RFR: 8281146: Replace StringCoding.hasNegatives with countPositives [v2]

[Claes Redestad]

On Tue, 15 Feb 2022 08:25:29 GMT, Lutz Schmidt <lucy at openjdk.org> wrote:

> Hi Claes, I'm working on the s390 implementation. 

Awesome, thanks!

> 
> Just for clarification: the return value must be the index of the first negative byte?

Yes, or the length if there are no such bytes. 

I've considered (and am still considering) writing the spec of `countPositives` to allow intrinsics to do an early return of a value that is less than the index if it's prohibitively expensive or complicated to implement the intrinsic to be precise in the case where it finds a negative byte. While it must be precise w.r.t. returning the full length if it's all positive bytes, no call site would break if the intrinsic returned 0 or some convenient number less than the first negative index (my first experiments with the x86 intrinsic did it like this, but since the semantics of the intrinsic would then differ from the java code I was asked to try and make it precise). The aarch64 algorithm is proving to be a challenge to work with and I might ask again for some leeway in a first implementation there.

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PR: https://git.openjdk.java.net/jdk/pull/7231