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Handle stores to projections correctly in liveness analysis

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Previously, we were too conservative and `x.field = 4` was treated as a
"use" of `x`.
This commit is contained in:
Dylan MacKenzie 2020-06-26 11:30:14 -07:00
parent 9672b5e95c
commit 558c8a8c3c
1 changed files with 25 additions and 2 deletions
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27
src/librustc_mir/dataflow/impls/liveness.rs
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@ -92,7 +92,27 @@ impl<'tcx, T> Visitor<'tcx> for TransferFunction<'_, T>
where
T: GenKill<Local>,
{
fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
let mir::Place { projection, local } = *place;
// We purposefully do not call `super_place` here to avoid calling `visit_local` for this
// place with one of the `Projection` variants of `PlaceContext`.
self.visit_projection(local, projection, context, location);
match DefUse::for_place(context) {
// Treat derefs as a use of the base local. `*p = 4` is not a def of `p` but a use.
Some(_) if place.is_indirect() => self.0.gen(local),
Some(DefUse::Def) if projection.is_empty() => self.0.kill(local),
Some(DefUse::Use) => self.0.gen(local),
_ => {}
}
}
fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
// Because we do not call `super_place` above, `visit_local` is only called for locals that
// do not appear as part of a `Place` in the MIR. This handles cases like the implicit use
// of the return place in a `Return` terminator or the index in an `Index` projection.
match DefUse::for_place(context) {
Some(DefUse::Def) => self.0.kill(local),
Some(DefUse::Use) => self.0.gen(local),
@ -126,7 +146,6 @@ impl DefUse {
| MutatingUseContext::AsmOutput
| MutatingUseContext::Borrow
| MutatingUseContext::Drop
| MutatingUseContext::Projection
| MutatingUseContext::Retag,
)
| PlaceContext::NonMutatingUse(
@ -134,11 +153,15 @@ impl DefUse {
| NonMutatingUseContext::Copy
| NonMutatingUseContext::Inspect
| NonMutatingUseContext::Move
| NonMutatingUseContext::Projection
| NonMutatingUseContext::ShallowBorrow
| NonMutatingUseContext::SharedBorrow
| NonMutatingUseContext::UniqueBorrow,
) => Some(DefUse::Use),
PlaceContext::MutatingUse(MutatingUseContext::Projection)
| PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
unreachable!("A projection could be a def or a use and must be handled separately")
}
}
}
}