mmtk/

mmtk.rs

//! MMTk instance.
use crate::global_state::{GcStatus, GlobalState};
use crate::plan::CreateGeneralPlanArgs;
use crate::plan::Plan;
use crate::policy::sft_map::{create_sft_map, SFTMap};
use crate::scheduler::GCWorkScheduler;

#[cfg(feature = "vo_bit")]
use crate::util::address::ObjectReference;
#[cfg(feature = "analysis")]
use crate::util::analysis::AnalysisManager;
use crate::util::finalizable_processor::FinalizableProcessor;
use crate::util::heap::gc_trigger::GCTrigger;
use crate::util::heap::layout::heap_parameters::MAX_SPACES;
use crate::util::heap::layout::vm_layout::{vm_layout, VMLayout};
use crate::util::heap::layout::{self, Mmapper, VMMap};
use crate::util::heap::HeapMeta;
use crate::util::opaque_pointer::*;
use crate::util::options::Options;
use crate::util::reference_processor::ReferenceProcessors;
#[cfg(feature = "sanity")]
use crate::util::sanity::sanity_checker::SanityChecker;
#[cfg(feature = "extreme_assertions")]
use crate::util::slot_logger::SlotLogger;
use crate::util::statistics::stats::Stats;
#[cfg(feature = "vm_space")]
use crate::vm::object_model::ObjectModel;
use crate::vm::ReferenceGlue;
use crate::vm::VMBinding;
use std::cell::UnsafeCell;
use std::collections::HashMap;
use std::default::Default;
#[cfg(feature = "sanity")]
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering;
use std::sync::Arc;
use std::sync::Mutex;

lazy_static! {
    // I am not sure if we should include these mmappers as part of MMTk struct.
    // The considerations are:
    // 1. We need VMMap and Mmapper to create spaces. It is natural that the mappers are not
    //    part of MMTK, as creating MMTK requires these mappers. We could use Rc/Arc for these mappers though.
    // 2. These mmappers are possibly global across multiple MMTk instances, as they manage the
    //    entire address space.
    // TODO: We should refactor this when we know more about how multiple MMTK instances work.

    /// A global VMMap that manages the mapping of spaces to virtual memory ranges.
    pub static ref VM_MAP: Box<dyn VMMap + Send + Sync> = layout::create_vm_map();

    /// A global Mmapper for mmaping and protection of virtual memory.
    pub static ref MMAPPER: Box<dyn Mmapper> = layout::create_mmapper();
}

use crate::util::rust_util::InitializeOnce;

// A global space function table that allows efficient dispatch space specific code for addresses in our heap.
pub static SFT_MAP: InitializeOnce<Box<dyn SFTMap>> = InitializeOnce::new();

/// MMTk builder. This is used to set options and other settings before actually creating an MMTk instance.
pub struct MMTKBuilder {
    /// The options for this instance.
    pub options: Options,
}

impl MMTKBuilder {
    /// Create an MMTK builder with options read from environment variables, or using built-in
    /// default if not overridden by environment variables.
    pub fn new() -> Self {
        let mut builder = Self::new_no_env_vars();
        builder.options.read_env_var_settings();
        builder
    }

    /// Create an MMTK builder with build-in default options, but without reading options from
    /// environment variables.
    pub fn new_no_env_vars() -> Self {
        MMTKBuilder {
            options: Options::default(),
        }
    }

    /// Set an option.
    pub fn set_option(&mut self, name: &str, val: &str) -> bool {
        self.options.set_from_string(name, val)
    }

    /// Set multiple options by a string. The string should be key-value pairs separated by white spaces,
    /// such as `threads=1 stress_factor=4096`.
    pub fn set_options_bulk_by_str(&mut self, options: &str) -> bool {
        self.options.set_bulk_from_string(options)
    }

    /// Custom VM layout constants. VM bindings may use this function for compressed or 39-bit heap support.
    /// This function must be called before MMTk::new()
    pub fn set_vm_layout(&mut self, constants: VMLayout) {
        VMLayout::set_custom_vm_layout(constants)
    }

    /// Build an MMTk instance from the builder.
    pub fn build<VM: VMBinding>(&self) -> MMTK<VM> {
        let mut options = self.options.clone();
        options.resolve_connected_options();
        MMTK::new(Arc::new(options))
    }
}

impl Default for MMTKBuilder {
    fn default() -> Self {
        Self::new()
    }
}

