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//! # `VMExternRef`
//!
//! `VMExternRef` is a reference-counted box for any kind of data that is
//! external and opaque to running Wasm. Sometimes it might hold a Wasmtime
//! thing, other times it might hold something from a Wasmtime embedder and is
//! opaque even to us. It is morally equivalent to `Rc<dyn Any>` in Rust, but
//! additionally always fits in a pointer-sized word. `VMExternRef` is
//! non-nullable, but `Option<VMExternRef>` is a null pointer.
//!
//! The one part of `VMExternRef` that can't ever be opaque to us is the
//! reference count. Even when we don't know what's inside an `VMExternRef`, we
//! need to be able to manipulate its reference count as we add and remove
//! references to it. And we need to do this from compiled Wasm code, so it must
//! be `repr(C)`!
//!
//! ## Memory Layout
//!
//! `VMExternRef` itself is just a pointer to an `VMExternData`, which holds the
//! opaque, boxed value, its reference count, and its vtable pointer.
//!
//! The `VMExternData` struct is *preceded* by the dynamically-sized value boxed
//! up and referenced by one or more `VMExternRef`s:
//!
//! ```text
//! ,-------------------------------------------------------.
//! | |
//! V |
//! +----------------------------+-----------+-----------+ |
//! | dynamically-sized value... | ref_count | value_ptr |---'
//! +----------------------------+-----------+-----------+
//! | VMExternData |
//! +-----------------------+
//! ^
//! +-------------+ |
//! | VMExternRef |-------------------+
//! +-------------+ |
//! |
//! +-------------+ |
//! | VMExternRef |-------------------+
//! +-------------+ |
//! |
//! ... ===
//! |
//! +-------------+ |
//! | VMExternRef |-------------------'
//! +-------------+
//! ```
//!
//! The `value_ptr` member always points backwards to the start of the
//! dynamically-sized value (which is also the start of the heap allocation for
//! this value-and-`VMExternData` pair). Because it is a `dyn` pointer, it is
//! fat, and also points to the value's `Any` vtable.
//!
//! The boxed value and the `VMExternRef` footer are held a single heap
//! allocation. The layout described above is used to make satisfying the
//! value's alignment easy: we just need to ensure that the heap allocation used
//! to hold everything satisfies its alignment. It also ensures that we don't
//! need a ton of excess padding between the `VMExternData` and the value for
//! values with large alignment.
//!
//! ## Reference Counting, Wasm Functions, and Garbage Collection
//!
//! For host VM code, we use plain reference counting, where cloning increments
//! the reference count, and dropping decrements it. We can avoid many of the
//! on-stack increment/decrement operations that typically plague the
//! performance of reference counting via Rust's ownership and borrowing system.
//! Moving a `VMExternRef` avoids mutating its reference count, and borrowing it
//! either avoids the reference count increment or delays it until if/when the
//! `VMExternRef` is cloned.
//!
//! When passing a `VMExternRef` into compiled Wasm code, we don't want to do
//! reference count mutations for every compiled `local.{get,set}`, nor for
//! every function call. Therefore, we use a variation of **deferred reference
//! counting**, where we only mutate reference counts when storing
//! `VMExternRef`s somewhere that outlives the activation: into a global or
//! table. Simultaneously, we over-approximate the set of `VMExternRef`s that
//! are inside Wasm function activations. Periodically, we walk the stack at GC
//! safe points, and use stack map information to precisely identify the set of
//! `VMExternRef`s inside Wasm activations. Then we take the difference between
//! this precise set and our over-approximation, and decrement the reference
//! count for each of the `VMExternRef`s that are in our over-approximation but
//! not in the precise set. Finally, the over-approximation is replaced with the
//! precise set.
//!
//! The `VMExternRefActivationsTable` implements the over-approximized set of
//! `VMExternRef`s referenced by Wasm activations. Calling a Wasm function and
//! passing it a `VMExternRef` moves the `VMExternRef` into the table, and the
//! compiled Wasm function logically "borrows" the `VMExternRef` from the
//! table. Similarly, `global.get` and `table.get` operations clone the gotten
//! `VMExternRef` into the `VMExternRefActivationsTable` and then "borrow" the
//! reference out of the table.
//!
//! When a `VMExternRef` is returned to host code from a Wasm function, the host
//! increments the reference count (because the reference is logically
//! "borrowed" from the `VMExternRefActivationsTable` and the reference count
//! from the table will be dropped at the next GC).
//!
