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diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs new file mode 100644 index 000000000..eb90169d0 --- /dev/null +++ b/compiler/rustc_middle/src/mir/syntax.rs @@ -0,0 +1,1168 @@ +//! This defines the syntax of MIR, i.e., the set of available MIR operations, and other definitions +//! closely related to MIR semantics. +//! This is in a dedicated file so that changes to this file can be reviewed more carefully. +//! The intention is that this file only contains datatype declarations, no code. + +use super::{BasicBlock, Constant, Field, Local, SwitchTargets, UserTypeProjection}; + +use crate::mir::coverage::{CodeRegion, CoverageKind}; +use crate::ty::adjustment::PointerCast; +use crate::ty::subst::SubstsRef; +use crate::ty::{self, List, Ty}; +use crate::ty::{Region, UserTypeAnnotationIndex}; + +use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece}; +use rustc_hir::def_id::DefId; +use rustc_hir::{self as hir}; +use rustc_hir::{self, GeneratorKind}; +use rustc_target::abi::VariantIdx; + +use rustc_ast::Mutability; +use rustc_span::def_id::LocalDefId; +use rustc_span::symbol::Symbol; +use rustc_span::Span; +use rustc_target::asm::InlineAsmRegOrRegClass; + +/// The various "big phases" that MIR goes through. +/// +/// These phases all describe dialects of MIR. Since all MIR uses the same datastructures, the +/// dialects forbid certain variants or values in certain phases. The sections below summarize the +/// changes, but do not document them thoroughly. The full documentation is found in the appropriate +/// documentation for the thing the change is affecting. +/// +/// Warning: ordering of variants is significant. +#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)] +#[derive(HashStable)] +pub enum MirPhase { + /// The dialect of MIR used during all phases before `DropsLowered` is the same. This is also + /// the MIR that analysis such as borrowck uses. + /// + /// One important thing to remember about the behavior of this section of MIR is that drop terminators + /// (including drop and replace) are *conditional*. The elaborate drops pass will then replace each + /// instance of a drop terminator with a nop, an unconditional drop, or a drop conditioned on a drop + /// flag. Of course, this means that it is important that the drop elaboration can accurately recognize + /// when things are initialized and when things are de-initialized. That means any code running on this + /// version of MIR must be sure to produce output that drop elaboration can reason about. See the + /// section on the drop terminatorss for more details. + Built = 0, + // FIXME(oli-obk): it's unclear whether we still need this phase (and its corresponding query). + // We used to have this for pre-miri MIR based const eval. + Const = 1, + /// This phase checks the MIR for promotable elements and takes them out of the main MIR body + /// by creating a new MIR body per promoted element. After this phase (and thus the termination + /// of the `mir_promoted` query), these promoted elements are available in the `promoted_mir` + /// query. + ConstsPromoted = 2, + /// After this projections may only contain deref projections as the first element. + Derefered = 3, + /// Beginning with this phase, the following variants are disallowed: + /// * [`TerminatorKind::DropAndReplace`] + /// * [`TerminatorKind::FalseUnwind`] + /// * [`TerminatorKind::FalseEdge`] + /// * [`StatementKind::FakeRead`] + /// * [`StatementKind::AscribeUserType`] + /// * [`Rvalue::Ref`] with `BorrowKind::Shallow` + /// + /// And the following variant is allowed: + /// * [`StatementKind::Retag`] + /// + /// Furthermore, `Drop` now uses explicit drop flags visible in the MIR and reaching a `Drop` + /// terminator means that the auto-generated drop glue will be invoked. Also, `Copy` operands + /// are allowed for non-`Copy` types. + DropsLowered = 4, + /// Beginning with this phase, the following variant is disallowed: + /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array` + /// + /// And the following variant is allowed: + /// * [`StatementKind::SetDiscriminant`] + Deaggregated = 5, + /// Before this phase, generators are in the "source code" form, featuring `yield` statements + /// and such. With this phase change, they are transformed into a proper state machine. Running + /// optimizations before this change can be potentially dangerous because the source code is to + /// some extent a "lie." In particular, `yield` terminators effectively make the value of all + /// locals visible to the caller. This means that dead store elimination before them, or code + /// motion across them, is not correct in general. This is also exasperated by type checking + /// having pre-computed a list of the types that it thinks are ok to be live across a yield + /// point - this is necessary to decide eg whether autotraits are implemented. Introducing new + /// types across a yield point will lead to ICEs becaues of this. + /// + /// Beginning with this phase, the following variants are disallowed: + /// * [`TerminatorKind::Yield`] + /// * [`TerminatorKind::GeneratorDrop`] + /// * [`ProjectionElem::Deref`] of `Box` + GeneratorsLowered = 6, + Optimized = 7, +} + +/////////////////////////////////////////////////////////////////////////// +// Borrow kinds + +#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)] +#[derive(Hash, HashStable)] +pub enum BorrowKind { + /// Data must be immutable and is aliasable. + Shared, + + /// The immediately borrowed place must be immutable, but projections from + /// it don't need to be. For example, a shallow borrow of `a.b` doesn't + /// conflict with a mutable borrow of `a.b.c`. + /// + /// This is used when lowering matches: when matching on a place we want to + /// ensure that place have the same value from the start of the match until + /// an arm is selected. This prevents this code from compiling: + /// ```compile_fail,E0510 + /// let mut x = &Some(0); + /// match *x { + /// None => (), + /// Some(_) if { x = &None; false } => (), + /// Some(_) => (), + /// } + /// ``` + /// This can't be a shared borrow because mutably borrowing (*x as Some).0 + /// should not prevent `if let None = x { ... }`, for example, because the + /// mutating `(*x as Some).0` can't affect the discriminant of `x`. + /// We can also report errors with this kind of borrow differently. + Shallow, + + /// Data must be immutable but not aliasable. This kind of borrow + /// cannot currently be expressed by the user and is used only in + /// implicit closure bindings. It is needed when the closure