RFR: 8327791: Optimization for new BigDecimal(String) [v4]

[Chen Liang]

On Mon, 11 Mar 2024 20:41:25 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> 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 one additional commit since the last revision:
> 
>   1. bug fix for CharBuffer catch IndexOutOfBoundsException
>   2. reorder if statement
>   3. Improve the performance of !isCompact branch

src/java.base/share/classes/java/math/BigDecimal.java line 762:

> 760:     }
> 761: 
> 762:     private BigDecimal(CharSequence val, MathContext mc) {

Can we move this constructor to before `parseExp`? This way there will be less diff and it will be easier to compare old and new code side-by-side.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18177#discussion_r1520792359