/// An MMTk instance. MMTk allows multiple instances to run independently, and each instance gives users a separate heap.
/// *Note that multi-instances is not fully supported yet*
pub struct MMTK<VM: VMBinding> {
    pub(crate) options: Arc<Options>,
    pub(crate) state: Arc<GlobalState>,
    pub(crate) plan: UnsafeCell<Box<dyn Plan<VM = VM>>>,
    pub(crate) reference_processors: ReferenceProcessors,
    pub(crate) finalizable_processor:
        Mutex<FinalizableProcessor<<VM::VMReferenceGlue as ReferenceGlue<VM>>::FinalizableType>>,
    pub(crate) scheduler: Arc<GCWorkScheduler<VM>>,
    #[cfg(feature = "sanity")]
    pub(crate) sanity_checker: Mutex<SanityChecker<VM::VMSlot>>,
    #[cfg(feature = "extreme_assertions")]
    pub(crate) slot_logger: SlotLogger<VM::VMSlot>,
    pub(crate) gc_trigger: Arc<GCTrigger<VM>>,
    pub(crate) stats: Arc<Stats>,
    #[cfg(feature = "sanity")]
    inside_sanity: AtomicBool,
    /// Analysis counters. The feature analysis allows us to periodically stop the world and collect some statistics.
    #[cfg(feature = "analysis")]
    pub(crate) analysis_manager: Arc<AnalysisManager<VM>>,
}

unsafe impl<VM: VMBinding> Sync for MMTK<VM> {}
unsafe impl<VM: VMBinding> Send for MMTK<VM> {}

impl<VM: VMBinding> MMTK<VM> {
    /// Create an MMTK instance. This is not public. Bindings should use [`MMTKBuilder::build`].
    pub(crate) fn new(options: Arc<Options>) -> Self {
        // Verify the Mmapper can handle the required address space size.
        vm_layout().validate_address_space();

        // Initialize SFT first in case we need to use this in the constructor.
        // The first call will initialize SFT map. Other calls will be blocked until SFT map is initialized.
        crate::policy::sft_map::SFTRefStorage::pre_use_check();
        SFT_MAP.initialize_once(&create_sft_map);

        let num_workers = if cfg!(feature = "single_worker") {
            1
        } else {
            *options.threads
        };

        let scheduler = GCWorkScheduler::new(num_workers, (*options.thread_affinity).clone());

        let state = Arc::new(GlobalState::default());

        let gc_trigger = Arc::new(GCTrigger::new(
            options.clone(),
            scheduler.clone(),
            state.clone(),
        ));

        let stats = Arc::new(Stats::new(&options));

        // We need this during creating spaces, but we do not use this once the MMTk instance is created.
        // So we do not save it in MMTK. This may change in the future.
        let mut heap = HeapMeta::new();

        // Create plan and spaces. Note that side metadata is not initialized yet. Plan creation should avoid using it.
        let mut plan = crate::plan::create_plan(
            *options.plan,
            CreateGeneralPlanArgs {
                vm_map: VM_MAP.as_ref(),
                mmapper: MMAPPER.as_ref(),
                options: options.clone(),
                state: state.clone(),
                gc_trigger: gc_trigger.clone(),
                scheduler: scheduler.clone(),
                stats: &stats,
                heap: &mut heap,
            },
        );

        // Initialize side metadata runtime state and reserve its address range after creating spaces.
        crate::util::metadata::side_metadata::initialize_side_metadata::<VM>(&options);

        // We haven't finished creating MMTk. No one is using the GC trigger. We cast the arc into a mutable reference.
        {
            // TODO: use Arc::get_mut_unchecked() when it is availble.
            let gc_trigger: &mut GCTrigger<VM> =
                unsafe { &mut *(Arc::as_ptr(&gc_trigger) as *mut _) };
            // We know the plan address will not change. Cast it to a static reference.
            let static_plan: &'static dyn Plan<VM = VM> = unsafe { &*(&*plan as *const _) };
            // Set the plan so we can trigger GC and check GC condition without using plan
            gc_trigger.set_plan(static_plan);
        }

        // TODO: This probably does not work if we have multiple MMTk instances.
        // This needs to be called after we create Plan. It needs to use HeapMeta, which is gradually built when we create spaces.
        VM_MAP.finalize_static_space_map(
            heap.get_discontig_start(),
            heap.get_discontig_end(),
            &mut |start_address| {
                plan.for_each_space_mut(&mut |space| {
                    // If the `VMMap` has a discontiguous memory range, we notify all discontiguous
                    // space that the starting address has been determined.
                    if let Some(pr) = space.maybe_get_page_resource_mut() {
                        pr.update_discontiguous_start(start_address);
                    }
                })
            },
        );

