先日、何かをどこまで最適化できるか試して遊んでいました。要素が存在するかどうかを線形検索するだけの単純なマップから始めて、そのほとんどを最適化することにしました。また、比較するために、std::map と std::vector で std::find を使用して同じことを行います。
マップの結果は予想どおりで、私のマップよりも作成と破壊が遅くなりますが、はるかに高速です (実際には、測定できませんでした。常に 0 を返します)。問題は std::vector にあります。私の実装よりも遅いと思っていましたが、そうではありません。私の実装は最悪のケースをスキップしているため（値がベクトルにない）、どうすれば同じまたは高速になるのか本当にわかりません。結果のキャッシュを使用します。
誰かがここで光を当てることができますか? stl の背後にいる連中が半神であることは知っていますが、それでも、これは意味がありません。
ベンチマーク結果 (i3、Windows 8.1 Pro 64、Visual Studio 2013):
std::vector :
Build : 85.0042 ms
Loop : 37.0011 ms
Find : 1.82259 ms -> First : Found, Second : Found, Third : Not Found
Release : 0 ms
--------------------
std::map :
Build : 6929.41 ms
Loop : 570.032 ms
Find : 0 ms -> First : Found, Second : Found, Third : Not Found
Release : 1425.08
--------------------
Linear Map V0:
Build : 194.012 ms
Loop : 49.0052 ms
Find : 1.88915 ms -> First : Found, Second : Found, Third : Not Found
Release : 109.004
マップのコードは次のとおりです。
template<typename T>
class LinearMap0
{
public:
LinearMap0()
{
_end = _root = new Node;
_prebuffer = nullptr;
prebufferCapacity = 0;
_alive = true;
prebufferMarker = 0;
_cache = _mm_set1_epi32(-1);
for (auto& ptr : _cacheBuffer) ptr = nullptr;
MinID = INT32_MAX - 1;
MaxID = -1;
}
void PreAllocate(int Count)
{
prebufferCapacity = Count;
_prebuffer = new Node[Count];
}
~LinearMap0()
{
if (_alive)
{
Release();
}
}
void Release()
{
Node* marker = _end;
while (marker->Prev)
{
marker = marker->Prev;
if (!marker->Next->IsPreAllocated) delete marker->Next;
}
if (!_root->IsPreAllocated) delete _root;
delete[] _prebuffer;
_alive = false;
}
void AddElement(int ID,T element)
{
Node* tmp = nullptr;
if (prebufferMarker < prebufferCapacity)
{
// Use a pre-allocated object
tmp = &_prebuffer[prebufferMarker];
prebufferMarker++;
tmp->IsPreAllocated = true;
}
else
{
tmp = new Node;
}
tmp->ID = ID;
tmp->Data = element;
// Update list
_end->Next = tmp;
Node* prevEnd = _end;
_end = tmp;
_end->Prev = prevEnd;
bool isMin = ID < MinID; MinID = ID * isMin + (1 - isMin) * MinID;
bool isMax = ID > MaxID; MaxID = ID * isMax + (1 - isMax) * MaxID;
}
void DeleteLast()
{
Node* tmp = _end;
_end = _end->Prev;
_end->Next = nullptr;
delete tmp;
}
template<class Function>
void Loop(Function&& f, bool Forward = true)
{
if (Forward)
{
Node* marker = _root;
while (marker->Next)
{
marker = marker->Next;
f(marker->Data);
}
}
else
{
Node* marker = _end;
while (marker->Prev)
{
marker = marker->Prev;
f(marker->Data);
}
}
}
T* Find(int ID)
{
// Bounds check
if (ID < MinID || ID > MaxID) return nullptr;
// Check it it's in the cache
// Compare the value to every value in the cache
__m128i idxSSE = _mm_set1_epi32(ID);
__m128i C = _mm_cmpeq_epi32(_cache, idxSSE);
// To change form -1 to 1
C = _mm_mul_epi32(C, _mm_set1_epi32(-1));
// Now C holds 1 if true, or 0 if false (in each of its 4 members). It should only be ONE set at 1
__m128i tmp = _mm_set1_epi32(1);
__m128i S = _mm_sub_epi32(tmp, C);
// Now find the index
int i = S.m128i_i32[0] * (C.m128i_i32[1] + S.m128i_i32[1] * (2 * C.m128i_i32[2] + S.m128i_i32[2] * (3 * C.m128i_i32[3] + S.m128i_i32[3] * -1)));
if (i != -1)
return _cacheBuffer[i];
// Traverse the list
Node* marker0 = _root;
T* obj = nullptr;
while (true)
{
