improve std::deque::_M_reallocate_map

[Jan Hubicka]

> > diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
> > index deb010a0ebb..e56cd0b9319 100644
> > --- a/libstdc++-v3/include/bits/deque.tcc
> > +++ b/libstdc++-v3/include/bits/deque.tcc
> > @@ -955,6 +955,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > 	  size_type __new_map_size = this->_M_impl._M_map_size
> > 				     + std::max(this->_M_impl._M_map_size,
> > 						__nodes_to_add) + 2;
> > +	  if (__new_map_size > ((max_size() + __deque_buf_size(sizeof(_Tp)) - 1)
> > +			        / __deque_buf_size(sizeof(_Tp))) * 2)
> > +	    __builtin_unreachable ();
> 
> I think it's worth avoiding the duplicate call, for -O0 -std=c++98.
> It makes the line shorter too:
> 
>   const size_t __bufsz = __deque_buf_size(sizeof(_Tp));
>   if (__new_map_size > ((max_size() + __bufsz - 1) / __bufsz) * 2)
>     __builtin_unreachable();

This looks better, in fact that what I inteded with that forgotten
veriable.  I think __deque_buf_size is a constexpr and gets replaced by
constant.

I wonder why "const" is useful here.  Also I wonder if there is any
difference between size_type and size_t, or it can be used
interchangeably?

For -O0 we do keep the cmp instruction around, but "optimize away" the
conditonal jump to unreachable, which is bit odd.  An option would be to
add ifdef __OPTIMIZE__ guards.

This is updated patch I am testing

diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
index deb010a0ebb..ee03c917a29 100644
--- a/libstdc++-v3/include/bits/deque.tcc
+++ b/libstdc++-v3/include/bits/deque.tcc
@@ -956,6 +956,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 				     + std::max(this->_M_impl._M_map_size,
 						__nodes_to_add) + 2;
 
+	  const size_t __bufsz = __deque_buf_size(sizeof(_Tp));
+	  if (__new_map_size > ((max_size() + __bufsz - 1) / __bufsz) * 2)
+	    __builtin_unreachable();
+
 	  _Map_pointer __new_map = this->_M_allocate_map(__new_map_size);
 	  __new_nstart = __new_map + (__new_map_size - __new_num_nodes) / 2
 			 + (__add_at_front ? __nodes_to_add : 0);
diff --git a/libstdc++-v3/include/bits/stl_deque.h b/libstdc++-v3/include/bits/stl_deque.h
index c617933bd81..dd1212793de 100644
--- a/libstdc++-v3/include/bits/stl_deque.h
+++ b/libstdc++-v3/include/bits/stl_deque.h
@@ -1266,7 +1266,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_NODISCARD
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return this->_M_impl._M_finish - this->_M_impl._M_start; }
+      {
+	size_type __sz = this->_M_impl._M_finish - this->_M_impl._M_start;
+	if (__sz > max_size ())
+	  __builtin_unreachable ();
+	return __sz;
+      }
 
       /**  Returns the size() of the largest possible %deque.  */
       _GLIBCXX_NODISCARD