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

[Raffaello Giulietti]

On Wed, 26 Apr 2023 01:47:07 GMT, Joe Darcy <darcy 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.
>
> src/java.base/share/classes/java/math/BigDecimal.java line 3749:
> 
>> 3747:      */
>> 3748:     @Override
>> 3749:     public float floatValue() {
> 
> If the total size of code were a concern, I wouldn't be adverse to floatValue() being implemented as
> 
> return (float)doubleValue();
> 
> Given the 2p+2 property between the float and double formats, BigDecimal -> double -> float and BigDecimal -> float round the same way.

No, that would not be correct. It would be subject to [double rounding](https://en.wikipedia.org/wiki/Rounding#Double_rounding), against the spec.

For example, `BigDecimal` 1.000000059604644775390626 should round to `float` `1.0000001f`.
When going to the closest `double` and then to the closest `float`, however, it first rounds to `double` `1.0000000596046448` and the to `float` `1.0f`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/9410#discussion_r1177530816