is + /// borrowing or mutating a mutable referent, e.g.: + /// ``` + /// let mut z = 3; + /// let x: &mut isize = &mut z; + /// let y = || *x += 5; + /// ``` + /// If we were to try to translate this closure into a more explicit + /// form, we'd encounter an error with the code as written: + /// ```compile_fail,E0594 + /// struct Env<'a> { x: &'a &'a mut isize } + /// let mut z = 3; + /// let x: &mut isize = &mut z; + /// let y = (&mut Env { x: &x }, fn_ptr); // Closure is pair of env and fn + /// fn fn_ptr(env: &mut Env) { **env.x += 5; } + /// ``` + /// This is then illegal because you cannot mutate an `&mut` found + /// in an aliasable location. To solve, you'd have to translate with + /// an `&mut` borrow: + /// ```compile_fail,E0596 + /// struct Env<'a> { x: &'a mut &'a mut isize } + /// let mut z = 3; + /// let x: &mut isize = &mut z; + /// let y = (&mut Env { x: &mut x }, fn_ptr); // changed from &x to &mut x + /// fn fn_ptr(env: &mut Env) { **env.x += 5; } + /// ``` + /// Now the assignment to `**env.x` is legal, but creating a + /// mutable pointer to `x` is not because `x` is not mutable. We + /// could fix this by declaring `x` as `let mut x`. This is ok in + /// user code, if awkward, but extra weird for closures, since the + /// borrow is hidden. + /// + /// So we introduce a "unique imm" borrow -- the referent is + /// immutable, but not aliasable. This solves the problem. For + /// simplicity, we don't give users the way to express this + /// borrow, it's just used when translating closures. + Unique, + + /// Data is mutable and not aliasable. + Mut { + /// `true` if this borrow arose from method-call auto-ref + /// (i.e., `adjustment::Adjust::Borrow`). + allow_two_phase_borrow: bool, + }, +} + +/////////////////////////////////////////////////////////////////////////// +// Statements + +/// The various kinds of statements that can appear in MIR. +/// +/// Not all of these are allowed at every [`MirPhase`]. Check the documentation there to see which +/// ones you do not have to worry about. The MIR validator will generally enforce such restrictions, +/// causing an ICE if they are violated. +#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)] +#[derive(TypeFoldable, TypeVisitable)] +pub enum StatementKind<'tcx> { + /// Assign statements roughly correspond to an assignment in Rust proper (`x = ...`) except + /// without the possibility of dropping the previous value (that must be done separately, if at + /// all). The *exact* way this works is undecided. It probably does something like evaluating + /// the LHS to a place and the RHS to a value, and then storing the value to the place. Various + /// parts of this may do type specific things that are more complicated than simply copying + /// bytes. + /// + /// **Needs clarification**: The implication of the above idea would be that assignment implies + /// that the resulting value is initialized. I believe we could commit to this separately from + /// committing to whatever part of the memory model we would need to decide on to make the above + /// paragragh precise. Do we want to? + /// + /// Assignments in which the types of the place and rvalue differ are not well-formed. + /// + /// **Needs clarification**: Do we ever want to worry about non-free (in the body) lifetimes for + /// the typing requirement in post drop-elaboration MIR? I think probably not - I'm not sure we + /// could meaningfully require this anyway. How about free lifetimes? Is ignoring this + /// interesting for optimizations? Do we want to allow such optimizations? + /// + /// **Needs clarification**: We currently require that the LHS place not overlap with any place + /// read as part of computation of the RHS for some rvalues (generally those not producing + /// primitives). This requirement is under discussion in [#68364]. As a part of this discussion, + /// it is also unclear in what order the components are evaluated. + /// + /// [#68364]: https://github.com/rust-lang/rust/issues/68364 + /// + /// See [`Rvalue`] documentation for details on each of those. + Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>), + + /// This represents all the reading that a pattern match may do (e.g., inspecting constants and + /// discriminant values), and the kind of pattern it comes from. This is in order to adapt + /// potential error messages to these specific patterns. + /// + /// Note that this also is emitted for regular `let` bindings to ensure that locals that are + /// never accessed still get some sanity checks for, e.g., `let x: ! = ..;` + /// + /// When executed at runtime this is a nop. + /// + /// Disallowed after drop elaboration. + FakeRead(Box<(FakeReadCause, Place<'tcx>)>), + + /// Write the discriminant for a variant to the enum Place. + /// + /// This is permitted for both generators and ADTs. This does not necessarily write to the + /// entire place; instead, it writes to the minimum set of bytes as required by the layout for + /// the type. + SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx }, + + /// Deinitializes the place. + /// + /// This writes `uninit` bytes to the entire place. + Deinit(Box<Place<'tcx>>), + + /// `StorageLive` and `StorageDead` statements mark the live range of a local. + /// + /// At any point during the execution of a function, each local is either allocated or + /// unallocated. Except as noted below, all locals except function parameters are initially + /// unallocated. `StorageLive` statements cause memory to be allocated for the local while + /// `StorageDead` statements cause the memory to be freed. Using a local in any way (not only + /// reading/writing from it) while it is unallocated is UB. + /// + /// Some locals have no `StorageLive` or `StorageDead` statements within the entire MIR body. + /// These locals are implicitly allocated for the full duration of the function. There is a + /// convenience method at `rustc_mir_dataflow::storage::always_storage_live_locals` for + /// computing these locals. + /// + /// If the local is already allocated, calling `StorageLive` again is UB. However, for an + /// unallocated local an additional `StorageDead` all is simply a nop. + StorageLive(Local), + + /// See `StorageLive` above. + StorageDead(Local), + + /// Retag references in the given place, ensuring they got fresh tags. + /// + /// This is part of the Stacked Borrows model. These statements are currently only interpreted + /// by miri and only generated when `-Z mir-emit-retag` is passed. See + /// <https://internals.rust-lang.org/t/stacked-borrows-an-aliasing-model-for-rust/8153/> for + /// more