AB
cpr:
> All destructors found in the C++ standard library are non-throwing
И что, optional в такой ситуации умывает руки?
> All destructors found in the C++ standard library are non-throwing
И что, optional в такой ситуации умывает руки?
1. App build with armv7a-linux-androideabi23-clang for Android is *100 times faster* than build for Linux for the same hardware with arm-linux-gnueabihf-g++ (gcc-7.0).
2. This platform is 32bit, so
sizeof(std::size_t) = 4.3. App uses cache which stores data in
std::unordered_map<TKey>, with sizeof(TKey) = 8.4.
std::unordered_map uses std::hash to calculate the hash of a key.5. Implementation
std::hash for unsigned long long (which is the same as TKey) in armv7a-linux-androideabi23-clang is the following:template <class _Tp, size_t = sizeof(_Tp) / sizeof(size_t)>6. Implementation
struct __scalar_hash;
template <class _Tp>
struct __scalar_hash<_Tp, 2>
: public unary_function<_Tp, size_t>
{
size_t operator()(_Tp __v) const _NOEXCEPT
{ /*...*./
return __murmur2_or_cityhash<size_t>()(&__u, sizeof(__u));
}
};
struct hash<unsigned long long>
: public __scalar_hash<unsigned long long>
{
};
std::hash for unsigned long long (which is the same as TKey) in arm-linux-gnueabihf-g++ is the following:#define _Cxx_hashtable_define_trivial_hash(_Tp) \which is equivalent to
template<> \
struct hash<_Tp> : public __hash_base<size_t, _Tp> \
{ \
size_t \
operator()(_Tp __val) const noexcept \
{ return static_cast<size_t>(__val); } \
};
_Cxx_hashtable_define_trivial_hash(unsigned long long)
return static_cast<std::size_t>(key).7. data keys have defined structure. Lower bits for nearby keys will be the same. Hence casting key to 32bit integer will produce more or less the same value for different nearby keys.
8. On armv7a linux almost all keys are stored in a single bucket of a hash table.
For such a hash table it will be equivalent of searching in a linked list with a few thousand elements. This explains such a difference in performance.
