玩了一段時間後導致了另一種方法,使我們可以用模板元編程結合C++ 11的可變參數模板和lambda功能展開的for循環所需的數字:
template <unsigned int DIMENSION>
inline unsigned long tuple_to_index_fixed_dimension(const_tup_t tup, const_tup_t shape) {
unsigned long res = 0; unsigned int k;
for (k=0; k<DIMENSION−1; ++k) {
res += tup[k];
res *= shape[k+1];
}
res += tup[k];
return res;
}
template <unsigned int DIMENSION, unsigned int CURRENT>
class ForEachFixedDimensionHelper {
public:
template <typename FUNCTION, typename ...TENSORS>
inline static void apply(tup_t counter, const_tup_t shape, FUNCTION function, TENSORS & ...args) {
for (counter[CURRENT]=0; counter[CURRENT]<shape[CURRENT]; ++counter[CURRENT])
ForEachFixedDimensionHelper<DIMENSION−1, CURRENT+1>::template apply<FUNCTION, TENSORS...>(counter, shape, function, args...);
}
};
template <unsigned int CURRENT>
class ForEachFixedDimensionHelper<1u, CURRENT> {
public:
template <typename FUNCTION, typename ...TENSORS>
inline static void apply(tup_t counter, const_tup_t shape, FUNCTION function, TENSORS & ...args) {
for (counter[CURRENT]=0; counter[CURRENT]<shape[CURRENT]; ++counter[CURRENT])
function(args[tuple_to_index_fixed_dimension<CURRENT+1>(counter, args.data_shape())]...); /* tensor.data_shape() is an accessor for returning the shape member. */
}
};
template <unsigned char DIMENSION>
class ForEachFixedDimension {
public:
template <typename FUNCTION, typename ...TENSORS>
inline static void apply(const_tup_t shape, FUNCTION function, TENSORS & ...args) {
unsigned long counter[DIMENSION];
memset(counter, 0, DIMENSION*sizeof(unsigned long));
ForEachFixedDimensionHelper<DIMENSION,0>::template apply<FUNCTION, TENSORS...>(counter, shape, function, args...);
}
};
還要注意元組值和形狀可以安全地聲明爲__restrict,這意味着它們指向不同的內存位置,因爲它們將專門構建用於迭代,然後解除分配。當另一個指針被解除引用和更改時(「指針別名」問題),由這些指針索引的值不需要從內存中重新讀取。當調用ForEachFixedDimension :: template apply時,可以在編譯時根據張量參數的內容推斷出typename FUNCTION(可能是一個lambda函數)和模板參數包typename ... TENSORS(variadic support)參數類型的功能。的展開for循環
所需的數量可以在運行時擡頭:
typedef unsigned int TEMPLATE_SEARCH_INT_TYPE;
template <TEMPLATE_SEARCH_INT_TYPE MINIMUM, TEMPLATE_SEARCH_INT_TYPE MAXIMUM, template <TEMPLATE_SEARCH_INT_TYPE> class WORKER>
class LinearTemplateSearch {
public:
template <typename...ARG_TYPES>
inline static void apply(TEMPLATE_SEARCH_INT_TYPE v, ARG_TYPES && ... args) {
if (v == MINIMUM)
WORKER<MINIMUM>::apply(std::forward<ARG_TYPES>(args)...);
else
LinearTemplateSearch<MINIMUM+1, MAXIMUM, WORKER>::apply(v, std::forward<ARG_TYPES>(args)...);
}
};
template <TEMPLATE_SEARCH_INT_TYPE MAXIMUM, template <TEMPLATE_SEARCH_INT_TYPE> class WORKER >
class LinearTemplateSearch<MAXIMUM, MAXIMUM, WORKER> {
public:
template <typename...ARG_TYPES>
inline static void apply(TEMPLATE_SEARCH_INT_TYPE v, ARG_TYPES && ... args) {
assert(v == MAXIMUM);
WORKER<MAXIMUM>::apply(std::forward<ARG_TYPES>(args)...);
}
};
注意,在這裏,儘管模板遞歸時,尺寸需要不直到運行時是已知的。這基本上是通過使用模板作爲即時(JIT)編譯形式,爲所有感興趣的維度預先計算策略,然後在運行時查找正確的策略來實現的。
所以這些方法用Benchmark進行了測試。在基準1中,數據被從形狀的張量(2 ,2 ,)複製到形狀的張量(2 ,2 ,2 )。在基準2,和形狀的兩個張量之間的內積(2 ,2 ,2 )(2 ,2 ,2 )被計算(僅來訪由兩者共享的元組索引)。將模板遞歸的實現與其他替代方法進行比較:元組迭代;元組迭代,其中維在編譯時已知;整數重新索引;整數重新索引,其中軸限制爲2的冪; numpy的; C風格for循環(硬編碼);矢量化的Fortran代碼; Go中的循環。
事實證明模板遞歸比元組索引,並且該方法通過升壓使用更快:
灰色數字表示平均運行時和誤差棒的分和最大。這裏是本方法的那些實施了用於基準1爲每個方法:
// Tuple iteration (DIMENSION must be compile−time constant): vector<unsigned long> t(DIMENSION);
