[zephen]

> I think this is not really true. Or rather, it depends on the UB you are talking about.

I mean, if you're going to argue that a compiler can do anything with any UB, then by all means make that argument.

Otherwise, then no, I don't think it's reasonable for a compiler to cause an infinite loop inside a function simply because that function itself doesn't return a value.

[fluoridation]

When you say "cause", do you mean insert on purpose, or do you mean cause by accident? I could see the latter happening, for example because the compiler doesn't generate a ret if the non-void function doesn't return anything, so control flow falls through to whatever code happens to be next in memory. I'm not aware of any compiler that does that, but it's something I could see happening, and the developers would have no reason to "fix" it, because it's perfectly up to spec.

[zephen]

According to the author of the second link I gave (here it is again):

https://www.quora.com/What-is-the-most-subtle-bug-you-have-h...

The problem was that the loop itself was altered, rather than that the function returned and then that somehow caused an infinite loop.

> I'm not aware of any compiler that does that, but it's something I could see happening, and the developers would have no reason to "fix" it, because it's perfectly up to spec.

This is where we disagree.

[uecker]

I am not sure what statement you are responding to. I am certainly not arguing that. I disagree with your claim that "it is practically impossible find a program without UB".

[zephen]

A study found that, for a particular subset of UB (code that had legal, detectable behavior changes at differing optimization levels), 40% of Debian Wheezy packages exhibited this UB.

https://people.csail.mit.edu/nickolai/papers/wang-stack.pdf

I submit that that's a small fraction of UB, that much of it would exist at any optimization level.

[uecker]

I know, but this still leaves 60% of programs without such UB which is far from "it is practically impossible find a program without UB". Also this this was a study from 2013 and many of those bugs found were fixed. Also GCC got UBSan in 2013 (so after this study).

[pseudohadamard]

That's "UB that was detected in this study". Since gcc will silently break code when it detects UB and you can't tell until you hit that specific case, the 40% is a lower bound. In practice it could be anything up to the full 100%.

[uecker]

In theory. But most C programs do not rely on UB. What is the basis for your claim?

[pseudohadamard]

Uhh... mathematics and logic? Since there's no perfect UB detector, one that detects UB in 40% of programs can only be presenting a lower bound. And I don't know why you think C programs rely on UB, they have it present without the programmer knowing about it.