Compiler panic when a slice is modified in multiple if statements.

[pventuzelo]

Aim

We (@FuzzingLabs) found an internal compiler error that occurs when a slice is modified in multiple if statements.

Expected Behavior

The compiler should not panic and handle the valid code properly.

Bug

The panic occurs when a slice is modified in two or more if statements, while these if statements make a comparison with the slice. As you can see in the code comments, every value and type has no impact.
The code attempted to be as minimalist as possible to highlight the source of the error, but it is still challenging to identify the reason without a thorough understanding of the compiler itself.

The error occurs with the latest compiler version of the main branch. The error message is as follows:

The application panicked (crashed).
Message:  ICE: load in merger should have store from outer block
Location: compiler/noirc_evaluator/src/ssa/opt/flatten_cfg/value_merger.rs:259

The panic comes from the line 259 of this file: https://github.com/noir-lang/noir/blob/master/acvm-repo/acir_field/src/generic_ark.rs

To Reproduce

1. Create a Noir project with the following command: nargo new test_issue
2. Paste the code below into the file main.nr
3. Compile the project using the following command: nargo compile

fn main() {

    // The compiler panics with this code.
    // The type, size, and value of the slice don't matter.
    // Indexes and values in the rest of the code don't matter as well.
    let mut slice1: [u32] = [1, 2, 3, 4];
    if slice1[0] == 3 {
        slice1[1] = 4;
    }

    if slice1[1] == 5 {
        slice1[3] = 6;
    }

    // The panic seems to only occur if the slice is modified in two different if statements that include a comparison on the slice.
    // For example, these codes don't panic:

    // Only one if statement
    let mut slice2: [u32] = [1, 2, 3, 4];
    if slice2[0] == 3 {
        slice2[1] = 4;
    } else {
        slice2[3] = 6;
    }

    // Only one slice modification
    let mut slice3: [u32] = [1, 2, 3, 4];
    if slice3[0] == 3 {
        slice3[1] = 4;
    }

    if slice3[1] == 5 {
        // It's a random instruction
        assert(slice3[3] == 6);
    }

    // if statement with no comparison on the slice
    let mut slice1: [u32] = [1, 2, 3, 4];
    if 1 == 3 {
        slice1[1] = 4;
    }

    if slice1[1] == 5 {
        slice1[3] = 6;
    }
}

Installation Method

None

Nargo Version

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response