//! For more general information on deferred reference counting, see *An
//! Examination of Deferred Reference Counting and Cycle Detection* by Quinane:
//! <https://openresearch-repository.anu.edu.au/bitstream/1885/42030/2/hon-thesis.pdf>
use crate::{Backtrace, SendSyncPtr, VMRuntimeLimits};
use std::alloc::Layout;
use std::any::Any;
use std::cell::UnsafeCell;
use std::cmp;
use std::collections::HashSet;
use std::hash::{Hash, Hasher};
use std::mem;
use std::ops::Deref;
use std::ptr::{self, NonNull};
use std::sync::atomic::{self, AtomicUsize, Ordering};
use wasmtime_environ::StackMap;
/// An external reference to some opaque data.
///
/// `VMExternRef`s dereference to their underlying opaque data as `dyn Any`.
///
/// Unlike the `externref` in the Wasm spec, `VMExternRef`s are non-nullable,
/// and always point to a valid value. You may use `Option<VMExternRef>` to
/// represent nullable references, and `Option<VMExternRef>` is guaranteed to
/// have the same size and alignment as a raw pointer, with `None` represented
/// with the null pointer.
///
/// `VMExternRef`s are reference counted, so cloning is a cheap, shallow
/// operation. It also means they are inherently shared, so you may not get a
/// mutable, exclusive reference to their inner contents, only a shared,
/// immutable reference. You may use interior mutability with `RefCell` or
/// `Mutex` to work around this restriction, if necessary.
///
/// `VMExternRef`s have pointer-equality semantics, not structural-equality
/// semantics. Given two `VMExternRef`s `a` and `b`, `a == b` only if `a` and
/// `b` point to the same allocation. `a` and `b` are considered not equal, even
/// if `a` and `b` are two different identical copies of the same data, if they
/// are in two different allocations. The hashing and ordering implementations
/// also only operate on the pointer.
///
/// # Example
///
/// ```
/// # fn foo() -> Result<(), Box<dyn std::error::Error>> {
/// use std::cell::RefCell;
/// use wasmtime_runtime::VMExternRef;
///
/// // Open a file. Wasm doesn't know about files, but we can let Wasm instances
/// // work with files via opaque `externref` handles.
/// let file = std::fs::File::create("some/file/path")?;
///
/// // Wrap the file up as an `VMExternRef` that can be passed to Wasm.
/// let extern_ref_to_file = VMExternRef::new(file);
///
/// // `VMExternRef`s dereference to `dyn Any`, so you can use `Any` methods to
/// // perform runtime type checks and downcasts.
///
/// assert!(extern_ref_to_file.is::<std::fs::File>());
/// assert!(!extern_ref_to_file.is::<String>());
///
/// if let Some(mut file) = extern_ref_to_file.downcast_ref::<std::fs::File>() {
/// use std::io::Write;
/// writeln!(&mut file, "Hello, `VMExternRef`!")?;
/// }
/// # Ok(())
/// # }
/// ```
#[derive(Debug)]
#[repr(transparent)]
pub struct VMExternRef(SendSyncPtr<VMExternData>);
impl std::fmt::Pointer for VMExternRef {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
std::fmt::Pointer::fmt(&self.0, f)
}
}
#[repr(C)]
pub(crate) struct VMExternData {
// Implicit, dynamically-sized member that always preceded an
// `VMExternData`.
//
// value: [u8],
//
/// The reference count for this `VMExternData` and value. When it reaches
/// zero, we can safely destroy the value and free this heap
/// allocation. This is an `UnsafeCell`, rather than plain `Cell`, because
/// it can be modified by compiled Wasm code.
///
/// Note: this field's offset must be kept in sync with
/// `wasmtime_environ::VMOffsets::vm_extern_data_ref_count()` which is
/// currently always zero.
ref_count: AtomicUsize,
/// Always points to the implicit, dynamically-sized `value` member that
/// precedes this `VMExternData`.
value_ptr: SendSyncPtr<dyn Any + Send + Sync>,
}
impl Clone for VMExternRef {
#[inline]
fn clone(&self) -> VMExternRef {
self.extern_data().increment_ref_count();
VMExternRef(self.0)
}
}
impl Drop for VMExternRef {
#[inline]
fn drop(&mut self) {
let data = self.extern_data();
// Note that the memory orderings here also match the standard library
// itself. Documentation is more available in the implementation of
// `Arc`, but the general idea is that this is a special pattern allowed
// by the C standard with atomic orderings where we "release" for all
// the decrements and only the final decrementer performs an acquire
// fence. This properly ensures that the final thread, which actually
// destroys the data, sees all the updates from all other threads.
if data.ref_count.fetch_sub(1, Ordering::Release) != 1 {
return;
}
atomic::fence(Ordering::Acquire);
unsafe {
VMExternData::drop_and_dealloc(self.0);
}
}
}
impl VMExternData {
/// Get the `Layout` for a value with the given size and alignment, and the
/// offset within that layout where the `VMExternData` footer resides.