details. + /// + /// For code that is not specific to stacked borrows, you should consider retags to read + /// and modify the place in an opaque way. + Retag(RetagKind, Box<Place<'tcx>>), + + /// Encodes a user's type ascription. These need to be preserved + /// intact so that NLL can respect them. For example: + /// ```ignore (illustrative) + /// let a: T = y; + /// ``` + /// The effect of this annotation is to relate the type `T_y` of the place `y` + /// to the user-given type `T`. The effect depends on the specified variance: + /// + /// - `Covariant` -- requires that `T_y <: T` + /// - `Contravariant` -- requires that `T_y :> T` + /// - `Invariant` -- requires that `T_y == T` + /// - `Bivariant` -- no effect + /// + /// When executed at runtime this is a nop. + /// + /// Disallowed after drop elaboration. + AscribeUserType(Box<(Place<'tcx>, UserTypeProjection)>, ty::Variance), + + /// Marks the start of a "coverage region", injected with '-Cinstrument-coverage'. A + /// `Coverage` statement carries metadata about the coverage region, used to inject a coverage + /// map into the binary. If `Coverage::kind` is a `Counter`, the statement also generates + /// executable code, to increment a counter variable at runtime, each time the code region is + /// executed. + Coverage(Box<Coverage>), + + /// Denotes a call to the intrinsic function `copy_nonoverlapping`. + /// + /// First, all three operands are evaluated. `src` and `dest` must each be a reference, pointer, + /// or `Box` pointing to the same type `T`. `count` must evaluate to a `usize`. Then, `src` and + /// `dest` are dereferenced, and `count * size_of::<T>()` bytes beginning with the first byte of + /// the `src` place are copied to the continguous range of bytes beginning with the first byte + /// of `dest`. + /// + /// **Needs clarification**: In what order are operands computed and dereferenced? It should + /// probably match the order for assignment, but that is also undecided. + /// + /// **Needs clarification**: Is this typed or not, ie is there a typed load and store involved? + /// I vaguely remember Ralf saying somewhere that he thought it should not be. + CopyNonOverlapping(Box<CopyNonOverlapping<'tcx>>), + + /// No-op. Useful for deleting instructions without affecting statement indices. + Nop, +} + +/// Describes what kind of retag is to be performed. +#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, Hash, HashStable)] +#[rustc_pass_by_value] +pub enum RetagKind { + /// The initial retag when entering a function. + FnEntry, + /// Retag preparing for a two-phase borrow. + TwoPhase, + /// Retagging raw pointers. + Raw, + /// A "normal" retag. + Default, +} + +/// The `FakeReadCause` describes the type of pattern why a FakeRead statement exists. +#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, Hash, HashStable, PartialEq)] +pub enum FakeReadCause { + /// Inject a fake read of the borrowed input at the end of each guards + /// code. + /// + /// This should ensure that you cannot change the variant for an enum while + /// you are in the midst of matching on it. + ForMatchGuard, + + /// `let x: !; match x {}` doesn't generate any read of x so we need to + /// generate a read of x to check that it is initialized and safe. + /// + /// If a closure pattern matches a Place starting with an Upvar, then we introduce a + /// FakeRead for that Place outside the closure, in such a case this option would be + /// Some(closure_def_id). + /// Otherwise, the value of the optional LocalDefId will be None. + // + // We can use LocaDefId here since fake read statements are removed + // before codegen in the `CleanupNonCodegenStatements` pass. + ForMatchedPlace(Option<LocalDefId>), + + /// A fake read of the RefWithinGuard version of a bind-by-value variable + /// in a match guard to ensure that its value hasn't change by the time + /// we create the OutsideGuard version. + ForGuardBinding, + + /// Officially, the semantics of + /// + /// `let pattern = <expr>;` + /// + /// is that `<expr>` is evaluated into a temporary and then this temporary is + /// into the pattern. + /// + /// However, if we see the simple pattern `let var = <expr>`, we optimize this to + /// evaluate `<expr>` directly into the variable `var`. This is mostly unobservable, + /// but in some cases it can affect the borrow checker, as in #53695. + /// Therefore, we insert a "fake read" here to ensure that we get + /// appropriate errors. + /// + /// If a closure pattern matches a Place starting with an Upvar, then we introduce a + /// FakeRead for that Place outside the closure, in such a case this option would be + /// Some(closure_def_id). + /// Otherwise, the value of the optional DefId will be None. + ForLet(Option<LocalDefId>), + + /// If we have an index expression like + /// + /// (*x)[1][{ x = y; 4}] + /// + /// then the first bounds check is invalidated when we evaluate the second + /// index expression. Thus we create a fake borrow of `x` across the second + /// indexer, which will cause a borrow check error. + ForIndex, +} + +#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)] +#[derive(TypeFoldable, TypeVisitable)] +pub struct Coverage { + pub kind: CoverageKind, + pub code_region: Option<CodeRegion>, +} + +#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)] +#[derive(TypeFoldable, TypeVisitable)] +pub struct CopyNonOverlapping<'tcx> { + pub src: Operand<'tcx>, + pub dst: Operand<'tcx>, + /// Number of elements to copy from src to dest, not bytes. + pub count: Operand<'tcx>, +} + +/////////////////////////////////////////////////////////////////////////// +// Terminators + +/// The various kinds of terminators, representing ways of exiting from a basic block. +/// +/// A note on unwinding: Panics may occur during the execution of some terminators. Depending on the +/// `-C panic` flag, this may either cause the program to abort or the call stack to unwind. Such +/// terminators have a `cleanup: Option<BasicBlock>` field on them. If stack unwinding occurs, then +/// once the current function is reached, execution continues at the given basic block, if any. If +/// `cleanup` is `None` then no cleanup is performed, and the stack continues unwinding. This is +/// equivalent to the execution of a `Resume` terminator. +/// +/// The basic block pointed to by a `cleanup` field must have its `cleanup` flag set. `cleanup` +/// basic blocks have a couple restrictions: +/// 1. All `cleanup` fields in them must be `None`. +/// 2. `Return` terminators are not allowed in them. `Abort` and `Unwind` terminators are. +/// 3. All other basic blocks (in the