RFR: 8327791: Optimization for new BigDecimal(String) [v13]

[Chen Liang]

On Wed, 13 Mar 2024 15:46:30 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:
> 
>   bug fix for CharArraySequence

src/java.base/share/classes/java/math/BigDecimal.java line 561:

> 559:             index += offset;
> 560:             if (index >= length)
> 561:                 throw new IndexOutOfBoundsException();

This logic is wrong: if offset is 3 and length is 2, aab*bc*c would be valid, but your code will IOOBE on `charAt(0)` because `index += offset` will be 3, 3 > 2.

You should use `Objects.checkIndex(index, length)` instead.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18177#discussion_r1523540101