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

[Raffaello Giulietti]

On Mon, 7 Oct 2024 19:48:20 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> `BigDecimal` was not designed to do astronomical computations. One would probably use other libraries for that, e.g., [GMP](https://gmplib.org/) and would not be interested too much in "stripping trailing zeros".
>> The focus of this class is mainly commercial applications, where decimal arithmetic is sometimes mandated by regulators.
>> 
>> So I'd expect that most usages, for example SQL or commercial application workloads, will process numbers with a few dozens of digits, maybe sometimes some hundreds digits for intermediate results. So I think that accelerating for these sizes makes the most practical sense.
>> 
>> With modest memory and CPU costs, I think even one million digits or so might be processed reasonably fast.
>> My point is that there's room for improvement in these range of sizes, but perhaps not for larger ones.
>> 
>> More importantly, though, I'd like to avoid one single computation to affect the resident memory footprint of a JVM instance for its entire lifetime.
>
> @rgiulietti FYI, I have found that `BigInteger` has already a cache for the repeated squares that uses in radix string conversion, and it is provided by the method `BigInteger.getRadixConversionCache(int radix, int exponent)`. Although, I don't know if it would be a good idea to use it, because it computes all the squares up to `radix^(2^exponent)`, and this may not be necessary but could cost a lot of running time and memory if the number to strip is very large...

Seems like `BigInteger`s division either takes care of removing common powers of 2 in the dividend and divisor (Knuth division), or removing them has no practical benefit (Burnikel-Ziegler).

Thus, I'm wondering if using powers of 10 rather than powers of 5 in the stripping logic could simplify the code while retaining comparable performance.

The powers of 10 are already indirectly kept in the class by method `bigTenToThe()` (although they affect resident memory footprint). However, since that method is there to stay and used in other places as well, it might be worthwhile to profit from it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1793603310