C++ for Scientists - Technische Universität Dresden

5.4. META-TUNING: WRITE YOUR OWN COMPILER OPTIMIZATION 175
{
};
typedef multi tmp sub type;
multi tmp(const Value& v) : value(v), sub(v) {}
Value value;
sub type sub;
template
struct multi tmp
{
multi tmp(const Value& v) {}
};
An object of this type can be recursively initialized so that we do not need a loop as for the
array. A functor can operate on the value member and pass a reference to the sub member to
its successor. This leads us to the implementation of our functor:
template
struct one norm ftor
{
template
void operator()(S& sum, const V& v, unsigned i)
{
using std::abs;
sum.value+= abs(v[i+Offset]);
one norm ftor()(sum.sub, v, i);
}
};
template
struct one norm ftor
{
template
void operator()(S& sum, const V& v, unsigned i) {}
};
The unrolled function that uses this functor reads:
template
typename Vector::value type
inline one norm(const Vector& v)
{
using std::abs;
typedef typename Vector::value type value type;
multi tmp multi sum(0);
unsigned s= size(v), sb= s / BSize ∗ BSize;
for (unsigned i= 0; i < sb; i+= BSize)
one norm ftor()(multi sum, v, i);
value type sum= multi sum.sum();
for (unsigned i= sb; i < s; i++)
sum+= abs(v[i]);