Add function ABI and type layout to StableMIR

This change introduces a new module to StableMIR named `abi` with
information from `rustc_target::abi` and `rustc_abi`, that allow users
to retrieve more low level information required to perform
bit-precise analysis.

The layout of a type can be retrieved via `Ty::layout`, and the instance
ABI can be retrieved via `Instance::fn_abi()`.

To properly handle errors while retrieve layout information, we had
to implement a few layout related traits.

compiler/stable_mir/src/abi.rs

@@ -0,0 +1,286 @@
+use crate::compiler_interface::with;
+use crate::mir::FieldIdx;
+use crate::ty::{Align, IndexedVal, Size, Ty, VariantIdx};
+use crate::Opaque;
+use std::num::NonZeroUsize;
+use std::ops::RangeInclusive;
+
+/// A function ABI definition.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct FnAbi {
+    /// The types of each argument.
+    pub args: Vec<ArgAbi>,
+
+    /// The expected return type.
+    pub ret: ArgAbi,
+
+    /// The count of non-variadic arguments.
+    ///
+    /// Should only be different from `args.len()` when a function is a C variadic function.
+    pub fixed_count: u32,
+
+    /// The ABI convention.
+    pub conv: CallConvention,
+}
+
+impl FnAbi {
+    pub fn is_c_variadic(&self) -> bool {
+        self.args.len() > self.fixed_count as usize
+    }
+}
+
+/// Information about the ABI of a function's argument, or return value.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct ArgAbi {
+    pub ty: Ty,
+    pub layout: Layout,
+    pub mode: PassMode,
+}
+
+/// How a function argument should be passed in to the target function.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum PassMode {
+    /// Ignore the argument.
+    ///
+    /// The argument is either uninhabited or a ZST.
+    Ignore,
+    /// Pass the argument directly.
+    ///
+    /// The argument has a layout abi of `Scalar` or `Vector`.
+    Direct,
+    /// Pass a pair's elements directly in two arguments.
+    ///
+    /// The argument has a layout abi of `ScalarPair`.
+    Pair,
+    /// Pass the argument after casting it.
+    Cast,
+    /// Pass the argument indirectly via a hidden pointer.
+    Indirect,
+}
+
+/// The layout of a type, alongside the type itself.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct TyAndLayout {
+    pub ty: Ty,
+    pub layout: Layout,
+}
+
+/// The layout of a type in memory.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct LayoutShape {
+    /// The fields location withing the layout
+    pub fields: FieldsShape,
+
+    /// Encodes information about multi-variant layouts.
+    /// Even with `Multiple` variants, a layout still has its own fields! Those are then
+    /// shared between all variants.
+    ///
+    /// To access all fields of this layout, both `fields` and the fields of the active variant
+    /// must be taken into account.
+    pub variants: VariantsShape,
+
+    /// The `abi` defines how this data is passed between functions.
+    pub abi: ValueAbi,
+
+    /// The ABI mandated alignment in bytes.
+    pub abi_align: Align,
+
+    /// The size of this layout in bytes.
+    pub size: Size,
+}
+
+impl LayoutShape {
+    /// Returns `true` if the layout corresponds to an unsized type.
+    #[inline]
+    pub fn is_unsized(&self) -> bool {
+        self.abi.is_unsized()
+    }
+
+    #[inline]
+    pub fn is_sized(&self) -> bool {
+        !self.abi.is_unsized()
+    }
+
+    /// Returns `true` if the type is sized and a 1-ZST (meaning it has size 0 and alignment 1).
+    pub fn is_1zst(&self) -> bool {
+        self.is_sized() && self.size == 0 && self.abi_align == 1
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Layout(usize);
+
+impl Layout {
+    pub fn shape(self) -> LayoutShape {
+        with(|cx| cx.layout_shape(self))
+    }
+}
+
+impl IndexedVal for Layout {
+    fn to_val(index: usize) -> Self {
+        Layout(index)
+    }
+    fn to_index(&self) -> usize {
+        self.0
+    }
+}
+
+/// Describes how the fields of a type are shaped in memory.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum FieldsShape {
+    /// Scalar primitives and `!`, which never have fields.
+    Primitive,
+
+    /// All fields start at no offset. The `usize` is the field count.
+    Union(NonZeroUsize),
+
+    /// Array/vector-like placement, with all fields of identical types.
+    Array { stride: Size, count: u64 },
+
+    /// Struct-like placement, with precomputed offsets.
+    ///
+    /// Fields are guaranteed to not overlap, but note that gaps
