Another argument for considering the abstract machine as the primary way to think about programs in a language is that it is very easy to end up with a set of optimizations that all look reasonable in isolation but are inconsistent, and lead to incorrect code when combined. Both GCC and LLVM suffer from this (and in fact MSVC had/has the exact same bug).

GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752

LLVM: https://bugs.llvm.org/show_bug.cgi?id=35229, https://bugs.llvm.org/show_bug.cgi?id=34548