Re: For which gcc release is going to be foreseen the support for the Coroutines TS extension?

[Florian Weimer <fweimer@redhat.com>]

* Richard Biener:

> On August 20, 2019 5:19:33 PM GMT+02:00, Nathan Sidwell <nathan@acm.org> wrote:
>>On 7/26/19 6:03 AM, Iain Sandoe wrote:
>>> Hello Sebastian,
>>> 
>>>> On 26 Jul 2019, at 10:19, Florian Weimer <fweimer@redhat.com> wrote:
>>
>>>> C++ coroutines are stackless.  I don't think any new low-level
>>run-time
>>>> support will be needed.
>>> 
>>> correct, C++20 coroutines and threading mechanisms are orthogonal
>>> facilities; one can use (IS C++20) coroutines on top of a threaded
>>system
>>> or in a single-threaded environment.
>>> 
>>> Two places I see them as being a go-to facility in embedded systems
>>are:
>>>   * co-operative multi-tasking UIs on single-threaded platforms.
>>>   * async I/O completion by continuations, rather than callbacks.
>>
>>There are cases where the overhead of threads is too expensive.  For 
>>instance hiding (cache-missing) load latencies by doing other work
>>while 
>>waiting -- a context switch at that point is far too expensive.
>
> But are coroutines so much lower latency (and a context switch does
> not involve cache misses on its own?). For doing useful work in this
> context CPU designers invented SMT...

I think the idea is that you don't have to worry about synchronizing
multiple threads to reap the benefits from hardware parallelism.  For
hiding memory access latency, that could be important because the
synchronization overhead could easily eat up any potential benefits.

Thanks,
Florian