RFR: 8327791: Optimization for new BigDecimal(String) [v3]

[Shaojin Wen]

> The current BigDecimal(String) constructor calls String#toCharArray, which has a memory allocation.
> 
> 
> public BigDecimal(String val) {
>     this(val.toCharArray(), 0, val.length()); // allocate char[]
> }
> 
> 
> When the length is greater than 18, create a char[]
> 
> 
> boolean isCompact = (len <= MAX_COMPACT_DIGITS); // 18
> if (!isCompact) {
>     // ...
> } else {
>     char[] coeff = new char[len]; // allocate char[]
>     // ...
> }
> 
> 
> This PR eliminates the two memory allocations mentioned above, resulting in an approximate 60% increase in performance..

Shaojin Wen has updated the pull request incrementally with two additional commits since the last revision:

 - less duplicate code
 - add more benchmark

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18177/files
  - new: https://git.openjdk.org/jdk/pull/18177/files/61b5531b..bdf9f4b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18177&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18177&range=01-02

  Stats: 372 lines in 3 files changed: 78 ins; 287 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18177.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18177/head:pull/18177

PR: https://git.openjdk.org/jdk/pull/18177