[aw1621107]

> I argued then that if instead noexcept violations were undefined, we could ignore all this, and instead just treat it as the pure optimization it was being marketed as (ie, help prove a region can't throw, so we can elide entire try/catch blocks etc).

Do you know if the reasoning for originally switching noexcept violations from UB to calling std::terminate was documented anywhere? The corresponding meeting minutes [0] describes the vote to change the behavior but not the reason(s). There's this bit, though:

> [Adamczyk] added that there was strong consensus that this approach did not add call overhead in quality exception handling implementations, and did not restrict optimization unnecessarily.

Did that view not pan out since that meeting?

[0]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n30...

[terrymah]

I think WG21 has been violently against adding additional UB to the language, because of some hacker news articles a decade ago about people being alarmed at null pointer checks being elided or things happening that didn’t match their expectation in signed int overflow or whatever. Generally it seems a view of spread that compiler implementers view undefined behavior as a license to party, that we’re generally having too much fun, and are not to be trusted.

In reality undefined behavior is useful in the sense that (like this case) it allows us to not have to write code to consider and handle certain situations - code which may make all situations slower, or allows certain optimizations to exist which work 99% of the time.

Regarding “not pan out”: I think the overhead of noexcept for the single function call case is fine, and inlining is and has always been the issue.

[aw1621107]

> I think WG21 has been violently against adding additional UB to the language, because of some hacker news articles a decade ago about people being alarmed at null pointer checks being elided or things happening that didn’t match their expectation in signed int overflow or whatever.

Huh, didn't expect the no-UB sentiment to have extended that far back!

> Regarding “not pan out”: I think the overhead of noexcept for the single function call case is fine, and inlining is and has always been the issue.

Do you know if the other major compilers also face similar issues?

[terrymah]

Things are much better in 2024 in MSVC than they were in 2014. The overhead today is mostly the additional metadata associated with tracking the state, and most of the inline compatibilities were worked through (with a ton of work by the compiler devs). So it's a binary size issue. We've even been working on that (I remember doing work to combine adjacent identical regions, etc). Not sure what the status is in GCC/LLVM today.

I'm just a little sore about it because it was being sold as a "hey here is an optimization!" and it very much was not, at least from where I was sitting. I thought this was a very very good case of having it be UB (I think the entire class of user source annotations like this should be UB if the runtime behavior violates the user annotation)

[aw1621107]

Sounds promising!

Do you think optimizations could eventually bring the std::terminate version of noexcept near/up to par with a hypothetical UB noexcept, or do you think that at least some overhead will always be present?

[account42]

Could the UB version of noexcept be provided as a compiler extension? Either a separate attribute or a compiler flag to switch the behavior would be fine.

[terrymah]

We had a UB version of noexcept for a very, very long time. __declspec(nothrow), the throw() function specifier, etc.

[flamedoge]

> did not restrict optimization unnecessarily.

well clearly there is a cost