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emmericp | 1 year ago

The time-travelling UB interpretation was popularized by this blog post about 10 years ago [1].

I'm not enough of a specification lawyer to say that this is definitely true, but the reasoning and example given there seems sound to me.

[1] https://devblogs.microsoft.com/oldnewthing/20140627-00/?p=63...

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uecker|1 year ago

Yes, random blog posts did a lot of damage here. Also broken compilers [1]. Note that blog post is correct about C++ but incorrectly assumes this is true for C as well.

[1]. https://developercommunity.visualstudio.com/t/Invalid-optimi...

kibwen|1 year ago

I'm inclined to trust Raymond Chen and John Regehr on these matters, so if you assert that they're incorrect here then a source to back up your assertion would help your argument.

quietbritishjim|1 year ago

> Also broken compilers [1].

The issue you linked to is not a counter example because, as the poster said, g may terminate the program in which case that snippet does not have undefined behaviour even if b is zero. The fact that they bothered to mention that g may terminate the program seems like an acknowledgement that it would be valid to do that time travelling if it didn't.

> Note that blog post is correct about C++ but incorrectly assumes this is true for C as well.

Presumably you're referring to this line of the C++ standard, which does not appear in the C standard:

> However, if any such execution contains an undefined operation, this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation).

I looked at every instance of the word "undefined" in the C standard and, granted, it definitely didn't have anything quite so clear about time travel as that. But it also didn't make any counter claims that operations before are valid. It pretty much just said that undefined behaviour causes behaviour that is undefined! So, without strong evidence, it seem presumptuous to assume that operations provably before undefined behaviour are well defined.