alt Hacker NewsHey! Author here :)
So let me start by saying that that blog post was written was 15 years ago and I don't even remember the details of it and what I've written there. But, I have a hot-take on this topic you've touched on!
From a programmer perspective, you are absolutely right. The behaviour is undefined, end of discussion. A programmer should never rely on what they observe as the effective behaviour of an UB. A programmer must avoid creating situations in code that could result in the execution flow venturing into the areas of UB. And - per C and C++ standards - results of UB can be anything (insert the old joke about UB formatting one's disk being a formally correct behaviour).
However, I'm a security researcher, and from the security point of view - especially on the offensive side - we need to know and understand the effective behaviours of UBs. This is because basically all "low-level" vulnerabilities in C/C++ are formally effects of UBs. As such, for the security crowd, it still makes sense to investigate, understand, and discuss the actual observed effects of UBs, especially why a compiler does this, what are the real-world actual variants of generated code (if any) for a given UB for this and other compilers, how can this be abused and exploited, and so on.
My point being - there are two sides to this coin.
Agreed.
As a programmer, the solution to "int a = 5; a = a++ + ++a;" is to decide what you result you wanted, and write code that will produce that result, and probably to pass options to the compiler that tell it to detect this kind of problem and print a warning. (On my system, the result happens to be 12; if that's what I want, I'll write "int a = 12;").
But if you have an existing program that includes that code, it can be useful to look into the actual behavior (for all the compilers that might be used to compile the code, with all possible options, on all possible target systems). Fixing the code should be part of that process, but you might still have running systems with the old bad code, and you need to understand the risks.
But producing some numeric result is not the only possible behavior, even in real life. Compilers can assume that the code being compiled does not have undefined behavior, and generate code based on that assumption. The results can be surprising.
As for formatting your disk, that's not just a theoretical risk. If a program has enough privileges that it can format your disk deliberately, it's possible that it could do so accidentally due to undefined behavior (for example, if a function pointer is corrupted).