RFR: 8284493: Fix rounding error in computeNextExponential [v2]

[openjdk-notifier[bot]]

On Thu, 2 Jun 2022 03:00:32 GMT, Chris Hennick <duke at openjdk.java.net> wrote:

>> Chris Hennick has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
>> 
>>   Fix rounding error in computeNextExponential; use FMA
>>   
>>   Repeatedly adding DoubleZigguratTables.exponentialX0 to extra causes a rounding error to accumulate at the tail of the distribution; this fixes that by tracking the multiple of exponentialX0 as a long.
>>   
>>   Add computeWinsorizedNextExponential for testability
>
> In addition to the changes discussed heretofore, I've also changed line 1382 to eliminate unneeded tail exploration; this should make `nextGaussian` faster at high percentiles (probably measurable at 99%ile; should definitely be measurable at  at 99.99%ile).

@Pr0methean Please do not rebase or force-push to an active PR as it invalidates existing review comments. All changes will be squashed into a single commit automatically when integrating. See [OpenJDK Developers’ Guide](https://openjdk.java.net/guide/#working-with-pull-requests) for more information.

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PR: https://git.openjdk.java.net/jdk/pull/8131