Is there a fast C or C++ standard library function for double precision inverse square root?

Question

I find myself typing

double foo=1.0/sqrt(...);

a lot, and I\'ve heard that modern processors have built-in inverse square root opcodes.

Answer 1

If your not afraid of using your own functions, try the following:

template 
T invsqrt(T x)
{
    return 1.0 / std::sqrt(x);
}

It should be just as fast as the orginal 1.0 / std::sqrt(x) in any modernly optimized compiler. Also, it can be used with doubles or floats.