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Auto merge of #98649 - RalfJung:guardians-of-mir, r=oli-obk

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move MIR syntax into a dedicated file and ping some people whenever it changes

Adding or changing MIR operations/statements/whatever should be under significant scrutiny wrt their wider impact, specified semantics, and so on. So let's start by putting all that into a dedicated file and pinging some people whenever that file changes.

This PR only moves definitions around, and then fiddles with imports until it all works again.
This commit is contained in:
bors 2022-06-30 03:50:35 +00:00
commit a9eb9c52f3
4 changed files with 1187 additions and 1149 deletions
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912
compiler/rustc_middle/src/mir/mod.rs
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1141
compiler/rustc_middle/src/mir/syntax.rs Normal file
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278
compiler/rustc_middle/src/mir/terminator.rs
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@ -1,16 +1,12 @@
use crate::mir;
use crate::mir::interpret::Scalar;
use crate::ty::{self, Ty, TyCtxt};
use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
use smallvec::{smallvec, SmallVec};
use super::{
AssertMessage, BasicBlock, InlineAsmOperand, Operand, Place, SourceInfo, Successors,
SuccessorsMut,
};
use super::{BasicBlock, InlineAsmOperand, Operand, SourceInfo, TerminatorKind};
use rustc_ast::InlineAsmTemplatePiece;
pub use rustc_ast::Mutability;
use rustc_macros::HashStable;
use rustc_span::Span;
use std::borrow::Cow;
use std::fmt::{self, Debug, Formatter, Write};
use std::iter;
@ -106,278 +102,16 @@ impl<'a> Iterator for SwitchTargetsIter<'a> {
impl<'a> ExactSizeIterator for SwitchTargetsIter<'a> {}
/// 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>,
},
}
#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
pub struct Terminator<'tcx> {
pub source_info: SourceInfo,
pub kind: TerminatorKind<'tcx>,
}
pub type Successors<'a> = impl Iterator<Item = BasicBlock> + 'a;
pub type SuccessorsMut<'a> =
iter::Chain<std::option::IntoIter<&'a mut BasicBlock>, slice::IterMut<'a, BasicBlock>>;
impl<'tcx> Terminator<'tcx> {
pub fn successors(&self) -> Successors<'_> {
self.kind.successors()