+    /// before, between and after all the fields are NOT always
+    /// padding, and as such their contents may not be discarded.
+    /// For example, enum variants leave a gap at the start,
+    /// where the discriminant field in the enum layout goes.
+    Arbitrary {
+        /// Offsets for the first byte of each field,
+        /// ordered to match the source definition order.
+        /// I.e.: It follows the same order as [crate::ty::VariantDef::fields()].
+        /// This vector does not go in increasing order.
+        offsets: Vec<Size>,
+    },
+}
+
+impl FieldsShape {
+    pub fn fields_by_offset_order(&self) -> Vec<FieldIdx> {
+        match self {
+            FieldsShape::Primitive => vec![],
+            FieldsShape::Union(_) | FieldsShape::Array { .. } => (0..self.count()).collect(),
+            FieldsShape::Arbitrary { offsets, .. } => {
+                let mut indices = (0..offsets.len()).collect::<Vec<_>>();
+                indices.sort_by_key(|idx| offsets[*idx]);
+                indices
+            }
+        }
+    }
+
+    pub fn count(&self) -> usize {
+        match self {
+            FieldsShape::Primitive => 0,
+            FieldsShape::Union(count) => count.get(),
+            FieldsShape::Array { count, .. } => *count as usize,
+            FieldsShape::Arbitrary { offsets, .. } => offsets.len(),
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum VariantsShape {
+    /// Single enum variants, structs/tuples, unions, and all non-ADTs.
+    Single { index: VariantIdx },
+
+    /// Enum-likes with more than one inhabited variant: each variant comes with
+    /// a *discriminant* (usually the same as the variant index but the user can
+    /// assign explicit discriminant values). That discriminant is encoded
+    /// as a *tag* on the machine. The layout of each variant is
+    /// a struct, and they all have space reserved for the tag.
+    /// For enums, the tag is the sole field of the layout.
+    Multiple {
+        tag: Scalar,
+        tag_encoding: TagEncoding,
+        tag_field: usize,
+        variants: Vec<LayoutShape>,
+    },
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum TagEncoding {
+    /// The tag directly stores the discriminant, but possibly with a smaller layout
+    /// (so converting the tag to the discriminant can require sign extension).
+    Direct,
+
+    /// Niche (values invalid for a type) encoding the discriminant:
+    /// Discriminant and variant index coincide.
+    /// The variant `untagged_variant` contains a niche at an arbitrary
+    /// offset (field `tag_field` of the enum), which for a variant with
+    /// discriminant `d` is set to
+    /// `(d - niche_variants.start).wrapping_add(niche_start)`.
+    ///
+    /// For example, `Option<(usize, &T)>`  is represented such that
+    /// `None` has a null pointer for the second tuple field, and
+    /// `Some` is the identity function (with a non-null reference).
+    Niche {
+        untagged_variant: VariantIdx,
+        niche_variants: RangeInclusive<VariantIdx>,
+        niche_start: u128,
+    },
+}
+
+/// Describes how values of the type are passed by target ABIs,
+/// in terms of categories of C types there are ABI rules for.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum ValueAbi {
+    Uninhabited,
+    Scalar(Scalar),
+    ScalarPair(Scalar, Scalar),
+    Vector {
+        element: Scalar,
+        count: u64,
+    },
+    Aggregate {
+        /// If true, the size is exact, otherwise it's only a lower bound.
+        sized: bool,
+    },
+}
+
+impl ValueAbi {
+    /// Returns `true` if the layout corresponds to an unsized type.
+    pub fn is_unsized(&self) -> bool {
+        match *self {
+            ValueAbi::Uninhabited
+            | ValueAbi::Scalar(_)
+            | ValueAbi::ScalarPair(..)
+            | ValueAbi::Vector { .. } => false,
+            ValueAbi::Aggregate { sized } => !sized,
+        }
+    }
+}
+
+/// We currently do not support `Scalar`, and use opaque instead.
+type Scalar = Opaque;
+
+/// General language calling conventions.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum CallConvention {
+    C,
+    Rust,
+
+    Cold,
+    PreserveMost,
+    PreserveAll,
+
+    // Target-specific calling conventions.
+    ArmAapcs,
+    CCmseNonSecureCall,
+
+    Msp430Intr,
+
+    PtxKernel,
+
+    X86Fastcall,
+    X86Intr,
+    X86Stdcall,
+    X86ThisCall,
+    X86VectorCall,
+
+    X86_64SysV,
+    X86_64Win64,
+
+    AmdGpuKernel,
+    AvrInterrupt,
+    AvrNonBlockingInterrupt,
+
+    RiscvInterrupt,
+}