current body) that are reachable from `cleanup` basic blocks +/// must also be `cleanup`. This is a part of the type system and checked statically, so it is +/// still an error to have such an edge in the CFG even if it's known that it won't be taken at +/// runtime. +#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)] +pub enum TerminatorKind<'tcx> { + /// Block has one successor; we continue execution there. + Goto { target: BasicBlock }, + + /// Switches based on the computed value. + /// + /// First, evaluates the `discr` operand. The type of the operand must be a signed or unsigned + /// integer, char, or bool, and must match the given type. Then, if the list of switch targets + /// contains the computed value, continues execution at the associated basic block. Otherwise, + /// continues execution at the "otherwise" basic block. + /// + /// Target values may not appear more than once. + SwitchInt { + /// The discriminant value being tested. + discr: Operand<'tcx>, + + /// The type of value being tested. + /// This is always the same as the type of `discr`. + /// FIXME: remove this redundant information. Currently, it is relied on by pretty-printing. + switch_ty: Ty<'tcx>, + + targets: SwitchTargets, + }, + + /// Indicates that the landing pad is finished and that the process should continue unwinding. + /// + /// Like a return, this marks the end of this invocation of the function. + /// + /// Only permitted in cleanup blocks. `Resume` is not permitted with `-C unwind=abort` after + /// deaggregation runs. + Resume, + + /// Indicates that the landing pad is finished and that the process should abort. + /// + /// Used to prevent unwinding for foreign items or with `-C unwind=abort`. Only permitted in + /// cleanup blocks. + Abort, + + /// Returns from the function. + /// + /// Like function calls, the exact semantics of returns in Rust are unclear. Returning very + /// likely at least assigns the value currently in the return place (`_0`) to the place + /// specified in the associated `Call` terminator in the calling function, as if assigned via + /// `dest = move _0`. It might additionally do other things, like have side-effects in the + /// aliasing model. + /// + /// If the body is a generator body, this has slightly different semantics; it instead causes a + /// `GeneratorState::Returned(_0)` to be created (as if by an `Aggregate` rvalue) and assigned + /// to the return place. + Return, + + /// Indicates a terminator that can never be reached. + /// + /// Executing this terminator is UB. + Unreachable, + + /// The behavior of this statement differs significantly before and after drop elaboration. + /// After drop elaboration, `Drop` executes the drop glue for the specified place, after which + /// it continues execution/unwinds at the given basic blocks. It is possible that executing drop + /// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an + /// issue tracking if drop glue has any interesting semantics in addition to those of a function + /// call?) + /// + /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the + /// `Drop` will be executed if... + /// + /// **Needs clarification**: End of that sentence. This in effect should document the exact + /// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure: + /// + /// > The drop glue is executed if, among all statements executed within this `Body`, an assignment to + /// > the place or one of its "parents" occurred more recently than a move out of it. This does not + /// > consider indirect assignments. + Drop { place: Place<'tcx>, target: BasicBlock, unwind: Option<BasicBlock> }, + + /// Drops the place and assigns a new value to it. + /// + /// This first performs the exact same operation as the pre drop-elaboration `Drop` terminator; + /// it then additionally assigns the `value` to the `place` as if by an assignment statement. + /// This assignment occurs both in the unwind and the regular code paths. The semantics are best + /// explained by the elaboration: + /// + /// ```ignore (MIR) + /// BB0 { + /// DropAndReplace(P <- V, goto BB1, unwind BB2) + /// } + /// ``` + /// + /// becomes + /// + /// ```ignore (MIR) + /// BB0 { + /// Drop(P, goto BB1, unwind BB2) + /// } + /// BB1 { + /// // P is now uninitialized + /// P <- V + /// } + /// BB2 { + /// // P is now uninitialized -- its dtor panicked + /// P <- V + /// } + /// ``` + /// + /// Disallowed after drop elaboration. + DropAndReplace { + place: Place<'tcx>, + value: Operand<'tcx>, + target: BasicBlock, + unwind: Option<BasicBlock>, + }, + + /// Roughly speaking, evaluates the `func` operand and the arguments, and starts execution of + /// the referred to function. The operand types must match the argument types of the function. + /// The return place type must match the return type. The type of the `func` operand must be + /// callable, meaning either a function pointer, a function type, or a closure type. + /// + /// **Needs clarification**: The exact semantics of this. Current backends rely on `move` + /// operands not aliasing the return place. It is unclear how this is justified in MIR, see + /// [#71117]. + /// + /// [#71117]: https://github.com/rust-lang/rust/issues/71117 + Call { + /// The function that’s being called. + func: Operand<'tcx>, + /// Arguments the function is called with. + /// These are owned by the callee, which is free to modify them. + /// This allows the memory occupied by "by-value" arguments to be + /// reused across function calls without duplicating the contents. + args: Vec<Operand<'tcx>>, + /// Where the returned value will be written + destination: Place<'tcx>, + /// Where to go after this call returns. If none, the call necessarily diverges. + target: Option<BasicBlock>, + /// Cleanups to be done if the call unwinds. + cleanup: Option<BasicBlock>, + /// `true` if this is from a call in HIR rather than from an overloaded + /// operator. True for overloaded function call. + from_hir_call: bool, + /// This `Span` is the span of the function, without the dot and receiver + /// (e.g. `foo(a, b)` in `x.foo(a, b)` + fn_span: Span, + }, + + /// Evaluates the operand, which must have type `bool`. If it is not equal to `expected`, + /// initiates a panic. Initiating a panic corresponds to a `Call` terminator with some + /// unspecified constant as the function to call, all the operands stored in the `AssertMessage` + /// as parameters, and `None` for the destination. Keep in mind that the `cleanup` path is not + /// necessarily executed even in the case of a panic, for example in `-C panic=abort`. If the + /// assertion does not