different ways of creating a default item in a dictionary if doesn't exist

gistfile1.txt

var dict = [String: [String: Int]]()

if dict["foo"] == nil {
    dict["foo"] = [:]
}

dict["foo"]?["bar"] = 5 // safe way to access dictionaries in general

dict["foo"]!["bar"] = 6 // we already tested for nil in the if statement, so force unwrap is safe, and maybe faster?

dict["baz", default: [:]]["boo"] = 7 // avoid if statement completely; no need to first test for "baz" in an if statement

// which of these is best? which has best performance? which is idiomatic swift?