charon_lib/transform/
remove_unit_locals.rs

1use derive_generic_visitor::*;
2use std::collections::HashSet;
3
4use crate::transform::TransformCtx;
5use crate::ullbc_ast::*;
6
7use super::ctx::UllbcPass;
8
9pub struct Transform;
10impl UllbcPass for Transform {
11    fn transform_body(&self, _ctx: &mut TransformCtx, body: &mut ExprBody) {
12        // Replace any copy/move of a unit local to a plain const assignment. Note: we don't touch
13        // other `Rvalue`s as they might have side-effects (e.g. reading through a pointer).
14        body.visit_statements(|st| {
15            if let RawStatement::Assign(_, rvalue) = &mut st.content
16                && let Rvalue::Use(Operand::Move(from) | Operand::Copy(from)) = rvalue
17                && from.is_local()
18                && from.ty().is_unit()
19            {
20                *rvalue = Rvalue::unit_value()
21            }
22        });
23
24        // Find the unused locals of unit type.
25        #[derive(Visitor)]
26        struct UnitLocalsVisitor {
27            unused_unit_locals: HashSet<LocalId>,
28        }
29        impl VisitBody for UnitLocalsVisitor {
30            fn enter_place(&mut self, x: &Place) {
31                if let Some(var_id) = x.as_local() {
32                    self.unused_unit_locals.remove(&var_id);
33                }
34            }
35            fn visit_ullbc_statement(
36                &mut self,
37                x: &ullbc_ast::Statement,
38            ) -> ControlFlow<Self::Break> {
39                match &x.content {
40                    RawStatement::Assign(place, rvalue) => {
41                        if place.is_local() && place.ty().is_unit() {
42                            // Don't count the assignment as a use.
43                        } else {
44                            self.visit(place)?;
45                        }
46                        self.visit(rvalue)?;
47                    }
48                    _ => self.visit_inner(x)?,
49                }
50                Continue(())
51            }
52        }
53        let unused_unit_locals = (UnitLocalsVisitor {
54            unused_unit_locals: body
55                .locals
56                .non_argument_locals()
57                .filter(|(_, var)| var.ty.is_unit())
58                .map(|(id, _)| id)
59                .collect(),
60        })
61        .visit_by_val_infallible(&*body)
62        .unused_unit_locals;
63
64        // Remove side-effect-free assignments into unused places.
65        body.visit_statements(|st| {
66            if let RawStatement::Assign(place, rvalue) = &st.content
67                && let Some(var_id) = place.as_local()
68                && unused_unit_locals.contains(&var_id)
69                && rvalue.is_aggregate()
70            {
71                st.content = RawStatement::Nop;
72            }
73        });
74    }
75}
charon_lib/transform/remove_unit_locals.rs

charon_lib/transform/
remove_unit_locals.rs
