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

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On 2016-05-05 15:39, Marc Glisse wrote:
On Thu, 5 May 2016, Christoph Hertzberg wrote:
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 evaluated to some __builtin_pow function and the
compiler decides depending on the size of the exponent how to evaluate

That is the case. Gcc has __builtin_pow (takes 2 double) and expands it
when the second argument is a small integer (though in this case it
produces (float)((double)x*(double)x)). More relevant builtins are
__builtin_powf (takes 2 floats) and __builtin_powif (takes a float and
an int).

Ok, my memory is not as bad as I thought. I recalled that something like that existed once. And as Ilja pointed out, at least gcc used to do that prior to C++11 (and apparently I missed the C++03 part when I looked it up).

However, writing pow(x,2) should really be avoided...

Yes, it is kind of annoying though to implement trivial square, cube, etc, functions for the few cases you need them (or to add a boost-dependency, unless you already do).


 Dipl. Inf., Dipl. Math. Christoph Hertzberg

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