[hzhou321]

> Indeed. UB in C doesn't mean "and then the program goes off the rails", it means that the entire program execution was meaningless, and no part of the toolchain is obligated to give any guarantees whatsoever if the program is ever executed, from the very first instruction.

This is the greatest sin modern compiler folks committed to abuse C. C as the language never says the compiler can change the code arbitrarily due to an UB statement. It is undefined. Most UB code in C, while not fully defined, has an obvious part of semantics that every one understands. For example, an integer overflow, while not defined on what should be the final value, it is understood that it is an operation of updating a value. It is definitely not, e.g., an assertion on the operand because UB can't happen.

Think about our natural language, which is full of undefined sentences. For example, "I'll lasso the moon for you". A compiler, which is a listener's brain, may not fully understand the sentence and it is perfectly fine to ignore the sentence. But if we interpret an undefined sentence as a license to misinterpret the entire conversation, then no one would dare to speak.

As computing goes beyond arithmetic and the program grows in complexity, I personally believe some amount of fuzziness is the key. This current narrow view from the compiler folks (and somehow gets accepted at large) is really, IMO, a setback in the computing evolution.

[kllrnohj]

> It is definitely not, e.g., an assertion on the operand because UB can't happen.

C specification says a program is ill-formed if any UB happens. So yes, the spec does say that compilers are allowed to assume UB doesn't happen. After all, a program with UB is ill-formed and therefore shouldn't exist!

I think you're conflating "unspecified behavior" and "undefined behavior" - the two have different meanings in the spec.

[hzhou321]

> C specification says a program is ill-formed if any UB happens. So yes, the spec does say that compilers are allowed to assume UB doesn't happen.

I disagree on the logic from "ill-formed" to "assume it doesn't happen".

> I think you're conflating "unspecified behavior" and "undefined behavior" - the two have different meanings in the spec.

I admit I don't differentiate those two words. I think they are just word-play.

[marssaxman]

> So yes, the spec does say that compilers are allowed to assume UB doesn't happen.

They are allowed to do so, but in practice this choice is not helpful.