wasmtime_environ/
gc.rs

1//! Target- and pointer-width-agnostic definitions of GC-related types and
2//! constants.
3//!
4//! These definitions are suitable for use both during compilation and at
5//! runtime.
6//!
7//! Note: We don't bother gating these on `cfg(feature = "gc")` because that
8//! makes downstream uses pretty annoying, and the primary thing we want to gate
9//! on our various `gc` cargo features is the actual garbage collection
10//! functions and their associated impact on binary size anyways.
11
12#[cfg(feature = "gc-drc")]
13pub mod drc;
14
15#[cfg(feature = "gc-null")]
16pub mod null;
17
18use crate::prelude::*;
19use crate::{
20    WasmArrayType, WasmCompositeInnerType, WasmCompositeType, WasmStorageType, WasmStructType,
21    WasmValType,
22};
23use core::alloc::Layout;
24
25/// Discriminant to check whether GC reference is an `i31ref` or not.
26pub const I31_DISCRIMINANT: u32 = 1;
27
28/// The size of the `VMGcHeader` in bytes.
29pub const VM_GC_HEADER_SIZE: u32 = 8;
30
31/// The minimum alignment of the `VMGcHeader` in bytes.
32pub const VM_GC_HEADER_ALIGN: u32 = 8;
33
34/// The offset of the `VMGcKind` field in the `VMGcHeader`.
35pub const VM_GC_HEADER_KIND_OFFSET: u32 = 0;
36
37/// The offset of the `VMSharedTypeIndex` field in the `VMGcHeader`.
38pub const VM_GC_HEADER_TYPE_INDEX_OFFSET: u32 = 4;
39
40/// Get the byte size of the given Wasm type when it is stored inside the GC
41/// heap.
42pub fn byte_size_of_wasm_ty_in_gc_heap(ty: &WasmStorageType) -> u32 {
43    use crate::{WasmHeapType::*, WasmRefType};
44    match ty {
45        WasmStorageType::I8 => 1,
46        WasmStorageType::I16 => 2,
47        WasmStorageType::Val(ty) => match ty {
48            WasmValType::Ref(WasmRefType {
49                nullable: _,
50                heap_type: ConcreteCont(_) | Cont,
51            }) => unimplemented!("Stack switching feature not compatbile with GC, yet"),
52            WasmValType::I32 | WasmValType::F32 | WasmValType::Ref(_) => 4,
53            WasmValType::I64 | WasmValType::F64 => 8,
54            WasmValType::V128 => 16,
55        },
56    }
57}
58
59/// Align `offset` up to `bytes`, updating `max_align` if `align` is the
60/// new maximum alignment, and returning the aligned offset.
61#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
62fn align_up(offset: &mut u32, max_align: &mut u32, align: u32) -> u32 {
63    debug_assert!(max_align.is_power_of_two());
64    debug_assert!(align.is_power_of_two());
65    *offset = offset.checked_add(align - 1).unwrap() & !(align - 1);
66    *max_align = core::cmp::max(*max_align, align);
67    *offset
68}
69
70/// Define a new field of size and alignment `bytes`, updating the object's
71/// total `size` and `align` as necessary. The offset of the new field is
72/// returned.
73#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
74fn field(size: &mut u32, align: &mut u32, bytes: u32) -> u32 {
75    let offset = align_up(size, align, bytes);
76    *size += bytes;
77    offset
78}
79
80/// Common code to define a GC array's layout, given the size and alignment of
81/// the collector's GC header and its expected offset of the array length field.
82#[cfg(any(feature = "gc-drc", feature = "gc-null"))]
83fn common_array_layout(
84    ty: &WasmArrayType,
85    header_size: u32,
86    header_align: u32,
87    expected_array_length_offset: u32,
88) -> GcArrayLayout {
89    use core::mem;
90
91    assert!(header_size >= crate::VM_GC_HEADER_SIZE);
92    assert!(header_align >= crate::VM_GC_HEADER_ALIGN);
93
94    let mut size = header_size;
95    let mut align = header_align;
96
97    let length_field_size = u32::try_from(mem::size_of::<u32>()).unwrap();
98    let length_field_offset = field(&mut size, &mut align, length_field_size);
99    assert_eq!(length_field_offset, expected_array_length_offset);
100
101    let elem_size = byte_size_of_wasm_ty_in_gc_heap(&ty.0.element_type);
102    let elems_offset = align_up(&mut size, &mut align, elem_size);
103    assert_eq!(elems_offset, size);
104
105    let elems_are_gc_refs = ty.0.element_type.is_vmgcref_type_and_not_i31();
106    if elems_are_gc_refs {
107        debug_assert_eq!(
108            length_field_offset + length_field_size,
109            elems_offset,
110            "DRC collector relies on GC ref elements appearing directly after the length field, without any padding",
111        );
112    }
113
114    GcArrayLayout {
115        base_size: size,
116        align,
117        elem_size,
118        elems_are_gc_refs,
119    }
120}
121
122/// Common code to define a GC struct's layout, given the size and alignment of
123/// the collector's GC header and its expected offset of the array length field.
