> > + // RAII type to destroy initialized elements.
>
> There's only one initialized element, not "elements".
>
> > + struct _Guard_elts
> > + {
> > + pointer _M_first, _M_last; // Elements to destroy
>
> We only need to store one pointer here, call it _M_p.
>
> > + _Tp_alloc_type& _M_alloc;
> > +
> > + _GLIBCXX20_CONSTEXPR
> > + _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> > + : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> > + { }
> > +
> > + _GLIBCXX20_CONSTEXPR
> > + ~_Guard_elts()
> > + { std::_Destroy(_M_first, _M_last, _M_alloc); }
>
> This should be either:
>
> std::_Destroy(_M_p, _M_p+1, _M_alloc);
>
> or avoid the loop that happens in that _Destroy function:
>
> _Alloc_traits::destroy(_M_alloc, _M_p);
>
> > +
> > + private:
> > + _Guard_elts(const _Guard_elts&);
> > + };
> > +
> > + // Guard the new element so it will be destroyed if anything
> > throws.
> > + _Guard_elts __guard_elts(__new_start + __elems, _M_impl);
> > +
> > + __new_finish = std::__uninitialized_move_if_noexcept_a(
> > + __old_start, __old_finish,
> > + __new_start, _M_get_Tp_allocator());
> > +
> > + ++__new_finish;
> > + // Guard everything before the new element too.
> > + __guard_elts._M_first = __new_start;
>
> This seems redundant, we're not doing any more insertions now, and so
> this store is dead.
I am attaching patch with this check removed. However I still think
Guard_elts needs to stay to be able to destroy the old ellements
>
> > +
> > + // New storage has been fully initialized, destroy the old
> > elements.
> > + __guard_elts._M_first = __old_start;
> > + __guard_elts._M_last = __old_finish;
... here
Does it look better? (I am not really that confidend with libstdc++).
I also was wondering if we do have some data on what libstdc++ functions
are perofrmance critical in practice. Given that the push_back can be
sped up very noticeably, I wonder if we don't have problems elsewhere?
Thank you,
Honza
diff --git a/libstdc++-v3/include/bits/stl_vector.h
b/libstdc++-v3/include/bits/stl_vector.h
index 5e18f6eedce..973f4d7e2e9 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -1288,7 +1288,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_ASAN_ANNOTATE_GREW(1);
}
else
- _M_realloc_insert(end(), __x);
+ _M_realloc_append(__x);
}
#if __cplusplus >= 201103L
@@ -1822,6 +1822,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
void
_M_realloc_insert(iterator __position, const value_type& __x);
+
+ void
+ _M_realloc_append(const value_type& __x);
#else
// A value_type object constructed with _Alloc_traits::construct()
// and destroyed with _Alloc_traits::destroy().
@@ -1871,6 +1874,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
void
_M_realloc_insert(iterator __position, _Args&&... __args);
+ template<typename... _Args>
+ _GLIBCXX20_CONSTEXPR
+ void
+ _M_realloc_append(_Args&&... __args);
+
// Either move-construct at the end, or forward to _M_insert_aux.
_GLIBCXX20_CONSTEXPR
iterator
diff --git a/libstdc++-v3/include/bits/vector.tcc
b/libstdc++-v3/include/bits/vector.tcc
index 80631d1e2a1..0ccef7911b3 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -120,7 +120,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_ASAN_ANNOTATE_GREW(1);
}
else
- _M_realloc_insert(end(), std::forward<_Args>(__args)...);
+ _M_realloc_append(std::forward<_Args>(__args)...);
#if __cplusplus > 201402L
return back();
#endif
@@ -459,6 +459,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
#endif
{
const size_type __len = _M_check_len(1u, "vector::_M_realloc_insert");
+ if (__len <= 0)
+ __builtin_unreachable ();
pointer __old_start = this->_M_impl._M_start;
pointer __old_finish = this->_M_impl._M_finish;
const size_type __elems_before = __position - begin();
@@ -571,6 +573,127 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
this->_M_impl._M_end_of_storage = __new_start + __len;
}
+#if __cplusplus >= 201103L
+ template<typename _Tp, typename _Alloc>
+ template<typename... _Args>
+ _GLIBCXX20_CONSTEXPR
+ void
+ vector<_Tp, _Alloc>::
+ _M_realloc_append(_Args&&... __args)
+#else
+ template<typename _Tp, typename _Alloc>
+ void
+ vector<_Tp, _Alloc>::
+ _M_realloc_append(const _Tp& __x)
+#endif
+ {
+ const size_type __len = _M_check_len(1u, "vector::_M_realloc_append");
+ if (__len <= 0)
+ __builtin_unreachable ();
+ pointer __old_start = this->_M_impl._M_start;
+ pointer __old_finish = this->_M_impl._M_finish;
+ const size_type __elems = end() - begin();
+ pointer __new_start(this->_M_allocate(__len));
+ pointer __new_finish(__new_start);
+
+ // RAII guard for allocated storage.