///
/// This doesn't take a `value: &T` because `VMExternRef::new_with` hasn't
/// constructed a `T` value yet, and it isn't generic over `T` because
/// `VMExternData::drop_and_dealloc` doesn't know what `T` to use, and has
/// to use `std::mem::{size,align}_of_val` instead.
unsafe fn layout_for(value_size: usize, value_align: usize) -> (Layout, usize) {
let extern_data_size = mem::size_of::<VMExternData>();
let extern_data_align = mem::align_of::<VMExternData>();
let value_and_padding_size = round_up_to_align(value_size, extern_data_align).unwrap();
let alloc_align = std::cmp::max(value_align, extern_data_align);
let alloc_size = value_and_padding_size + extern_data_size;
debug_assert!(
Layout::from_size_align(alloc_size, alloc_align).is_ok(),
"should create a `Layout` for size={} and align={} okay",
alloc_size,
alloc_align,
);
(
Layout::from_size_align_unchecked(alloc_size, alloc_align),
value_and_padding_size,
)
}
/// Drop the inner value and then free this `VMExternData` heap allocation.
pub(crate) unsafe fn drop_and_dealloc(mut data: SendSyncPtr<VMExternData>) {
log::trace!("Dropping externref data @ {:p}", data);
// Note: we introduce a block scope so that we drop the live
// reference to the data before we free the heap allocation it
// resides within after this block.
let (alloc_ptr, layout) = {
let data = data.as_mut();
debug_assert_eq!(data.ref_count.load(Ordering::SeqCst), 0);
// Same thing, but for the dropping the reference to `value` before
// we drop it itself.
let (layout, _) = {
let value = data.value_ptr.as_ref();
Self::layout_for(mem::size_of_val(value), mem::align_of_val(value))
};
ptr::drop_in_place(data.value_ptr.as_ptr());
let alloc_ptr = data.value_ptr.as_ptr().cast::<u8>();
(alloc_ptr, layout)
};
ptr::drop_in_place(data.as_ptr());
std::alloc::dealloc(alloc_ptr, layout);
}
#[inline]
fn increment_ref_count(&self) {
// This is only using during cloning operations, and like the standard
// library we use `Relaxed` here. The rationale is better documented in
// libstd's implementation of `Arc`, but the general gist is that we're
// creating a new pointer for our own thread, so there's no need to have
// any synchronization with orderings. The synchronization with other
// threads with respect to orderings happens when the pointer is sent to
// another thread.
self.ref_count.fetch_add(1, Ordering::Relaxed);
}
}
#[inline]
fn round_up_to_align(n: usize, align: usize) -> Option<usize> {
debug_assert!(align.is_power_of_two());
let align_minus_one = align - 1;
Some(n.checked_add(align_minus_one)? & !align_minus_one)
}
impl VMExternRef {
/// Wrap the given value inside an `VMExternRef`.
pub fn new<T>(value: T) -> VMExternRef
where
T: 'static + Any + Send + Sync,
{
VMExternRef::new_with(|| value)
}
/// Construct a new `VMExternRef` in place by invoking `make_value`.
pub fn new_with<T>(make_value: impl FnOnce() -> T) -> VMExternRef
where
T: 'static + Any + Send + Sync,
{
unsafe {
let (layout, footer_offset) =
VMExternData::layout_for(mem::size_of::<T>(), mem::align_of::<T>());
let alloc_ptr = std::alloc::alloc(layout);
let alloc_ptr = NonNull::new(alloc_ptr).unwrap_or_else(|| {
std::alloc::handle_alloc_error(layout);
});
let value_ptr = alloc_ptr.cast::<T>();
ptr::write(value_ptr.as_ptr(), make_value());
let extern_data_ptr =
alloc_ptr.cast::<u8>().as_ptr().add(footer_offset) as *mut VMExternData;
ptr::write(
extern_data_ptr,
VMExternData {
ref_count: AtomicUsize::new(1),
// Cast from `*mut T` to `*mut dyn Any` here.
value_ptr: SendSyncPtr::new(NonNull::new_unchecked(value_ptr.as_ptr())),
},
);
log::trace!("New externref data @ {:p}", extern_data_ptr);
VMExternRef(NonNull::new_unchecked(extern_data_ptr).into())
}
}
/// Turn this `VMExternRef` into a raw, untyped pointer.
