RFR: 8315585: Optimization for new BigDecimal(String)

[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..

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

Commit messages:
 - fix code style
 - optimization for new BigDecimal(char[])
 - optimization for new BigDecimal(String)

Changes: https://git.openjdk.org/jdk/pull/18177/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18177&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315585
  Stats: 407 lines in 4 files changed: 331 ins; 29 del; 47 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