All good remarks as usual, here is a new version.
I took the time to leverage on the new method to replace a usage of
_M_src_hash_code.
libstdc++: [_Hashtable] Fix hash code cache usage when stateful
hash functor
It is wrong to reuse a cached hash code from another container when
this code depends
on the state of the container's Hash functor.
Add checks that Hash functor is stateless before reusing the cached
hash code.
libstdc++-v3/ChangeLog:
* include/bits/hashtable_policy.h
(_Hash_code_base::_M_copy_code,
_Hash_code_base::_M_stored_code): Remove.
* include/bits/hashtable.h (_M_hash_code_ext): New.
(_M_copy_code): New.
(_M_assign): Use latter.
(_M_bucket_index_ex): New.
(_M_equals): Use latter.
(_M_store_code): New.
* testsuite/23_containers/unordered_map/modifiers/merge.cc
(test10): New
test case.
Tested under Linux x64.
Ok to commit ?
François
On 19/02/2025 14:33, Jonathan Wakely wrote:
On 20/01/25 22:12 +0100, François Dumont wrote:
Hi
In my work on fancy pointer support I've decided to always cache the
hash code.
Doing so I spotted a bug in the management of this cache when hash
functor is stateful.
libstdc++: [_Hashtable] Fix hash code cache usage when hash
functor is stateful
It is wrong to reuse a cached hash code when this code depends
then on the state
of the Hash functor.
Add checks that Hash functor is stateless before reusing the
cached hash code.
libstdc++-v3/ChangeLog:
* include/bits/hashtable_policy.h
(_Hash_code_base::_M_copy_code): Remove.
* include/bits/hashtable.h (_M_copy_code): New.
I like replacing the _M_copy_code overloads with one function, thanks.
We could do the same for _M_store_code too.
(_M_assign): Use latter.
(_M_bucket_index_ex): New.
I assume "ex" means "external"? That doesn't seem very clear to me
though. Maybe "ext" would be better.
(_M_equals): Use latter.
*
testsuite/23_containers/unordered_map/modifiers/merge.cc (test10): New
test case.
Tested under Linux x64, ok to commit ?
François
diff --git a/libstdc++-v3/include/bits/hashtable.h
b/libstdc++-v3/include/bits/hashtable.h
index d6d76a743bb..b3c1d7aac24 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -808,6 +808,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_bucket_index(__hash_code __c) const
{ return __hash_code_base::_M_bucket_index(__c,
_M_bucket_count); }
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
+ // Like _M_bucket_index but when the node is coming from another
+ // container instance.
+ size_type
+ _M_bucket_index_ex(const __node_value_type& __n) const
+ {
+ if constexpr (__hash_cached::value)
+ if constexpr (std::is_empty<_Hash>::value)
+ return _RangeHash{}(__n._M_hash_code, _M_bucket_count);
+
+ return _RangeHash{}
+ (this->_M_hash_code(_ExtractKey{}(__n._M_v())), _M_bucket_count);
+ }
+
+ void
+ _M_copy_code(__node_value_type& __to,
+ const __node_value_type& __from) const
+ {
+ if constexpr (__hash_cached::value)
+ {
+ if constexpr (std::is_empty<_Hash>::value)
+ __to._M_hash_code = __from._M_hash_code;
+ else
+ __to._M_hash_code =
+ this->_M_hash_code(_ExtractKey{}(__from._M_v()));
+ }
+ }
These two functions are doing similar work, would it make sense to
factor out the common part to a new function:
// Get hash code for a node that comes from another _Hashtable.
// Reuse a cached hash code if the hash function is stateless,
// otherwise recalculate it using our own hash function.
size_t
_M_hash_code_ext(const __node_value_type& __from) const
{
if constexpr (__and_<__hash_cached, is_empty<_Hash>>::value)
return __from._M_hash_code;
else
this->_M_hash_code(_ExtractKey{}(__from._M_v()));
}
// Like _M_bucket_index but when the node is coming from another
// container instance.
size_type
_M_bucket_index_ext(const __node_value_type& __n) const
{ return _M_bucket_index(_M_hash_code_ext(__n)); }
void
_M_copy_code(__node_value_type& __to,
const __node_value_type& __from) const
{
if constexpr (__hash_cached::value)
__to._M_hash_code = _M_hash_code_ext(__n);
}
+#pragma GCC diagnostic pop
+
// Find and insert helper functions and types
// Find the node before the one matching the criteria.
