[Itanium ABI] If we can make __cxa_end_catch calls nounwind conditionally

[ChuanqiXu9]

This was discussed in https://reviews.llvm.org/D108905.

The simple background is:

void bar();
void foo() {
    try {
        bar();
    } catch(...) {}
}

From the perspective of programmers, foo() should be clearly noexcept (although this is not completely correct). And the root cause here is that __cxa_end_catch is not unwind.

In D108905 we want to make __cxa_end_catch calls nounwind unconditionally. But it gets blocked since the behavior is wrong in certain cases if the destroying exception can throw an exception: https://godbolt.org/z/PYnj85Pdq. Like the case shows, foo() in the above case is not 100% noexcept. So the current behavior is correct.

However, we know it is really rare that the destroying exception can throw and this problem is relatively important in C++ coroutines. Since C++ coroutines are nearly wrapped in a big try catch generated by compiler:

try {
   coroutine function body
} catch (...) {
   promise.unhandled_exception()
}
final_suspend: // final suspend is guaranteed to not throw

So if we can make __cxa_end_catch as nounwind, we can make many coroutines as noexcept automatically.

Given the above two reasons (the exceptional case is rare and it is helpful for a lot of functions), I think it makes sense to make __cxa_end_catch calls nounwind conditionally (e.g., by a compilation flag) at least.

[llvmbot]

@llvm/issue-subscribers-c-1

[yuanfang-chen]

@rjmccall

[rjmccall]

Conditional on what? Honestly, what you've written comes across as "it would be really convenient if this property were true, but it isn't, but maybe we can get our optimization potential back by just ignoring that".

I agree that it's really bad that catch (...) {} can throw, but I think you need to take it up with the C++ committee.

[ChuanqiXu9]

Yeah, it is best to get a consensus with the C++ committee. But due to it may break existing codes, it may be a long time or not approved. What I imaged is to add a flag -fno-throw-destructor or -fno-throw-exception-destructor. So we can solve the problem and not violate the standard language.

[mclow]

try  { foo (); }
catch (...) {
    log("exception thrown by Foo");
    throw;
}

[ChuanqiXu9]

try { foo (); }
catch (...) {
log("exception thrown by Foo");
throw;
}

May I ask the intention?

[mclow]

To log the fact that foo exited via an exception, and then continue processing the exception elsewhere.

[ChuanqiXu9]

To log the fact that foo exited via an exception, and then continue processing the exception elsewhere.

I still don't get your point. Do you mean https://godbolt.org/z/PYnj85Pdq?

[rjmccall]

Marshall, I think you're missing Chuanqi's point. Of course it's possible to throw an exception out of the body of a catch (...) clause, either by rethrowing the caught exception or just throwing a new exception. Chuanqi's complaint arises from the fact that merely exiting a catch (...) clause can throw because of the possibility that you've caught an exception type with a throwing destructor.

Chuanqi, the targeted flag is an interesting idea. We could also emit an error if you try to throw an exception with a throwing destructor, just to make the dialect as self-consistent as possible. (I think we'd want to allow catching such types with an explicit clause, to allow programmers to work around problems if they run into them because of separate compilation.)

[ChuanqiXu9]

Thanks for the suggestion. I'll try to make it.

[smeenai]

We would appreciate such an option as well. I ran across this recently while looking into exception metadata size, and found it surprising given that C++11 make destructors implicitly noexcept. I've read through all the discussions and understand the rationale now, but an option to opt-in to this behavior would be useful while the standardization aspects are being sorted out.