if (marker0->ID == ID)
{
obj = &marker0->Data;
}
if (marker0->Next) marker0 = marker0->Next; else break;
}
// Cache value and return
_cache.m128i_i32[cacheMarker] = ID;
_cacheBuffer[cacheMarker] = obj;
cacheMarker = (cacheMarker + 1) & 3; // x & 3 = x % 4
return obj;
}
private:
struct Node
{
Node()
{
Prev = nullptr;
Next = nullptr;
IsPreAllocated = false;
ID = -1;
}
T Data;
Node* Prev;
Node* Next;
bool IsPreAllocated;
int ID;
};
Node* _root;
Node* _end;
Node* _prebuffer;
int prebufferCapacity;
int prebufferMarker;
bool _alive;
__m128i _cache;
T* _cacheBuffer[4];
int cacheMarker;
int MinID, MaxID;
};
そして、これがベンチマークです：
// Initialize seeds
const __int64 ecount = 5 * 1000*1000;
vector<__int64> seed(ecount);
for (__int64 i = 0; i < ecount; i++)
{
seed[i] = i;
}
random_shuffle(seed.begin(), seed.end());
///////////// std::vector
vector<__int64> v;
cout << "--------------------" << endl;
cout << "std::vector :" << endl;
cout << " Build : " << time_call([&]()
{
v.resize(ecount/2);
for (__int64 i = 0; i < ecount; i++)
{
if (i < (ecount / 2))
v[i] = seed[i];
else
v.push_back(seed[i]);
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
for (auto& n : v)
n /= 2;
}) << " ms" << endl;
bool found1, found2, found3;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = find(v.begin(), v.end(), seed[5] / 2) != v.end();//find(seed[5]) != m.end();
found2 = find(v.begin(), v.end(), seed[1000] / 2) != v.end();
// Shouldn't exist
found3 = find(v.begin(), v.end(), -1234) != v.end();
}
})) / 15.0) / 3.0;
cout << " ms " << " -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
v.clear();
}) << " ms" << endl;
///////////// std::map
map<__int64, __int64> m;
cout << "--------------------" << endl;
cout << "std::map :" << endl;
cout << " Build : " << time_call([&]()
{
for (__int64 i = 0; i < ecount; i++)
{
m[seed[i]] = seed[i];
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
for (auto& n : m)
n.second /= 2;
}) << " ms" << endl;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = m.find(seed[5]) != m.end();
found2 = m.find(seed[1000]) != m.end();
// Shouldn't exist
found3 = m.find(-1234) != m.end();
}
})) / 15.0) / 3.0;
cout << " ms " << " -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
m.clear();
}) << endl;
///////////// Linear Map V0
LinearMap0<__int64> c;
cout << "--------------------" << endl;
cout << "Linear Map V0:" << endl;
cout << " Build : " << time_call([&]()
{
c.PreAllocate(ecount / 2);
for (__int64 i = 0; i < ecount; i++)
{
c.AddElement(seed[i],seed[i]);
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
c.Loop([](__int64& Data)
{
Data /= 2;
});
}) << " ms" << endl;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = c.Find(seed[5]);
found2 = c.Find(seed[1000]);
// Shouldn't exist
found3 = c.Find(-1234);
}
})) / 15.0) / 3.0;
cout << " ms -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
c.Release();
}) << endl;
編集: time_call は:
template <class Function>
double time_call(Function&& f)
{
chrono::time_point<chrono::high_resolution_clock> start, end;
start = chrono::high_resolution_clock::now();
f();
end = chrono::high_resolution_clock::now();
return ((double)(chrono::duration_cast<chrono::nanoseconds>(end - start).count())) / 1000000.0;
}