@@ -1587,7 +1617,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__node_ptr __ht_n = __ht._M_begin();
__node_ptr __this_n
= __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
- this->_M_copy_code(*__this_n, *__ht_n);
+ _M_copy_code(*__this_n, *__ht_n);
_M_update_bbegin(__this_n);
// Then deal with other nodes.
@@ -1596,7 +1626,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
__this_n = __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
__prev_n->_M_nxt = __this_n;
- this->_M_copy_code(*__this_n, *__ht_n);
+ _M_copy_code(*__this_n, *__ht_n);
size_type __bkt = _M_bucket_index(*__this_n);
if (!_M_buckets[__bkt])
_M_buckets[__bkt] = __prev_n;
@@ -2851,7 +2881,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if constexpr (__unique_keys::value)
for (auto __x_n = _M_begin(); __x_n; __x_n = __x_n->_M_next())
{
- std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+ std::size_t __ybkt = __other._M_bucket_index_ex(*__x_n);
auto __prev_n = __other._M_buckets[__ybkt];
if (!__prev_n)
return false;
@@ -2878,7 +2908,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__x_n_end = __x_n_end->_M_next())
++__x_count;
- std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+ std::size_t __ybkt = __other._M_bucket_index_ex(*__x_n);
auto __y_prev_n = __other._M_buckets[__ybkt];
if (!__y_prev_n)
return false;
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h
b/libstdc++-v3/include/bits/hashtable_policy.h
index 1fa8c01d5e8..61c57651c4a 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1108,19 +1108,9 @@ namespace __detail
_M_store_code(_Hash_node_code_cache<false>&, __hash_code) const
{ }
- void
- _M_copy_code(_Hash_node_code_cache<false>&,
- const _Hash_node_code_cache<false>&) const
- { }
-
void
_M_store_code(_Hash_node_code_cache<true>& __n, __hash_code
__c) const
{ __n._M_hash_code = __c; }
-
- void
- _M_copy_code(_Hash_node_code_cache<true>& __to,
- const _Hash_node_code_cache<true>& __from) const
- { __to._M_hash_code = __from._M_hash_code; }
};
/// Partial specialization used when nodes contain a cached hash code.
diff --git
a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
index 10b61464243..010181f7038 100644
---
a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
+++
b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
@@ -417,6 +417,51 @@ test09()
VERIFY( c2.size() == 3 );
}
+struct slow_stateful_hash
+{
+ size_t seed = 0;
+
+ auto operator()(const int& i) const noexcept
+ { return std::hash<int>()(i) + seed; }
+};
+
+namespace std
+{
+ template<>
+ struct __is_fast_hash<slow_stateful_hash> : public std::false_type
+ { };
+}
+
+void
+test10()
+{
+ using map_type = std::unordered_map<int, int, slow_stateful_hash>;
+ map_type c1({ {1, 1}, {3, 3}, {5, 5} }, 0, slow_stateful_hash{1});
+ map_type c2({ {2, 2}, {4, 4}, {6, 6} }, 0, slow_stateful_hash{2});
+ const auto c3 = c2;
+
+ c1.merge(c2);
+ VERIFY( c1.size() == 6 );
+ VERIFY( c2.empty() );
+
+ c2 = c3;
+ c1.clear();
+ c1.merge(std::move(c2));
+ VERIFY( c1 == c3 );
+ VERIFY( c2.empty() );
+
+ c2.merge(std::move(c1));
+ VERIFY( c1.empty() );
+ VERIFY( c2 == c3 );
+
+ c2.merge(c1);
+ VERIFY( c1.empty() );
+ VERIFY( c2 == c3 );
+
+ c2.merge(c2);
+ VERIFY( c2 == c3 );
+}
+
int
main()
{
@@ -429,4 +474,5 @@ main()
test07();
test08();
test09();
+ test10();
}
diff --git a/libstdc++-v3/include/bits/hashtable.h
b/libstdc++-v3/include/bits/hashtable.h
index b17a314a11d..246e62afc6a 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -808,6 +808,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_bucket_index(__hash_code __c) const
{ return __hash_code_base::_M_bucket_index(__c, _M_bucket_count); }
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
+ // Get hash code for a node that comes from another _Hashtable.
