charon_lib/transform/remove_dynamic_checks.rs
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//! # Micro-pass: remove the dynamic checks for array/slice bounds, overflow, and division by zero.
//! Note that from a semantic point of view, an out-of-bound access or a division by zero
//! must lead to a panic in Rust (which is why those checks are always present, even when
//! compiling for release). In our case, we take this into account in the semantics of our
//! array/slice manipulation and arithmetic functions, on the verification side.
use crate::ast::*;
use crate::transform::TransformCtx;
use crate::ullbc_ast::{ExprBody, RawStatement, Statement};
use super::ctx::UllbcPass;
/// Rustc inserts dybnamic checks during MIR lowering. They all end in an `Assert` statement (and
/// this is the only use of this statement).
fn remove_dynamic_checks(_ctx: &mut TransformCtx, statements: &mut [Statement]) {
// We return the statements we want to keep, which must be a prefix of `block.statements`.
let statements_to_keep = match statements {
// Bounds checks for arrays/slices. They look like:
// l := len(a)
// b := copy x < copy l
// assert(move b == true)
[Statement {
content: RawStatement::Assign(len, Rvalue::Len(..)),
..
}, Statement {
content:
RawStatement::Assign(
is_in_bounds,
Rvalue::BinaryOp(BinOp::Lt, _, Operand::Copy(lt_op2)),
),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, rest @ ..]
if lt_op2 == len && cond == is_in_bounds && *expected == true =>
{
rest
}
// Zero checks for division and remainder. They look like:
// b := copy x == const 0
// assert(move b == false)
[Statement {
content:
RawStatement::Assign(is_zero, Rvalue::BinaryOp(BinOp::Eq, _, Operand::Const(_zero))),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, rest @ ..]
if cond == is_zero && *expected == false =>
{
rest
}
// Overflow checks for signed division and remainder. They look like:
// is_neg_1 := y == (-1)
// is_min := x == INT::min
// has_overflow := move (is_neg_1) & move (is_min)
// assert(move has_overflow == false)
[Statement {
content: RawStatement::Assign(is_neg_1, Rvalue::BinaryOp(BinOp::Eq, _y_op, _minus_1)),
..
}, Statement {
content: RawStatement::Assign(is_min, Rvalue::BinaryOp(BinOp::Eq, _x_op, _int_min)),
..
}, Statement {
content:
RawStatement::Assign(
has_overflow,
Rvalue::BinaryOp(BinOp::BitAnd, Operand::Move(and_op1), Operand::Move(and_op2)),
),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, rest @ ..]
if and_op1 == is_neg_1
&& and_op2 == is_min
&& cond == has_overflow
&& *expected == false =>
{
rest
}
// Overflow checks for right/left shift. They can look like:
// x := ...;
// b := move x < const 32; // or another constant
// assert(move b == true);
[Statement {
content: RawStatement::Assign(x, _),
..
}, Statement {
content:
RawStatement::Assign(
has_overflow,
Rvalue::BinaryOp(BinOp::Lt, Operand::Move(lt_op2), Operand::Const(..)),
),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, rest @ ..]
if lt_op2 == x && cond == has_overflow && *expected == true =>
{
rest
}
// They can also look like:
// b := const c < const 32; // or another constant
// assert(move b == true);
[Statement {
content:
RawStatement::Assign(
has_overflow,
Rvalue::BinaryOp(BinOp::Lt, Operand::Const(..), Operand::Const(..)),
),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, rest @ ..]
if cond == has_overflow && *expected == true =>
{
rest
}
// Overflow checks for addition/subtraction/multiplication. They look like:
// r := x checked.+ y;
// assert(move r.1 == false);
// They only happen in constants because we compile with `-C opt-level=3`. They span two
// blocks so we remove them in a later pass.
[Statement {
content:
RawStatement::Assign(
result,
Rvalue::BinaryOp(BinOp::CheckedAdd | BinOp::CheckedSub | BinOp::CheckedMul, ..),
),
..
}, Statement {
content:
RawStatement::Assert(Assert {
cond: Operand::Move(cond),
expected,
}),
..
}, ..]
if result.is_local()
&& let Some((sub, ProjectionElem::Field(FieldProjKind::Tuple(2), p_id))) =
cond.as_projection()
&& sub.is_local()
&& cond.var_id() == result.var_id()
&& p_id.index() == 1
&& *expected == false =>
{
// We leave this assert intact; it will be simplified in
// [`remove_arithmetic_overflow_checks`].
return;
}
_ => return,
};
// Remove the statements.
let keep_len = statements_to_keep.len();
for i in 0..statements.len() - keep_len {
statements[i].content = RawStatement::Nop;
}
}
pub struct Transform;
impl UllbcPass for Transform {
fn transform_body(&self, ctx: &mut TransformCtx, b: &mut ExprBody) {
for block in b.body.iter_mut() {
block.transform_sequences(|seq| {
remove_dynamic_checks(ctx, seq);
Vec::new()
});
}
}
}