RFR: 8327791: Optimization for new BigDecimal(String) [v12]

[Claes Redestad]

On Wed, 13 Mar 2024 13:12:54 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> If the input is "+" or "-" an exception will be thrown on line 583
>> 
>>             boolean isneg = c == '-'; // leading minus means negative
>>             if (isneg || c == '+') {
>>                 c = val.charAt(++offset); 
>>                 len--;
>>             }
>
> if the input is "" an exception will be throw on line 579
> 
>             int offset = 0;
>             char c = val.charAt(offset);

Relying on the upper bounds check of `charAt` doesn't work well with the `CharArraySequence` whose `charAt` deliberately does not throw an IIOBE if the array is longer than the provided length, ie, it'll look at chars beyond the provided range. The examples I tested still end up as a NFE, but it's clear from the cause that we're running past the length:

jshell> new BigDecimal(new char[] { '-', '1', 'e'}, 0, 1);
|  Exception java.lang.NumberFormatException
|        at BigDecimal.<init> (BigDecimal.java:754)
|        at BigDecimal.<init> (BigDecimal.java:543)
|        at BigDecimal.<init> (BigDecimal.java:518)
|        at (#4:1)
|  Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
|        at BigDecimal$CharArraySequence.charAt (BigDecimal.java:559)
|        at BigDecimal.parseExp (BigDecimal.java:772)
|        at BigDecimal.<init> (BigDecimal.java:619)
|        ...

Baseline/expected:

jshell> new BigDecimal(new char[] { '-', '1', 'e'}, 0, 1);
|  Exception java.lang.NumberFormatException: No digits found.
|        at BigDecimal.<init> (BigDecimal.java:635)
|        at BigDecimal.<init> (BigDecimal.java:518)
|        at (#1:1)

Having a check on `len > 0` is more robust - and I'd be surprised if avoiding a redundant check on the loop entry is affecting performance?

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PR Review Comment: https://git.openjdk.org/jdk/pull/18177#discussion_r1523301252