Re: Re: Adding a new thread model to GCC

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 17 April 2016 at 17:56, lh_mouse <lh_mouse@126.com> wrote:
> A glance over gthr.h reminds me __gthread_time_t. There seem few requirements documented in gthr.h.
> I discussed this with Adrien Nader on mingw-w64's mailing list a few days ago.
>
> Specifically, here are the two questions:
> 0) Should __gthread_time_t be a struct or a plain integral type?
> The 'struct timespec' used by pthread is a struct introduced in POSIX.
> However my implementation uses a plain uint64_t.

I don't see why it has to be a struct, it just has to be suitable as
an argument to the relevant __gthread functions.

If the current code assumes a struct and the Windows API calls need an
integer then either the existing code needs to be made more flexible,
or you need to define it as a struct and then convert to an integer
inside your new gthread wrapper functions.


> 1) How to obtain a __gthread_time_t representing the current time?
> According to Linux man pages, the timeout parameter of pthread_cond_timedwait() is the same as gettimeofday() - that is, it uses the wall clock.
> My implementation uses GetTickCount64() - that is, my implementation uses a monotonic clock.

std::condition_variable::__clock_t must be a typedef for the clock
used by the underlying implementation, so it sounds like you should
use std::chrono::steady_clock for your thread model.


> Quoting from ISO/IEC WG21 Draft N4582 (C++1z):
> [quote]
> 30.4.1.3.1 Class timed_mutex [thread.timedmutex.class]
> ...
> template <class Rep, class Period>
> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
> template <class Clock, class Duration>
> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
> ...
> [/quote]
> the std::timed_mutex::try_lock_for() function template shall accept any clock type, hence we have to do timestamp translation. It is also important to know how libstdc++ handles this.

All conversions are done using the std::chrono facilities, before any
conversion to __gthread_time_t. That means the conversions are
portable.