> No. This is what I call the "portable assembler"-understanding of undefined behavior and it is entirely false. Clang does not need to pretend that i is initialized with a value larger than 10, there is no requirement in the C standard that undefined behavior has to be explainable by looking at it like a portable assembler. Clang is free to produce whatever output it wants because the behavior of that code is literally _undefined_.

You're kind of lawyering this. Sure, it's "undefined", but is that useful to anyone outside of compiler writers? How useful is it to have a program that's very fast but entirely wrong? If the behavior is undefined, I want an error, not a free license for the compiler to do whatever the hell it wants.