fail, execution continues at the specified basic block. + Assert { + cond: Operand<'tcx>, + expected: bool, + msg: AssertMessage<'tcx>, + target: BasicBlock, + cleanup: Option<BasicBlock>, + }, + + /// Marks a suspend point. + /// + /// Like `Return` terminators in generator bodies, this computes `value` and then a + /// `GeneratorState::Yielded(value)` as if by `Aggregate` rvalue. That value is then assigned to + /// the return place of the function calling this one, and execution continues in the calling + /// function. When next invoked with the same first argument, execution of this function + /// continues at the `resume` basic block, with the second argument written to the `resume_arg` + /// place. If the generator is dropped before then, the `drop` basic block is invoked. + /// + /// Not permitted in bodies that are not generator bodies, or after generator lowering. + /// + /// **Needs clarification**: What about the evaluation order of the `resume_arg` and `value`? + Yield { + /// The value to return. + value: Operand<'tcx>, + /// Where to resume to. + resume: BasicBlock, + /// The place to store the resume argument in. + resume_arg: Place<'tcx>, + /// Cleanup to be done if the generator is dropped at this suspend point. + drop: Option<BasicBlock>, + }, + + /// Indicates the end of dropping a generator. + /// + /// Semantically just a `return` (from the generators drop glue). Only permitted in the same situations + /// as `yield`. + /// + /// **Needs clarification**: Is that even correct? The generator drop code is always confusing + /// to me, because it's not even really in the current body. + /// + /// **Needs clarification**: Are there type system constraints on these terminators? Should + /// there be a "block type" like `cleanup` blocks for them? + GeneratorDrop, + + /// A block where control flow only ever takes one real path, but borrowck needs to be more + /// conservative. + /// + /// At runtime this is semantically just a goto. + /// + /// Disallowed after drop elaboration. + FalseEdge { + /// The target normal control flow will take. + real_target: BasicBlock, + /// A block control flow could conceptually jump to, but won't in + /// practice. + imaginary_target: BasicBlock, + }, + + /// A terminator for blocks that only take one path in reality, but where we reserve the right + /// to unwind in borrowck, even if it won't happen in practice. This can arise in infinite loops + /// with no function calls for example. + /// + /// At runtime this is semantically just a goto. + /// + /// Disallowed after drop elaboration. + FalseUnwind { + /// The target normal control flow will take. + real_target: BasicBlock, + /// The imaginary cleanup block link. This particular path will never be taken + /// in practice, but in order to avoid fragility we want to always + /// consider it in borrowck. We don't want to accept programs which + /// pass borrowck only when `panic=abort` or some assertions are disabled + /// due to release vs. debug mode builds. This needs to be an `Option` because + /// of the `remove_noop_landing_pads` and `abort_unwinding_calls` passes. + unwind: Option<BasicBlock>, + }, + + /// Block ends with an inline assembly block. This is a terminator since + /// inline assembly is allowed to diverge. + InlineAsm { + /// The template for the inline assembly, with placeholders. + template: &'tcx [InlineAsmTemplatePiece], + + /// The operands for the inline assembly, as `Operand`s or `Place`s. + operands: Vec<InlineAsmOperand<'tcx>>, + + /// Miscellaneous options for the inline assembly. + options: InlineAsmOptions, + + /// Source spans for each line of the inline assembly code. These are + /// used to map assembler errors back to the line in the source code. + line_spans: &'tcx [Span], + + /// Destination block after the inline assembly returns, unless it is + /// diverging (InlineAsmOptions::NORETURN). + destination: Option<BasicBlock>, + + /// Cleanup to be done if the inline assembly unwinds. This is present + /// if and only if InlineAsmOptions::MAY_UNWIND is set. + cleanup: Option<BasicBlock>, + }, +} + +/// Information about an assertion failure. +#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, PartialOrd)] +pub enum AssertKind<O> { + BoundsCheck { len: O, index: O }, + Overflow(BinOp, O, O), + OverflowNeg(O), + DivisionByZero(O), + RemainderByZero(O), + ResumedAfterReturn(GeneratorKind), + ResumedAfterPanic(GeneratorKind), +} + +#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)] +#[derive(TypeFoldable, TypeVisitable)] +pub enum InlineAsmOperand<'tcx> { + In { + reg: InlineAsmRegOrRegClass, + value: Operand<'tcx>, + }, + Out { + reg: InlineAsmRegOrRegClass, + late: bool, + place: Option<Place<'tcx>>, + }, + InOut { + reg: InlineAsmRegOrRegClass, + late: bool, + in_value: Operand<'tcx>, + out_place: Option<Place<'tcx>>, + }, + Const { + value: Box<Constant<'tcx>>, + }, + SymFn { + value: Box<Constant<'tcx>>, + }, + SymStatic { + def_id: DefId, + }, +} + +/// Type for MIR `Assert` terminator error messages. +pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>; + +/////////////////////////////////////////////////////////////////////////// +// Places + +/// Places roughly correspond to a "location in memory." Places in MIR are the same mathematical +/// object as places in Rust. This of course means that what exactly they are is undecided and part +/// of the Rust memory model. However, they will likely contain at least the following pieces of +/// information in some form: +/// +/// 1. The address in memory that the place refers to. +/// 2. The provenance with which the place is being accessed. +/// 3. The type of the place and an optional variant index. See [`PlaceTy`][super::tcx::PlaceTy]. +/// 4. Optionally, some metadata. This exists if and only if the type of the place is not `Sized`. +/// +/// We'll give a description below of how all pieces of the place except for the provenance are +/// calculated. We cannot give a description of the provenance, because that is part of the +/// undecided aliasing model - we only include it here at all to acknowledge its existence. +/// +/// Each local naturally corresponds to the place `Place { local, projection: [] }`. This place has +/// the address of the local's allocation and the type of the local. +/// +/// **Needs clarification:** Unsized locals seem to present a bit of an issue. Their allocation +/// can't actually be created on `StorageLive`, because it's unclear how big to make the allocation. +/// Furthermore, MIR produces assignments to unsized locals, although that is not permitted under +/// `#![feature(unsized_locals)]` in Rust. Besides just putting "unsized locals are special and +/// different" in a bunch of places, I (JakobDegen) don't know how to incorporate this behavior into +/// the current MIR semantics in a clean way - possibly this needs some design work first. +/// +/// For places that are not locals, ie they have a non-empty list of projections, we define the +/// values as a function of the parent place, that is the place with its last [`ProjectionElem`] +/// stripped. The way this is computed of course depends on the kind of that last projection +/// element: +/// +/// - [`Downcast`](ProjectionElem::Downcast): This projection sets the place's variant index to the +/// given one, and makes no other changes. A `Downcast` projection on a place with its variant +/// index already set is not well-formed. +/// - [`Field`](ProjectionElem::Field): `Field` projections take their parent place and create a +/// place referring to one of the fields of the type. The resulting address is the parent +/// address, plus the offset of the field. The type becomes the type of the field. If the parent +/// was unsized and so had metadata associated with it, then the metadata is retained if the +/// field is unsized and thrown out if it is sized. +/// +/// These projections are only legal for tuples, ADTs, closures, and generators. If the ADT or +/// generator has more than one variant, the parent place's variant index must be set, indicating +/// which variant is being used. If it has just one variant, the variant index may or may not be +/// included - the single possible variant is inferred if it is not included. +/// - [`ConstantIndex`](ProjectionElem::ConstantIndex): Computes an offset in units of `T` into the +/// place as described in the documentation for the `ProjectionElem`. The resulting address is +/// the parent's address plus that offset, and the type is `T`. This is only legal if the parent +/// place has type `[T; N]` or `[T]` (*not* `&[T]`). Since such a `T` is always sized, any +/// resulting metadata is thrown out. +/// - [`Subslice`](ProjectionElem::Subslice): This projection calculates an offset and a new +/// address in a similar manner as `ConstantIndex`. It is also only legal on `[T; N]` and `[T]`. +/// However, this yields a `Place` of type `[T]`, and additionally sets the metadata to be the +/// length of the subslice. +/// - [`Index`](ProjectionElem::Index): Like `ConstantIndex`, only legal on `[T; N]` or `[T]`. +/// However, `Index` additionally takes a local from which the value of the index is computed at +/// runtime. Computing the value of the index involves interpreting the `Local` as a +/// `Place { local, projection: [] }`, and then computing its value as if done via +/// [`Operand::Copy`]. The array/slice is then indexed with the resulting value. The local must +/// have type `usize`. +/// - [`Deref`](ProjectionElem::Deref): Derefs are the last type of projection, and the most +/// complicated. They are only legal on parent places that are references, pointers, or `Box`. A +/// `Deref` projection begins by loading a value from the parent place, as if by +/// [`Operand::Copy`]. It then dereferences the resulting pointer, creating a place of the +/// pointee's type. The resulting address is the address that was stored in the pointer. If the +/// pointee type is unsized, the pointer additionally stored the value of the metadata. +/// +/// Computing a place may cause UB. One possibility is that the pointer used for a `Deref` may not +/// be suitably aligned. Another possibility is that the place is not in bounds, meaning it does not +/// point to an actual allocation. +/// +/// However, if this is actually UB and when the UB kicks in is undecided. This is being discussed +/// in [UCG#319]. The options include that every place must obey those rules, that only some places +/// must obey them, or that places impose no rules of their own. +/// +/// [UCG#319]: https://github.com/rust-lang/unsafe-code-guidelines/issues/319 +/// +/// Rust currently requires that every place obey those two rules. This is checked by MIRI and taken +/// advantage of by codegen (via `gep inbounds`). That is possibly subject to change. +#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, HashStable)] +pub struct Place<'tcx> { + pub local: Local, + + /// projection out of a place (access a field, deref a pointer, etc) + pub projection: &'tcx List<PlaceElem<'tcx>>, +} + +#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)] +#[derive(TyEncodable, TyDecodable, HashStable)] +pub enum ProjectionElem<V, T> { + Deref, + Field(Field, T), + /// Index into a slice/array. + /// + /// Note that this does not also dereference, and so it does not exactly correspond to slice + /// indexing in Rust. In other words, in the below Rust code: + /// + /// ```rust + /// let x = &[1, 2, 3, 4]; + /// let i = 2; + /// x[i]; + /// ``` + /// + /// The `x[i]` is turned into a `Deref` followed by an `Index`, not just an `Index`. The same + /// thing is true of the `ConstantIndex` and `Subslice` projections below. + Index(V), + + /// These indices are generated by slice patterns. Easiest to explain + /// by example: + /// + /// ```ignore (illustrative) + /// [X, _, .._, _, _] => { offset: 0, min_length: 4, from_end: false }, + /// [_, X, .._, _, _] => { offset: 1, min_length: 4, from_end: false }, + /// [_, _, .._, X, _] => { offset: 2, min_length: 4, from_end: true }, + /// [_, _, .._, _, X] => { offset: 1, min_length: 4, from_end: true }, + /// ``` + ConstantIndex { + /// index or -index (in Python terms), depending on from_end + offset: u64, + /// The thing being indexed must be at least this long. For arrays this + /// is always the exact length. + min_length: u64, + /// Counting backwards from end? This is always false when indexing an + /// array. + from_end: bool, + }, + + /// These indices are generated by slice patterns. + /// + /// If `from_end` is true `slice[from..slice.len() - to]`. + /// Otherwise `array[from..to]`. + Subslice { + from: u64, + to: u64, + /// Whether `to` counts from the start or end of the array/slice. + /// For `PlaceElem`s this is `true` if and only if the base is a slice. + /// For `ProjectionKind`, this can also be `true` for arrays. + from_end: bool, + }, + + /// "Downcast" to a variant of an enum or a generator. + /// + /// The included Symbol is the name of the variant, used for printing MIR. + Downcast(Option<Symbol>, VariantIdx), +} + +/// Alias for projections as they appear in places, where the base is a place +/// and the index is a