124#[cfg(any(feature = "gc-null", feature = "gc-drc"))]
125fn common_struct_layout(
126    ty: &WasmStructType,
127    header_size: u32,
128    header_align: u32,
129) -> GcStructLayout {
130    assert!(header_size >= crate::VM_GC_HEADER_SIZE);
131    assert!(header_align >= crate::VM_GC_HEADER_ALIGN);
132
133    // Process each field, aligning it to its natural alignment.
134    //
135    // We don't try and do any fancy field reordering to minimize padding (yet?)
136    // because (a) the toolchain probably already did that and (b) we're just
137    // doing the simple thing first, and (c) this is tricky in the presence of
138    // subtyping where we need a subtype's fields to be assigned the same
139    // offsets as its supertype's fields. We can come back and improve things
140    // here if we find that (a) isn't actually holding true in practice.
141    let mut size = header_size;
142    let mut align = header_align;
143
144    let fields = ty
145        .fields
146        .iter()
147        .map(|f| {
148            let field_size = byte_size_of_wasm_ty_in_gc_heap(&f.element_type);
149            let offset = field(&mut size, &mut align, field_size);
150            let is_gc_ref = f.element_type.is_vmgcref_type_and_not_i31();
151            GcStructLayoutField { offset, is_gc_ref }
152        })
153        .collect();
154
155    // Ensure that the final size is a multiple of the alignment, for
156    // simplicity.
157    let align_size_to = align;
158    align_up(&mut size, &mut align, align_size_to);
159
160    GcStructLayout {
161        size,
162        align,
163        fields,
164    }
165}
166
167/// A trait for getting the layout of a Wasm GC struct or array inside a
168/// particular collector.
169pub trait GcTypeLayouts {
170    /// The offset of an array's length field.
171    ///
172    /// This must be the same for all arrays in the heap, regardless of their
173    /// element type.
174    fn array_length_field_offset(&self) -> u32;
175
176    /// Get this collector's layout for the given composite type.
177    ///
178    /// Returns `None` if the type is a function type, as functions are not
179    /// managed by the GC.
180    fn gc_layout(&self, ty: &WasmCompositeType) -> Option<GcLayout> {
181        assert!(!ty.shared);
182        match &ty.inner {
183            WasmCompositeInnerType::Array(ty) => Some(self.array_layout(ty).into()),
184            WasmCompositeInnerType::Struct(ty) => Some(self.struct_layout(ty).into()),
185            WasmCompositeInnerType::Func(_) => None,
186            WasmCompositeInnerType::Cont(_) => {
187                unimplemented!("Stack switching feature not compatbile with GC, yet")
188            }
189        }
190    }
191
192    /// Get this collector's layout for the given array type.
193    fn array_layout(&self, ty: &WasmArrayType) -> GcArrayLayout;
194
195    /// Get this collector's layout for the given struct type.
196    fn struct_layout(&self, ty: &WasmStructType) -> GcStructLayout;
197}
198
199/// The layout of a GC-managed object.
200#[derive(Clone, Debug)]
201pub enum GcLayout {
202    /// The layout of a GC-managed array object.
203    Array(GcArrayLayout),
204
205    /// The layout of a GC-managed struct object.
206    Struct(GcStructLayout),
207}
208
209impl From<GcArrayLayout> for GcLayout {
210    fn from(layout: GcArrayLayout) -> Self {
211        Self::Array(layout)
212    }
213}
214
215impl From<GcStructLayout> for GcLayout {
216    fn from(layout: GcStructLayout) -> Self {
217        Self::Struct(layout)
218    }
219}
220
221impl GcLayout {
222    /// Get the underlying `GcStructLayout`, or panic.
