charon_lib/transform/reconstruct_boxes.rs
1//! # Micro-pass: reconstruct piecewise box allocations using `malloc` and `ShallowInitBox`.
2
3use crate::register_error;
4use crate::transform::TransformCtx;
5use crate::ullbc_ast::*;
6
7use super::ctx::UllbcPass;
8
9pub struct Transform;
10
11/// The special `alloc::boxed::box_new(x)` intrinsic becomes the following:
12///
13/// ```text
14/// @2 := size_of<i32>
15/// @3 := align_of<i32>
16/// @4 := alloc::alloc::exchange_malloc(move (@2), move (@3))
17/// storage_live(@5)
18/// @5 := shallow_init_box::<i32>(move (@4))
19/// // possibly some intermediate statements
20/// *(@5) := x
21/// ```
22///
23/// We reconstruct this into a call to `Box::new(x)`.
24impl UllbcPass for Transform {
25 fn transform_body(&self, ctx: &mut TransformCtx, b: &mut ExprBody) {
26 // We need to find a block that has exchange_malloc as the following terminator:
27 // ```text
28 // @4 := alloc::alloc::exchange_malloc(move (@2), move (@3))
29 // ```
30 // We then chekc that that this block ends with two assignments:
31 // ```text
32 // @2 := size_of<i32>
33 // @3 := align_of<i32>
34 // ```
35 // If that is the case, we look at the target block and check that it starts with`
36 // ```text
37 // storage_live(@5)
38 // @5 := shallow_init_box::<i32>(move (@4))
39 // ```
40 // We then look for the assignment into the box and take a not of its index.
41 // ```text
42 // *(@5) := x
43 // ```
44 // Finally, we replace all these assignments with a call to `@5 = Box::new(x)`
45 // We do so by replacing the terminator (exchange_malloc) with the correct call
46 // and replacing the assignment @3 := align_of<i32> with the storage live.
47 // Everything else becomes Nop.
48
49 for candidate_block_idx in b.body.all_indices() {
50 let second_block;
51 let at_5;
52 let box_generics;
53 let value_to_write;
54 let old_assign_idx;
55 let assign_span;
56 let unwind_target;
57
58 if let Some(candidate_block) = b.body.get(candidate_block_idx)
59 // If the terminator is a call
60 && let RawTerminator::Call {
61 target: target_block_idx,
62 call:
63 Call {
64 args: malloc_args,
65 func: _, // TODO: once we have a system to recognize intrinsics, check the call is to exchange_malloc.
66 dest: malloc_dest,
67 },
68 on_unwind,
69 } = &candidate_block.terminator.content
70 // The call has two move arguments
71 && let [Operand::Move(arg0), Operand::Move(arg1)] = malloc_args.as_slice()
72 && let [ .., Statement {
73 content: RawStatement::Assign(size, Rvalue::NullaryOp(NullOp::SizeOf, _)),
74 ..
75 }, Statement {
76 content: RawStatement::Assign(align, Rvalue::NullaryOp(NullOp::AlignOf, _)),
77 ..
78 }] = candidate_block.statements.as_slice()
79 && arg0 == size && arg1 == align
80 && let Some(target_block) = b.body.get(*target_block_idx)
81 && let [Statement {
82 content: RawStatement::StorageLive(target_var),
83 ..
84 }, Statement {
85 content:
86 RawStatement::Assign(box_make, Rvalue::ShallowInitBox(Operand::Move(alloc_use), _)),
87 ..
88 }, rest @ ..] = target_block.statements.as_slice()
89 && alloc_use == malloc_dest
90 && box_make.is_local()
91 && box_make.local_id() == *target_var
92 && let TyKind::Adt(TypeId::Builtin(BuiltinTy::Box), generics) =
93 b.locals[*target_var].ty.kind()
94 && let Some((assign_idx_in_rest, val, span)) = rest.iter().enumerate().find_map(|(idx, st)| {
95 if let Statement {
96 content: RawStatement::Assign(box_deref, val),
97 span,
98 ..
99 } = st
100 && let Some((sub, ProjectionElem::Deref)) = box_deref.as_projection()
101 && sub == box_make
102 {
103 Some((idx, val, span))
104 } else {
105 None
106 }
107 })
108 {
109 at_5 = box_make.clone();
110 old_assign_idx = assign_idx_in_rest + 2; // +2 because rest skips the first two statements
111 value_to_write = val.clone();
112 box_generics = generics.clone();
113 second_block = *target_block_idx;
114 assign_span = *span;
115 unwind_target = *on_unwind;
116 } else {
117 continue;
118 }
119
120 let first_block = b.body.get_mut(candidate_block_idx).unwrap();
121 let number_statements = first_block.statements.len();
122 let value_to_write = match value_to_write {
123 Rvalue::Use(op) => {
124 first_block
125 .statements
126 .get_mut(number_statements - 2)
127 .unwrap()
128 .content = RawStatement::Nop;
129 op
130 }
131 _ => {
132 // We need to create a new variable to store the value.
133 let name = b.locals[at_5.local_id()].name.clone();
134 let ty = box_generics.types[0].clone();
135 let var = b.locals.new_var(name, ty);
136 let st = Statement::new(
137 assign_span,
138 RawStatement::Assign(var.clone(), value_to_write),
139 );
140 // We overide the @2 := size_of<i32> statement with the rvalue assignment
141 *first_block
142 .statements
143 .get_mut(number_statements - 2)
144 .unwrap() = st;
145 Operand::Move(var)
146 }
147 };
148 first_block
149 .statements
150 .get_mut(number_statements - 1)
151 .unwrap()
152 .content = RawStatement::StorageLive(at_5.local_id());
153 first_block.terminator.content = RawTerminator::Call {
154 call: Call {
155 func: FnOperand::Regular(FnPtr {
156 func: Box::new(FunIdOrTraitMethodRef::Fun(FunId::Builtin(
157 BuiltinFunId::BoxNew,
158 ))),
159 generics: Box::new(box_generics),
160 }),
161 args: vec![value_to_write],
162 dest: at_5,
163 },
164 target: second_block,
165 on_unwind: unwind_target,
166 };
167
168 // We now update the statements in the second block.
169 let second_block = b.body.get_mut(second_block).unwrap();
170 second_block.statements.get_mut(0).unwrap().content = RawStatement::Nop;
171 second_block.statements.get_mut(1).unwrap().content = RawStatement::Nop;
172 second_block
173 .statements
174 .get_mut(old_assign_idx)
175 .unwrap()
176 .content = RawStatement::Nop;
177 }
178
179 // Make sure we got all the `ShallowInitBox`es.
180 b.body.dyn_visit_in_body(|rvalue: &Rvalue| {
181 if rvalue.is_shallow_init_box() {
182 register_error!(
183 ctx,
184 b.span,
185 "Could not reconstruct `Box` initialization; \
186 branching during `Box` initialization is not supported."
187 );
188 }
189 });
190 }
191}