local. +pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>>; + +/////////////////////////////////////////////////////////////////////////// +// Operands + +/// An operand in MIR represents a "value" in Rust, the definition of which is undecided and part of +/// the memory model. One proposal for a definition of values can be found [on UCG][value-def]. +/// +/// [value-def]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/value-domain.md +/// +/// The most common way to create values is via loading a place. Loading a place is an operation +/// which reads the memory of the place and converts it to a value. This is a fundamentally *typed* +/// operation. The nature of the value produced depends on the type of the conversion. Furthermore, +/// there may be other effects: if the type has a validity constraint loading the place might be UB +/// if the validity constraint is not met. +/// +/// **Needs clarification:** Ralf proposes that loading a place not have side-effects. +/// This is what is implemented in miri today. Are these the semantics we want for MIR? Is this +/// something we can even decide without knowing more about Rust's memory model? +/// +/// **Needs clarifiation:** Is loading a place that has its variant index set well-formed? Miri +/// currently implements it, but it seems like this may be something to check against in the +/// validator. +#[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum Operand<'tcx> { + /// Creates a value by loading the given place. + /// + /// Before drop elaboration, the type of the place must be `Copy`. After drop elaboration there + /// is no such requirement. + Copy(Place<'tcx>), + + /// Creates a value by performing loading the place, just like the `Copy` operand. + /// + /// This *may* additionally overwrite the place with `uninit` bytes, depending on how we decide + /// in [UCG#188]. You should not emit MIR that may attempt a subsequent second load of this + /// place without first re-initializing it. + /// + /// [UCG#188]: https://github.com/rust-lang/unsafe-code-guidelines/issues/188 + Move(Place<'tcx>), + + /// Constants are already semantically values, and remain unchanged. + Constant(Box<Constant<'tcx>>), +} + +/////////////////////////////////////////////////////////////////////////// +// Rvalues + +/// The various kinds of rvalues that can appear in MIR. +/// +/// Not all of these are allowed at every [`MirPhase`] - when this is the case, it's stated below. +/// +/// Computing any rvalue begins by evaluating the places and operands in some order (**Needs +/// clarification**: Which order?). These are then used to produce a "value" - the same kind of +/// value that an [`Operand`] produces. +#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)] +pub enum Rvalue<'tcx> { + /// Yields the operand unchanged + Use(Operand<'tcx>), + + /// Creates an array where each element is the value of the operand. + /// + /// This is the cause of a bug in the case where the repetition count is zero because the value + /// is not dropped, see [#74836]. + /// + /// Corresponds to source code like `[x; 32]`. + /// + /// [#74836]: https://github.com/rust-lang/rust/issues/74836 + Repeat(Operand<'tcx>, ty::Const<'tcx>), + + /// Creates a reference of the indicated kind to the place. + /// + /// There is not much to document here, because besides the obvious parts the semantics of this + /// are essentially entirely a part of the aliasing model. There are many UCG issues discussing + /// exactly what the behavior of this operation should be. + /// + /// `Shallow` borrows are disallowed after drop lowering. + Ref(Region<'tcx>, BorrowKind, Place<'tcx>), + + /// Creates a pointer/reference to the given thread local. + /// + /// The yielded type is a `*mut T` if the static is mutable, otherwise if the static is extern a + /// `*const T`, and if neither of those apply a `&T`. + /// + /// **Note:** This is a runtime operation that actually executes code and is in this sense more + /// like a function call. Also, eliminating dead stores of this rvalue causes `fn main() {}` to + /// SIGILL for some reason that I (JakobDegen) never got a chance to look into. + /// + /// **Needs clarification**: Are there weird additional semantics here related to the runtime + /// nature of this operation? + ThreadLocalRef(DefId), + + /// Creates a pointer with the indicated mutability to the place. + /// + /// This is generated by pointer casts like `&v as *const _` or raw address of expressions like + /// `&raw v` or `addr_of!(v)`. + /// + /// Like with references, the semantics of this operation are heavily dependent on the aliasing + /// model. + AddressOf(Mutability, Place<'tcx>), + + /// Yields the length of the place, as a `usize`. + /// + /// If the type of the place is an array, this is the array length. For slices (`[T]`, not + /// `&[T]`) this accesses the place's metadata to determine the length. This rvalue is + /// ill-formed for places of other types. + Len(Place<'tcx>), + + /// Performs essentially all of the casts that can be performed via `as`. + /// + /// This allows for casts from/to a variety of types. + /// + /// **FIXME**: Document exactly which `CastKind`s allow which types of casts. Figure out why + /// `ArrayToPointer` and `MutToConstPointer` are special. + Cast(CastKind, Operand<'tcx>, Ty<'tcx>), + + /// * `Offset` has the same semantics as [`offset`](pointer::offset), except that the second + /// parameter may be a `usize` as well. + /// * The comparison operations accept `bool`s, `char`s, signed or unsigned integers, floats, + /// raw pointers, or function pointers and return a `bool`. The types of the operands must be + /// matching, up to the usual caveat of the lifetimes in function pointers. + /// * Left and right shift operations accept signed or unsigned integers not necessarily of the + /// same type and return a value of the same type as their LHS. Like in Rust, the RHS is + /// truncated as needed. + /// * The `Bit*` operations accept signed integers, unsigned integers, or bools with matching + /// types and return a value of that type. + /// * The remaining operations accept signed integers, unsigned integers, or floats with + /// matching types and return a value of that type. + BinaryOp(BinOp, Box<(Operand<'tcx>, Operand<'tcx>)>), + + /// Same as `BinaryOp`, but yields `(T, bool)` with a `bool` indicating an error condition. + /// + /// When overflow checking is disabled and we are generating run-time code, the error condition + /// is false. Otherwise, and always during CTFE, the error condition is determined as described + /// below. + /// + /// For addition, subtraction, and