mmtk/policy/

sft.rs

use crate::plan::tracing::OptionObjectQueue;
use crate::scheduler::GCWorker;
use crate::util::*;
use crate::vm::VMBinding;
use std::marker::PhantomData;

/// Space Function Table (SFT).
///
/// This trait captures functions that reflect _space-specific per-object
/// semantics_.   These functions are implemented for each object via a special
/// space-based dynamic dispatch mechanism where the semantics are _not_
/// determined by the object's _type_, but rather, are determined by the _space_
/// that the object is in.
///
/// The underlying mechanism exploits the fact that spaces use the address space
/// at an MMTk chunk granularity with the consequence that each chunk maps to
/// exactluy one space, so knowing the chunk for an object reveals its space.
/// The dispatch then works by performing simple address arithmetic on the object
/// reference to find a chunk index which is used to index a table which returns
/// the space.   The relevant function is then dispatched against that space
/// object.
///
/// We use the SFT trait to simplify typing for Rust, so our table is a
/// table of SFT rather than Space.
pub trait SFT {
    /// The space name
    fn name(&self) -> &'static str;

    /// Get forwarding pointer if the object is forwarded.
    fn get_forwarded_object(&self, _object: ObjectReference) -> Option<ObjectReference> {
        None
    }

    /// Is the object live, determined by the policy?
    fn is_live(&self, object: ObjectReference) -> bool;

    /// Is the object reachable, determined by the policy?
    /// Note: Objects in ImmortalSpace may have `is_live = true` but are actually unreachable.
    fn is_reachable(&self, object: ObjectReference) -> bool {
        self.is_live(object)
    }

    // Functions for pinning/unpining and checking if an object is pinned
    // For non moving policies, all the objects are considered as forever pinned,
    // thus attempting to pin or unpin them will not succeed and will always return false.
    // For policies where moving is compusory, pin/unpin is impossible and will panic (is_object_pinned will return false).
    // For policies that support pinning (eg. Immix), pin/unpin will return a boolean indicating that the
    // pinning/unpinning action has been performed by the function, and is_object_pinned will return whether the object
    // is currently pinned.
    #[cfg(feature = "object_pinning")]
    fn pin_object(&self, object: ObjectReference) -> bool;
    #[cfg(feature = "object_pinning")]
    fn unpin_object(&self, object: ObjectReference) -> bool;
    #[cfg(feature = "object_pinning")]
    fn is_object_pinned(&self, object: ObjectReference) -> bool;

    /// Is the object movable, determined by the policy? E.g. the policy is non-moving,
    /// or the object is pinned.
    fn is_movable(&self) -> bool;

    /// Is the object sane? A policy should return false if there is any abnormality about
    /// object - the sanity checker will fail if an object is not sane.
    #[cfg(feature = "sanity")]
    fn is_sane(&self) -> bool;

    /// Is the object managed by MMTk? For most cases, if we find the sft for an object, that means
    /// the object is in the space and managed by MMTk. However, for some spaces, like MallocSpace,
    /// we mark the entire chunk in the SFT table as a malloc space, but only some of the addresses
    /// in the space contain actual MMTk objects. So they need a further check.
    fn is_in_space(&self, _object: ObjectReference) -> bool {
        true
    }

    /// Is `addr` a valid object reference to an object allocated in this space?
    /// This default implementation works for all spaces that use MMTk's mapper to allocate memory.
    /// Some spaces, like `MallocSpace`, use third-party libraries to allocate memory.
    /// Such spaces needs to override this method.
    #[cfg(feature = "vo_bit")]
    fn is_mmtk_object(&self, addr: Address) -> Option<ObjectReference>;

    #[cfg(feature = "vo_bit")]
    fn find_object_from_internal_pointer(
        &self,
        ptr: Address,
        max_search_bytes: usize,
    ) -> Option<ObjectReference>;

    /// Initialize object metadata (in the header, or in the side metadata).
    ///
    /// This method is called after an object is allocated.  Specifically,
    /// -   The VM binding calls [`crate::MMTK::initialize_vm_space_object`] which calls this method
    ///     to set the metadata for the VM space.
    /// -   Objects in other spaces are allocated by mutators using an MMTk allocator.
    ///     `Mutator::post_alloc` will call this method after allocation.
    fn initialize_object_metadata(&self, object: ObjectReference, _bytes: usize);