///
/// Unlike `into_raw`, this does not consume and forget `self`. It is *not*
/// safe to use `from_raw` on pointers returned from this method; only use
/// `clone_from_raw`!
///
/// Nor does this method increment the reference count. You must ensure
/// that `self` (or some other clone of `self`) stays alive until
/// `clone_from_raw` is called.
#[inline]
pub fn as_raw(&self) -> *mut u8 {
let ptr = self.0.as_ptr().cast::<u8>();
ptr
}
/// Consume this `VMExternRef` into a raw, untyped pointer.
///
/// # Safety
///
/// This method forgets self, so it is possible to create a leak of the
/// underlying reference counted data if not used carefully.
///
/// Use `from_raw` to recreate the `VMExternRef`.
pub unsafe fn into_raw(self) -> *mut u8 {
let ptr = self.0.as_ptr().cast::<u8>();
std::mem::forget(self);
ptr
}
/// Recreate a `VMExternRef` from a pointer returned from a previous call to
/// `as_raw`.
///
/// # Safety
///
/// Unlike `clone_from_raw`, this does not increment the reference count of the
/// underlying data. It is not safe to continue to use the pointer passed to this
/// function.
pub unsafe fn from_raw(ptr: *mut u8) -> Self {
debug_assert!(!ptr.is_null());
VMExternRef(NonNull::new_unchecked(ptr).cast().into())
}
/// Recreate a `VMExternRef` from a pointer returned from a previous call to
/// `as_raw`.
///
/// # Safety
///
/// Wildly unsafe to use with anything other than the result of a previous
/// `as_raw` call!
///
/// Additionally, it is your responsibility to ensure that this raw
/// `VMExternRef`'s reference count has not dropped to zero. Failure to do
/// so will result in use after free!
pub unsafe fn clone_from_raw(ptr: *mut u8) -> Self {
debug_assert!(!ptr.is_null());
let x = VMExternRef(NonNull::new_unchecked(ptr).cast().into());
x.extern_data().increment_ref_count();
x
}
/// Get the strong reference count for this `VMExternRef`.
///
/// Note that this loads with a `SeqCst` ordering to synchronize with other
/// threads.
pub fn strong_count(&self) -> usize {
self.extern_data().ref_count.load(Ordering::SeqCst)
}
#[inline]
fn extern_data(&self) -> &VMExternData {
unsafe { self.0.as_ref() }
}
}
/// Methods that would normally be trait implementations, but aren't to avoid
/// potential footguns around `VMExternRef`'s pointer-equality semantics.
///
/// Note that none of these methods are on `&self`, they all require a
/// fully-qualified `VMExternRef::foo(my_ref)` invocation.
impl VMExternRef {
/// Check whether two `VMExternRef`s point to the same inner allocation.
///
/// Note that this uses pointer-equality semantics, not structural-equality
/// semantics, and so only pointers are compared, and doesn't use any `Eq`
/// or `PartialEq` implementation of the pointed-to values.
#[inline]
pub fn eq(a: &Self, b: &Self) -> bool {
ptr::eq(a.0.as_ptr(), b.0.as_ptr())
}
/// Hash a given `VMExternRef`.
///
/// Note that this just hashes the pointer to the inner value, it does *not*
/// use the inner value's `Hash` implementation (if any).
#[inline]
pub fn hash<H>(externref: &Self, hasher: &mut H)
where
H: Hasher,
{
ptr::hash(externref.0.as_ptr(), hasher);
}
/// Compare two `VMExternRef`s.
///
/// Note that this uses pointer-equality semantics, not structural-equality
/// semantics, and so only pointers are compared, and doesn't use any `Cmp`
/// or `PartialCmp` implementation of the pointed-to values.
#[inline]
pub fn cmp(a: &Self, b: &Self) -> cmp::Ordering {
let a = a.0.as_ptr() as usize;
let b = b.0.as_ptr() as usize;
a.cmp(&b)
}
}
impl Deref for VMExternRef {
type Target = dyn Any;
fn deref(&self) -> &dyn Any {
unsafe { self.extern_data().value_ptr.as_ref() }
}
}
/// A wrapper around a `VMExternRef` that implements `Eq` and `Hash` with
/// pointer semantics.
///
/// We use this so that we can morally put `VMExternRef`s inside of `HashSet`s
/// even though they don't implement `Eq` and `Hash` to avoid foot guns.
