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Add new MutatatingUseContexts for deinit and SetDiscriminant

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This commit is contained in:
Jakob Degen
2022-04-11 06:04:53 -04:00
parent f7ca97a209
commit 48b01a0d0e
9 changed files with 57 additions and 41 deletions
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32
compiler/rustc_mir_dataflow/src/impls/liveness.rs
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@@ -18,30 +18,6 @@ use crate::{AnalysisDomain, Backward, CallReturnPlaces, GenKill, GenKillAnalysis
/// such an assignment is currently marked as a "use" of `x` in an attempt to be maximally
/// conservative.
///
/// ## Enums and `SetDiscriminant`
///
/// Assigning a literal value to an `enum` (e.g. `Option<i32>`), does not result in a simple
/// assignment of the form `_1 = /*...*/` in the MIR. For example, the following assignment to `x`:
///
/// ```
/// x = Some(4);
/// ```
///
/// compiles to this MIR
///
/// ```
/// ((_1 as Some).0: i32) = const 4_i32;
/// discriminant(_1) = 1;
/// ```
///
/// However, `MaybeLiveLocals` **does** mark `x` (`_1`) as "killed" after a statement like this.
/// That's because it treats the `SetDiscriminant` operation as a definition of `x`, even though
/// the writes that actually initialized the locals happened earlier.
///
/// This makes `MaybeLiveLocals` unsuitable for certain classes of optimization normally associated
/// with a live variables analysis, notably dead-store elimination. It's a dirty hack, but it works
/// okay for the generator state transform (currently the main consumer of this analysis).
///
/// [`MaybeBorrowedLocals`]: super::MaybeBorrowedLocals
/// [flow-test]: https://github.com/rust-lang/rust/blob/a08c47310c7d49cbdc5d7afb38408ba519967ecd/src/test/ui/mir-dataflow/liveness-ptr.rs
/// [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
@@ -161,7 +137,13 @@ impl DefUse {
match context {
PlaceContext::NonUse(_) => None,
PlaceContext::MutatingUse(MutatingUseContext::Store) => Some(DefUse::Def),
PlaceContext::MutatingUse(MutatingUseContext::Store | MutatingUseContext::Deinit) => {
Some(DefUse::Def)
}
// Setting the discriminant is not a use because it does no reading, but it is also not
// a def because it does not overwrite the whole place
PlaceContext::MutatingUse(MutatingUseContext::SetDiscriminant) => None,
// `MutatingUseContext::Call` and `MutatingUseContext::Yield` indicate that this is the
// destination place for a `Call` return or `Yield` resume respectively. Since this is