    /// Trace objects through SFT. This along with [`crate::plan::tracing::SFTTrace`]
    /// provides an easy way for most plans to trace objects without the need to implement any plan-specific
    /// code. However, tracing objects for some policies are more complicated, and they do not provide an
    /// implementation of this method. For example, mark compact space requires trace twice in each GC.
    /// Immix has defrag trace and fast trace.
    fn sft_trace_object(
        &self,
        // We use `OptionObjectQueue`, the simplest `ObjectQueue` implementation, for `queue`
        // because SFT doesn't support generic parameters.  The generic `SFTTrace::trace_object`
        // method wraps `SFT::sft_trace_object` and forwards the enqueued object to the actual
        // queue.
        queue: &mut OptionObjectQueue,
        object: ObjectReference,
        worker: GCWorkerMutRef,
    ) -> ObjectReference;

    /// Print debug info for the object. The implementer should print one line at a time so in case of an unexpected error,
    /// we still print something.
    fn debug_print_object_info(&self, _object: ObjectReference) {
        println!("This policy does not implement debug_print_object_info.");
    }
}

// Create erased VM refs for these types that will be used in `sft_trace_object()`.
// In this way, we can store the refs with <VM> in SFT (which cannot have parameters with generic type parameters)

use crate::util::erase_vm::define_erased_vm_mut_ref;
define_erased_vm_mut_ref!(GCWorkerMutRef = GCWorker<VM>);

/// Print debug info for SFT. Should be false when committed.
pub const DEBUG_SFT: bool = cfg!(debug_assertions) && false;

/// An empty entry for SFT.
#[derive(Debug)]
pub struct EmptySpaceSFT {}

pub const EMPTY_SFT_NAME: &str = "empty";
pub const EMPTY_SPACE_SFT: EmptySpaceSFT = EmptySpaceSFT {};

impl SFT for EmptySpaceSFT {
    fn name(&self) -> &'static str {
        EMPTY_SFT_NAME
    }
    fn is_live(&self, object: ObjectReference) -> bool {
        panic!(
            "Called is_live() on {:x}, which maps to an empty space",
            object
        )
    }
    #[cfg(feature = "sanity")]
    fn is_sane(&self) -> bool {
        warn!("Object in empty space!");
        false
    }
    #[cfg(feature = "object_pinning")]
    fn pin_object(&self, _object: ObjectReference) -> bool {
        panic!("Cannot pin/unpin objects of EmptySpace.")
    }
    #[cfg(feature = "object_pinning")]
    fn unpin_object(&self, _object: ObjectReference) -> bool {
        panic!("Cannot pin/unpin objects of EmptySpace.")
    }
    #[cfg(feature = "object_pinning")]
    fn is_object_pinned(&self, _object: ObjectReference) -> bool {
        false
    }
    fn is_movable(&self) -> bool {
        /*
         * FIXME steveb I think this should panic (ie the function should not
         * be invoked on an empty space).   However, JikesRVM currently does
         * call this in an unchecked way and expects 'false' for out of bounds
         * addresses.  So until that is fixed upstream, we'll return false here.
         *
         * panic!("called is_movable() on empty space")
         */
        false
    }
    fn is_in_space(&self, _object: ObjectReference) -> bool {
        false
    }
    #[cfg(feature = "vo_bit")]
    fn is_mmtk_object(&self, _addr: Address) -> Option<ObjectReference> {
        None
    }
    #[cfg(feature = "vo_bit")]
    fn find_object_from_internal_pointer(
        &self,
        _ptr: Address,
        _max_search_bytes: usize,
    ) -> Option<ObjectReference> {
        None
    }

    fn initialize_object_metadata(&self, object: ObjectReference, _bytes: usize) {
        panic!(
            "Called initialize_object_metadata() on {:x}, which maps to an empty space",
            object
        )
    }

    fn sft_trace_object(
        &self,
        _queue: &mut OptionObjectQueue,
        object: ObjectReference,
        _worker: GCWorkerMutRef,
    ) -> ObjectReference {
        // We do not have the `VM` type parameter here, so we cannot forward the call to the VM.
        panic!(
            "Call trace_object() on {}, which maps to an empty space. SFTTrace does not support the fallback to vm_trace_object().",
            object,
        )
    }
}