rustc_mir_dataflow/impls/
liveness.rs

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1use rustc_index::bit_set::DenseBitSet;
2use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
3use rustc_middle::mir::{
4    self, CallReturnPlaces, Local, Location, Place, StatementKind, TerminatorEdges,
5};
6
7use crate::{Analysis, Backward, GenKill};
8
9/// A [live-variable dataflow analysis][liveness].
10///
11/// This analysis considers references as being used only at the point of the
12/// borrow. In other words, this analysis does not track uses because of references that already
13/// exist. See [this `mir-dataflow` test][flow-test] for an example. You almost never want to use
14/// this analysis without also looking at the results of [`MaybeBorrowedLocals`].
15///
16/// ## Field-(in)sensitivity
17///
18/// As the name suggests, this analysis is field insensitive. If a projection of a variable `x` is
19/// assigned to (e.g. `x.0 = 42`), it does not "define" `x` as far as liveness is concerned. In fact,
20/// such an assignment is currently marked as a "use" of `x` in an attempt to be maximally
21/// conservative.
22///
23/// [`MaybeBorrowedLocals`]: super::MaybeBorrowedLocals
24/// [flow-test]: https://github.com/rust-lang/rust/blob/a08c47310c7d49cbdc5d7afb38408ba519967ecd/src/test/ui/mir-dataflow/liveness-ptr.rs
25/// [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
26pub struct MaybeLiveLocals;
27
28impl<'tcx> Analysis<'tcx> for MaybeLiveLocals {
29    type Domain = DenseBitSet<Local>;
30    type Direction = Backward;
31
32    const NAME: &'static str = "liveness";
33
34    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
35        // bottom = not live
36        DenseBitSet::new_empty(body.local_decls.len())
37    }
38
39    fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
40        // No variables are live until we observe a use
41    }
42
43    fn apply_primary_statement_effect(
44        &mut self,
45        state: &mut Self::Domain,
46        statement: &mir::Statement<'tcx>,
47        location: Location,
48    ) {
49        TransferFunction(state).visit_statement(statement, location);
50    }
51
52    fn apply_primary_terminator_effect<'mir>(
53        &mut self,
54        state: &mut Self::Domain,
55        terminator: &'mir mir::Terminator<'tcx>,
56        location: Location,
57    ) -> TerminatorEdges<'mir, 'tcx> {
58        TransferFunction(state).visit_terminator(terminator, location);
59        terminator.edges()
60    }
61
62    fn apply_call_return_effect(
63        &mut self,
64        state: &mut Self::Domain,
65        _block: mir::BasicBlock,
66        return_places: CallReturnPlaces<'_, 'tcx>,
67    ) {
68        if let CallReturnPlaces::Yield(resume_place) = return_places {
69            YieldResumeEffect(state).visit_place(
70                &resume_place,
71                PlaceContext::MutatingUse(MutatingUseContext::Yield),
72                Location::START,
73            )
74        } else {
75            return_places.for_each(|place| {
76                if let Some(local) = place.as_local() {
77                    state.kill(local);
78                }
79            });
80        }
81    }
82}
83
84pub struct TransferFunction<'a>(pub &'a mut DenseBitSet<Local>);
85
86impl<'tcx> Visitor<'tcx> for TransferFunction<'_> {
87    fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
88        if let PlaceContext::MutatingUse(MutatingUseContext::Yield) = context {
89            // The resume place is evaluated and assigned to only after coroutine resumes, so its
90            // effect is handled separately in `call_resume_effect`.
91            return;
92        }
93
94        match DefUse::for_place(*place, context) {
95            Some(DefUse::Def) => {
96                if let PlaceContext::MutatingUse(
97                    MutatingUseContext::Call | MutatingUseContext::AsmOutput,
98                ) = context
99                {
100                    // For the associated terminators, this is only a `Def` when the terminator
101                    // returns "successfully." As such, we handle this case separately in
102                    // `call_return_effect` above. However, if the place looks like `*_5`, this is
103                    // still unconditionally a use of `_5`.
104                } else {
105                    self.0.kill(place.local);
106                }
107            }
108            Some(DefUse::Use) => self.0.gen_(place.local),
109            None => {}
110        }
111
112        self.visit_projection(place.as_ref(), context, location);
113    }
114
115    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
116        DefUse::apply(self.0, local.into(), context);
117    }
118}
119
120struct YieldResumeEffect<'a>(&'a mut DenseBitSet<Local>);
121
122impl<'tcx> Visitor<'tcx> for YieldResumeEffect<'_> {
123    fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
124        DefUse::apply(self.0, *place, context);
125        self.visit_projection(place.as_ref(), context, location);
126    }
127
128    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
129        DefUse::apply(self.0, local.into(), context);
130    }
131}
132
133#[derive(Eq, PartialEq, Clone)]
134enum DefUse {
135    Def,
136    Use,
137}
138
139impl DefUse {
140    fn apply(state: &mut DenseBitSet<Local>, place: Place<'_>, context: PlaceContext) {
141        match DefUse::for_place(place, context) {
142            Some(DefUse::Def) => state.kill(place.local),
143            Some(DefUse::Use) => state.gen_(place.local),
144            None => {}
145        }
146    }
147
148    fn for_place(place: Place<'_>, context: PlaceContext) -> Option<DefUse> {
149        match context {
150            PlaceContext::NonUse(_) => None,
151
152            PlaceContext::MutatingUse(
153                MutatingUseContext::Call
154                | MutatingUseContext::Yield
155                | MutatingUseContext::AsmOutput
156                | MutatingUseContext::Store
157                | MutatingUseContext::Deinit,
158            ) => {
159                if place.is_indirect() {
160                    // Treat derefs as a use of the base local. `*p = 4` is not a def of `p` but a
161                    // use.
