charon_lib/transform/
index_to_function_calls.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
//! Desugar array/slice index operations to function calls.

use derive_visitor::{DriveMut, VisitorMut};

use crate::llbc_ast::*;
use crate::transform::TransformCtx;

use super::ctx::LlbcPass;

/// Visitor to transform the operands by introducing intermediate let
/// statements.
///
/// We explore the statements without diving into substatements, and in particular explore
/// the places and operands. Places always appear as destinations we are writing to.
/// While we explore the places/operands present in a statement, We temporarily
/// store the new statements inside the visitor. Once we've finished exploring
/// the statement, we insert those before the statement.
#[derive(VisitorMut)]
#[visitor(Place(exit), Operand, Call, FnOperand, Rvalue)]
struct Visitor<'a> {
    locals: &'a mut Locals,
    statements: Vec<Statement>,
    // When we encounter a place, we remember when a given place is accessed mutably in this
    // stack. Unfortunately this requires us to be very careful to catch all the cases where we
    // see places.
    place_mutability_stack: Vec<bool>,
    // Span information of the statement
    span: Span,
}

impl<'a> Visitor<'a> {
    fn fresh_var(&mut self, name: Option<String>, ty: Ty) -> Place {
        self.locals.new_var(name, ty)
    }

    fn transform_place(&mut self, mut_access: bool, place: &mut Place) {
        use ProjectionElem::*;
        let Some((subplace, pe @ (Index { .. } | Subslice { .. }))) = place.as_projection() else {
            return;
        };
        let TyKind::Adt(TypeId::Builtin(builtin_ty), generics) = subplace.ty().kind() else {
            unreachable!()
        };

        // The built-in function to call.
        let indexing_function = {
            let builtin_fun = BuiltinFunId::Index(BuiltinIndexOp {
                is_array: matches!(builtin_ty, BuiltinTy::Array),
                mutability: RefKind::mutable(mut_access),
                is_range: matches!(pe, Subslice { .. }),
            });
            // Same generics as the array/slice type, except for the extra lifetime.
            let generics = GenericArgs {
                regions: vec![Region::Erased].into(),
                ..generics.clone()
            };
            FnOperand::Regular(FnPtr {
                func: FunIdOrTraitMethodRef::mk_builtin(builtin_fun),
                generics,
            })
        };

        let input_ty = TyKind::Ref(
            Region::Erased,
            subplace.ty().clone(),
            RefKind::mutable(mut_access),
        )
        .into_ty();

        let elem_ty = generics.types[0].clone();
        let output_inner_ty = if matches!(pe, Index { .. }) {
            elem_ty
        } else {
            TyKind::Adt(
                TypeId::Builtin(BuiltinTy::Slice),
                GenericArgs::new_from_types(vec![elem_ty].into()),
            )
            .into_ty()
        };
        let output_ty = {
            TyKind::Ref(
                Region::Erased,
                output_inner_ty.clone(),
                RefKind::mutable(mut_access),
            )
            .into_ty()
        };

        // Push the statement:
        //`tmp0 = &{mut}p`
        let input_var = {
            let input_var = self.fresh_var(None, input_ty);
            let kind = RawStatement::Assign(
                input_var.clone(),
                Rvalue::Ref(subplace.clone(), BorrowKind::mutable(mut_access)),
            );
            self.statements.push(Statement::new(self.span, kind));
            input_var
        };

