RFR: 8327791: Optimization for new BigDecimal(String) [v8]

[Claes Redestad]

On Tue, 12 Mar 2024 10:34:33 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:
> 
>   refactor CharArraySequence

src/java.base/share/classes/java/math/BigInteger.java line 608:

> 606:      * Constructs a new BigInteger using a char array with radix=10.
> 607:      * Sign is precalculated outside and not allowed in the val. The {@code val}
> 608:      * array is assumed to be unchanged for the duration of the constructor

This removed comment about the array being assumed to be unchanged is still applicable to the `CharSequence` (since it'll be backed by the passed in `char[]`).

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PR Review Comment: https://git.openjdk.org/jdk/pull/18177#discussion_r1521309524