#[derive(Clone, Debug)]
struct VMExternRefWithTraits(VMExternRef);
impl Hash for VMExternRefWithTraits {
fn hash<H>(&self, hasher: &mut H)
where
H: Hasher,
{
VMExternRef::hash(&self.0, hasher)
}
}
impl PartialEq for VMExternRefWithTraits {
fn eq(&self, other: &Self) -> bool {
VMExternRef::eq(&self.0, &other.0)
}
}
impl Eq for VMExternRefWithTraits {}
type TableElem = UnsafeCell<Option<VMExternRef>>;
/// A table that over-approximizes the set of `VMExternRef`s that any Wasm
/// activation on this thread is currently using.
///
/// Under the covers, this is a simple bump allocator that allows duplicate
/// entries. Deduplication happens at GC time.
#[repr(C)] // `alloc` must be the first member, it's accessed from JIT code.
pub struct VMExternRefActivationsTable {
/// Structures used to perform fast bump allocation of storage of externref
/// values.
///
/// This is the only member of this structure accessed from JIT code.
alloc: VMExternRefTableAlloc,
/// When unioned with `chunk`, this is an over-approximation of the GC roots
/// on the stack, inside Wasm frames.
///
/// This is used by slow-path insertion, and when a GC cycle finishes, is
/// re-initialized to the just-discovered precise set of stack roots (which
/// immediately becomes an over-approximation again as soon as Wasm runs and
/// potentially drops references).
over_approximated_stack_roots: HashSet<VMExternRefWithTraits>,
/// The precise set of on-stack, inside-Wasm GC roots that we discover via
/// walking the stack and interpreting stack maps.
///
/// This is *only* used inside the `gc` function, and is empty otherwise. It
/// is just part of this struct so that we can reuse the allocation, rather
/// than create a new hash set every GC.
precise_stack_roots: HashSet<VMExternRefWithTraits>,
/// A debug-only field for asserting that we are in a region of code where
/// GC is okay to preform.
#[cfg(debug_assertions)]
gc_okay: bool,
}
#[repr(C)] // This is accessed from JIT code.
struct VMExternRefTableAlloc {
/// Bump-allocation finger within the `chunk`.
///
/// NB: this is an `UnsafeCell` because it is written to by compiled Wasm
/// code.
next: UnsafeCell<NonNull<TableElem>>,
/// Pointer to just after the `chunk`.
///
/// This is *not* within the current chunk and therefore is not a valid
/// place to insert a reference!
end: NonNull<TableElem>,
/// Bump allocation chunk that stores fast-path insertions.
///
/// This is not accessed from JIT code.
chunk: Box<[TableElem]>,
}
// This gets around the usage of `UnsafeCell` throughout the internals of this
// allocator, but the storage should all be Send/Sync and synchronization isn't
// necessary since operations require `&mut self`.
unsafe impl Send for VMExternRefTableAlloc {}
unsafe impl Sync for VMExternRefTableAlloc {}
fn _assert_send_sync() {
fn _assert<T: Send + Sync>() {}
_assert::<VMExternRefActivationsTable>();
_assert::<VMExternRef>();
}
impl VMExternRefActivationsTable {
const CHUNK_SIZE: usize = 4096 / mem::size_of::<usize>();
/// Create a new `VMExternRefActivationsTable`.
pub fn new() -> Self {
// Start with an empty chunk in case this activations table isn't used.
// This means that there's no space in the bump-allocation area which
// will force any path trying to use this to the slow gc path. The first
// time this happens, though, the slow gc path will allocate a new chunk
// for actual fast-bumping.
let mut chunk: Box<[TableElem]> = Box::new([]);
let next = chunk.as_mut_ptr();
let end = unsafe { next.add(chunk.len()) };
VMExternRefActivationsTable {
alloc: VMExternRefTableAlloc {
next: UnsafeCell::new(NonNull::new(next).unwrap()),
end: NonNull::new(end).unwrap(),
chunk,
},
over_approximated_stack_roots: HashSet::new(),
precise_stack_roots: HashSet::new(),
#[cfg(debug_assertions)]
gc_okay: true,
}
}
fn new_chunk(size: usize) -> Box<[UnsafeCell<Option<VMExternRef>>]> {
assert!(size >= Self::CHUNK_SIZE);
(0..size).map(|_| UnsafeCell::new(None)).collect()
}
/// Get the available capacity in the bump allocation chunk.
#[inline]
pub fn bump_capacity_remaining(&self) -> usize {
let end = self.alloc.end.as_ptr() as usize;
let next = unsafe { *self.alloc.next.get() };
end - next.as_ptr() as usize
}
/// Try and insert a `VMExternRef` into this table.
///
/// This is a fast path that only succeeds when the bump chunk has the
/// capacity for the requested insertion.
///
/// If the insertion fails, then the `VMExternRef` is given back. Callers
/// may attempt a GC to free up space and try again, or may call
/// `insert_slow_path` to infallibly insert the reference (potentially
/// allocating additional space in the table to hold it).