+ // Reuse a cached hash code if the hash function is stateless,
+ // otherwise recalculate it using our own hash function.
+ __hash_code
+ _M_hash_code_ext(const __node_value_type& __from) const
+ {
+ if constexpr (__and_<__hash_cached, is_empty<_Hash>>::value)
+ return __from._M_hash_code;
+ else
+ return this->_M_hash_code(_ExtractKey{}(__from._M_v()));
+ }
+
+ // Like _M_bucket_index but when the node is coming from another
+ // container instance.
+ size_type
+ _M_bucket_index_ext(const __node_value_type& __from) const
+ { return _RangeHash{}(_M_hash_code_ext(__from), _M_bucket_count); }
+
+ void
+ _M_copy_code(__node_value_type& __to,
+ const __node_value_type& __from) const
+ {
+ if constexpr (__hash_cached::value)
+ __to._M_hash_code = _M_hash_code_ext(__from);
+ }
+
+ void
+ _M_store_code(__node_value_type& __to, __hash_code __code) const
+ {
+ if constexpr (__hash_cached::value)
+ __to._M_hash_code = __code;
+ }
+#pragma GCC diagnostic pop
+
// Find and insert helper functions and types
// Find the node before the one matching the criteria.
@@ -1210,16 +1246,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// with a hash function that might not match this->hash_function().
template<typename _H2>
__hash_code
- _M_src_hash_code(const _H2&, const key_type& __k,
- const __node_value_type& __src_n) const
+ _M_src_hash_code(const _H2&, const __node_value_type& __src_n) const
{
- if constexpr (__hash_cached::value)
- if constexpr (std::is_same_v<_H2, _Hash>)
- if constexpr (std::is_empty_v<_Hash>)
- // If the node has a cached hash code, it's OK to use it.
- return this->_M_hash_code(__src_n);
-
- return this->_M_hash_code(__k);
+ if constexpr (__and_<__hash_cached,
+ is_same<_H2, _Hash>, is_empty<_Hash>>::value)
+ // If the node has a cached hash code, it's OK to use it.
+ return __src_n._M_hash_code;
+ else
+ return this->_M_hash_code(_ExtractKey{}(__src_n._M_v()));
}
public:
@@ -1328,9 +1362,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
do
{
const auto& __node = static_cast<__node_type&>(*__prev->_M_nxt);
- const key_type& __k = _ExtractKey{}(__node._M_v());
- // Hash code from this->hash_function():
- auto __code = _M_src_hash_code(__src.hash_function(), __k, __node);
+ // Hash code from this:
+ auto __code = _M_hash_code_ext(__node);
// Bucket index in __src, using code from __src.hash_function():
size_type __src_bkt = __src._M_bucket_index(__node);
auto __nh = __src._M_extract_node(__src_bkt, __prev);
@@ -1356,9 +1389,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
for (auto __i = __src.cbegin(), __end = __src.cend(); __i != __end;)
{
auto __pos = __i++;
- const key_type& __k = _ExtractKey{}(*__pos);
__hash_code __code
- = _M_src_hash_code(__src.hash_function(), __k, *__pos._M_cur);
+ = _M_src_hash_code(__src.hash_function(), *__pos._M_cur);
auto __nh = __src.extract(__pos);
__hint = _M_insert_multi_node(__hint, __code, __nh._M_ptr)._M_cur;
__nh.release();
@@ -1588,7 +1620,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__node_ptr __ht_n = __ht._M_begin();
__node_ptr __this_n
= __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
- this->_M_copy_code(*__this_n, *__ht_n);
+ _M_copy_code(*__this_n, *__ht_n);
_M_update_bbegin(__this_n);
// Then deal with other nodes.
