Live variable analysis

src/librustc_mir/dataflow/impls/liveness.rs

@@ -0,0 +1,137 @@
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
+use rustc_middle::mir::{self, Local, Location};
+
+use crate::dataflow::{AnalysisDomain, Backward, BottomValue, GenKill, GenKillAnalysis};
+
+/// A [live-variable dataflow analysis][liveness].
+///
+/// [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 BottomValue for MaybeLiveLocals {
+    // bottom = not live
+    const BOTTOM_VALUE: bool = false;
+}
+
+impl AnalysisDomain<'tcx> for MaybeLiveLocals {
+    type Idx = Local;
+    type Direction = Backward;
+
+    const NAME: &'static str = "liveness";
+
+    fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
+        body.local_decls.len()
+    }
+
+    fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut BitSet<Self::Idx>) {
+        // No variables are live until we observe a use
+    }
+}
+
+impl GenKillAnalysis<'tcx> for MaybeLiveLocals {
+    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_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
+        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 succesfully, 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::Projection
+                | MutatingUseContext::Retag,
+            )
+            | PlaceContext::NonMutatingUse(
+                NonMutatingUseContext::AddressOf
+                | NonMutatingUseContext::Copy
+                | NonMutatingUseContext::Inspect
+                | NonMutatingUseContext::Move
+                | NonMutatingUseContext::Projection
+                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::SharedBorrow
+                | NonMutatingUseContext::UniqueBorrow,
+            ) => Some(DefUse::Use),
+        }
+    }
+}

src/librustc_mir/dataflow/impls/mod.rs

@@ -21,9 +21,11 @@ use super::on_lookup_result_bits;
 use crate::dataflow::drop_flag_effects;
 
 mod borrowed_locals;
+mod liveness;
 mod storage_liveness;
 
 pub use self::borrowed_locals::*;
+pub use self::liveness::MaybeLiveLocals;
 pub use self::storage_liveness::*;
 
 pub(super) mod borrows;

src/librustc_mir/dataflow/mod.rs

@@ -10,8 +10,8 @@ pub use self::framework::{
 };
 pub use self::impls::{
     borrows::Borrows, DefinitelyInitializedPlaces, EverInitializedPlaces, MaybeBorrowedLocals,
-    MaybeInitializedPlaces, MaybeMutBorrowedLocals, MaybeRequiresStorage, MaybeStorageLive,
-    MaybeUninitializedPlaces,
+    MaybeInitializedPlaces, MaybeLiveLocals, MaybeMutBorrowedLocals, MaybeRequiresStorage,
+    MaybeStorageLive, MaybeUninitializedPlaces,
 };
 
 use self::move_paths::MoveData;