#[inline]
pub fn try_insert(&mut self, externref: VMExternRef) -> Result<(), VMExternRef> {
unsafe {
let next = *self.alloc.next.get();
if next == self.alloc.end {
return Err(externref);
}
debug_assert!(
(*next.as_ref().get()).is_none(),
"slots >= the `next` bump finger are always `None`"
);
ptr::write(next.as_ptr(), UnsafeCell::new(Some(externref)));
let next = NonNull::new_unchecked(next.as_ptr().add(1));
debug_assert!(next <= self.alloc.end);
*self.alloc.next.get() = next;
Ok(())
}
}
/// Insert a reference into the table, falling back on a GC to clear up
/// space if the table is already full.
///
/// # Unsafety
///
/// The same as `gc`.
#[inline]
pub unsafe fn insert_with_gc(
&mut self,
limits: *const VMRuntimeLimits,
externref: VMExternRef,
module_info_lookup: &dyn ModuleInfoLookup,
) {
#[cfg(debug_assertions)]
assert!(self.gc_okay);
if let Err(externref) = self.try_insert(externref) {
self.gc_and_insert_slow(limits, externref, module_info_lookup);
}
}
#[inline(never)]
unsafe fn gc_and_insert_slow(
&mut self,
limits: *const VMRuntimeLimits,
externref: VMExternRef,
module_info_lookup: &dyn ModuleInfoLookup,
) {
gc(limits, module_info_lookup, self);
// Might as well insert right into the hash set, rather than the bump
// chunk, since we are already on a slow path and we get de-duplication
// this way.
self.over_approximated_stack_roots
.insert(VMExternRefWithTraits(externref));
}
/// Insert a reference into the table, without ever performing GC.
#[inline]
pub fn insert_without_gc(&mut self, externref: VMExternRef) {
if let Err(externref) = self.try_insert(externref) {
self.insert_slow_without_gc(externref);
}
}
#[inline(never)]
fn insert_slow_without_gc(&mut self, externref: VMExternRef) {
self.over_approximated_stack_roots
.insert(VMExternRefWithTraits(externref));
}
fn num_filled_in_bump_chunk(&self) -> usize {
let next = unsafe { *self.alloc.next.get() };
let bytes_unused = (self.alloc.end.as_ptr() as usize) - (next.as_ptr() as usize);
let slots_unused = bytes_unused / mem::size_of::<TableElem>();
self.alloc.chunk.len().saturating_sub(slots_unused)
}
fn elements(&self, mut f: impl FnMut(&VMExternRef)) {
for elem in self.over_approximated_stack_roots.iter() {
f(&elem.0);
}
// The bump chunk is not all the way full, so we only iterate over its
// filled-in slots.
let num_filled = self.num_filled_in_bump_chunk();
for slot in self.alloc.chunk.iter().take(num_filled) {
if let Some(elem) = unsafe { &*slot.get() } {
f(elem);
}
}
}
fn insert_precise_stack_root(
precise_stack_roots: &mut HashSet<VMExternRefWithTraits>,
root: NonNull<VMExternData>,
) {
let root = unsafe { VMExternRef::clone_from_raw(root.as_ptr().cast()) };
log::trace!("Found externref on stack: {:p}", root);
precise_stack_roots.insert(VMExternRefWithTraits(root));
}
/// Sweep the bump allocation table after we've discovered our precise stack
/// roots.
fn sweep(&mut self) {
log::trace!("begin GC sweep");
// Sweep our bump chunk.
let num_filled = self.num_filled_in_bump_chunk();
unsafe {
*self.alloc.next.get() = self.alloc.end;
}
for slot in self.alloc.chunk.iter().take(num_filled) {
unsafe {
*slot.get() = None;
}
}
debug_assert!(
self.alloc
.chunk
.iter()
.all(|slot| unsafe { (*slot.get()).as_ref().is_none() }),
"after sweeping the bump chunk, all slots should be `None`"
);
// If this is the first instance of gc then the initial chunk is empty,
// so we lazily allocate space for fast bump-allocation in the future.
if self.alloc.chunk.is_empty() {
self.alloc.chunk = Self::new_chunk(Self::CHUNK_SIZE);
self.alloc.end =
NonNull::new(unsafe { self.alloc.chunk.as_mut_ptr().add(self.alloc.chunk.len()) })
.unwrap();
}
// Reset our `next` finger to the start of the bump allocation chunk.
unsafe {
let next = self.alloc.chunk.as_mut_ptr();
debug_assert!(!next.is_null());
*self.alloc.next.get() = NonNull::new_unchecked(next);
}
// The current `precise_stack_roots` becomes our new over-appoximated
// set for the next GC cycle.
mem::swap(
&mut self.precise_stack_roots,
&mut self.over_approximated_stack_roots,
);
// And finally, the new `precise_stack_roots` should be cleared and
// remain empty until the next GC cycle.
