On Tue, 22 Jun 2021 at 20:51, Jonathan Wakely wrote:
>
> On Tue, 22 Jun 2021 at 17:03, Matthias Kretz <m.kretz@gsi.de> wrote:
> >
> > On Dienstag, 22. Juni 2021 17:20:41 CEST Jonathan Wakely wrote:
> > > On Tue, 22 Jun 2021 at 14:21, Matthias Kretz wrote:
> > > > This does a try_lock on all lockabes even if any of them fails. I think
> > > > that's
> > > > not only more expensive but also non-conforming. I think you need to defer
> > > > locking and then loop from beginning to end to break the loop on the first
> > > > unsuccessful try_lock.
> > >
> > > Oops, good point. I'll add a test for that too. Here's the fixed code:
> > >
> > >     template<typename _L0, typename... _Lockables>
> > >       inline int
> > >       __try_lock_impl(_L0& __l0, _Lockables&... __lockables)
> > >       {
> > > #if __cplusplus >= 201703L
> > >         if constexpr ((is_same_v<_L0, _Lockables> && ...))
> > >           {
> > >             constexpr int _Np = 1 + sizeof...(_Lockables);
> > >             unique_lock<_L0> __locks[_Np] = {
> > >                 {__l0, defer_lock}, {__lockables, defer_lock}...
> > >             };
> > >             for (int __i = 0; __i < _Np; ++__i)
> >
> > I thought coding style requires a { here?
>
> Maybe for the compiler, but I don't think libstdc++ has such a rule. I
> can add the braces though, it's probably better.
>
> >
> > >               if (!__locks[__i].try_lock())
> > >                 {
> > >                   const int __failed = __i;
> > >                   while (__i--)
> > >                     __locks[__i].unlock();
> > >                   return __i;
> >
> > You meant `return __failed`?
>
> Yep, copy&paste error while trying to avoid the TABs in the real code
> screwing up the gmail formatting :-(
>
>
> > >                 }
> > >             for (auto& __l : __locks)
> > >               __l.release();
> > >             return -1;
> > >           }
> > >         else
> > > #endif
> > >
> > > > [...]
> > > > Yes, if only we had a wrapping integer type that wraps at an arbitrary N.
> > > > Like
> > > >
> > > > unsigned int but with parameter, like:
> > > >   for (__wrapping_uint<_Np> __k = __idx; __k != __first; --__k)
> > > >
> > > >     __locks[__k - 1].unlock();
> > > >
> > > > This is the loop I wanted to write, except --__k is simpler to write and
> > > > __k -
> > > > 1 would also wrap around to _Np - 1 for __k == 0. But if this is the only
> > > > place it's not important enough to abstract.
> > >
> > > We might be able to use __wrapping_uint in std::seed_seq::generate too, and
> > > maybe some other places in <random>. But we can add that later if we decide
> > > it's worth it.
> >
> > OK.
> >
> > > > I also considered moving it down here. Makes sense unless you want to call
> > > > __detail::__lock_impl from other functions. And if we want to make it work
> > > > for
> > > > pre-C++11 we could do
> > > >
> > > >   using __homogeneous
> > > >
> > > >     = __and_<is_same<_L1, _L2>, is_same<_L1, _L3>...>;
> > > >
> > > >   int __i = 0;
> > > >   __detail::__lock_impl(__homogeneous(), __i, 0, __l1, __l2, __l3...);
> > >
> > > We don't need tag dispatching, we could just do:
> > >
> > > if _GLIBCXX17_CONSTEXPR (homogeneous::value)
> > >  ...
> > > else
> > >  ...
> > >
> > > because both branches are valid for the homogeneous case, i.e. we aren't
> > > using if-constexpr to avoid invalid instantiations.
> >
> > But for the inhomogeneous case the homogeneous code is invalid (initialization
> > of C-array of unique_lock<_L1>).
>
> Oops, yeah of course.
>
> >
> > > But given that the default -std option is gnu++17 now, I'm OK with the
> > > iterative version only being used for C++17.
> >
> > Fair enough.

Here's what I've tested and pushed to trunk. Thanks for the
improvement and comments.