RFR: 8315585: Optimization for decimal to string

[温绍锦]

BigDecimal is a commonly used class in business development, It is often necessary to perform toString or toPlainString operations on BigDecimal.

The current version uses StringBuilder resulting in multiple memory allocations, I made a modification to improve performance.

Because BigDecimal uses stringCache to cache the result of toString, the performance of toString needs special treatment before testing, such as clearing stringCache by unsafe operation before each test, The performance of toString is similar to that of toEngineering.

The performance data is as follows: 

## 1. benchmark script

sh make/devkit/createJMHBundle.sh
bash configure --with-jmh=build/jmh/jars
make test TEST="micro:java.math.BigDecimals.*ToPlainString"
make test TEST="micro:java.math.BigDecimals.*ToEngineering"


## 2. benchmark environment
* virtual machine : [aliyun_ecs_c8i.xlarge](https://help.aliyun.com/zh/ecs/user-guide/overview-of-instance-families#c8i)
* cpu intel xeon sapphire rapids (x64)

## 3. benchmark result

-BigDecimals.testHugeToPlainString         avgt   15   188.691 ±  0.822  ns/op  (baseline)
-BigDecimals.testLargeToPlainString        avgt   15    36.656 ±  0.065  ns/op
-BigDecimals.testSmallToPlainString        avgt   15    34.342 ±  0.068  ns/op
-BigDecimals.testToPlainString             avgt   15  1719.494 ± 24.886  ns/op

+Benchmark                                 Mode  Cnt     Score    Error  Units (optimize)
+BigDecimals.testHugeToPlainString         avgt   15   133.972 ?  0.328  ns/op (+40.84%)
+BigDecimals.testLargeToPlainString        avgt   15    14.957 ?  0.047  ns/op (145.07%)
+BigDecimals.testSmallToPlainString        avgt   15    12.045 ?  0.036  ns/op (+185.11)
+BigDecimals.testToPlainString             avgt   15  1643.500 ?  3.217  ns/op (+4.62%)

-Benchmark                                 Mode  Cnt     Score    Error  Units (baseline)
-BigDecimals.testHugeToEngineeringString   avgt   15   207.621 ±  5.018  ns/op
-BigDecimals.testLargeToEngineeringString  avgt   15    35.658 ±  3.144  ns/op
-BigDecimals.testSmallToEngineeringString  avgt   15    15.142 ±  0.053  ns/op
-BigDecimals.testToEngineeringString       avgt   15  1813.959 ± 12.842  ns/op

+Benchmark                                 Mode  Cnt     Score    Error  Units (optimize)
+BigDecimals.testHugeToEngineeringString   avgt   15   142.110 ?  0.987  ns/op (+45.09%)
+BigDecimals.testLargeToEngineeringString  avgt   15    12.509 ?  0.056  ns/op (+185.05%)
+BigDecimals.testSmallToEngineeringString  avgt   15    10.579 ?  0.066  ns/op (+43.13%)
+BigDecimals.testToEngineeringString       avgt   15  1646.783 ?  3.060  ns/op (+10.15%)

-------------

Commit messages:
 - update copyright info
 - optimization for decimal to string

Changes: https://git.openjdk.org/jdk/pull/15555/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15555&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315585
  Stats: 748 lines in 4 files changed: 522 ins; 33 del; 193 mod
  Patch: https://git.openjdk.org/jdk/pull/15555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15555/head:pull/15555

PR: https://git.openjdk.org/jdk/pull/15555