[PATCH] D96418: [clang] Refactor mustprogress handling, add it to all loops in c++11+.

[Johannes Doerfert via Phabricator via cfe-commits]

jdoerfert added a comment.

In D96418#2783541 <https://reviews.llvm.org/D96418#2783541>, @leonardchan wrote:

>> Well, no, I'm afraid it is actually clear that that code does have UB according to the C++ standard.  Perhaps you mean that it *shouldn't* have UB, or that Clang should define its behavior despite the standard.
>>
>> I might agree with you that I don't see the value in using this stronger rule in C++, but I think it would help to understand the problem a little better.  I assume this is causing problems for a less trivial test case?  Or do you really have code that's relying on that loop not terminating?
>
> I see. I guess it's good (and bad) that this discovered this UB in our code base. The example I posted is almost exactly what's in our test case. For this particular one, we need a non-terminating thread, so a thread_create function is passed `do_nothing`. Locally, we could get around this by using something like inline assembly to avoid UB.

FWIW, you want an atomic access, something that is explicitly avoiding the UB while "busy waiting". Not just some inline assembly that tries to "hide the fact" it's UB.


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