multiplication on integers the error condition is set when + /// the infinite precision result would be unequal to the actual result. + /// + /// For shift operations on integers the error condition is set when the value of right-hand + /// side is greater than or equal to the number of bits in the type of the left-hand side, or + /// when the value of right-hand side is negative. + /// + /// Other combinations of types and operators are unsupported. + CheckedBinaryOp(BinOp, Box<(Operand<'tcx>, Operand<'tcx>)>), + + /// Computes a value as described by the operation. + NullaryOp(NullOp, Ty<'tcx>), + + /// Exactly like `BinaryOp`, but less operands. + /// + /// Also does two's-complement arithmetic. Negation requires a signed integer or a float; + /// bitwise not requires a signed integer, unsigned integer, or bool. Both operation kinds + /// return a value with the same type as their operand. + UnaryOp(UnOp, Operand<'tcx>), + + /// Computes the discriminant of the place, returning it as an integer of type + /// [`discriminant_ty`]. Returns zero for types without discriminant. + /// + /// The validity requirements for the underlying value are undecided for this rvalue, see + /// [#91095]. Note too that the value of the discriminant is not the same thing as the + /// variant index; use [`discriminant_for_variant`] to convert. + /// + /// [`discriminant_ty`]: crate::ty::Ty::discriminant_ty + /// [#91095]: https://github.com/rust-lang/rust/issues/91095 + /// [`discriminant_for_variant`]: crate::ty::Ty::discriminant_for_variant + Discriminant(Place<'tcx>), + + /// Creates an aggregate value, like a tuple or struct. + /// + /// This is needed because dataflow analysis needs to distinguish + /// `dest = Foo { x: ..., y: ... }` from `dest.x = ...; dest.y = ...;` in the case that `Foo` + /// has a destructor. + /// + /// Disallowed after deaggregation for all aggregate kinds except `Array` and `Generator`. After + /// generator lowering, `Generator` aggregate kinds are disallowed too. + Aggregate(Box<AggregateKind<'tcx>>, Vec<Operand<'tcx>>), + + /// Transmutes a `*mut u8` into shallow-initialized `Box<T>`. + /// + /// This is different from a normal transmute because dataflow analysis will treat the box as + /// initialized but its content as uninitialized. Like other pointer casts, this in general + /// affects alias analysis. + ShallowInitBox(Operand<'tcx>, Ty<'tcx>), + + /// A CopyForDeref is equivalent to a read from a place at the + /// codegen level, but is treated specially by drop elaboration. When such a read happens, it + /// is guaranteed (via nature of the mir_opt `Derefer` in rustc_mir_transform/src/deref_separator) + /// that the only use of the returned value is a deref operation, immediately + /// followed by one or more projections. Drop elaboration treats this rvalue as if the + /// read never happened and just projects further. This allows simplifying various MIR + /// optimizations and codegen backends that previously had to handle deref operations anywhere + /// in a place. + CopyForDeref(Place<'tcx>), +} + +#[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum CastKind { + /// An exposing pointer to address cast. A cast between a pointer and an integer type, or + /// between a function pointer and an integer type. + /// See the docs on `expose_addr` for more details. + PointerExposeAddress, + /// An address-to-pointer cast that picks up an exposed provenance. + /// See the docs on `from_exposed_addr` for more details. + PointerFromExposedAddress, + /// All sorts of pointer-to-pointer casts. Note that reference-to-raw-ptr casts are + /// translated into `&raw mut/const *r`, i.e., they are not actually casts. + Pointer(PointerCast), + /// Remaining unclassified casts. + Misc, +} + +#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum AggregateKind<'tcx> { + /// The type is of the element + Array(Ty<'tcx>), + Tuple, + + /// The second field is the variant index. It's equal to 0 for struct + /// and union expressions. The fourth field is + /// active field number and is present only for union expressions + /// -- e.g., for a union expression `SomeUnion { c: .. }`, the + /// active field index would identity the field `c` + Adt(DefId, VariantIdx, SubstsRef<'tcx>, Option<UserTypeAnnotationIndex>, Option<usize>), + + // Note: We can use LocalDefId since closures and generators a deaggregated + // before codegen. + Closure(LocalDefId, SubstsRef<'tcx>), + Generator(LocalDefId, SubstsRef<'tcx>, hir::Movability), +} + +#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum NullOp { + /// Returns the size of a value of that type + SizeOf, + /// Returns the minimum alignment of a type + AlignOf, +} + +#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum UnOp { + /// The `!` operator for logical inversion + Not, + /// The `-` operator for negation + Neg, +} + +#[derive(Copy, Clone, Debug, PartialEq, PartialOrd, Eq, TyEncodable, TyDecodable, Hash, HashStable)] +pub enum BinOp { + /// The `+` operator (addition) + Add, + /// The `-` operator (subtraction) + Sub, + /// The `*` operator (multiplication) + Mul, + /// The `/` operator (division) + /// + /// Division by zero is UB, because the compiler should have inserted checks + /// prior to this. + Div, + /// The `%` operator (modulus) + /// + /// Using zero as the modulus (second operand) is UB, because the compiler + /// should have inserted checks prior to this. + Rem, + /// The `^` operator (bitwise xor) + BitXor, + /// The `&` operator (bitwise and) + BitAnd, + /// The `|` operator (bitwise or) + BitOr, + /// The `<<` operator (shift left) + /// + /// The offset is truncated to the size of the first operand before shifting. + Shl, + /// The `>>` operator (shift right) + /// + /// The offset is truncated to the size of the first operand before shifting. + Shr, + /// The `==` operator (equality) + Eq, + /// The `<` operator (less than) + Lt, + /// The `<=` operator (less than or equal to) + Le, + /// The `!=` operator (not equal to) + Ne, + /// The `>=` operator (greater than or equal to) + Ge, + /// The `>` operator (greater than) + Gt, + /// The `ptr.offset` operator + Offset, +} + +// Some nodes are used a lot. Make sure they don't unintentionally get bigger. +#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))] +mod size_asserts { + use super::*; + // These are in alphabetical order, which is easy to maintain. + static_assert_size!(AggregateKind<'_>, 48); + static_assert_size!(Operand<'_>, 24); + static_assert_size!(Place<'_>, 16); + static_assert_size!(PlaceElem<'_>, 24); + static_assert_size!(Rvalue<'_>, 40); +} |