RFR: 8327791: Optimization for new BigDecimal(String) [v18]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 22 additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into optim_dec_new
 - Merge remote-tracking branch 'upstream/master' into optim_dec_new
 - use while instead for
 - Update src/java.base/share/classes/java/math/BigDecimal.java
   
   Co-authored-by: Claes Redestad <claes.redestad at oracle.com>
 - bug fix for CharArraySequence#charAt
 - bug fix for CharArraySequence
 - fix benchmark
 - one CharArraySequence
 - restore comment
 - easier to compare
 - ... and 12 more: https://git.openjdk.org/jdk/compare/1701be36...bb620ba3

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18177/files
  - new: https://git.openjdk.org/jdk/pull/18177/files/e516b580..bb620ba3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18177&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18177&range=16-17

  Stats: 6763 lines in 129 files changed: 5031 ins; 1483 del; 249 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