RFR: 8315585: Optimization for decimal to string

[温绍锦]

I submitted PR #15555 before, and there were too many changes. I split it into multiple PRs with small changes. This one is one of them.

this PR removed the duplicate code for getChars in BigDecimal#StringBuilderHelper, i also make performance faster, Here are the numbers run on a MacBook M1 Pro:


-Benchmark                                 Mode  Cnt     Score    Error  Units (baseline)
-BigDecimals.testHugeToEngineeringString   avgt   15   228.102 ? 12.568  ns/op
-BigDecimals.testLargeToEngineeringString  avgt   15    53.814 ?  0.180  ns/op
-BigDecimals.testSmallToEngineeringString  avgt   15    17.521 ?  0.521  ns/op
-BigDecimals.testToEngineeringString       avgt   15  1814.858 ? 64.432  ns/op

+Benchmark                                 Mode  Cnt     Score    Error  Units ()
+BigDecimals.testHugeToEngineeringString   avgt   15   209.579 ?  5.037  ns/op (+8.84)
+BigDecimals.testLargeToEngineeringString  avgt   15    29.617 ?  0.557  ns/op (+81.70)
+BigDecimals.testSmallToEngineeringString  avgt   15    11.230 ?  0.075  ns/op (+56.02)
+BigDecimals.testToEngineeringString       avgt   15  1732.913 ? 33.629  ns/op (+4.73)


Please review and don't hesitate to critique my approach and patch.

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Commit messages:
 - optimize BigDecimal.toString

Changes: https://git.openjdk.org/jdk/pull/16006/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16006&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315585
  Stats: 198 lines in 6 files changed: 121 ins; 59 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/16006.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16006/head:pull/16006

PR: https://git.openjdk.org/jdk/pull/16006