wasmtime_environ/gc.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410
//! 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-drc")]
pub mod drc;
#[cfg(feature = "gc-null")]
pub mod null;
use crate::prelude::*;
use crate::{
WasmArrayType, WasmCompositeInnerType, 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;
/// The size of the `VMGcHeader` in bytes.
pub const VM_GC_HEADER_SIZE: u32 = 8;
/// The minimum alignment of the `VMGcHeader` in bytes.
pub const VM_GC_HEADER_ALIGN: u32 = 8;
/// The offset of the `VMGcKind` field in the `VMGcHeader`.
pub const VM_GC_HEADER_KIND_OFFSET: u32 = 0;
/// The offset of the `VMSharedTypeIndex` field in the `VMGcHeader`.
pub const VM_GC_HEADER_TYPE_INDEX_OFFSET: u32 = 4;
/// 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,
},
}
}
/// Align `offset` up to `bytes`, updating `max_align` if `align` is the
/// new maximum alignment, and returning the aligned offset.
#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
fn align_up(offset: &mut u32, max_align: &mut u32, align: u32) -> u32 {
debug_assert!(max_align.is_power_of_two());
debug_assert!(align.is_power_of_two());
*offset = offset.checked_add(align - 1).unwrap() & !(align - 1);
*max_align = core::cmp::max(*max_align, align);
*offset
}
/// Define a new field of size and alignment `bytes`, updating the object's
/// total `size` and `align` as necessary. The offset of the new field is
/// returned.
#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
fn field(size: &mut u32, align: &mut u32, bytes: u32) -> u32 {
let offset = align_up(size, align, bytes);
*size += bytes;
offset
}
/// Common code to define a GC array's layout, given the size and alignment of
/// the collector's GC header and its expected offset of the array length field.
#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
fn common_array_layout(
ty: &WasmArrayType,
header_size: u32,
header_align: u32,
expected_array_length_offset: u32,
) -> GcArrayLayout {
assert!(header_size >= crate::VM_GC_HEADER_SIZE);
assert!(header_align >= crate::VM_GC_HEADER_ALIGN);
let mut size = header_size;
let mut align = header_align;
let length_field_offset = field(&mut size, &mut align, 4);
assert_eq!(length_field_offset, expected_array_length_offset);
let elem_size = byte_size_of_wasm_ty_in_gc_heap(&ty.0.element_type);
let elems_offset = align_up(&mut size, &mut align, elem_size);
assert_eq!(elems_offset, size);
GcArrayLayout {
base_size: size,
align,
elem_size,
}
}
/// Common code to define a GC struct's layout, given the size and alignment of
/// the collector's GC header and its expected offset of the array length field.
#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
fn common_struct_layout(
ty: &WasmStructType,
header_size: u32,
header_align: u32,
) -> GcStructLayout {
assert!(header_size >= crate::VM_GC_HEADER_SIZE);
assert!(header_align >= crate::VM_GC_HEADER_ALIGN);
// Process each field, aligning it to its natural alignment.
//
// We don't try and do any fancy field reordering to minimize padding
// (yet?) because (a) the toolchain probably already did that and (b)
// we're just doing the simple thing first. We can come back and improve
// things here if we find that (a) isn't actually holding true in
// practice.
let mut size = header_size;
let mut align = header_align;
let fields = ty
.fields
.iter()
.map(|f| {
let field_size = byte_size_of_wasm_ty_in_gc_heap(&f.element_type);
field(&mut size, &mut align, field_size)
})
.collect();
// Ensure that the final size is a multiple of the alignment, for
// simplicity.
let align_size_to = align;
align_up(&mut size, &mut align, align_size_to);
GcStructLayout {
size,
align,
fields,
}
}
/// 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> {
assert!(!ty.shared);
match &ty.inner {
WasmCompositeInnerType::Array(ty) => Some(self.array_layout(ty).into()),
WasmCompositeInnerType::Struct(ty) => Some(self.struct_layout(ty).into()),
WasmCompositeInnerType::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 27 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 = 0b01000 << 27,
AnyRef = 0b10000 << 27,
EqRef = 0b10100 << 27,
ArrayRef = 0b10101 << 27,
StructRef = 0b10110 << 27,
}
impl VMGcKind {
/// Mask this value with a `u32` to get just the bits that `VMGcKind` uses.
pub const MASK: u32 = 0b11111 << 27;
/// 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::AnyRef.as_u32() => Self::AnyRef,
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, AnyRef, EqRef, ArrayRef, StructRef];
for (sup, subs) in [
(ExternRef, vec![]),
(AnyRef, vec![EqRef, ArrayRef, StructRef]),
(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));
}
}
}
}