        // Construct the arguments to pass to the indexing function.
        let mut args = vec![Operand::Move(input_var)];
        if let Subslice { from, .. } = &pe {
            args.push(from.as_ref().clone());
        }
        let (last_arg, from_end) = match &pe {
            Index {
                offset: x,
                from_end,
                ..
            }
            | Subslice {
                to: x, from_end, ..
            } => (x.as_ref().clone(), *from_end),
            _ => unreachable!(),
        };
        if from_end {
            let usize_ty = TyKind::Literal(LiteralTy::Integer(IntegerTy::Usize)).into_ty();
            let len_var = self.fresh_var(None, usize_ty.clone());
            let kind = RawStatement::Assign(
                len_var.clone(),
                Rvalue::Len(
                    subplace.clone(),
                    subplace.ty().clone(),
                    generics.const_generics.get(0.into()).cloned(),
                ),
            );
            self.statements.push(Statement::new(self.span, kind));
            // `index_var = len(p) - last_arg`
            let index_var = self.fresh_var(None, usize_ty);
            let kind = RawStatement::Assign(
                index_var.clone(),
                Rvalue::BinaryOp(BinOp::Sub, Operand::Copy(len_var), last_arg),
            );
            self.statements.push(Statement::new(self.span, kind));
            args.push(Operand::Copy(index_var));
        } else {
            args.push(last_arg);
        }

        // Call the indexing function:
        // `tmp1 = {Array,Slice}{Mut,Shared}{Index,SubSlice}(move tmp0, <other args>)`
        let output_var = {
            let output_var = self.fresh_var(None, output_ty);
            let index_call = Call {
                func: indexing_function,
                args,
                dest: output_var.clone(),
            };
            let kind = RawStatement::Call(index_call);
            self.statements.push(Statement::new(self.span, kind));
            output_var
        };

        // Update the place.
        *place = output_var.project(ProjectionElem::Deref, output_inner_ty);
    }
}

/// The visitor methods.
impl<'a> Visitor<'a> {
    /// We explore places from the inside-out.
    fn exit_place(&mut self, place: &mut Place) {
        // We intercept every traversal that would reach a place and push the correct mutability on
        // the stack.
        let mut_access = *self.place_mutability_stack.last().unwrap();
        self.transform_place(mut_access, place);
    }

    fn enter_operand(&mut self, op: &mut Operand) {
        match op {
            Operand::Move(_) => {
                self.place_mutability_stack.push(true);
            }
            Operand::Copy(_) => {
                self.place_mutability_stack.push(false);
            }
            Operand::Const(..) => {}
        }
    }

    fn exit_operand(&mut self, op: &mut Operand) {
        match op {
            Operand::Move(_) | Operand::Copy(_) => {
                self.place_mutability_stack.pop();
            }
            Operand::Const(..) => {}
        }
    }

    fn enter_call(&mut self, _c: &mut Call) {
        self.place_mutability_stack.push(true);
    }

    fn exit_call(&mut self, _c: &mut Call) {
        self.place_mutability_stack.pop();
    }

    fn enter_fn_operand(&mut self, fn_op: &mut FnOperand) {
        match fn_op {
            FnOperand::Regular(_) => {}
            FnOperand::Move(_) => {
                self.place_mutability_stack.push(true);
            }
        }
    }

    fn exit_fn_operand(&mut self, fn_op: &mut FnOperand) {
        match fn_op {
            FnOperand::Regular(_) => {}
            FnOperand::Move(_) => {
                self.place_mutability_stack.pop();
            }
        }
    }

    fn enter_rvalue(&mut self, rv: &mut Rvalue) {
        use Rvalue::*;
        match rv {
            Use(_) | NullaryOp(..) | UnaryOp(..) | BinaryOp(..) | Aggregate(..) | Global(..)
            | GlobalRef(..) | Repeat(..) | ShallowInitBox(..) => {}
            RawPtr(_, ptrkind) => match *ptrkind {
                RefKind::Mut => {
                    self.place_mutability_stack.push(true);
                }
                RefKind::Shared => {
                    self.place_mutability_stack.push(false);
                }
            },
            Ref(_, bkind) => match *bkind {
                // `UniqueImmutable` de facto gives mutable access and only shows up if there is
                // nested mutable access.
                BorrowKind::Mut | BorrowKind::TwoPhaseMut | BorrowKind::UniqueImmutable => {
                    self.place_mutability_stack.push(true);
                }
                BorrowKind::Shared | BorrowKind::Shallow => {
                    self.place_mutability_stack.push(false);
                }
            },
            Discriminant(..) | Len(..) => {
                // We access places, but those places are used to access
                // elements without mutating them
                self.place_mutability_stack.push(false);
            }
        }
    }

