===================================================================
@@ -195,6 +195,18 @@
// @} group hashes
+ // Hint about performance of hash functor. If not fast the hash based
+ // containers will cache the hash code.
+ // Default behavior is to consider that hasher are fast unless specified
+ // otherwise.
+ template<typename _Hash>
+ struct __is_fast_hash : public std::true_type
+ { };
+
+ template<>
+ struct __is_fast_hash<hash<long double>> : public std::false_type
+ { };
+
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
===================================================================
@@ -3053,6 +3053,10 @@
{ return std::_Hash_impl::hash(__s.data(), __s.length()); }
};
+ template<>
+ struct __is_fast_hash<hash<string>> : std::false_type
+ { };
+
#ifdef _GLIBCXX_USE_WCHAR_T
/// std::hash specialization for wstring.
template<>
@@ -3064,6 +3068,10 @@
{ return std::_Hash_impl::hash(__s.data(),
__s.length() * sizeof(wchar_t)); }
};
+
+ template<>
+ struct __is_fast_hash<hash<wstring>> : std::false_type
+ { };
#endif
#endif /* _GLIBCXX_COMPATIBILITY_CXX0X */
@@ -3079,6 +3087,10 @@
__s.length() * sizeof(char16_t)); }
};
+ template<>
+ struct __is_fast_hash<hash<u16string>> : std::false_type
+ { };
+
/// std::hash specialization for u32string.
template<>
struct hash<u32string>
@@ -3089,6 +3101,10 @@
{ return std::_Hash_impl::hash(__s.data(),
__s.length() * sizeof(char32_t)); }
};
+
+ template<>
+ struct __is_fast_hash<hash<u32string>> : std::false_type
+ { };
#endif
_GLIBCXX_END_NAMESPACE_VERSION
===================================================================
@@ -40,9 +40,8 @@
template<typename _Tp, typename _Hash>
using __cache_default
- = __not_<__and_<// Do not cache for builtin integral types having trivial
- // hasher.
- is_integral<_Tp>,
+ = __not_<__and_<// Do not cache for fast hasher.
+ __is_fast_hash<_Hash>,
// Mandatory to make local_iterator default
// constructible.
is_default_constructible<_Hash>,
===================================================================
@@ -26,6 +26,7 @@
# error "hash_c++0x.cc must be compiled with -std=gnu++0x"
#endif
+#include <type_traits>
#include <bits/functional_hash.h>
namespace std _GLIBCXX_VISIBILITY(default)
Hi Here is the last patch I can think of for 4.8. Thanks to it default performance reported in performance/23_containers/insert/54075.cc and performance/23_containers/insert_erase/41975.cc are always the best: 54075.cc std::unordered_set without hash code cached 300000 insertion attempts, 300000 inserted 10r 8u 1s 13761936mem 0pf 54075.cc std::unordered_set without hash code cached 10 times insertion of 300000 elements 31r 31u 0s 0mem 0pf 54075.cc std::unordered_set with hash code cached 300000 insertion attempts, 300000 inserted 10r 9u 1s 18562000mem 0pf 54075.cc std::unordered_set with hash code cached 10 times insertion of 300000 elements 34r 35u 0s 0mem 0pf 54075.cc std::unordered_set default cache 300000 insertion attempts, 300000 inserted 9r 8u 0s 13761936mem 0pf 54075.cc std::unordered_set default cache 10 times insertion of 300000 elements 31r 32u 0s 0mem 0pf 41975.cc std::unordered_set<string> without hash code cached: first insert 9r 9u 0s 8450336mem 0pf 41975.cc std::unordered_set<string> without hash code cached: erase from iterator 6r 5u 0s -6400096mem 0pf 41975.cc std::unordered_set<string> without hash code cached: second insert 6r 5u 0s 6400000mem 0pf 41975.cc std::unordered_set<string> without hash code cached: erase from key 5r 5u 0s -6400000mem 0pf 41975.cc std::unordered_set<string> with hash code cached: first insert 5r 5u 1s 8450336mem 0pf 41975.cc std::unordered_set<string> with hash code cached: erase from iterator 4r 3u 0s -6400096mem 0pf 41975.cc std::unordered_set<string> with hash code cached: second insert 3r 3u 0s 6400016mem 0pf 41975.cc std::unordered_set<string> with hash code cached: erase from key 4r 3u 0s -6400016mem 0pf 41975.cc std::unordered_set<string> default cache: first insert 5r 5u 1s 8450336mem 0pf 41975.cc std::unordered_set<string> default cache: erase from iterator 4r 3u 0s -6400096mem 0pf 41975.cc std::unordered_set<string> default cache: second insert 3r 3u 0s 6400000mem 0pf 41975.cc std::unordered_set<string> default cache: erase from key 4r 3u 0s -6400000mem 0pf 2013-02-02 François Dumont <fdumont@gcc.gnu.org> * include/bits/functional_hash.h (std::__is_fast_hash<>): New. * include/bits/basic_string.h: Specialize previous to mark std::hash for string types as slow. * include/bits/hashtable.h (__cache_default): Replace is_integral with __is_fast_hash. * src/c++11/hash_c++0x.cc: Add type_traits include. Tested under Linux x86_64. Ok to commit ? François