162                    Some(DefUse::Use)
163                } else if place.projection.is_empty() {
164                    Some(DefUse::Def)
165                } else {
166                    None
167                }
168            }
169
170            // Setting the discriminant is not a use because it does no reading, but it is also not
171            // a def because it does not overwrite the whole place
172            PlaceContext::MutatingUse(MutatingUseContext::SetDiscriminant) => {
173                place.is_indirect().then_some(DefUse::Use)
174            }
175
176            // All other contexts are uses...
177            PlaceContext::MutatingUse(
178                MutatingUseContext::RawBorrow
179                | MutatingUseContext::Borrow
180                | MutatingUseContext::Drop
181                | MutatingUseContext::Retag,
182            )
183            | PlaceContext::NonMutatingUse(
184                NonMutatingUseContext::RawBorrow
185                | NonMutatingUseContext::Copy
186                | NonMutatingUseContext::Inspect
187                | NonMutatingUseContext::Move
188                | NonMutatingUseContext::PlaceMention
189                | NonMutatingUseContext::FakeBorrow
190                | NonMutatingUseContext::SharedBorrow,
191            ) => Some(DefUse::Use),
192
193            PlaceContext::MutatingUse(MutatingUseContext::Projection)
194            | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
195                unreachable!("A projection could be a def or a use and must be handled separately")
196            }
197        }
198    }
199}
200
201/// Like `MaybeLiveLocals`, but does not mark locals as live if they are used in a dead assignment.
202///
203/// This is basically written for dead store elimination and nothing else.
204///
205/// All of the caveats of `MaybeLiveLocals` apply.
206pub struct MaybeTransitiveLiveLocals<'a> {
207    always_live: &'a DenseBitSet<Local>,
208}
209
210impl<'a> MaybeTransitiveLiveLocals<'a> {
211    /// The `always_alive` set is the set of locals to which all stores should unconditionally be
212    /// considered live.
213    ///
214    /// This should include at least all locals that are ever borrowed.
215    pub fn new(always_live: &'a DenseBitSet<Local>) -> Self {
216        MaybeTransitiveLiveLocals { always_live }
217    }
218}
219
220impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
221    type Domain = DenseBitSet<Local>;
222    type Direction = Backward;
223
224    const NAME: &'static str = "transitive liveness";
225
226    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
227        // bottom = not live
228        DenseBitSet::new_empty(body.local_decls.len())
229    }
230
231    fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
232        // No variables are live until we observe a use
233    }
234
235    fn apply_primary_statement_effect(
236        &mut self,
237        state: &mut Self::Domain,
238        statement: &mir::Statement<'tcx>,
239        location: Location,
240    ) {
241        // Compute the place that we are storing to, if any
242        let destination = match &statement.kind {
243            StatementKind::Assign(assign) => assign.1.is_safe_to_remove().then_some(assign.0),
244            StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
245                Some(**place)
246            }
247            StatementKind::FakeRead(_)
248            | StatementKind::StorageLive(_)
249            | StatementKind::StorageDead(_)
250            | StatementKind::Retag(..)
251            | StatementKind::AscribeUserType(..)
252            | StatementKind::PlaceMention(..)
253            | StatementKind::Coverage(..)
254            | StatementKind::Intrinsic(..)
255            | StatementKind::ConstEvalCounter
256            | StatementKind::BackwardIncompatibleDropHint { .. }
257            | StatementKind::Nop => None,
258        };
259        if let Some(destination) = destination {
260            if !destination.is_indirect()
261                && !state.contains(destination.local)
262                && !self.always_live.contains(destination.local)
263            {
264                // This store is dead
265                return;
266            }
267        }
268        TransferFunction(state).visit_statement(statement, location);
269    }
270
271    fn apply_primary_terminator_effect<'mir>(
272        &mut self,
273        state: &mut Self::Domain,
274        terminator: &'mir mir::Terminator<'tcx>,
275        location: Location,
276    ) -> TerminatorEdges<'mir, 'tcx> {
277        TransferFunction(state).visit_terminator(terminator, location);
278        terminator.edges()
279    }
280
281    fn apply_call_return_effect(
282        &mut self,
283        state: &mut Self::Domain,
284        _block: mir::BasicBlock,
285        return_places: CallReturnPlaces<'_, 'tcx>,
286    ) {
287        if let CallReturnPlaces::Yield(resume_place) = return_places {
288            YieldResumeEffect(state).visit_place(
289                &resume_place,
290                PlaceContext::MutatingUse(MutatingUseContext::Yield),
291                Location::START,
292            )
293        } else {
294            return_places.for_each(|place| {
295                if let Some(local) = place.as_local() {
296                    state.remove(local);
297                }
298            });
299        }
300    }
301}
rustc_mir_dataflow/impls/liveness.rs

rustc_mir_dataflow/impls/
liveness.rs
