Chapel: Should we keep compareExchange and compareExchangeStrong?

[ronawho]

We implement compareExchange(), which is just an alias of compareExchangeStrong(). It seems odd to me to have both, and I propose we deprecate one of them. I don't have a strong preference on which one we deprecate. C/C++ have compare_exchange_weak() and compare_exchange_strong(). Rust went with compare_exchange_weak() and compare_exchange()

For some additional context compare_exchange_weak() can spuriously fail, and is intended to be used in situations where you'd be looping anyways as it can offer better performance on some platforms. https://stackoverflow.com/questions/25199838/understanding-stdatomiccompare-exchange-weak-in-c11 has a pretty good explanation of weak vs. strong CAS.

[mppf]

compareExchange stong should be the "default" since compareExchangeWeak is an optimization. I think deprecating compareExchangeStrong is reasonable.

[bradcray]

I feel inclined to keep compareExchangeStrong() so that someone coming from C++ will find the familiar name easily and to keep compareExchange() for convenience. I might also consider adding a param strong = true argument to the compareExchange() overload so that (1) someone could switch between weak and strong via a config param or somesuch without having to have a param conditional that called one of two functions (i.e., for brevity) and (2) so that there's slightly more reason for both compareExchange() and compareExchangeStrong() to exist (i.e., it's not just the same function twice with two different names).

[ronawho]

I think my preference is to just have compareExchange() and compareExchangeWeak().

Adding compareExchangeStrong() just to match C++ doesn't seem very compelling to me since we're already different with our atomic read()/write() (called load()/store() in C/C++).

Plus it wouldn't be a breaking change to add compareExchangeStrong() later if there is demand for it.

[ronawho]

FWIW Rust went with compare_exchange and compare_exchange_weak

[mppf]

I feel inclined to keep compareExchangeStrong() so that someone coming from C++ will find the familiar name easily and to keep compareExchange() for convenience.

Suppose we did that. Suppose such a user coming from C++ is reading some Chapel code. That Chapel code might use compareExchange (which we are supposing exists only for convenience). In this case there is no benefit to that user to keeping compareExchangeStrong, right? At that point isn't it just a matter of writing something they can find with grep/search in the documentation?

[ronawho]

Vote for compareExchange() (and compareExchangeWeak())

[ronawho]

Vote for compareExchange() and compareExchangeStrong() (and compareExchangeWeak())

[ronawho]

Vote for compareExchange() (and compareExchangeWeak())

This is what's on master with https://github.com/chapel-lang/chapel/pull/14883.

@bradcray you had originally been in favor of having a compareExchangeStrong() -- do you still feel that way?

[bradcray]

Not enough to fight against this tide. I still find the notion of a param arg on compareExchange() intriguing, but not enough to fight for that either (and it can always be added later if someone wants to argue for it).

[ronawho]

Ok, thanks

Since compareExchange() and compareExchangeWeak() are on master, and nobody is currently fighting for compareExchangeStrong() I'm going to close this.