[PATCH v2] Add powf bench tests

[Joseph Myers]

On Mon, 12 Jun 2017, Paul Clarke wrote:

> +## name: negative base to even exponent
> +-0x1.fffffep+127, -0x1.8p+3
> +-0x1.fffffep+127, -0x1.cccccep+2
> +-0x1.fffffep+127, -0x1.33333p+1
> +-0x1.fffffep+127, 0x1.333338p+1
> +-0x1.fffffep+127, 0x1.ccccdp+2

Many of these are obviously not integer exponents.

> +## name: negative base to odd exponent
> +-0x1.fffffep+127, -0x1.6p+3
> +-0x1.fffffep+127, -0x1.8cccccp+2
> +-0x1.fffffep+127, -0x1.66666p+0
> +-0x1.fffffep+127, 0x1.b33338p+1
> +-0x1.fffffep+127, 0x1.066668p+3

Likewise.

I don't actually think the cases with NaN results (and other special 
cases) are particularly interesting for benchmarking; they're thoroughly 
covered in the tests of correctness, but if NaN results are common enough 
in your application that it's worth optimizing for that case, there's 
something wrong with your application.  That is, there should be a lot 
more of the ordinary cases (ones where powf actually has to compute a log 
and do exponentiation) in the benchmark than of the special cases.

-- 
Joseph S. Myers
joseph@codesourcery.com