RFR: 8341470: BigDecimal.stripTrailingZeros() optimization [v12]

[Raffaello Giulietti]

On Thu, 10 Oct 2024 07:32:15 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> The class Javadoc mentions, in at least two places, that the two's complement representation conceptually has an infinite number of leading virtual sign bits.
>> Once this point is clear, IMO the method's own Javadoc is then clear in turn. Maybe it could be improved as in "the bits that differ _in value_ from its sign bit". (Changing the spec requires another process and another, separate issue, but it can be done if needed.)
>
> @rgiulietti So it must have been my limitation, but I didn't realize it, even though I knew that there are virtually infinite sign bits...

Turns out that the table maintained by `expandBigIntegerTenPowers()` never stores more than 19 * 16 = 304 entries, totaling some 20 kB of resident storage in the worst case. Nor is the iterative multiplication by 10 a problem for such a small cache.

I tried to reduce the size of the `FIVE_TO_2_TO` array to a length of 16 + 1 instead of 20. The size is then around 80 kB, about the same order of magnitude as for the cache above.
The impact on performance is barely noticeable on large number of trailing zeros, with a slowdown of about 5%.


Benchmark                            Mode  Cnt          Score         Error  Units
BigDecimalStripTrailingZeros.testL   avgt   15    2733190.412 ?   51858.195  ns/op
BigDecimalStripTrailingZeros.testM   avgt   15       9262.737 ?      18.105  ns/op
BigDecimalStripTrailingZeros.testS   avgt   15        153.792 ?       1.234  ns/op
BigDecimalStripTrailingZeros.testXL  avgt   15  466326466.667 ? 6076783.818  ns/op
BigDecimalStripTrailingZeros.testXS  avgt   15         21.336 ?       0.058  ns/op
Finished running test 'micro:java.math.BigDecimalStripTrailingZeros'

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PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1795735351