compiler/stable_mir/src/compiler_interface.rs

@@ -5,6 +5,7 @@
 
 use std::cell::Cell;
 
+use crate::abi::{FnAbi, Layout, LayoutShape};
 use crate::mir::alloc::{AllocId, GlobalAlloc};
 use crate::mir::mono::{Instance, InstanceDef, StaticDef};
 use crate::mir::Body;
@@ -173,6 +174,15 @@ pub trait Context {
 
     /// Return information about the target machine.
     fn target_info(&self) -> MachineInfo;
+
+    /// Get an instance ABI.
+    fn instance_abi(&self, def: InstanceDef) -> Result<FnAbi, Error>;
+
+    /// Get the layout of a type.
+    fn ty_layout(&self, ty: Ty) -> Result<Layout, Error>;
+
+    /// Get the layout shape.
+    fn layout_shape(&self, id: Layout) -> LayoutShape;
 }
 
 // A thread local variable that stores a pointer to the tables mapping between TyCtxt

compiler/stable_mir/src/lib.rs

@@ -33,6 +33,7 @@ use crate::mir::Body;
 use crate::mir::Mutability;
 use crate::ty::{ImplDef, ImplTrait, IndexedVal, Span, TraitDecl, TraitDef, Ty};
 
+pub mod abi;
 #[macro_use]
 pub mod crate_def;
 pub mod compiler_interface;

compiler/stable_mir/src/mir/mono.rs

@@ -1,3 +1,4 @@
+use crate::abi::FnAbi;
 use crate::crate_def::CrateDef;
 use crate::mir::Body;
 use crate::ty::{Allocation, ClosureDef, ClosureKind, FnDef, GenericArgs, IndexedVal, Ty};
@@ -56,6 +57,11 @@ impl Instance {
         with(|context| context.instance_ty(self.def))
     }
 
+    /// Retrieve information about this instance binary interface.
+    pub fn fn_abi(&self) -> Result<FnAbi, Error> {
+        with(|cx| cx.instance_abi(self.def))
+    }
+
     /// Retrieve the instance's mangled name used for calling the given instance.
     ///
     /// This will also look up the correct name of instances from upstream crates.

compiler/stable_mir/src/ty.rs

@@ -3,6 +3,7 @@ use super::{
     mir::{Body, Mutability},
     with, DefId, Error, Symbol,
 };
+use crate::abi::Layout;
 use crate::crate_def::CrateDef;
 use crate::mir::alloc::{read_target_int, read_target_uint, AllocId};
 use crate::target::MachineInfo;
@@ -85,6 +86,11 @@ impl Ty {
     pub fn unsigned_ty(inner: UintTy) -> Ty {
         Ty::from_rigid_kind(RigidTy::Uint(inner))
     }
+
+    /// Get a type layout.
+    pub fn layout(self) -> Result<Layout, Error> {
+        with(|cx| cx.ty_layout(self))
+    }
 }
 
 impl Ty {