+ struct _Guard
+ {
+ pointer _M_storage; // Storage to deallocate
+ size_type _M_len;
+ _Tp_alloc_type& _M_alloc;
+
+ _GLIBCXX20_CONSTEXPR
+ _Guard(pointer __s, size_type __l, _Tp_alloc_type& __a)
+ : _M_storage(__s), _M_len(__l), _M_alloc(__a)
+ { }
+
+ _GLIBCXX20_CONSTEXPR
+ ~_Guard()
+ {
+ if (_M_storage)
+ __gnu_cxx::__alloc_traits<_Tp_alloc_type>::
+ deallocate(_M_alloc, _M_storage, _M_len);
+ }
+
+ private:
+ _Guard(const _Guard&);
+ };
+
+ {
+ _Guard __guard(__new_start, __len, _M_impl);
+
+ // The order of the three operations is dictated by the C++11
+ // case, where the moves could alter a new element belonging
+ // to the existing vector. This is an issue only for callers
+ // taking the element by lvalue ref (see last bullet of C++11
+ // [res.on.arguments]).
+
+ // If this throws, the existing elements are unchanged.
+#if __cplusplus >= 201103L
+ _Alloc_traits::construct(this->_M_impl,
+ std::__to_address(__new_start + __elems),
+ std::forward<_Args>(__args)...);
+#else
+ _Alloc_traits::construct(this->_M_impl,
+ __new_start + __elems,
+ __x);
+#endif
+
+#if __cplusplus >= 201103L
+ if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
+ {
+ // Relocation cannot throw.
+ __new_finish = _S_relocate(__old_start, __old_finish,
+ __new_start, _M_get_Tp_allocator());
+ ++__new_finish;
+ }
+ else
+#endif
+ {
+ // RAII type to destroy initialized elements.
+ struct _Guard_elts
+ {
+ pointer _M_first, _M_last; // Elements to destroy
+ _Tp_alloc_type& _M_alloc;
+
+ _GLIBCXX20_CONSTEXPR
+ _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
+ : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
+ { }
+
+ _GLIBCXX20_CONSTEXPR
+ ~_Guard_elts()
+ { std::_Destroy(_M_first, _M_last, _M_alloc); }
+
+ private:
+ _Guard_elts(const _Guard_elts&);
+ };
+
+ // Guard the new element so it will be destroyed if anything throws.
+ _Guard_elts __guard_elts(__new_start + __elems, _M_impl);
+
+ __new_finish = std::__uninitialized_move_if_noexcept_a(
+ __old_start, __old_finish,
+ __new_start, _M_get_Tp_allocator());
+
+ ++__new_finish;
+
+ // New storage has been fully initialized, destroy the old elements.
+ __guard_elts._M_first = __old_start;
+ __guard_elts._M_last = __old_finish;
+ }
+ __guard._M_storage = __old_start;
+ __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
+ }
+ // deallocate should be called before assignments to _M_impl,
+ // to avoid call-clobbering
+
+ this->_M_impl._M_start = __new_start;
+ this->_M_impl._M_finish = __new_finish;
+ this->_M_impl._M_end_of_storage = __new_start + __len;
+ }
+
template<typename _Tp, typename _Alloc>
_GLIBCXX20_CONSTEXPR
void
