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Add cycle checking to cleanup control flow validation

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Jakob Degen 2023-01-13 08:32:54 -08:00
parent f49126e3d6
commit ec3d993410
2 changed files with 54 additions and 22 deletions
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11
compiler/rustc_middle/src/mir/syntax.rs
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@ -512,13 +512,16 @@ pub struct CopyNonOverlapping<'tcx> {
/// 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.
/// 4. The induced subgraph on cleanup blocks must look roughly like an upside down tree. This is
/// necessary to ensure that landing pad information can be correctly codegened. More precisely:
/// 4. The control flow between cleanup blocks must look like an upside down tree. Roughly
/// speaking, this means that control flow that looks like a V is allowed, while control flow
/// that looks like a W is not. This is necessary to ensure that landing pad information can be
/// correctly codegened on MSVC. More precisely:
///
/// Begin with the standard control flow graph `G`. Modify `G` as follows: for any two cleanup
/// vertices `u` and `v` such that `u` dominates `v`, contract `u` and `v` into a single vertex,
/// deleting self edges and duplicate edges in the process. The cleanup blocks of the resulting
/// graph must form an inverted forest.
/// deleting self edges and duplicate edges in the process. Now remove all vertices from `G`
/// that are not cleanup vertices or are not reachable. The resulting graph must be an inverted
/// tree, that is each vertex may have at most one successor and there may be no cycles.
#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, TypeFoldable, TypeVisitable)]
pub enum TerminatorKind<'tcx> {
/// Block has one successor; we continue execution there.