The implementation of stdlib necessarily contains code which invokes implementation defined behavior, and the reason that the compiler implementation doesn't tr

The implementation of stdlib necessarily contains code which invokes implementation defined behavior, and the reason that the compiler implementation doesn't treat it in like manner is simply that it doesn't identify it as such or is prevented from doing so. So the treatment of libraries doesn't need to be consistent with that of user programs. Practices that ensure that such code is not optimized away include scattering the information needed to do so across multiple translation units, so that the compiler does not put them together.