        // The order here is important:
        plan.initialize_side_metadata();
        // Initialize side metadat sanity first
        plan.verify_side_metadata_sanity();
        // Then intiialize SFT because it may use side metadata
        plan.initialize_sft();

        MMTK {
            options,
            state,
            plan: UnsafeCell::new(plan),
            reference_processors: ReferenceProcessors::new(),
            finalizable_processor: Mutex::new(FinalizableProcessor::<
                <VM::VMReferenceGlue as ReferenceGlue<VM>>::FinalizableType,
            >::new()),
            scheduler,
            #[cfg(feature = "sanity")]
            sanity_checker: Mutex::new(SanityChecker::new()),
            #[cfg(feature = "sanity")]
            inside_sanity: AtomicBool::new(false),
            #[cfg(feature = "extreme_assertions")]
            slot_logger: SlotLogger::new(),
            #[cfg(feature = "analysis")]
            analysis_manager: Arc::new(AnalysisManager::new(stats.clone())),
            gc_trigger,
            stats,
        }
    }

    /// Initialize the GC worker threads that are required for doing garbage collections.
    /// This is a mandatory call for a VM during its boot process once its thread system
    /// is ready.
    ///
    /// Internally, this function will invoke [`Collection::spawn_gc_thread()`] to spawn GC worker
    /// threads.
    ///
    /// # Arguments
    ///
    /// *   `tls`: The thread that wants to enable the collection. This value will be passed back
    ///     to the VM in [`Collection::spawn_gc_thread()`] so that the VM knows the context.
    ///
    /// [`Collection::spawn_gc_thread()`]: crate::vm::Collection::spawn_gc_thread()
    pub fn initialize_collection(&'static self, tls: VMThread) {
        assert!(
            !self.state.is_initialized(),
            "MMTk collection has been initialized (was initialize_collection() already called before?)"
        );
        self.scheduler.spawn_gc_threads(self, tls);
        self.state.gc_status.set_initialized();
        probe!(mmtk, collection_initialized);
    }

    /// Shut down all GC worker threads.
    pub fn shutdown(&'static self) {
        if self.state.is_initialized() {
            self.scheduler.shutdown_gc_threads();
            self.state.gc_status.set_uninitialized();
        }
    }

    /// Prepare an MMTk instance for calling the `fork()` system call.
    ///
    /// The `fork()` system call is available on Linux and some UNIX variants, and may be emulated
    /// on other platforms by libraries such as Cygwin.  The properties of the `fork()` system call
    /// requires the users to do some preparation before calling it.
    ///
    /// -   **Multi-threading**:  If `fork()` is called when the process has multiple threads, it
    ///     will only duplicate the current thread into the child process, and the child process can
    ///     only call async-signal-safe functions, notably `exec()`.  For VMs that that use
    ///     multi-process concurrency, it is imperative that when calling `fork()`, only one thread may
    ///     exist in the process.
    ///
    /// -   **File descriptors**: The child process inherits copies of the parent's set of open
    ///     file descriptors.  This may or may not be desired depending on use cases.
    ///
    /// This function helps VMs that use `fork()` for multi-process concurrency.  It instructs all
    /// GC threads to save their contexts and return from their entry-point functions.  Currently,
    /// such threads only include GC workers, and the entry point is
    /// [`crate::memory_manager::start_worker`].  A subsequent call to `MMTK::after_fork()` will
    /// re-spawn the threads using their saved contexts.  The VM must not allocate objects in the
    /// MMTk heap before calling `MMTK::after_fork()`.
    ///
    /// TODO: Currently, the MMTk core does not keep any files open for a long time.  In the
    /// future, this function and the `after_fork` function may be used for handling open file
    /// descriptors across invocations of `fork()`.  One possible use case is logging GC activities
    /// and statistics to files, such as performing heap dumps across multiple GCs.
    ///
    /// If a VM intends to execute another program by calling `fork()` and immediately calling
    /// `exec`, it may skip this function because the state of the MMTk instance will be irrelevant
    /// in that case.
    ///
    /// # Caution!
    ///
    /// This function sends an asynchronous message to GC threads and returns immediately, but it
    /// is only safe for the VM to call `fork()` after the underlying **native threads** of the GC
    /// threads have exited.  After calling this function, the VM should wait for their underlying
    /// native threads to exit in VM-specific manner before calling `fork()`.
    pub fn prepare_to_fork(&'static self) {
        assert!(
            self.state.is_initialized(),
            "MMTk collection has not been initialized, yet (was initialize_collection() called before?)"
        );
        probe!(mmtk, prepare_to_fork);
        self.scheduler.stop_gc_threads_for_forking();
    }