t.fill(0);
unsigned long k;
for (k=0; k<x.flat.size(); advance_tuple_fixed_dimension<DIMENSION>(&t[0], &x.data_shape()[0]), ++k)
x[k] = y[tuple_to_index_fixed_dimension<DIMENSION>(&t[0], &y.data_shape()[0])];
// boost:
x[ boost::indices[range(0, x.shape[0])][range(0,x.shape[1])][range(0,x.shape[2])] ] = y[ boost::indices[range(0,x.shape[0])][range(0,x.shape[1])][range(0,x.shape[2])] ];
! Fortran 95
x = y(1:2**5,1:2**9,1:2**9)
// Hard−coded for loops in C: unsigned long k;
for (k=0; k<x.data_shape()[0]; ++k) {
for (unsigned long j=0; j<x.data_shape()[1]; ++j) {
unsigned long x_bias = (k*x.data_shape()[1] + j)*x.data_shape()[2];
unsigned long y_bias = (k*y.data_shape()[1] + j)*y.data_shape()[2];
for (unsigned long i=0; i<x.data_shape()[2]; ++i)
x[x_bias + i] = y[y_bias + i];
}
}
// Integer reindexing:
unsigned long k;
for (k=0; k<x.flat.size(); ++k)
x[k] = y[reindex(k, &x.data_shape()[0], &y.data_shape()[0], DIMENSION)];
// Integer reindexing (axes are powers of 2):
unsigned long k;
for (k=0; k<x.flat.size(); ++k)
x[k] = y[reindex_powers_of_2(k, &x_log_shape[0], &y_log_shape[0], DIMENSION)];
// Tuple iteration (DIMENSION unknown at compile time):
vector<unsigned long> t(DIMENSION);
t.fill(0);
unsigned long k;
for (k=0; k<x.flat_size(); advance_tuple(&t[0], &x.data_shape()[0], DIMENSION), ++k)
x[k] = y[t];
# numpy (python):
x_sh = x.shape.
x = np.array(y[:x_sh[0], :x_sh[1], :x_sh[2]])
// Go:
for i:=0; i<1<<9; i++ {
for j:=0; j<1<<9; j++{
for k:=0; k<1<<5; k++{
x[i][j][k] = y[i][j][k]
}
}
}
// TRIOT (DIMENSION unknown at compile time):
apply_tensors([](double & xV, double yV) {
xV = yV;
},
x.data_shape(),
x, y);
令人驚訝地,整數重新索引(即使軸是2的冪)基本上不是使一個元組計數器慢。與模板遞歸版本有時更快(包括比boost更快30%,即使boost :: multi_array必須知道編譯時的維數)。
這裏是你將如何使用這個嵌套的循環招用模板遞歸一個又一個例子:
double dot_product(const Tensor & x<double>, const Tensor<double> & y) { // This function written for homogeneous types, but not unnecessary
double tot = 0.0;
for_each_tensors([&tot](double xV, double yV) {
tot += xV * yV;
},
x.data_shape(), /* Iterate over valid tuples for x.data_shape(); as written, this line assumes x has smaller shape*/
x, y);
return tot;
}
,並通過元組迭代多維卷積的實現,該版本與模板遞歸和numpy的進行了比較通過卷積兩個矩陣,每個矩陣具有形狀(2 ,2 )。
Tensor<double> triot_naive_convolve(const Tensor<double> & lhs, const Tensor<double> & rhs) {
assert(lhs.dimension() == rhs.dimension());
Tensor<double> result(lhs.data_shape() + rhs.data_shape() − 1ul);
result.flat().fill(0.0);
Vector<unsigned long> counter_result(result.dimension());
enumerate_for_each_tensors([&counter_result, &result, &rhs](const_tup_t counter_lhs, const unsigned int dim_lhs, double lhs_val) {
enumerate_for_each_tensors([&counter_result, &result, &rhs, &counter_lhs, &lhs_val](const_tup_t counter_rhs, const unsigned int dim_rhs, double rhs_val) {
for (unsigned int i=0; i<dim_rhs; ++i)
counter_result[i] = counter_lhs[i] + counter_rhs[i];
unsigned long result_flat = tuple_to_index(counter_result, result.data_shape(), dim_rhs);
result.flat()[result_flat] += lhs_val * rhs_val;
},
rhs.data_shape(),
rhs);
},
lhs.data_shape(), lhs);
return result;
}
這些基準是定時的2.0 GHz的英特爾Core i7的芯片上的優化(-std = C++ 11 -Ofast -march =天然 - mtune中=天然-fomit幀指針)。所有Fortran實現都以相反順序使用軸,並以緩存優化方式訪問數據,因爲Fortran使用列主數組格式。 可在此small journal article中找到詳細信息和源代碼(一個簡單的多維數組庫,其中在編譯時不需要知道維數)。