RFR: 8205592: BigDecimal.doubleValue() is depressingly slow

[Raffaello Giulietti]

On Thu, 7 Jul 2022 15:20:32 GMT, Raffaello Giulietti <duke at openjdk.org> wrote:

> A reimplementation of `BigDecimal.[double|float]Value()` to enhance performance, avoiding an intermediate string and its subsequent parsing on the slow path.

These are the improvements over the current implementation:

- Much more cases are processed by the fast path.
- Most values that will either produce 0 or infinity are detected early in a fast way to avoid expensive computations.
- If neither of the above applies, the conversion to `String` and subsequent parsing, as currently done, is replaced by `BigInteger` arithmetic. There's at most one division between `BigInteger`s. Of course, no need for `toString()` nor parsing.
- Extensive comments explain all the details.

JMH benchmarks show that, on the fast path, the new implementation is on par or way better (>200x for the cases not currently covered).
Cases where 0 or infinity are detected early contribute with speedup factors of >200x.
`BigInteger` arithmetic contributes with speed factors of 2x-8x on "typical" `BigDecimal`s of precision 18 to 24 and scale 2 to 6.

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PR: https://git.openjdk.org/jdk/pull/9410