C++ for Scientists - Technische Universität Dresden

174 CHAPTER 5. META-PROGRAMMING
}
unsigned s= size(v), sb= s / BSize ∗ BSize;
for (unsigned i= 0; i < sb; i+= BSize)
one norm ftor()(sum, v, i);
for (unsigned i= 1; i < BSize; i++)
sum[0]+= sum[i];
for (unsigned i= sb; i < s; i++)
sum[0]+= abs(v[i]);
return sum[0];
The according functor must refer the right element in the sum array:
template
struct one norm ftor
{
template
void operator()(S∗ sum, const V& v, unsigned i)
{
using std::abs;
sum[Offset]+= abs(v[i+Offset]);
one norm ftor()(sum, v, i);
}
};
template
struct one norm ftor
{
template
void operator()(S∗ sum, const V& v, unsigned i) {}
};
On the test machine this took:
Compute time one_norm(v) is 7.33 µs.
Compute time one_norm(v) is 5.15 µs.
Compute time one_norm(v) is 2 µs.
Compute time one_norm(v) is 1.4 µs.
Compute time one_norm(v) is 1.16 µs.
This is even a bit slower than the version with one variable. Maybe an array is more expensive
to pass as argument even in an inline function. Let us try something else.
Reducing on a Nested Class Object
⇒ reduction unroll nesting example.cpp
To avoid arrays, we can define a class for n temporary variables where n is a template argument.
Such a class is designed more consistently with the recursive scheme of the functors:
template
struct multi tmp