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unord: Extract key to avoid preemptive mallocs in insert/emplace

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unordered_set::emplace and unordered_map::emplace construct a node, then
try to insert it.  If insertion fails, the node gets deleted.

To avoid this unnecessary malloc traffic, check to see if the argument
to emplace has the appropriate key_type.  If so, we can use that key
directly and delay the malloc until we're sure we're inserting something
new.

Test updates by Eric Fiselier, who rewrote the old allocation tests to
include the new cases.

There are two orthogonal future directions:

1. Apply the same optimization to set and map.

2. Extend the optimization to when the argument is not key_type, but can
   be converted to it without side effects.  Ideally, we could do this
   whenever key_type is trivially destructible and the argument is
   trivially convertible to key_type, but in practise the relevant type
   traits "blow up sometimes".  At least, we should catch a few simple
   cases (such as when both are primitive types).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@263746 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan P. N. Exon Smith
2016-03-17 20:45:20 +00:00
parent 735026e3ee
commit 798ec8436d
4 changed files with 240 additions and 50 deletions
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58
include/__hash_table
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@@ -100,6 +100,23 @@ __next_hash_pow2(size_t __n)
return size_t(1) << (std::numeric_limits<size_t>::digits - __clz(__n-1));
}
#ifndef _LIBCPP_CXX03_LANG
struct __extract_key_fail_tag {};
struct __extract_key_self_tag {};
struct __extract_key_first_tag {};
template <class _ValTy, class _Key,
class _RawValTy = typename __unconstref<_ValTy>::type>
struct __can_extract_key
: conditional<is_same<_RawValTy, _Key>::value, __extract_key_self_tag,
__extract_key_fail_tag>::type {};
template <class _Pair, class _Key, class _First, class _Second>
struct __can_extract_key<_Pair, _Key, pair<_First, _Second>>
: conditional<is_same<typename remove_const<_First>::type, _Key>::value,
__extract_key_first_tag, __extract_key_fail_tag>::type {};
#endif
template <class _Tp, class _Hash, class _Equal, class _Alloc> class __hash_table;
template <class _NodePtr> class _LIBCPP_TYPE_VIS_ONLY __hash_iterator;
@@ -903,6 +920,7 @@ public:
typedef typename _NodeTypes::__node_value_type __node_value_type;
typedef typename _NodeTypes::__container_value_type __container_value_type;
typedef typename _NodeTypes::key_type key_type;
typedef value_type& reference;
typedef const value_type& const_reference;
typedef typename __alloc_traits::pointer pointer;
@@ -1041,13 +1059,49 @@ public:
#ifndef _LIBCPP_CXX03_LANG
template <class _Key, class ..._Args>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool> __emplace_unique_key_args(_Key const& __k, _Args&&... __args);
template <class... _Args>
pair<iterator, bool> __emplace_unique(_Args&&... __args);
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool> __emplace_unique_impl(_Args&&... __args);
template <class _Pp>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool> __emplace_unique(_Pp&& __x) {
return __emplace_unique_extract_key(_VSTD::forward<_Pp>(__x),
__can_extract_key<_Pp, key_type>());
}
template <class... _Args>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool> __emplace_unique(_Args&&... __args) {
return __emplace_unique_impl(_VSTD::forward<_Args>(__args)...);
}
template <class _Pp>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool>
__emplace_unique_extract_key(_Pp&& __x, __extract_key_fail_tag) {
return __emplace_unique_impl(_VSTD::forward<_Pp>(__x));
}
template <class _Pp>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool>
__emplace_unique_extract_key(_Pp&& __x, __extract_key_self_tag) {
return __emplace_unique_key_args(__x, _VSTD::forward<_Pp>(__x));
}
template <class _Pp>
_LIBCPP_INLINE_VISIBILITY
pair<iterator, bool>
__emplace_unique_extract_key(_Pp&& __x, __extract_key_first_tag) {
return __emplace_unique_key_args(__x.first, _VSTD::forward<_Pp>(__x));
}
template <class... _Args>
_LIBCPP_INLINE_VISIBILITY
iterator __emplace_multi(_Args&&... __args);
template <class... _Args>
_LIBCPP_INLINE_VISIBILITY
iterator __emplace_hint_multi(const_iterator __p, _Args&&... __args);
@@ -1989,7 +2043,7 @@ __done:
template <class _Tp, class _Hash, class _Equal, class _Alloc>
template <class... _Args>
pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
__hash_table<_Tp, _Hash, _Equal, _Alloc>::__emplace_unique(_Args&&... __args)
__hash_table<_Tp, _Hash, _Equal, _Alloc>::__emplace_unique_impl(_Args&&... __args)
{
__node_holder __h = __construct_node(_VSTD::forward<_Args>(__args)...);
pair<iterator, bool> __r = __node_insert_unique(__h.get());