//
// Note that this may run arbitrary code as we run externref
// destructors. Because of our `&mut` borrow above on this table,
// though, we're guaranteed that nothing will touch this table.
self.precise_stack_roots.clear();
log::trace!("end GC sweep");
}
/// Set whether it is okay to GC or not right now.
///
/// This is provided as a helper for enabling various debug-only assertions
/// and checking places where the `wasmtime-runtime` user expects there not
/// to be any GCs.
#[inline]
pub fn set_gc_okay(&mut self, okay: bool) -> bool {
#[cfg(debug_assertions)]
{
return std::mem::replace(&mut self.gc_okay, okay);
}
#[cfg(not(debug_assertions))]
{
let _ = okay;
return true;
}
}
}
/// Used by the runtime to lookup information about a module given a
/// program counter value.
pub trait ModuleInfoLookup {
/// Lookup the module information from a program counter value.
fn lookup(&self, pc: usize) -> Option<&dyn ModuleInfo>;
}
/// Used by the runtime to query module information.
pub trait ModuleInfo {
/// Lookup the stack map at a program counter value.
fn lookup_stack_map(&self, pc: usize) -> Option<&StackMap>;
}
#[derive(Debug, Default)]
struct DebugOnly<T> {
inner: T,
}
impl<T> std::ops::Deref for DebugOnly<T> {
type Target = T;
fn deref(&self) -> &T {
if cfg!(debug_assertions) {
&self.inner
} else {
panic!(
"only deref `DebugOnly` when `cfg(debug_assertions)` or \
inside a `debug_assert!(..)`"
)
}
}
}
impl<T> std::ops::DerefMut for DebugOnly<T> {
fn deref_mut(&mut self) -> &mut T {
if cfg!(debug_assertions) {
&mut self.inner
} else {
panic!(
"only deref `DebugOnly` when `cfg(debug_assertions)` or \
inside a `debug_assert!(..)`"
)
}
}
}
/// Perform garbage collection of `VMExternRef`s.
///
/// # Unsafety
///
/// You must have called `VMExternRefActivationsTable::set_stack_canary` for at
/// least the oldest host-->Wasm stack frame transition on this thread's stack
/// (it is idempotent to call it more than once) and keep its return value alive
/// across the duration of that host-->Wasm call.
///
/// Additionally, you must have registered the stack maps for every Wasm module
/// that has frames on the stack with the given `stack_maps_registry`.
pub unsafe fn gc(
limits: *const VMRuntimeLimits,
module_info_lookup: &dyn ModuleInfoLookup,
externref_activations_table: &mut VMExternRefActivationsTable,
) {
log::debug!("start GC");
#[cfg(debug_assertions)]
assert!(externref_activations_table.gc_okay);
debug_assert!({
// This set is only non-empty within this function. It is built up when
// walking the stack and interpreting stack maps, and then drained back
// into the activations table's bump-allocated space at the
// end. Therefore, it should always be empty upon entering this
// function.
externref_activations_table.precise_stack_roots.is_empty()
});
// This function proceeds by:
//
// * walking the stack,
//
// * finding the precise set of roots inside Wasm frames via our stack maps,
// and
//
// * resetting our bump-allocated table's over-approximation to the
// newly-discovered precise set.
// The `activations_table_set` is used for `debug_assert!`s checking that
// every reference we read out from the stack via stack maps is actually in
// the table. If that weren't true, than either we forgot to insert a
// reference in the table when passing it into Wasm (a bug) or we are
// reading invalid references from the stack (another bug).
let mut activations_table_set: DebugOnly<HashSet<_>> = Default::default();
if cfg!(debug_assertions) {
externref_activations_table.elements(|elem| {
activations_table_set.insert(elem.as_raw() as *mut VMExternData);
});
}
log::trace!("begin GC trace");
Backtrace::trace(limits, |frame| {
let pc = frame.pc();
debug_assert!(pc != 0, "we should always get a valid PC for Wasm frames");
let fp = frame.fp();
debug_assert!(
fp != 0,
"we should always get a valid frame pointer for Wasm frames"
);
let module_info = module_info_lookup
.lookup(pc)
.expect("should have module info for Wasm frame");
let stack_map = match module_info.lookup_stack_map(pc) {
Some(sm) => sm,
None => {
log::trace!("No stack map for this Wasm frame");
return std::ops::ControlFlow::Continue(());
}
};
log::trace!(
"We have a stack map that maps {} words in this Wasm frame",
stack_map.mapped_words()
);
let sp = fp - stack_map.mapped_words() as usize * mem::size_of::<usize>();
for i in 0..(stack_map.mapped_words() as usize) {
// Stack maps have one bit per word in the frame, and the
// zero^th bit is the *lowest* addressed word in the frame,
// i.e. the closest to the SP. So to get the `i`^th word in
// this frame, we add `i * sizeof(word)` to the SP.
