RFR: 8327791: Optimization for new BigDecimal(String) [v10]

[Shaojin Wen]

On Tue, 12 Mar 2024 13:07:26 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:
> 
>   restore comment

I agree with you, the char[] constructors is probably less performance sensitive than the others.

The performance numbers under MacBookPro M1 Max are as follows:


-Benchmark                                      Mode  Cnt   Score   Error  Units (one CharArraySequence)
-BigDecimals.testConstructorWithSmallCharArray  avgt   15  19.157 ? 0.074  ns/op

+Benchmark                                      Mode  Cnt   Score   Error  Units (two CharArraySequence)
+BigDecimals.testConstructorWithSmallCharArray  avgt   15  17.833 ? 0.124  ns/op +7.42

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PR Comment: https://git.openjdk.org/jdk/pull/18177#issuecomment-1991781940