> 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.
> How useful is it to have a program that's very fast but entirely wrong?
This only affects you if your program has undefined behavior, at that point your program was wrong anyway, you were just lucky that the compiler compiled it into something that works. Taking a different compiler or a different target architecture and you might not be lucky. So even the old "portable assembly" kind of undefined behavior is actually the opposite of portable.
> Sure, it's "undefined", but is that useful to anyone outside of compiler writers?
It may surprise you that compiler writers are not evil people that take joy in the suffering of their users. They implement these aggressive optimizations because they actually produce better results for valid programs. It just happens that this coincides with more "absurd" results if the program has undefined behavior. But at that point your program was wrong anyway.
If you absolutely want "portable assembler"-semantics for your C program you can do that by using (very) old C compilers. Your program will not get optimized as aggressively, but "portable assembler"-semantics and optimizations are inconsistent with each other anyway.
Not always - in some cases you need -Wextra or even -Weverything to get a warning. [Edit: was thinking of GCC here, and yesterday's similar thread - clang may be more forthcoming with warnings, I don't know.]
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.