Re: [boost] Non-allocating future-promise

On 17 Oct 2014 at 16:07, Hartmut Kaiser wrote:

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__attribute__((noinline)) int test1() { promise<int> p; auto f(p.get_future()); p.set_value(5); return f.get(); }

... reduces to exactly:

_Z5test1v: # @_Z5test1v .cfi_startproc # BB#0: # %_ZN7promiseIiJEED2Ev.exit2 movl $5, %eax ret

That's cool! I assume all of the necessary synchronization is done lock-free?

Yep. Using my new spinlock library, the one I built the safe erasing concurrent_unordered_map with. I can absolutely guarantee that this sequence always reduces to nothing: basic_promise<int> p; p.set_value(5); auto f(p.get_future()); return f.get(); This is because we can detach the future from the promise in get_future() and therefore skip use of atomics, and it turns out that clang and GCC are clever here and spot the opportunity to reduce. I am currently deciding what to do about this sequence: basic_promise<int> p; auto f(p.get_future()); p.set_value(5); return f.get(); The big problem is that you can't have non-trivial destructors with constexpr which rules that out (we need destructors to zero out the pointer to self in the other side). We cannot switch on atomic use if we wish to retain constexpr reduction, so the default safe choice is: 1. Switch on atomic use in get_future(), so get_future() halts constexpr onwards. 2. We add an atomic_basic_future which turns on atomics like this: basic_promise<int> p; auto f(make_atomic(p.get_future())); // switches on locking // Locks promise, locks future, sets value, detaches relationship, unlocks p.set_value(5); // constexpr return f.get(); The future<T> convenience template alias would therefore always alias to atomic_basic_future<> for compatibility.

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The non-allocating future-promise is intended to be compatible with the Concurrency TS extensions (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4123.html) where futures can have continuations scheduled against them via a future.then(callable). However, this involves an unavoidable memory allocation, which while fine for dynamically assigned continuations is overkill for statically assigned continuations.

There is no reason why one could not also have a method of statically adding continuations which uses type extension to store the additions. Let us say we have a template class promise and that this promise also provides a .then(callable) like this:

FWIW, N4123 adds .then to the future<> type, not the promise<>.

Sure. That's because future.then() executes its continuations at the point of someone doing a future.get(). As promise.then() attaches continuations to the promise instead, it would execute its continuations at the point of someone doing a promise.set_value() or promise.set_exception(). (Some might wonder why not subclass basic_promise<> and override the set_value()/set_exception() with custom behaviour? Unlike promise, basic_promise is designed to be customised and subclassed in a family of future-ish promise-ish types. It's a very valid point, but I worry about generic extension, see below).

I don't think it makes a lot of sense to add it to the promise in the first place as continuations are something local to the consumer, not the producer.

There is a very valid use case for continuations being executed at both places: at the point of value release AND/OR at the point of value acquire. AFIO, for example, schedules its continuations in the thread where an op completes, not in the thread which gets the result of an op (which may be never). I currently have a struct async_io_op containing a shared_future and a routine which wraps set_value() to launch the continuations, I would much rather prefer a native afio::future<> with customised behaviour. The really key part is that all futures belonging to the future family can work together. If I therefore feed to when_all() a heterogenous sequence of future family types, that "just works" because every future turns into a permit object fundamentally. Right now one has to hoop jump a bit mixing when_all() with AFIO because AFIO needs to store extra stuff with each future, hence the struct async_io_op wrapping the future when ideally we'd have the future wrap the extra stuff. However, it is exactly implementing things like when_all() where being able to statically hook arbitrary continuations to either side of the future-promise relationship *from the outside* can make life vastly easier. One simply needs to accept that your basic_future<...> may contain a very long sequence of variadic lambda types, and therefore you can only ever use auto or decltype/result_of for its storage declaration. Obviously if you don't use static continuations, one need not worry about unwritable type declarators.

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template<class F> promise then(F &&c) { return promise(std::move(*this), std::forward<F>(c)); }

... and therefore to statically add continuations to the future promise, one simply iterates .then(callable) on the promise like this:

auto p=promise<int>().then([]).then([]) ...; auto f=p.get_future(); p.set_value(5); // executes all the continuations

My first question to the Boost community is this: is there some less ugly way? I'm thinking maybe the promise's constructor could take a set of continuations, and then a make_promise(callable, callable, ...) could construct a statically continued future-promise?

You seldom want to chain .then() calls, in my experience.

You're right for non-generic code. The trouble with families of futures is that the metaprogramming can explode. I don't want bad design choices now to break the genericity later.

Most of the time to attach one continuation and pass the resulting future to some other place where you might attach another continuation, etc.

I have no issue with this and dynamic continuations. With static continuations I can't see how to do static continuations without either (a) elaborating the type of both future and promise to store those continuations or (b) type slice the pointer from promise to future, and have a virtual function be invoked to set values. The (a) scenario forces the programmer to add all static continuations to promise before fetching a future. The (b) scenario is a big vote in favour of an atomic_basic_future<> which turns on atomics and contains a virtual type uneraser function for promise to set the value through. It would all still be no alloc and still very, very fast - we are talking dozens of opcodes versus thousands with std::future. Some might wonder why do we need constexpr reduction of future promise anyway? Well, monadically speaking, a promise suspends execution and a future resumes execution. When combined with expected, you effectively get coroutined monadic programming via future promise because you can squirrel away a monadic operation into a promise, and resume execution from a future. This isn't as good as C++ 17 resumable functions. But it may be handy enough, and we get this now not later.

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My second question to the Boost community is this: should static continuations be able to see the value being set? Or indeed, to modify it before it gets sent to future.get()?

What would be the point of having a continuation which does not see the value once set?

I was thinking that basic_future and basic_promise wouldn't force any particular continuations policy on subclasses - basic_future<void> is a good example why. I was thinking that basic_future<T>.then(callable) would do the following outcomes: 1. If callable(basic_future<T>), the future just set/got is passed to the continuation. As future<U> is template aliased to basic_future> this matches the then() in the Concurrency TS. 2. If callable(T) i.e. the type carried by the basic_future, this is equivalent to calling the callable with the future.value(). If T were expected, that means one resumes a monadic sequence. 3. If callable(), the callable is simply invoked. It costs me nothing to add this, so you might as well. I can see atexit_future() type use cases. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/