FYI. Here is a summary of some time travel information
I collected and my position.


First, here is a quote from Jeff Law, the person who wrote the
optimization that eliminates code paths with UB for GCC:

> Right. This is similar to some of the restrictions we deal with in
> the path isolation pass. Essentially we have a path, when traversed,
> would result in a *0. We would like to be able to find the edge
> upon-which the *0 is control dependent and optimize the test so that
> it always went to the valid path rather than the *0 path.
>
> The problem is there may be observable side effects on the *0 path
> between the test and the actual *0 -- including calls to nonreturning
> functions, setjmp/longjmp, things that could trap, etc. This case is
> similar. We can't back-propagate the non-null status through any
> statements with observable side effects.


Nevertheless, I found this example where GCC reorders a volatile
store with respect to a potentially trapping instruction:

https://godbolt.org/z/5vx9cPfM7

But the comment I got from Richard Biener about this was that
they did not consider volatile stores to be observable.
(Martin Sebor produced a one-line fix for this, but it was
not clear whether such cases should/will be fixed)


MSVC appears to do time-travel:

https://godbolt.org/z/1cd388sPj

But then it does this also when the UB is not guaranteed
to happen, so this seems to be either a compiler bug
or some non-standard compilation mode:

https://godbolt.org/z/8a8fTW8qP


The C standard says:

"behavior, upon use of a nonportable or erroneous program
construct or of erroneous data, for which this document
imposes no requirements"

To me the term "behavior" clearly implies something
happening at some point of time (either at run-time or
compile-time), and consequently "no requirements" only
applies to something at a point of time because the
words "for which" restrict to what this applies.


Martin