JDK 9 RFR of JDK-8134795: Port fdlibm pow to Java

[joe darcy]

Hi Jeff,

Thanks for the bug report; I'll take a look into this,

-Joe

On 9/17/2015 3:12 PM, Jeff Hain wrote:
> Hi.
>
>
>
> >At long last, I've started the port of the C version of FDLIBM (freely
> >distributable math library) from C to Java, beginning with the pow 
> method:
>
>
> I ran it through tests of my math lib (jafama),
> and found the following issue:
>
> // ok
>     StrictMath.pow(1.0000000000000004, 2.1E9) = 1.0000009325877754
> FdLibm.Pow.compute(1.0000000000000004, 2.1E9) = 1.0000009325877754
>
> // overflows early
>     StrictMath.pow(1.0000000000000004, 2.2E9) = 1.0000009769967388
> FdLibm.Pow.compute(1.0000000000000004, 2.2E9) = Infinity
>
> // actual overflow is much further
>     StrictMath.pow(1.0000000000000004, 1.59828858065033216E18) = 
> 1.7976931348621944E308
>     StrictMath.pow(1.0000000000000004, 1.59828858065033242E18) = Infinity
>
>
>
> Comparing the behavior of your code with that of Eric Blake's StrictMath
> (the version I got has other issues, but not that one),
> it could be corrected by replacing
>
> if (x_abs > 1.0) {
>
> with
>
> if (x_abs >= 1.0000009536743164) {
>
>
>
> After that modification, tests didn't show any difference with StrictMath.
>
>
>
> -Jeff
>