    /// Call this function after the VM called the `fork()` system call.
    ///
    /// This function will re-spawn MMTk threads from saved contexts.
    ///
    /// # Arguments
    ///
    /// *   `tls`: The thread that wants to respawn MMTk threads after forking. This value will be
    ///     passed back to the VM in `Collection::spawn_gc_thread()` so that the VM knows the
    ///     context.
    pub fn after_fork(&'static self, tls: VMThread) {
        assert!(
            self.state.is_initialized(),
            "MMTk collection has not been initialized, yet (was initialize_collection() called before?)"
        );
        probe!(mmtk, after_fork);
        self.scheduler.respawn_gc_threads_after_forking(tls);
    }

    /// Generic hook to allow benchmarks to be harnessed. MMTk will trigger a GC
    /// to clear any residual garbage and start collecting statistics for the benchmark.
    /// This is usually called by the benchmark harness as its last step before the actual benchmark.
    pub fn harness_begin(&self, tls: VMMutatorThread) {
        probe!(mmtk, harness_begin);
        let gc_triggered = self.handle_user_collection_request(tls, true, true);
        // Since handle_user_collection_request may not trigger GC if tls is null, we add a
        // block_for_gc to compensate for this because we force a GC in harness begin.
        //
        // Only do this if a GC was actually triggered. A plan that does not collect garbage
        // (NoGC) ignores the request and never schedules a GC, so blocking here would wait
        // forever and deadlock the VM.
        //
        // FIXME: Fix the API of handle_user_collection_request so that we won't need this
        // workaround.
        if gc_triggered && tls.0 .0.is_null() {
            use crate::vm::Collection;
            VM::VMCollection::block_for_gc(tls);
        }
        self.state.inside_harness.store(true, Ordering::SeqCst);
        self.stats.start_all();
        self.scheduler.enable_stat();
    }

    /// Generic hook to allow benchmarks to be harnessed. MMTk will stop collecting
    /// statistics, and print out the collected statistics in a defined format.
    /// This is usually called by the benchmark harness right after the actual benchmark.
    pub fn harness_end(&'static self) {
        self.stats.stop_all(self);
        self.state.inside_harness.store(false, Ordering::SeqCst);
        probe!(mmtk, harness_end);
    }

    #[cfg(feature = "sanity")]
    pub(crate) fn sanity_begin(&self) {
        self.inside_sanity.store(true, Ordering::Relaxed)
    }

    #[cfg(feature = "sanity")]
    pub(crate) fn sanity_end(&self) {
        self.inside_sanity.store(false, Ordering::Relaxed)
    }

    #[cfg(feature = "sanity")]
    #[allow(unused)]
    pub(crate) fn is_in_sanity(&self) -> bool {
        self.inside_sanity.load(Ordering::Relaxed)
    }

    /// Get the current GC status for MMTk.
    pub fn get_gc_status(&self) -> GcStatus {
        self.state.gc_status.load()
    }

    /// Disable collection. On success, returns `Ok(true)` if this call actually switched
    /// collection from enabled to disabled, `Ok(false)` if it only increased the nesting depth of
    /// an already-disabled status. If MMTk is unable to disable GC right now (possibly a GC is in
    /// progress, or a GC has been requested), returns `Err` with the status that prevented it;
    /// users should invoke runtime safepoints or other mechanisms to prepare for a GC pause, and
    /// then call this function again.
    ///
    /// This call is nestable. Each call must be paired with a matching call to
    /// [`MMTK::enable_collection`].
    pub fn disable_collection(&self) -> Result<bool, GcStatus> {
        self.gc_trigger.disable_collection()
    }

    /// Enable collection. If collection is not currently disabled (e.g. there was no prior
    /// matching call to [`MMTK::disable_collection`]), this is a no-op.
    /// Returns `true` if this call actually re-enabled collection (i.e. it was the outermost
    /// matching call), `false` if it only decremented the nesting depth, or if collection was
    /// already enabled.
    pub fn enable_collection(&self) -> bool {
        self.gc_trigger.enable_collection()
    }

    /// Return whether collection is currently enabled.
    pub fn is_collection_enabled(&self) -> bool {
        self.gc_trigger.is_collection_enabled()
    }

