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fix(ssa): fix the compile-time check for equality in acir-gen #904

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Feb 24, 2023
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fix(ssa): fix the compile-time check for equality in acir-gen #904

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37 changes: 22 additions & 15 deletions crates/noirc_evaluator/src/ssa/acir_gen/operations/cmp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,11 @@ pub(crate) fn evaluate_neq(
ctx: &SsaContext,
evaluator: &mut Evaluator,
) -> Expression {
// Check whether the `lhs` and `rhs` are trivially equal
if lhs == rhs {
return Expression::zero();
}

// Check whether the `lhs` and `rhs` are Arrays
if let (Some(a), Some(b)) = (Memory::deref(ctx, lhs), Memory::deref(ctx, rhs)) {
let array_a = &ctx.mem[a];
Expand Down Expand Up @@ -64,28 +69,30 @@ pub(crate) fn evaluate_neq(

return Expression::from(&constraints::evaluate_zero_equality(x_witness, evaluator));
}

// Arriving here means that `lhs` and `rhs` are not Arrays
let l_c = l_c.expect("ICE: unexpected array pointer");
let r_c = r_c.expect("ICE: unexpected array pointer");
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// Arriving here means that `lhs` and `rhs` are not Arrays
//
// Check if `lhs` and `rhs` are constants. If so, we can evaluate whether
// they are equal at compile time.
if let (Some(l), Some(r)) = (l_c.to_const(), r_c.to_const()) {
if l == r {
return Expression::default();
} else {
return Expression::one();
}
}
let mut x = InternalVar::from(constraints::subtract(
l_c.expression(),
FieldElement::one(),
r_c.expression(),
));
//todo we need a witness because of the directive, but we should use an expression
let x_witness =
x.get_or_compute_witness(evaluator, false).expect("unexpected constant expression");
Expression::from(&constraints::evaluate_zero_equality(x_witness, evaluator))

// Check if `x` is constant. If so, we can evaluate whether
// it is zero at compile time.
if let Some(x_const) = x.to_const() {
if x_const.is_zero() {
Expression::zero()
} else {
Expression::one()
}
} else {
//todo we need a witness because of the directive, but we should use an expression
let x_witness =
x.get_or_compute_witness(evaluator, false).expect("unexpected constant expression");
Expression::from(&constraints::evaluate_zero_equality(x_witness, evaluator))
}
}

pub(crate) fn evaluate_eq(
Expand Down