On Wed, 26 Jan 2022 12:51:31 GMT, Claes Redestad <redes...@openjdk.org> wrote:
> I'm requesting comments and, hopefully, some help with this patch to replace > `StringCoding.hasNegatives` with `countPositives`. The new method does a very > similar pass, but alters the intrinsic to return the number of leading bytes > in the `byte[]` range which only has positive bytes. This allows for dealing > much more efficiently with those `byte[]`s that has a ASCII prefix, with no > measurable cost on ASCII-only or latin1/UTF16-mostly input. > > Microbenchmark results: > https://jmh.morethan.io/?gists=428b487e92e3e47ccb7f169501600a88,3c585de7435506d3a3bdb32160fe8904 > > - Only implemented on x86 for now, but I want to verify that implementations > of `countPositives` can be implemented with similar efficiency on all > platforms that today implement a `hasNegatives` intrinsic (aarch64, ppc etc) > before moving ahead. This pretty much means holding up this until it's > implemented on all platforms, which can either contributed to this PR or as > dependent follow-ups. > > - An alternative to holding up until all platforms are on board is to allow > the implementation of `StringCoding.hasNegatives` and `countPositives` to be > implemented so that the non-intrinsified method calls into the intrinsified. > This requires structuring the implementations differently based on which > intrinsic - if any - is actually implemented. One way to do this could be to > mimic how `java.nio` handles unaligned accesses and expose which intrinsic is > available via `Unsafe` into a `static final` field. > > - There are a few minor regressions (~5%) in the x86 implementation on > `encode-/decodeLatin1Short`. Those regressions disappear when mixing inputs, > for example `encode-/decodeShortMixed` even see a minor improvement, which > makes me consider those corner case regressions with little real world > implications (if you have latin1 Strings, you're likely to also have > ASCII-only strings in your mix). > Hi Claes, it can get implemented similarly on PPC64: #7430 You can integrate > it if you prefer that, but better after it got a Review. Hi Martin, perfect! Ideally we can get all platforms that has a `hasNegatives` intrinsic moved over so we can just switch it over big-bang style: remove the `@IntrinsicCandidate`, avoid contortions to pick the "right" implementation on the Java level based on which intrinsic is available and drop all VM-internal scaffolding for `hasNegatives`. Then it makes perfect sense to fold your patch into this PR, rather than have a tail of follow-ups. ------------- PR: https://git.openjdk.java.net/jdk/pull/7231