    /// Return true if the current GC is an emergency GC.
    ///
    /// An emergency GC happens when a normal GC cannot reclaim enough memory to satisfy allocation
    /// requests.  Plans may do full-heap GC, defragmentation, etc. during emergency GCs in order to
    /// free up more memory.
    ///
    /// VM bindings can call this function during GC to check if the current GC is an emergency GC.
    /// If it is, the VM binding is recommended to retain fewer objects than normal GCs, to the
    /// extent allowed by the specification of the VM or the language.  For example, the VM binding
    /// may choose not to retain objects used for caching.  Specifically, for Java virtual machines,
    /// that means not retaining referents of [`SoftReference`][java-soft-ref] which is primarily
    /// designed for implementing memory-sensitive caches.
    ///
    /// [java-soft-ref]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/ref/SoftReference.html
    pub fn is_emergency_collection(&self) -> bool {
        self.state.is_emergency_collection()
    }

    /// Return true if the current GC is trigger manually by the user/binding.
    pub fn is_user_triggered_collection(&self) -> bool {
        self.state.is_user_triggered_collection()
    }

    /// The application code has requested a collection. This is just a GC hint, and
    /// we may ignore it.
    ///
    /// Returns whether a GC was ran or not. If MMTk triggers a GC, this method will block the
    /// calling thread and return true when the GC finishes. Otherwise, this method returns
    /// false immediately.
    ///
    /// # Arguments
    /// * `tls`: The mutator thread that requests the GC
    /// * `force`: The request cannot be ignored (except for NoGC)
    /// * `exhaustive`: The requested GC should be exhaustive. This is also a hint.
    pub fn handle_user_collection_request(
        &self,
        tls: VMMutatorThread,
        force: bool,
        exhaustive: bool,
    ) -> bool {
        if self
            .gc_trigger
            .handle_user_collection_request(force, exhaustive)
        {
            use crate::vm::Collection;
            // Do not block for GC if the `tls` does not represent a valid mutator thread. This
            // allows non-mutator threads to trigger GC but not block for GC.
            //
            // FIXME: Make a proper API that allows `handle_user_collection_request` to be called by
            // non-mutators and/or not trigger GC.
            if !tls.0 .0.is_null() {
                VM::VMCollection::block_for_gc(tls);
            }
            true
        } else {
            false
        }
    }

    /// MMTK has requested stop-the-world activity (e.g., stw within a concurrent gc).
    #[allow(unused)]
    pub fn trigger_internal_collection_request(&self) {
        self.gc_trigger.trigger_internal_collection_request();
    }

    /// Get a reference to the plan.
    pub fn get_plan(&self) -> &dyn Plan<VM = VM> {
        unsafe { &**(self.plan.get()) }
    }

    /// Get the plan as mutable reference.
    ///
    /// # Safety
    ///
    /// This is unsafe because the caller must ensure that the plan is not used by other threads.
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn get_plan_mut(&self) -> &mut dyn Plan<VM = VM> {
        &mut **(self.plan.get())
    }

    /// Get the run time options.
    pub fn get_options(&self) -> &Options {
        &self.options
    }

    /// Enumerate objects in all spaces in this MMTK instance.
    ///
    /// The call-back function `f` is called for every object that has the valid object bit (VO
    /// bit), i.e. objects that are allocated in the heap of this MMTK instance, but has not been
    /// reclaimed, yet.
    ///
    /// # Notes about object initialization and finalization
    ///
    /// When this function visits an object, it only guarantees that its VO bit must have been set.
    /// It is not guaranteed if the object has been "fully initialized" in the sense of the
    /// programming language the VM is implementing.  For example, the object header and the type
    /// information may not have been written.
    ///
    /// It will also visit objects that have been "finalized" in the sense of the programming
    /// langauge the VM is implementing, as long as the object has not been reclaimed by the GC,
    /// yet.  Be careful.  If the object header is destroyed, it may not be safe to access such
    /// objects in the high-level language.
    ///
    /// # Interaction with allocation and GC
    ///
    /// This function does not mutate the heap.  It is safe if multiple threads execute this
    /// function concurrently during mutator time.
    ///
    /// It has *undefined behavior* if allocation or GC happens while this function is being
    /// executed.  The VM binding must ensure no threads are allocating and GC does not start while
    /// executing this function.  One way to do this is stopping all mutators before calling this
    /// function.
    ///
    /// Some high-level languages may provide an API that allows the user to allocate objects and
    /// trigger GC while enumerating objects.  One example is [`ObjectSpace::each_object`][os_eo] in
    /// Ruby.  The VM binding may use the callback of this function to save all visited object
    /// references and let the user visit those references after this function returns.  Make sure
    /// those saved references are in the root set or in an object that will live through GCs before
    /// the high-level language finishes visiting the saved object references.
    ///
    /// [os_eo]: https://docs.ruby-lang.org/en/master/ObjectSpace.html#method-c-each_object
    #[cfg(feature = "vo_bit")]
    pub fn enumerate_objects<F>(&self, f: F)
    where
        F: FnMut(ObjectReference),
    {
        use crate::util::object_enum;

