C++ for Scientists - Technische Universität Dresden

158 CHAPTER 5. META-PROGRAMMING
A state-of-the-art compiler will recognize that all iterations are independent one from each
other, e.g., data[2]= that[2]; is independent of data[1]= that[1];. The compiler will also determine
the size of loop during compilation. As a consequence, the generated binary of a type with size
3 will be equivalent to:
template
class fsize vector
{
template
self& operator=(const self& that)
{
data[0]= that[0];
data[1]= that[1];
data[2]= that[2];
}
};
The right-hand-side vector that might be an expression template § 5.3 for say alpha ∗ x + y and
its evaluation will be also inlined:
template
class fsize vector
{
template
self& operator=(const self& that)
{
data[0]= alpha ∗ x[0] + y[0];
data[1]= alpha ∗ x[1] + y[1];
data[2]= alpha ∗ x[2] + y[2];
}
};
To make the unrolling more explicit and for the sake of step-wise introducing meta-tuning we
develop a functor that computes the assignment:
template
struct fsize assign
{
void operator()(Target& tar, const Source& src)
{
fsize assign()(tar, src);
std::cout ≪ ”assign entry ” ≪ N ≪ ’\n’;
tar[N]= src[N];
}
};
template
struct fsize assign
{
void operator()(Target& tar, const Source& src)
{
std::cout ≪ ”assign entry ” ≪ 0 ≪ ’\n’;
tar[0]= src[0];
}
};