# Will Julia ever fix its "using ..." latency problems?

**URL:** <https://discourse.julialang.org/t/will-julia-ever-fix-its-using-latency-problems/50659>\
**Category:** Performance\
**Created:** [November 23, 2020, 11:57pm UTC](https://discourse.julialang.org/t/will-julia-ever-fix-its-using-latency-problems/50659 "2020-11-23T23:57:55Z")\
**Posts on this page:** 1\
**Showing post:** 92

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**Author:** ![FedericoStra](https://avatars.discourse-cdn.com/v4/letter/f/76d3ee/32.png) [@FedericoStra](https://discourse.julialang.org/u/FedericoStra)\
**Post date:** [November 26, 2020, 7:26pm UTC](https://discourse.julialang.org/t/will-julia-ever-fix-its-using-latency-problems/50659/92 "2020-11-26T19:26:57Z")

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> [@Henrique\_Becker](#):
>
> The basic idea is: […], I can try my luck […] It is a luck/chance/probability/bayesian thing. You have the possibility of play dice with the compiler and get some not-entirely-trustworthy information.

I get what you mean, and I agree that it might be “sufficient” in some circumstances, but please consider that this might affect some tests that silently pass instead of failing. I don’t particularly like this chance-based programming, and I would not like to set foot on an airplane whose [fly-by-wire](https://en.wikipedia.org/wiki/Fly-by-wire) system has been tested this way.

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> [@Henrique\_Becker](#):
>
> I am not sure if the possible changes in behavior are restricted to just to that one.

No no, definitely that one is not the only possiblility: it was just an example of an alternative to full UB.

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> [@Henrique\_Becker](#):
>
> In particular, I am not sure if changing the `const` variable to an object of another type will always error, or it may accept it and end up with a segmentation fault when the variable is referenced, as the object address may not be inlined but the type information can be, for example.

This would be particularly nasty. If reassigning to a `const` is UB (especially if the type is different), I don’t see any reason not to make it a hard error instead, since anything that follows it would be completely meaningless (actually, in C, UB is “retroactive” too, so also what _precedes_ the invalid operation becomes meaningless: see [this example](https://godbolt.org/z/5soe1r)).

* * *

In any case, I’ve opened [this issue](https://github.com/JuliaLang/julia/issues/38584), where I’m not expecting plenty of spam regarding what UB is or isn’t (as witnessed in this thread), but rather a clarifying answer from the core developers.

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