223    #[track_caller]
224    pub fn unwrap_struct(&self) -> &GcStructLayout {
225        match self {
226            Self::Struct(s) => s,
227            _ => panic!("GcLayout::unwrap_struct on non-struct GC layout"),
228        }
229    }
230
231    /// Get the underlying `GcArrayLayout`, or panic.
232    #[track_caller]
233    pub fn unwrap_array(&self) -> &GcArrayLayout {
234        match self {
235            Self::Array(a) => a,
236            _ => panic!("GcLayout::unwrap_array on non-array GC layout"),
237        }
238    }
239}
240
241/// The layout of a GC-managed array.
242///
243/// This layout is only valid for use with the GC runtime that created it. It is
244/// not valid to use one GC runtime's layout with another GC runtime, doing so
245/// is memory safe but will lead to general incorrectness like panics and wrong
246/// results.
247///
248/// All offsets are from the start of the object; that is, the size of the GC
249/// header (for example) is included in the offset.
250///
251/// All arrays are composed of the generic `VMGcHeader`, followed by
252/// collector-specific fields, followed by the contiguous array elements
253/// themselves. The array elements must be aligned to the element type's natural
254/// alignment.
255#[derive(Clone, Debug)]
256pub struct GcArrayLayout {
257    /// The size of this array object, without any elements.
258    ///
259    /// The array's elements, if any, must begin at exactly this offset.
260    pub base_size: u32,
261
262    /// The alignment of this array.
263    pub align: u32,
264
265    /// The size and natural alignment of each element in this array.
266    pub elem_size: u32,
267
268    /// Whether or not the elements of this array are GC references or not.
269    pub elems_are_gc_refs: bool,
270}
271
272impl GcArrayLayout {
273    /// Get the total size of this array for a given length of elements.
274    #[inline]
275    pub fn size_for_len(&self, len: u32) -> u32 {
276        self.elem_offset(len)
277    }
278
279    /// Get the offset of the `i`th element in an array with this layout.
280    #[inline]
281    pub fn elem_offset(&self, i: u32) -> u32 {
282        self.base_size + i * self.elem_size
283    }
284
285    /// Get a `core::alloc::Layout` for an array of this type with the given
286    /// length.
287    pub fn layout(&self, len: u32) -> Layout {
288        let size = self.size_for_len(len);
289        let size = usize::try_from(size).unwrap();
290        let align = usize::try_from(self.align).unwrap();
291        Layout::from_size_align(size, align).unwrap()
292    }
293}
294
295/// The layout for a GC-managed struct type.
296///
297/// This layout is only valid for use with the GC runtime that created it. It is
298/// not valid to use one GC runtime's layout with another GC runtime, doing so
299/// is memory safe but will lead to general incorrectness like panics and wrong
300/// results.
301///
302/// All offsets are from the start of the object; that is, the size of the GC
303/// header (for example) is included in the offset.
304#[derive(Clone, Debug)]
305pub struct GcStructLayout {
306    /// The size (in bytes) of this struct.
307    pub size: u32,
308
309    /// The alignment (in bytes) of this struct.
310    pub align: u32,
311
312    /// The fields of this struct. The `i`th entry contains information about
313    /// the `i`th struct field's layout.
314    pub fields: Vec<GcStructLayoutField>,
315}
316
317impl GcStructLayout {
318    /// Get a `core::alloc::Layout` for a struct of this type.
319    pub fn layout(&self) -> Layout {
320        let size = usize::try_from(self.size).unwrap();
321        let align = usize::try_from(self.align).unwrap();
322        Layout::from_size_align(size, align).unwrap()
323    }
324}
325
326/// A field in a `GcStructLayout`.
327#[derive(Clone, Copy, Debug)]
328pub struct GcStructLayoutField {
329    /// The offset (in bytes) of this field inside instances of this type.
330    pub offset: u32,
331
332    /// Whether or not this field might contain a reference to another GC
333    /// object.
334    ///
335    /// Note: it is okay for this to be `false` for `i31ref`s, since they never
336    /// actually reference another GC object.
337    pub is_gc_ref: bool,
338}
339
340/// The kind of an object in a GC heap.
341///
342/// Note that this type is accessed from Wasm JIT code.
