That word is carrying a lot of weight here. Compilers are unbelievably complex these days, and it's impossible for any one human to fully understand the entire compilation process, including the effects of any arbitrary combination of compiler flags.

Any assumptions you have about what the compiler does in the face of UB will collapse on the next patch release of that compiler, or the moment somebody changes the compiler flags, or the moment somebody tries to compile the code for a slightly different OS, not to mention architecture.

There is no other way to understand what C compilers do than reading the standard.