rustc_mir_transform/
copy_prop.rs

1use rustc_index::IndexSlice;
2use rustc_index::bit_set::DenseBitSet;
3use rustc_middle::mir::visit::*;
4use rustc_middle::mir::*;
5use rustc_middle::ty::TyCtxt;
6use tracing::{debug, instrument};
7
8use crate::ssa::SsaLocals;
9
10/// Unify locals that copy each other.
11///
12/// We consider patterns of the form
13///   _a = rvalue
14///   _b = move? _a
15///   _c = move? _a
16///   _d = move? _c
17/// where each of the locals is only assigned once.
18///
19/// We want to replace all those locals by `_a`, either copied or moved.
20pub(super) struct CopyProp;
21
22impl<'tcx> crate::MirPass<'tcx> for CopyProp {
23    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
24        sess.mir_opt_level() >= 1
25    }
26
27    #[instrument(level = "trace", skip(self, tcx, body))]
28    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
29        debug!(def_id = ?body.source.def_id());
30
31        let typing_env = body.typing_env(tcx);
32        let ssa = SsaLocals::new(tcx, body, typing_env);
33        debug!(borrowed_locals = ?ssa.borrowed_locals());
34        debug!(copy_classes = ?ssa.copy_classes());
35
36        let mut any_replacement = false;
37        let mut storage_to_remove = DenseBitSet::new_empty(body.local_decls.len());
38        for (local, &head) in ssa.copy_classes().iter_enumerated() {
39            if local != head {
40                any_replacement = true;
41                storage_to_remove.insert(head);
42            }
43        }
44
45        if !any_replacement {
46            return;
47        }
48
49        let fully_moved = fully_moved_locals(&ssa, body);
50        debug!(?fully_moved);
51
52        Replacer { tcx, copy_classes: ssa.copy_classes(), fully_moved, storage_to_remove }
53            .visit_body_preserves_cfg(body);
54
55        crate::simplify::remove_unused_definitions(body);
56    }
57
58    fn is_required(&self) -> bool {
59        false
60    }
61}
62
63/// `SsaLocals` computed equivalence classes between locals considering copy/move assignments.
64///
65/// This function also returns whether all the `move?` in the pattern are `move` and not copies.
66/// A local which is in the bitset can be replaced by `move _a`. Otherwise, it must be
67/// replaced by `copy _a`, as we cannot move multiple times from `_a`.
68///
69/// If an operand copies `_c`, it must happen before the assignment `_d = _c`, otherwise it is UB.
70/// This means that replacing it by a copy of `_a` if ok, since this copy happens before `_c` is
71/// moved, and therefore that `_d` is moved.
72#[instrument(level = "trace", skip(ssa, body))]
73fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
74    let mut fully_moved = DenseBitSet::new_filled(body.local_decls.len());
75
76    for (_, rvalue, _) in ssa.assignments(body) {
77        let (Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
78        | Rvalue::CopyForDeref(place)) = rvalue
79        else {
80            continue;
81        };
82
83        let Some(rhs) = place.as_local() else { continue };
84        if !ssa.is_ssa(rhs) {
85            continue;
86        }
87
88        if let Rvalue::Use(Operand::Copy(_)) | Rvalue::CopyForDeref(_) = rvalue {
89            fully_moved.remove(rhs);
90        }
91    }
92
93    ssa.meet_copy_equivalence(&mut fully_moved);
94
95    fully_moved
96}
97
98/// Utility to help performing substitution of `*pattern` by `target`.
99struct Replacer<'a, 'tcx> {
100    tcx: TyCtxt<'tcx>,
101    fully_moved: DenseBitSet<Local>,
102    storage_to_remove: DenseBitSet<Local>,
103    copy_classes: &'a IndexSlice<Local, Local>,
104}
105
106impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
107    fn tcx(&self) -> TyCtxt<'tcx> {
108        self.tcx
109    }
110
111    #[tracing::instrument(level = "trace", skip(self))]
112    fn visit_local(&mut self, local: &mut Local, ctxt: PlaceContext, _: Location) {
113        let new_local = self.copy_classes[*local];
114        match ctxt {
115            // Do not modify the local in storage statements.
116            PlaceContext::NonUse(NonUseContext::StorageLive | NonUseContext::StorageDead) => {}
117            // We access the value.
118            _ => *local = new_local,
119        }
120    }
121
122    #[tracing::instrument(level = "trace", skip(self))]
123    fn visit_operand(&mut self, operand: &mut Operand<'tcx>, loc: Location) {
124        if let Operand::Move(place) = *operand
125            // A move out of a projection of a copy is equivalent to a copy of the original
126            // projection.
127            && !place.is_indirect_first_projection()
128            && !self.fully_moved.contains(place.local)
129        {
130            *operand = Operand::Copy(place);
131        }
132        self.super_operand(operand, loc);
133    }
134
135    #[tracing::instrument(level = "trace", skip(self))]
136    fn visit_statement(&mut self, stmt: &mut Statement<'tcx>, loc: Location) {
137        // When removing storage statements, we need to remove both (#107511).
138        if let StatementKind::StorageLive(l) | StatementKind::StorageDead(l) = stmt.kind
139            && self.storage_to_remove.contains(l)
140        {
141            stmt.make_nop();
142            return;
143        }
144
145        self.super_statement(stmt, loc);
146
147        // Do not leave tautological assignments around.
148        if let StatementKind::Assign(box (lhs, ref rhs)) = stmt.kind
149            && let Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)) | Rvalue::CopyForDeref(rhs) =
150                *rhs
151            && lhs == rhs
152        {
153            stmt.make_nop();
154        }
155    }
156}
rustc_mir_transform/copy_prop.rs

rustc_mir_transform/
copy_prop.rs
