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Diffstat (limited to 'compiler/rustc_mir_dataflow/src/impls/liveness.rs')
| -rw-r--r-- | compiler/rustc_mir_dataflow/src/impls/liveness.rs | 165 |
1 files changed, 165 insertions, 0 deletions
diff --git a/compiler/rustc_mir_dataflow/src/impls/liveness.rs b/compiler/rustc_mir_dataflow/src/impls/liveness.rs new file mode 100644 index 00000000000..0039d3188d5 --- /dev/null +++ b/compiler/rustc_mir_dataflow/src/impls/liveness.rs @@ -0,0 +1,165 @@ +use rustc_index::bit_set::BitSet; +use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor}; +use rustc_middle::mir::{self, Local, Location}; + +use crate::{AnalysisDomain, Backward, GenKill, GenKillAnalysis}; + +/// A [live-variable dataflow analysis][liveness]. +/// +/// This analysis considers references as being used only at the point of the +/// borrow. In other words, this analysis does not track uses because of references that already +/// exist. See [this `mir-dataflow` test][flow-test] for an example. You almost never want to use +/// this analysis without also looking at the results of [`MaybeBorrowedLocals`]. +/// +/// [`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 +pub struct MaybeLiveLocals; + +impl MaybeLiveLocals { + fn transfer_function<T>(&self, trans: &'a mut T) -> TransferFunction<'a, T> { + TransferFunction(trans) + } +} + +impl AnalysisDomain<'tcx> for MaybeLiveLocals { + type Domain = BitSet<Local>; + type Direction = Backward; + + const NAME: &'static str = "liveness"; + + fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain { + // bottom = not live + BitSet::new_empty(body.local_decls.len()) + } + + fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) { + // No variables are live until we observe a use + } +} + +impl GenKillAnalysis<'tcx> for MaybeLiveLocals { + type Idx = Local; + + fn statement_effect( + &self, + trans: &mut impl GenKill<Self::Idx>, + statement: &mir::Statement<'tcx>, + location: Location, + ) { + self.transfer_function(trans).visit_statement(statement, location); + } + + fn terminator_effect( + &self, + trans: &mut impl GenKill<Self::Idx>, + terminator: &mir::Terminator<'tcx>, + location: Location, + ) { + self.transfer_function(trans).visit_terminator(terminator, location); + } + + fn call_return_effect( + &self, + trans: &mut impl GenKill<Self::Idx>, + _block: mir::BasicBlock, + _func: &mir::Operand<'tcx>, + _args: &[mir::Operand<'tcx>], + dest_place: mir::Place<'tcx>, + ) { + if let Some(local) = dest_place.as_local() { + trans.kill(local); + } + } + + fn yield_resume_effect( + &self, + trans: &mut impl GenKill<Self::Idx>, + _resume_block: mir::BasicBlock, + resume_place: mir::Place<'tcx>, + ) { + if let Some(local) = resume_place.as_local() { + trans.kill(local); + } + } +} + +struct TransferFunction<'a, T>(&'a mut T); + +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(place.as_ref(), 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), + _ => {} + } + } +} + +#[derive(Eq, PartialEq, Clone)] +enum DefUse { + Def, + Use, +} + +impl DefUse { + fn for_place(context: PlaceContext) -> Option<DefUse> { + match context { + PlaceContext::NonUse(_) => None, + + PlaceContext::MutatingUse(MutatingUseContext::Store) => Some(DefUse::Def), + + // `MutatingUseContext::Call` and `MutatingUseContext::Yield` indicate that this is the + // destination place for a `Call` return or `Yield` resume respectively. Since this is + // only a `Def` when the function returns successfully, we handle this case separately + // in `call_return_effect` above. + PlaceContext::MutatingUse(MutatingUseContext::Call | MutatingUseContext::Yield) => None, + + // All other contexts are uses... + PlaceContext::MutatingUse( + MutatingUseContext::AddressOf + | MutatingUseContext::AsmOutput + | MutatingUseContext::Borrow + | MutatingUseContext::Drop + | MutatingUseContext::Retag, + ) + | PlaceContext::NonMutatingUse( + NonMutatingUseContext::AddressOf + | NonMutatingUseContext::Copy + | NonMutatingUseContext::Inspect + | NonMutatingUseContext::Move + | 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") + } + } + } +} |