABLE(h->h_refcount >= 0);
        DUK_HEAPHDR_PREINC_REFCOUNT(h);
    }
}
#endif
 
#if 0  /* unused */
DUK_INTERNAL void duk_tval_incref_allownull(duk_tval *tv) {
    if (tv == NULL) {
        return;
    }
    if (DUK_TVAL_NEEDS_REFCOUNT_UPDATE(tv)) {
        duk_heaphdr *h = DUK_TVAL_GET_HEAPHDR(tv);
        DUK_ASSERT(h != NULL);
        DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
        DUK_ASSERT_DISABLE(h->h_refcount >= 0);
        DUK_HEAPHDR_PREINC_REFCOUNT(h);
    }
}
#endif
 
DUK_INTERNAL void duk_tval_decref(duk_hthread *thr, duk_tval *tv) {
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(tv != NULL);
 
    if (DUK_TVAL_NEEDS_REFCOUNT_UPDATE(tv)) {
        duk_heaphdr *h = DUK_TVAL_GET_HEAPHDR(tv);
        DUK_ASSERT(h != NULL);
        DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
        duk_heaphdr_decref(thr, h);
    }
}
 
#if 0  /* unused */
DUK_INTERNAL void duk_tval_decref_allownull(duk_hthread *thr, duk_tval *tv) {
    DUK_ASSERT(thr != NULL);
 
    if (tv == NULL) {
        return;
    }
    if (DUK_TVAL_NEEDS_REFCOUNT_UPDATE(tv)) {
        duk_heaphdr *h = DUK_TVAL_GET_HEAPHDR(tv);
        DUK_ASSERT(h != NULL);
        DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
        duk_heaphdr_decref(thr, h);
    }
}
#endif
 
#if !defined(DUK_USE_FAST_REFCOUNT_DEFAULT)
DUK_INTERNAL void duk_heaphdr_incref(duk_heaphdr *h) {
    DUK_ASSERT(h != NULL);
    DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
    DUK_ASSERT_DISABLE(DUK_HEAPHDR_GET_REFCOUNT(h) >= 0);
 
#if defined(DUK_USE_ROM_OBJECTS)
    if (DUK_HEAPHDR_HAS_READONLY(h)) {
        return;
    }
#endif
 
    DUK_HEAPHDR_PREINC_REFCOUNT(h);
}
#endif
 
#if 0  /* unused */
DUK_INTERNAL void duk_heaphdr_incref_allownull(duk_heaphdr *h) {
    if (h == NULL) {
        return;
    }
    DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
    DUK_ASSERT_DISABLE(DUK_HEAPHDR_GET_REFCOUNT(h) >= 0);
 
    DUK_HEAPHDR_PREINC_REFCOUNT(h);
}
#endif
 
DUK_INTERNAL void duk_heaphdr_decref(duk_hthread *thr, duk_heaphdr *h) {
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(thr->heap != NULL);
    DUK_ASSERT(h != NULL);
    DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
    DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(h) >= 1);
 
#if defined(DUK_USE_ROM_OBJECTS)
    if (DUK_HEAPHDR_HAS_READONLY(h)) {
        return;
    }
#endif
    if (DUK_HEAPHDR_PREDEC_REFCOUNT(h) != 0) {
        return;
    }
    duk_heaphdr_refzero(thr, h);
}
 
DUK_INTERNAL void duk_heaphdr_decref_allownull(duk_hthread *thr, duk_heaphdr *h) {
    DUK_ASSERT(thr != NULL);
    DUK_ASSERT(thr->heap != NULL);
 
    if (h == NULL) {
        return;
    }
    DUK_ASSERT(DUK_HEAPHDR_HTYPE_VALID(h));
 
#if defined(DUK_USE_ROM_OBJECTS)
    if (DUK_HEAPHDR_HAS_READONLY(h)) {
        return;
    }
#endif
    DUK_ASSERT(DUK_HEAPHDR_GET_REFCOUNT(h) >= 1);
    if (DUK_HEAPHDR_PREDEC_REFCOUNT(h) != 0) {
        return;
    }
    duk_heaphdr_refzero(thr, h);
}
 
#else
 
/* no refcounting */
 
#endif  /* DUK_USE_REFERENCE_COUNTING */