RFR: 8271732: Regression in StringBuilder.charAt bounds checking

[Naoto Sato]

On Wed, 11 Aug 2021 14:26:32 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`.

LGTM.

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Marked as reviewed by naoto (Reviewer).

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