RFR: 8302204: Optimize BigDecimal.divide

[Raffaello Giulietti]

On Fri, 10 Feb 2023 10:00:05 GMT, Xiaowei Lu <duke at openjdk.org> wrote:

> [JDK-8269667](https://bugs.openjdk.org/browse/JDK-8269667) has uncovered the poor performance of BigDecimal.divide under certain circumstance.
> 
> We confront similar situations when benchmarking Spark3 on TPC-DS test kit. According to the flame-graph below, it is StripZeros that spends most of the time of BigDecimal.divide. Hence we propose this patch to optimize stripping zeros.
> ![图片 1](https://user-images.githubusercontent.com/39413832/218062061-53cd0220-776e-4b72-8b9a-6b0f11707986.png)
> 
> Currently, createAndStripZerosToMatchScale() is performed linearly. That is, the target value is parsed from back to front, each time stripping out single ‘0’. To optimize, we can adopt the method of binary search. That is, each time we try to strip out ${scale/2} ‘0’s. 
> 
> The performance looks good. Therotically, time complexity of our method is O(log n), while the current one is O(n). In practice, benchmarks on Spark3 show that 1/3 less time (102s->68s) is spent on TPC-DS query4. We also runs Jtreg and JCK to check correctness, and it seems fine.
> 
> More about environment: 
> we run Spark3.3.0 on Openjdk11, but it seems jdk version doesn’t have much impact on BigDecimal. Spark cluster consists of a main node and 2 core nodes, each has 4cores, 16g memory and 4x500GB storage.

After making sure that `intVal` is even, and before attempting a division by a power of 10, it might help to check if 5 divides `intVal` in the first place. If it doesn't, there no point in performing the division.

It can be shown that 5 divides `intVal` if and only if it divides the `long` sum of all `int`s in the `mag` array of `intVal`.

I didn't try out if this contributes to improve the overall performance, but it might be worth giving a try.

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PR: https://git.openjdk.org/jdk/pull/12509