RFR: 8326227: Rounding error that may distort computeNextGaussian results [v13]

[Chris Hennick]

> This provides a slightly more accurate bounding limit for `computeNextExponentialSoftCapped`. Currently, if the maximum is specified as 12.0, it won't actually return a value larger than `1.5 * exponentialX0` (11.353912041222094); and the error gets worse as we go further into the tail. (This affects slightly less than 12 outputs per million for an ideal RNG.) This could cause the `while (computeNextExponentialSoftCapped(rng, limit) < limit)` check in `computeNextGaussian` on line 1402 to always be true, making `nextGaussian` loop infinitely in the worst case (with probability $$e^{-2^{63} exponentialX0} \approx 10^{-3.022 \times 10^{19}}$$ for an ideal RNG); but more likely, it would give a result that was truncated too close to zero.
> 
> This change is being tested prior to submission to OpenJDK by https://github.com/openjdk/jdk/pull/17703/commits/b8be051cbf40a6a05fafc6a2c76942e9e0b11fdf.

Chris Hennick has updated the pull request incrementally with one additional commit since the last revision:

  Fix?

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17703/files
  - new: https://git.openjdk.org/jdk/pull/17703/files/ddccfa6f..080f3ea8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17703&range=12
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17703&range=11-12

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17703.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17703/head:pull/17703

PR: https://git.openjdk.org/jdk/pull/17703