RFR: JDK-8302028: Port fdlibm atan2 to Java [v2]

[Joe Darcy]

On Fri, 17 Feb 2023 19:33:07 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/FdLibm.java line 458:
>> 
>>> 456:                 case 0, 1 ->  y;              // atan(+/-0, +anything)  = +/-0
>>> 457:                 case 2    ->  Math.PI + tiny; // atan(+0,   -anything)  =  pi
>>> 458:                 default   -> -Math.PI - tiny; // atan(-0,   -anything)  = -pi
>> 
>> The original switch statement and this switch expression are semantically equivalent only because of our knowledge that `m` can only assume values 0, 1, 2, or 3. This requires more reasoning than a more verbatim copy of the original statement. Not sure if it is worth.
>> 
>> The same holds for the switch expressions below.
>
> Yes, the use of switch expressions here is certainly not strictly necessary.
> 
> Something that helped convince me it was okay was that the final switch on m in atan2 uses the set of case labels {0, 1, 2, default}., further implying that m can only take on the values {0, 1, 2, 3}.

Reverted to switch statements rather than switch expressions.

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PR: https://git.openjdk.org/jdk/pull/12608