charon_lib/transform/
remove_arithmetic_overflow_checks.rs

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//! # Micro-pass: remove the overflow checks for arithmetic operations we couldn't remove in
//! [`remove_dynamic_checks`]. See comments there for more details.
use crate::transform::TransformCtx;
use crate::ullbc_ast::*;

use super::ctx::UllbcPass;

pub struct Transform;

impl Transform {
    /// After `remove_dynamic_checks`, the only remaining dynamic checks are overflow checks. We
    /// couldn't remove them in ullbc because the generated code spans two basic blocks. They are
    /// inserted only in constants since we otherwise compile in release mode. These assertions
    /// look like:
    /// ```text
    /// r := x checked.+ y;
    /// assert(move r.1 == false);
    /// z := move r.0;
    /// ```
    /// We replace that with:
    /// ```text
    /// z := x + y;
    /// ```
    fn update_statements(seq: &mut [Statement]) {
        if let [Statement {
            content:
                RawStatement::Assign(
                    binop,
                    Rvalue::BinaryOp(
                        op @ (BinOp::CheckedAdd | BinOp::CheckedSub | BinOp::CheckedMul),
                        _,
                        _,
                    ),
                ),
            ..
        }, Statement {
            content:
                RawStatement::Assert(Assert {
                    cond: Operand::Move(assert_cond),
                    expected: false,
                }),
            ..
        }, Statement {
            content: RawStatement::Assign(final_value, Rvalue::Use(Operand::Move(assigned))),
            ..
        }, ..] = seq
        {
            // assigned should be: binop.0
            // assert_cond should be: binop.1
            if let Some((sub0, ProjectionElem::Field(FieldProjKind::Tuple(..), fid0))) =
                assigned.as_projection()
                && fid0.index() == 0
                && let Some((sub1, ProjectionElem::Field(FieldProjKind::Tuple(..), fid1))) =
                    assert_cond.as_projection()
                && fid1.index() == 1
                && binop.is_local()
                && *sub0 == *binop
                && *sub1 == *binop
            {
                // Switch to the unchecked operation.
                *op = match op {
                    BinOp::CheckedAdd => BinOp::Add,
                    BinOp::CheckedSub => BinOp::Sub,
                    BinOp::CheckedMul => BinOp::Mul,
                    _ => unreachable!(),
                };
                // Assign to the correct value in the first statement.
                std::mem::swap(binop, final_value);
                // Remove the other two statements.
                seq[1].content = RawStatement::Nop;
                seq[2].content = RawStatement::Nop;
            }
        }
    }
}

impl UllbcPass for Transform {
    fn transform_body(&self, _ctx: &mut TransformCtx, b: &mut ExprBody) {
        b.transform_sequences(|_, seq| {
            Transform::update_statements(seq);
            Vec::new()
        })
    }
}