RFR: 8271732: Regression in StringBuilder.charAt bounds checking [v4]

[Alan Bateman]

On Wed, 11 Aug 2021 21:40:52 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> In #4738 we removed the `checkIndex(value, count)` call in `ASB.charAt` to avoid potentially getting two bounds checks in the UTF-16 case. Problem is this optimization cause a regression since `StringUTF16.charAt(..)` checks `index` against the length of the `value` array and not `count`.
>> 
>> A correct fix that still maintain the possible performance advantage reported by #4738 is to reinstate the `checkIndex(value, count)` and call `StringUTF16.getChar` instead of `charAt`.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix copy-paste error

Marked as reviewed by alanb (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/5086