Re: [eigen] fix double-promotion in 3.3 Core/SpecialFunctions.h

[Ilja Honkonen <ilja.j.honkonen@xxxxxxxx>]

> > > std::pow(float, int) actually gets promoted to std::pow(double, double).
> > Note that this is correct as of c++11:
> > http://en.cppreference.com/w/cpp/numeric/math/pow
> Yes, I looked that up as well. I don't know what gave me the idea that
> there were pow(float, int) and pow(double, int) overloads.

Those used to exist but c++11 removed them. According to stackoverflow 
and others the old overloads could have been faster. Don't know how 
intentional this was of the c++ standards committee.

> > > This is especially bad for std::pow(x, 2). Inside Eigen we can use
> > Does pow(x, 2.0) fix it? If not, pow(x, float(2)) at least should...
> Yes that would work, but it is inefficient. I (wrongfully) expected
> pow(x, 2) to be evaluated to x*x, at least if the exponent is known at
> compile time and small. That could work, if pow(float, int) got

Looks like boost has this covered: 
http://www.boost.org/doc/libs/1_60_0/libs/math/doc/html/math_toolkit/powers/ct_pow.html 
This overload would be nice for Eigen types also.
Ilja