On Tue, 22 Jun 2021 at 14:21, Matthias Kretz wrote:
> On Tuesday, 22 June 2021 14:51:26 CEST Jonathan Wakely wrote:
> > With your suggestion
> > to also drop std::tuple the number of parameters decides which
> > function we call. And we don't instantiate std::tuple. And we can also
> > get rid of the __try_to_lock function, which was only used to deduce
> > the lock type rather than use tuple_element to get it. That's much
> > nicer.
>
> 👍
>
> > > How about optimizing a likely common case where all lockables have the
> > > same
> > > type? In that case we don't require recursion and can manage stack
> usage
> > > much
> > > simpler:
> > The stack usage is bounded by the number of mutexes being locked,
> > which is unlikely to get large, but we can do that.
>
> Right. I meant simpler, because it takes a while of staring at the
> recursive
> implementation to understand how it works. :)
>
> > We can do it for try_lock too:
> >
> > template<typename _L1, typename _L2, typename... _L3>
> > int
> > try_lock(_L1& __l1, _L2& __l2, _L3&... __l3)
> > {
> > #if __cplusplus >= 201703L
> > if constexpr (is_same_v<_L1, _L2>
> > && (is_same_v<_L1, _L3> && ...))
> > {
> > constexpr int _Np = 2 + sizeof...(_L3);
> > unique_lock<_L1> __locks[_Np] = {
> > {__l1, try_to_lock}, {__l2, try_to_lock}, {__l3,
> > try_to_lock}... };
>
> 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)
if (!__locks[__i].try_lock())
{
const int __failed = __i;
while (__i--)
__locks[__i].unlock();
return __i;
}
for (auto& __l : __locks)
__l.release();
return -1;
}
else
#endif
> > for (int __i = 0; __i < _Np; ++__i)
> > if (!__locks[__i])
> > return __i;
> > for (auto& __l : __locks)
> > __l.release();
> > return -1;
> > }
> > else
> > #endif
> > return __detail::__try_lock_impl(__l1, __l2, __l3...);
> > }
> >
> > > if constexpr ((is_same_v<_L0, _L1> && ...))
> > > {
> > > constexpr int _Np = 1 + sizeof...(_L1);
> > > std::array<unique_lock<_L0>, _Np> __locks = {
> > > {__l0, defer_lock}, {__l1, defer_lock}...
> > > };
> > > int __first = 0;
> > > do {
> > > __locks[__first].lock();
> > > for (int __j = 1; __j < _Np; ++__j)
> > > {
> > > const int __idx = (__first + __j) % _Np;
> > > if (!__locks[__idx].try_lock())
> > > {
> > > for (int __k = __idx; __k != __first;
> > > __k = __k == 1 ? _Np : __k - 1)
> > > __locks[__k - 1].unlock();
> >
> > This loop doesn't work if any try_lock fails when first==0, because
> > the loop termination condition is never reached.
>
> Uh yes. Which is the same reason why the __j loop doesn't start from
> __first +
> 1.
>
> > I find this a bit easier to understand than the loop above, and
> > correct (I think):
> >
> > for (int __k = __j; __k != 0; --__k)
> > __locks[(__first + __k - 1) % _Np].unlock();
>
> 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.
> > [...]
> > @@ -620,15 +632,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > * @post All arguments are locked.
> > *
> > * All arguments are locked via a sequence of calls to lock(),
> > try_lock() - * and unlock(). If the call exits via an exception any
> > locks that were - * obtained will be released.
> > + * and unlock(). If this function exits via an exception any locks
> that
> > + * were obtained will be released.
> > */
> > template<typename _L1, typename _L2, typename... _L3>
> > void
> > lock(_L1& __l1, _L2& __l2, _L3&... __l3)
> > {
> > - int __i = 0;
> > - __detail::__lock_impl(__i, 0, __l1, __l2, __l3...);
> > +#if __cplusplus >= 201703L
>
> 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. So for pre-C++17 it
would still compile both branches (and instantiate the recursive function
templates) but would use the iterative code at run time.
But given that the default -std option is gnu++17 now, I'm OK with the
iterative version only being used for C++17.
