It can completely break applications. Particularly those that require a particular level of floating point precision (in which case a fixed precision library is

It can completely break applications. Particularly those that require a particular level of floating point precision (in which case a fixed precision library is actually a better choice and those that use abundant aliasing (which plenty of modern static analyzers can help with). But for the most part I've been using -Ofast for years in a wide variety of applications with no issues.