C++ for Scientists - Technische Universität Dresden

176 CHAPTER 5. META-PROGRAMMING
}
return sum;
There is one piece still missing. We need to reduce the partial sums in multi sum. Unfortunately
we cannot write a loop over the members of multi sum. So, we need a recursive function that
dives down into multi sum. This would be a bit cumbersome as free function, especially as we
try to avoid partial specialization of template. As a member function, it is much easier and the
specialization happens more safely on the class level:
template
struct multi tmp
{
Value sum() const { return value + sub.sum(); }
};
template
struct multi tmp
{
Value sum() const { return 0; }
};
Note that we started the summation with 0 not the innermost value member. We could do this
but then we need another specialization for multi tmp. Likewise we can implement a
general reduction but we need as in std::accumulate an initial element:
template
struct multi tmp
{
template
Value reduce(Op op, const Value& init) const { return op(value, sub.reduce(op, init)); }
};
template
struct multi tmp
{
template
Value reduce(Op, const Value& init) const { return init; }
};
The compute time of this version is:
Compute time one_norm(v) is 7.47 µs.
Compute time one_norm(v) is 1.14 µs.
Compute time one_norm(v) is 0.71 µs.
Compute time one_norm(v) is 0.75 µs.
Compute time one_norm(v) is 1.01 µs.
Pushing Temporaries into Registers
⇒ reduction unroll registers example.cpp