    fn exit_rvalue(&mut self, rv: &mut Rvalue) {
        use Rvalue::*;
        match rv {
            Use(_) | NullaryOp(..) | UnaryOp(..) | BinaryOp(..) | Aggregate(..) | Global(..)
            | GlobalRef(..) | Repeat(..) | ShallowInitBox(..) => {}
            RawPtr(..) | Ref(..) | Discriminant(..) | Len(..) => {
                self.place_mutability_stack.pop();
            }
        }
    }
}

pub struct Transform;

/// We do the following.
///
/// If `p` is a projection (for instance: `var`, `*var`, `var.f`, etc.), we
/// detect:
/// - whether it operates on a slice or an array (we keep track of the types)
/// - whether the access might mutate the value or not (it is
///   the case if it is in a `move`, `&mut` or at the lhs of an assignment),
///   and do the following transformations
///
/// ```text
///   // If array and mutable access:
///   ... p[i] ...
///      ~~>
///   tmp0 = &mut p
///   tmp1 = ArrayIndexMut(move p, i)
///   ... *tmp1 ...
///
///   // If array and non-mutable access:
///   ... p[i] ...
///      ~~>
///   tmp0 := & p
///   tmp1 := ArrayIndexShared(move tmp0, i)
///   ... *tmp1 ...
///
///   // Omitting the slice cases, which are similar
/// ```
///
/// For instance, it leads to the following transformations:
/// ```text
///   // x : [u32; N]
///   y : u32 = copy x[i]
///      ~~>
///   tmp0 : & [u32; N] := &x
///   tmp1 : &u32 = ArrayIndexShared(move tmp0, i)
///   y : u32 = copy (*tmp1)
///
///   // x : &[T; N]
///   y : &T = & (*x)[i]
///      ~~>
///   tmp0 : & [T; N] := & (*x)
///   tmp1 : &T = ArrayIndexShared(move tmp0, i)
///   y : &T = & (*tmp1)
///
///   // x : [u32; N]
///   y = &mut x[i]
///      ~~>
///   tmp0 : &mut [u32; N] := &mut x
///   tmp1 : &mut u32 := ArrayIndexMut(move tmp0, i)
///   y = &mut (*tmp)
///
///   // When using an index on the lhs:
///   // y : [T; N]
///   y[i] = x
///      ~~>
///   tmp0 : &mut [T; N] := &mut y;
///   tmp1 : &mut T = ArrayIndexMut(move y, i)
///   *tmp1 = x
/// ```
impl LlbcPass for Transform {
    fn transform_body(&self, _ctx: &mut TransformCtx<'_>, b: &mut ExprBody) {
        b.body.transform(&mut |st: &mut Statement| {
            let mut visitor = Visitor {
                locals: &mut b.locals,
                statements: Vec::new(),
                place_mutability_stack: Vec::new(),
                span: st.span,
            };

            // We don't explore sub-statements.
            use llbc_ast::Switch::*;
            use RawStatement::*;
            match &mut st.content {
                Loop(..) => {}
                Switch(If(op, ..) | SwitchInt(op, ..)) => op.drive_mut(&mut visitor),
                Switch(Match(place, ..)) => {
                    visitor.place_mutability_stack.push(false); // Unsure why we do this
                    place.drive_mut(&mut visitor)
                }
                Abort(..) | Return | Break(..) | Continue(..) | Nop | Error(..) | Assert(..)
                | Call(..) => {
                    st.drive_mut(&mut visitor);
                }
                FakeRead(place) => {
                    visitor.place_mutability_stack.push(false);
                    place.drive_mut(&mut visitor);
                }
                Assign(..) | SetDiscriminant(..) | Drop(..) => {
                    visitor.place_mutability_stack.push(true);
                    st.drive_mut(&mut visitor);
                }
            }
            visitor.statements
        });
    }
}