let stack_slot = sp + i * mem::size_of::<usize>();
if !stack_map.get_bit(i) {
log::trace!(
"Stack slot @ {:p} does not contain externrefs",
stack_slot as *const (),
);
continue;
}
let stack_slot = stack_slot as *const *mut VMExternData;
let r = std::ptr::read(stack_slot);
log::trace!("Stack slot @ {:p} = {:p}", stack_slot, r);
debug_assert!(
r.is_null() || activations_table_set.contains(&r),
"every on-stack externref inside a Wasm frame should \
have an entry in the VMExternRefActivationsTable; \
{:?} is not in the table",
r
);
if let Some(r) = NonNull::new(r) {
VMExternRefActivationsTable::insert_precise_stack_root(
&mut externref_activations_table.precise_stack_roots,
r,
);
}
}
std::ops::ControlFlow::Continue(())
});
log::trace!("end GC trace");
externref_activations_table.sweep();
log::debug!("end GC");
}
#[cfg(test)]
mod tests {
use super::*;
use std::convert::TryInto;
#[test]
fn extern_ref_is_pointer_sized_and_aligned() {
assert_eq!(mem::size_of::<VMExternRef>(), mem::size_of::<*mut ()>());
assert_eq!(mem::align_of::<VMExternRef>(), mem::align_of::<*mut ()>());
assert_eq!(
mem::size_of::<Option<VMExternRef>>(),
mem::size_of::<*mut ()>()
);
assert_eq!(
mem::align_of::<Option<VMExternRef>>(),
mem::align_of::<*mut ()>()
);
}
#[test]
fn ref_count_is_at_correct_offset() {
let s = "hi";
let s: &(dyn Any + Send + Sync) = &s as _;
let s: *const (dyn Any + Send + Sync) = s as _;
let s: *mut (dyn Any + Send + Sync) = s as _;
let extern_data = VMExternData {
ref_count: AtomicUsize::new(0),
value_ptr: NonNull::new(s).unwrap().into(),
};
let extern_data_ptr = &extern_data as *const _;
let ref_count_ptr = &extern_data.ref_count as *const _;
let actual_offset = (ref_count_ptr as usize) - (extern_data_ptr as usize);
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
ptr: 8,
num_imported_functions: 0,
num_imported_tables: 0,
num_imported_memories: 0,
num_imported_globals: 0,
num_defined_tables: 0,
num_defined_memories: 0,
num_owned_memories: 0,
num_defined_globals: 0,
num_escaped_funcs: 0,
});
assert_eq!(
offsets.vm_extern_data_ref_count(),
actual_offset.try_into().unwrap(),
);
}
#[test]
fn table_next_is_at_correct_offset() {
let table = VMExternRefActivationsTable::new();
let table_ptr = &table as *const _;
let next_ptr = &table.alloc.next as *const _;
let actual_offset = (next_ptr as usize) - (table_ptr as usize);
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
ptr: 8,
num_imported_functions: 0,
num_imported_tables: 0,
num_imported_memories: 0,
num_imported_globals: 0,
num_defined_tables: 0,
num_defined_memories: 0,
num_owned_memories: 0,
num_defined_globals: 0,
num_escaped_funcs: 0,
});
assert_eq!(
offsets.vm_extern_ref_activation_table_next() as usize,
actual_offset
);
}
#[test]
fn table_end_is_at_correct_offset() {
let table = VMExternRefActivationsTable::new();
let table_ptr = &table as *const _;
let end_ptr = &table.alloc.end as *const _;
let actual_offset = (end_ptr as usize) - (table_ptr as usize);
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
ptr: 8,
num_imported_functions: 0,
num_imported_tables: 0,
num_imported_memories: 0,
num_imported_globals: 0,
num_defined_tables: 0,
num_defined_memories: 0,
num_owned_memories: 0,
num_defined_globals: 0,
num_escaped_funcs: 0,
});
assert_eq!(
offsets.vm_extern_ref_activation_table_end() as usize,
actual_offset
);
}
}