Deque rotate on current node

[Jonathan Wakely]

On 20/05/19 07:39 +0200, François Dumont wrote:
>Hi
>
>  std::deque is supposed to be like a double-queue that you can attack 
>from front and back. But currrently its implementation makes it behave 
>differently when starting from a fresh deque. If push_back then all 
>goes well, it is copy/move to the current internal 'node'. If 
>push_front then a new 'node' is created and on next pop_back the 
>initial node will be deallocated without having ever be used.
>
>  This patch changes this behavior. As long as deque is empty we can 
>play with its state to make it push_back- or push_front-ready. It will 
>also benefit to the usage of deque in the std::stack.
>
>  More generally it could really improve scenario in which deque is 
>used as queue of elements exchanged between different threads. As you 
>need to make sure that consumers are faster than producers then your 
>deque should remain empty most of the time and so this proposed patch 
>should avoid nodes to be allocated/deallocated all the time.

This benefits the case where you always push at the same end. But with
your patch if I do push_front then push_back,
we still allocate a second node even though the first one is nearly
empty.

Would it be better to point into the middle of the node, not right at
the end or right at the beginning?

>    * include/bits/deque.tcc (deque<>::_M_push_back_aux):
>    Rotate on current node if deque is empty.
>    (deue<>::_M_push_front_aux): Likewise.
>
>Tested under Linux x86_64, ok to commit ?
>
>François
>

>diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
>index 2053afe1d69..245e3e712d8 100644
>--- a/libstdc++-v3/include/bits/deque.tcc
>+++ b/libstdc++-v3/include/bits/deque.tcc
>@@ -507,6 +507,19 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       _M_push_back_aux(const value_type& __t)
> #endif
>       {
>+	if (empty())
>+	  {
>+	    // Move iterators to point to the current node begin.
>+	    this->_M_impl._M_start._M_cur = this->_M_impl._M_start._M_first;
>+	    this->_M_impl._M_finish._M_cur = this->_M_impl._M_finish._M_first;
>+#if __cplusplus >= 201103L
>+	    emplace_back(std::forward<_Args>(__args)...);
>+#else
>+	    push_back(__t);
>+#endif
>+	    return;
>+	  }
>+
> 	if (size() == max_size())
> 	  __throw_length_error(
> 	      __N("cannot create std::deque larger than max_size()"));
>@@ -546,6 +559,19 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       _M_push_front_aux(const value_type& __t)
> #endif
>       {
>+	if (empty())
>+	  {
>+	    // Move iterators to point to the current node end.
>+	    this->_M_impl._M_finish._M_cur = this->_M_impl._M_finish._M_last - 1;
>+	    this->_M_impl._M_start._M_cur = this->_M_impl._M_start._M_last - 1;
>+#if __cplusplus >= 201103L
>+	    emplace_front(std::forward<_Args>(__args)...);
>+#else
>+	    push_front(__t);
>+#endif
>+	    return;
>+	  }
>+
> 	if (size() == max_size())
> 	  __throw_length_error(
> 	      __N("cannot create std::deque larger than max_size()"));