343///
344/// `VMGcKind` is a bitset where to test if `a` is a subtype of an
345/// "abstract-ish" type `b`, we can simply use a single bitwise-and operation:
346///
347/// ```ignore
348/// a <: b   iff   a & b == b
349/// ```
350///
351/// For example, because `VMGcKind::AnyRef` has the high bit set, every kind
352/// representing some subtype of `anyref` also has its high bit set.
353///
354/// We say "abstract-ish" type because in addition to the abstract heap types
355/// (other than `i31`) we also have variants for `externref`s that have been
356/// converted into an `anyref` via `extern.convert_any` and `externref`s that
357/// have been converted into an `anyref` via `any.convert_extern`. Note that in
358/// the latter case, because `any.convert_extern $foo` produces a value that is
359/// not an instance of `eqref`, `VMGcKind::AnyOfExternRef & VMGcKind::EqRef !=
360/// VMGcKind::EqRef`.
361///
362/// Furthermore, this type only uses the highest 6 bits of its `u32`
363/// representation, allowing the lower 27 bytes to be bitpacked with other stuff
364/// as users see fit.
365#[repr(u32)]
366#[derive(Clone, Copy, Debug, PartialEq, Eq)]
367#[rustfmt::skip]
368#[expect(missing_docs, reason = "self-describing variants")]
369pub enum VMGcKind {
370    ExternRef      = 0b01000 << 27,
371    AnyRef         = 0b10000 << 27,
372    EqRef          = 0b10100 << 27,
373    ArrayRef       = 0b10101 << 27,
374    StructRef      = 0b10110 << 27,
375}
376
377/// The size of the `VMGcKind` in bytes.
378pub const VM_GC_KIND_SIZE: u8 = 4;
379
380const _: () = assert!(VM_GC_KIND_SIZE as usize == core::mem::size_of::<VMGcKind>());
381
382impl VMGcKind {
383    /// Mask this value with a `u32` to get just the bits that `VMGcKind` uses.
384    pub const MASK: u32 = 0b11111 << 27;
385
386    /// Mask this value with a `u32` that potentially contains a `VMGcKind` to
387    /// get the bits that `VMGcKind` doesn't use.
388    pub const UNUSED_MASK: u32 = !Self::MASK;
389
390    /// Does the given value fit in the unused bits of a `VMGcKind`?
391    #[inline]
392    pub fn value_fits_in_unused_bits(value: u32) -> bool {
393        (value & Self::UNUSED_MASK) == value
394    }
395
396    /// Convert the given value into a `VMGcKind` by masking off the unused
397    /// bottom bits.
398    #[inline]
399    pub fn from_high_bits_of_u32(val: u32) -> VMGcKind {
400        let masked = val & Self::MASK;
401        match masked {
402            x if x == Self::ExternRef.as_u32() => Self::ExternRef,
403            x if x == Self::AnyRef.as_u32() => Self::AnyRef,
404            x if x == Self::EqRef.as_u32() => Self::EqRef,
405            x if x == Self::ArrayRef.as_u32() => Self::ArrayRef,
406            x if x == Self::StructRef.as_u32() => Self::StructRef,
407            _ => panic!("invalid `VMGcKind`: {masked:#032b}"),
408        }
409    }
410
411    /// Does this kind match the other kind?
412    ///
413    /// That is, is this kind a subtype of the other kind?
414    #[inline]
415    pub fn matches(self, other: Self) -> bool {
416        (self.as_u32() & other.as_u32()) == other.as_u32()
417    }
418
419    /// Get this `VMGcKind` as a raw `u32`.
420    #[inline]
421    pub fn as_u32(self) -> u32 {
422        self as u32
423    }
424}
425
426#[cfg(test)]
427mod tests {
428    use super::VMGcKind::*;
429    use crate::prelude::*;
430
431    #[test]
432    fn kind_matches() {
433        let all = [ExternRef, AnyRef, EqRef, ArrayRef, StructRef];
434
435        for (sup, subs) in [
436            (ExternRef, vec![]),
437            (AnyRef, vec![EqRef, ArrayRef, StructRef]),
438            (EqRef, vec![ArrayRef, StructRef]),
439            (ArrayRef, vec![]),
440            (StructRef, vec![]),
441        ] {
442            assert!(sup.matches(sup));
443            for sub in &subs {
444                assert!(sub.matches(sup));
445            }
446            for kind in all.iter().filter(|k| **k != sup && !subs.contains(k)) {
447                assert!(!kind.matches(sup));
448            }
449        }
450    }
451}