RFR: 8302863: Speed up String::encodeASCII using countPositives

[Brett Okken]

On Sat, 18 Feb 2023 23:26:08 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> When encoding Strings to US-ASCII we can speed up the happy path significantly by using `StringCoding.countPositives` as a speculative check for whether there are any chars that needs to be replaced by `'?'`. Once a non-ASCII char is encountered we fall back to the slow loop and replace as needed.
> 
> An alternative could be unrolling or using a byte array VarHandle, as show-cased by Brett Okken here: https://mail.openjdk.org/pipermail/core-libs-dev/2023-February/100573.html Having to replace chars with `?` is essentially an encoding error so it might be safe to assume this case is exceptional in practice.

For the happy path of no encoding failures, this "trivial" change is a great improvement.

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Marked as reviewed by bokken at github.com (no known OpenJDK username).

PR: https://git.openjdk.org/jdk/pull/12640