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by uecker 720 days ago
The poster is me. You are right that this is not an example for time-travel. There aren't really good examples for true time travel because compilers generally do not do this. But my point is that with compilers behaving like this, people might confuse this for time-traveling UB. I have certainly met some who did and the blog posts seems to have similar examples (but I haven't looked closely now).

Note that I am a member of WG14. We added more clarification to C23 to make clear that this is not a valid interpretation of UB, see here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf
2 comments
AlotOfReading 720 days ago
I didn't notice that section when I last read through C23, but I'm very glad to see it. Reining in UB is one of the hardest problems I've had to deal with, and being able to say operations are defined up to the point of UB makes my job so much easier.

The lack of clarity in earlier standards made it impossible to deal with code incrementally, since all the unknown execution paths could potentially breach back in time and smash your semantics.

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uecker 720 days ago
Thank you. This was my motivation. It is only a small step... much more work to do.
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quietbritishjim 720 days ago
Ok, fair enough. I must admit I was looking at C99 as I thought that was most generally relevant, I don't follow recent C standards (as much as I do those for C++) and C23 hasn't been ratified yet. I've found your new snippet:

> In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program.

I consider that a change in the standard but, of course, that's allowed, especially as it's backwards compatible for well defined programs.

The wording is a little odd: it makes it sound a like you need some undefined behaviour in order to make the operations beforehand work, and, taken very literally, that operations between two undefined behaviours will work (because they're still "before an operation with undefined behavior"). But I suppose the intention is clear.

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uecker 720 days ago
The definition of UB (which hasn't changed) is: "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirement."

Note that the "for which" IMHO already makes this clear that this can not travel in time. When everything could be affected these words ("for which") would be meaningless.

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