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//! Target- and pointer-width-agnostic definitions of GC-related types and
//! constants.
//!
//! These definitions are suitable for use both during compilation and at
//! runtime.
//!
//! Note: We don't bother gating these on `cfg(feature = "gc")` because that
//! makes downstream uses pretty annoying, and the primary thing we want to gate
//! on our various `gc` cargo features is the actual garbage collection
//! functions and their associated impact on binary size anyways.
#[cfg(feature = "gc")]
pub mod drc;
use crate::prelude::*;
use crate::{WasmArrayType, WasmCompositeType, WasmStorageType, WasmStructType, WasmValType};
use core::alloc::Layout;
/// Discriminant to check whether GC reference is an `i31ref` or not.
pub const I31_DISCRIMINANT: u64 = 1;
/// A mask that can be used to check for non-null and non-i31ref GC references
/// with a single bitwise-and operation.
pub const NON_NULL_NON_I31_MASK: u64 = !I31_DISCRIMINANT;
/// Get the byte size of the given Wasm type when it is stored inside the GC
/// heap.
pub fn byte_size_of_wasm_ty_in_gc_heap(ty: &WasmStorageType) -> u32 {
match ty {
WasmStorageType::I8 => 1,
WasmStorageType::I16 => 2,
WasmStorageType::Val(ty) => match ty {
WasmValType::I32 | WasmValType::F32 | WasmValType::Ref(_) => 4,
WasmValType::I64 | WasmValType::F64 => 8,
WasmValType::V128 => 16,
},
}
}
/// A trait for getting the layout of a Wasm GC struct or array inside a
/// particular collector.
pub trait GcTypeLayouts {
/// The offset of an array's length field.
///
/// This must be the same for all arrays in the heap, regardless of their
/// element type.
fn array_length_field_offset(&self) -> u32;
/// Get this collector's layout for the given composite type.
///
/// Returns `None` if the type is a function type, as functions are not
/// managed by the GC.
fn gc_layout(&self, ty: &WasmCompositeType) -> Option<GcLayout> {
match ty {
WasmCompositeType::Array(ty) => Some(self.array_layout(ty).into()),
WasmCompositeType::Struct(ty) => Some(self.struct_layout(ty).into()),
WasmCompositeType::Func(_) => None,
}
}
/// Get this collector's layout for the given array type.
fn array_layout(&self, ty: &WasmArrayType) -> GcArrayLayout;
/// Get this collector's layout for the given struct type.
fn struct_layout(&self, ty: &WasmStructType) -> GcStructLayout;
}
/// The layout of a GC-managed object.
#[derive(Clone, Debug)]
pub enum GcLayout {
/// The layout of a GC-managed array object.
Array(GcArrayLayout),
/// The layout of a GC-managed struct object.
Struct(GcStructLayout),
}
impl From<GcArrayLayout> for GcLayout {
fn from(layout: GcArrayLayout) -> Self {
Self::Array(layout)
}
}
impl From<GcStructLayout> for GcLayout {
fn from(layout: GcStructLayout) -> Self {
Self::Struct(layout)
}
}
impl GcLayout {
/// Get the underlying `GcStructLayout`, or panic.
#[track_caller]
pub fn unwrap_struct(&self) -> &GcStructLayout {
match self {
Self::Struct(s) => s,
_ => panic!("GcLayout::unwrap_struct on non-struct GC layout"),
}
}
/// Get the underlying `GcArrayLayout`, or panic.
#[track_caller]
pub fn unwrap_array(&self) -> &GcArrayLayout {
match self {
Self::Array(a) => a,
_ => panic!("GcLayout::unwrap_array on non-array GC layout"),
}
}
}
/// The layout of a GC-managed array.
///
/// This layout is only valid for use with the GC runtime that created it. It is
/// not valid to use one GC runtime's layout with another GC runtime, doing so
/// is memory safe but will lead to general incorrectness like panics and wrong
/// results.
///
/// All offsets are from the start of the object; that is, the size of the GC
/// header (for example) is included in the offset.
///
/// All arrays are composed of the generic `VMGcHeader`, followed by
/// collector-specific fields, followed by the contiguous array elements
/// themselves. The array elements must be aligned to the element type's natural
/// alignment.
#[derive(Clone, Debug)]
#[allow(dead_code)] // Not used yet, but added for completeness.
pub struct GcArrayLayout {
/// The size of this array object, without any elements.
///
/// The array's elements, if any, must begin at exactly this offset.
pub base_size: u32,
/// The alignment of this array.
pub align: u32,
/// The size and natural alignment of each element in this array.
pub elem_size: u32,
}
impl GcArrayLayout {
/// Get the total size of this array for a given length of elements.
#[inline]
pub fn size_for_len(&self, len: u32) -> u32 {
self.elem_offset(len)
}
/// Get the offset of the `i`th element in an array with this layout.
#[inline]
pub fn elem_offset(&self, i: u32) -> u32 {
self.base_size + i * self.elem_size
}
/// Get a `core::alloc::Layout` for an array of this type with the given
/// length.
pub fn layout(&self, len: u32) -> Layout {
let size = self.size_for_len(len);
let size = usize::try_from(size).unwrap();
let align = usize::try_from(self.align).unwrap();
Layout::from_size_align(size, align).unwrap()
}
}
/// The layout for a GC-managed struct type.
