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?
> if (!__locks[__i].try_lock())
> {
> const int __failed = __i;
> while (__i--)
> __locks[__i].unlock();
> return __i;
You meant `return __failed`?
> }
> 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>).
> 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.
--
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Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Centre for Heavy Ion Research https://gsi.de
std::experimental::simd https://github.com/VcDevel/std-simd
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