        let mut enumerator = object_enum::ClosureObjectEnumerator::<_, VM>::new(f);
        let plan = self.get_plan();
        plan.for_each_space(&mut |space| {
            space.enumerate_objects(&mut enumerator);
        })
    }

    /// Aggregate a hash map of live bytes per space with the space stats to produce
    /// a map of live bytes stats for the spaces.
    pub(crate) fn aggregate_live_bytes_in_last_gc(
        &self,
        live_bytes_per_space: [usize; MAX_SPACES],
    ) -> HashMap<&'static str, crate::LiveBytesStats> {
        use crate::policy::space::Space;
        let mut ret = HashMap::new();
        self.get_plan().for_each_space(&mut |space: &dyn Space<VM>| {
            let space_name = space.get_name();
            let space_idx = space.get_descriptor().get_index();
            let used_pages = space.reserved_pages();
            if used_pages != 0 {
                let used_bytes = crate::util::conversions::pages_to_bytes(used_pages);
                let live_bytes = live_bytes_per_space[space_idx];
                debug_assert!(
                    live_bytes <= used_bytes,
                    "Live bytes of objects in {} ({} bytes) is larger than used pages ({} bytes), something is wrong.",
                    space_name, live_bytes, used_bytes
                );
                ret.insert(space_name, crate::LiveBytesStats {
                    live_bytes,
                    used_pages,
                    used_bytes,
                });
            }
        });
        ret
    }

    /// Print VM maps.  It will print the memory ranges used by spaces as well as some attributes of
    /// the spaces.
    ///
    /// -   "I": The space is immortal.  Its objects will never die.
    /// -   "N": The space is non-movable.  Its objects will never move.
    ///
    /// Arguments:
    /// *   `out`: the place to print the VM maps.
    /// *   `space_name`: If `None`, print all spaces;
    ///     if `Some(n)`, only print the space whose name is `n`.
    pub fn debug_print_vm_maps(
        &self,
        out: &mut impl std::fmt::Write,
        space_name: Option<&str>,
    ) -> Result<(), std::fmt::Error> {
        let mut result_so_far = Ok(());
        self.get_plan().for_each_space(&mut |space| {
            if result_so_far.is_ok()
                && (space_name.is_none() || space_name == Some(space.get_name()))
            {
                result_so_far = crate::policy::space::print_vm_map(space, out);
            }
        });
        result_so_far
    }

    /// Initialize object metadata for a VM space object.
    /// Objects in the VM space are allocated/managed by the binding. This function provides a way for
    /// the binding to set object metadata in MMTk for an object in the space.
    #[cfg(feature = "vm_space")]
    pub fn initialize_vm_space_object(&self, object: crate::util::ObjectReference) {
        use crate::policy::sft::SFT;
        let bytes = VM::VMObjectModel::get_current_size(object);
        self.get_plan()
            .base()
            .vm_space
            .initialize_object_metadata(object, bytes)
    }
}

/// A non-mangled function to print object information for debugging purposes. This function can be directly
/// called from a debugger.
#[no_mangle]
pub fn mmtk_debug_print_object(object: crate::util::ObjectReference) {
    // If the address is unmapped, we cannot access its metadata. Just quit.
    if !object.to_raw_address().is_mapped() {
        println!("{} is not mapped in MMTk", object);
        return;
    }

    // If the address is not aligned to the object reference size, it is not an object reference.
    if !object
        .to_raw_address()
        .is_aligned_to(crate::util::ObjectReference::ALIGNMENT)
    {
        println!(
            "{} is not properly aligned. It is not an object reference.",
            object
        );
    }

    // Forward to the space
    let sft = SFT_MAP.get_checked(object.to_raw_address());
    // Print the space name
    println!("In {}:", sft.name());
    // Print object information
    sft.debug_print_object_info(object);
}