///
/// This layout is only valid for use with the GC runtime that created it. It is
/// not valid to use one GC runtime's layout with another GC runtime, doing so
/// is memory safe but will lead to general incorrectness like panics and wrong
/// results.
///
/// All offsets are from the start of the object; that is, the size of the GC
/// header (for example) is included in the offset.
#[derive(Clone, Debug)]
pub struct GcStructLayout {
/// The size (in bytes) of this struct.
pub size: u32,
/// The alignment (in bytes) of this struct.
pub align: u32,
/// The fields of this struct. The `i`th entry is the `i`th struct field's
/// offset (in bytes) in the struct.
pub fields: Vec<u32>,
}
impl GcStructLayout {
/// Get a `core::alloc::Layout` for a struct of this type.
pub fn layout(&self) -> Layout {
let size = usize::try_from(self.size).unwrap();
let align = usize::try_from(self.align).unwrap();
Layout::from_size_align(size, align).unwrap()
}
}
/// The kind of an object in a GC heap.
///
/// Note that this type is accessed from Wasm JIT code.
///
/// `VMGcKind` is a bitset where to test if `a` is a subtype of an
/// "abstract-ish" type `b`, we can simply use a single bitwise-and operation:
///
/// ```ignore
/// a <: b iff a & b == b
/// ```
///
/// For example, because `VMGcKind::AnyRef` has the high bit set, every kind
/// representing some subtype of `anyref` also has its high bit set.
///
/// We say "abstract-ish" type because in addition to the abstract heap types
/// (other than `i31`) we also have variants for `externref`s that have been
/// converted into an `anyref` via `extern.convert_any` and `externref`s that
/// have been converted into an `anyref` via `any.convert_extern`. Note that in
/// the latter case, because `any.convert_extern $foo` produces a value that is
/// not an instance of `eqref`, `VMGcKind::AnyOfExternRef & VMGcKind::EqRef !=
/// VMGcKind::EqRef`.
///
/// Furthermore, this type only uses the highest 6 bits of its `u32`
/// representation, allowing the lower 26 bytes to be bitpacked with other stuff
/// as users see fit.
#[repr(u32)]
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
#[rustfmt::skip]
#[allow(missing_docs)]
pub enum VMGcKind {
ExternRef = 0b010000 << 26,
ExternOfAnyRef = 0b011000 << 26,
AnyRef = 0b100000 << 26,
AnyOfExternRef = 0b100100 << 26,
EqRef = 0b101000 << 26,
ArrayRef = 0b101001 << 26,
StructRef = 0b101010 << 26,
}
impl VMGcKind {
/// Mask this value with a `u32` to get just the bits that `VMGcKind` uses.
pub const MASK: u32 = 0b111111 << 26;
/// Mask this value with a `u32` that potentially contains a `VMGcKind` to
/// get the bits that `VMGcKind` doesn't use.
pub const UNUSED_MASK: u32 = !Self::MASK;
/// Does the given value fit in the unused bits of a `VMGcKind`?
#[inline]
pub fn value_fits_in_unused_bits(value: u32) -> bool {
(value & Self::UNUSED_MASK) == value
}
/// Convert the given value into a `VMGcKind` by masking off the unused
/// bottom bits.
#[inline]
pub fn from_high_bits_of_u32(val: u32) -> VMGcKind {
let masked = val & Self::MASK;
match masked {
x if x == Self::ExternRef.as_u32() => Self::ExternRef,
x if x == Self::ExternOfAnyRef.as_u32() => Self::ExternOfAnyRef,
x if x == Self::AnyRef.as_u32() => Self::AnyRef,
x if x == Self::AnyOfExternRef.as_u32() => Self::AnyOfExternRef,
x if x == Self::EqRef.as_u32() => Self::EqRef,
x if x == Self::ArrayRef.as_u32() => Self::ArrayRef,
x if x == Self::StructRef.as_u32() => Self::StructRef,
_ => panic!("invalid `VMGcKind`: {masked:#032b}"),
}
}
/// Does this kind match the other kind?
///
/// That is, is this kind a subtype of the other kind?
#[inline]
pub fn matches(self, other: Self) -> bool {
(self.as_u32() & other.as_u32()) == other.as_u32()
}
/// Get this `VMGcKind` as a raw `u32`.
#[inline]
pub fn as_u32(self) -> u32 {
self as u32
}
}
#[cfg(test)]
mod tests {
use super::VMGcKind::*;
use crate::prelude::*;
#[test]
fn kind_matches() {
let all = [
ExternRef,
ExternOfAnyRef,
AnyRef,
AnyOfExternRef,
EqRef,
ArrayRef,
StructRef,
];
for (sup, subs) in [
(ExternRef, vec![ExternOfAnyRef]),
(ExternOfAnyRef, vec![]),
(AnyRef, vec![AnyOfExternRef, EqRef, ArrayRef, StructRef]),
(AnyOfExternRef, vec![]),
(EqRef, vec![ArrayRef, StructRef]),
(ArrayRef, vec![]),
(StructRef, vec![]),
] {
assert!(sup.matches(sup));
for sub in &subs {
assert!(sub.matches(sup));
}
for kind in all.iter().filter(|k| **k != sup && !subs.contains(k)) {
assert!(!kind.matches(sup));
}
}
}
}