@@ -1597,7 +1629,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
__this_n = __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
__prev_n->_M_nxt = __this_n;
- this->_M_copy_code(*__this_n, *__ht_n);
+ _M_copy_code(*__this_n, *__ht_n);
size_type __bkt = _M_bucket_index(*__this_n);
if (!_M_buckets[__bkt])
_M_buckets[__bkt] = __prev_n;
@@ -2438,7 +2470,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
__rehash_guard._M_guarded_obj = nullptr;
- this->_M_store_code(*__node, __code);
+ _M_store_code(*__node, __code);
// Always insert at the beginning of the bucket.
_M_insert_bucket_begin(__bkt, __node);
@@ -2465,7 +2497,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_rehash(__do_rehash.second, false_type{});
__rehash_guard._M_guarded_obj = nullptr;
- this->_M_store_code(*__node, __code);
+ _M_store_code(*__node, __code);
const key_type& __k = _ExtractKey{}(__node->_M_v());
size_type __bkt = _M_bucket_index(__code);
@@ -2852,7 +2884,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if constexpr (__unique_keys::value)
for (auto __x_n = _M_begin(); __x_n; __x_n = __x_n->_M_next())
{
- std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+ std::size_t __ybkt = __other._M_bucket_index_ext(*__x_n);
auto __prev_n = __other._M_buckets[__ybkt];
if (!__prev_n)
return false;
@@ -2879,7 +2911,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__x_n_end = __x_n_end->_M_next())
++__x_count;
- std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+ std::size_t __ybkt = __other._M_bucket_index_ext(*__x_n);
auto __y_prev_n = __other._M_buckets[__ybkt];
if (!__y_prev_n)
return false;
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h
b/libstdc++-v3/include/bits/hashtable_policy.h
index 1fa8c01d5e8..a9567cc3c74 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1103,24 +1103,6 @@ namespace __detail
_M_bucket_index(const _Hash_node_value<_Value, true>& __n,
size_t __bkt_count) const noexcept
{ return _RangeHash{}(__n._M_hash_code, __bkt_count); }
-
- void
- _M_store_code(_Hash_node_code_cache<false>&, __hash_code) const
- { }
-
- void
- _M_copy_code(_Hash_node_code_cache<false>&,
- const _Hash_node_code_cache<false>&) const
- { }
-
- void
- _M_store_code(_Hash_node_code_cache<true>& __n, __hash_code __c) const
- { __n._M_hash_code = __c; }
-
- void
- _M_copy_code(_Hash_node_code_cache<true>& __to,
- const _Hash_node_code_cache<true>& __from) const
- { __to._M_hash_code = __from._M_hash_code; }
};
/// Partial specialization used when nodes contain a cached hash code.
diff --git
a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
index 10b61464243..010181f7038 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
@@ -417,6 +417,51 @@ test09()
VERIFY( c2.size() == 3 );
}
+struct slow_stateful_hash
+{
+ size_t seed = 0;
+
+ auto operator()(const int& i) const noexcept
+ { return std::hash<int>()(i) + seed; }
+};
+
+namespace std
+{
+ template<>
+ struct __is_fast_hash<slow_stateful_hash> : public std::false_type
+ { };
+}
+
+void
+test10()
+{
+ using map_type = std::unordered_map<int, int, slow_stateful_hash>;
+ map_type c1({ {1, 1}, {3, 3}, {5, 5} }, 0, slow_stateful_hash{1});
+ map_type c2({ {2, 2}, {4, 4}, {6, 6} }, 0, slow_stateful_hash{2});
+ const auto c3 = c2;
+
+ c1.merge(c2);
+ VERIFY( c1.size() == 6 );
+ VERIFY( c2.empty() );
+
+ c2 = c3;
+ c1.clear();
+ c1.merge(std::move(c2));
+ VERIFY( c1 == c3 );
+ VERIFY( c2.empty() );
+
+ c2.merge(std::move(c1));
+ VERIFY( c1.empty() );
+ VERIFY( c2 == c3 );
+
+ c2.merge(c1);
+ VERIFY( c1.empty() );
+ VERIFY( c2 == c3 );
+
+ c2.merge(c2);
+ VERIFY( c2 == c3 );
+}
+
int
main()
{
@@ -429,4 +474,5 @@ main()
test07();
test08();
test09();
+ test10();
}
