On Mon, 6 Sep 2021 06:45:07 GMT, q2q-2q2
<[email protected]> wrote:
>> Shortcut String equality checks by checking equality of the value array
>
> q2q-2q2 has updated the pull request incrementally with one additional commit
> since the last revision:
>
> JDK-8272192 Shortcut String equality checks by checking equality of the
> value array
David is right that many of these methods have VM intrinsics that'd have to be
fixed to see much benefit of this.
I can imagine some of these to be profitable in edge cases with string
de-duplication enabled. Otherwise the backing arrays will typically never be
shared and the identity test always be false (`""` is an exception to that rule
that interestingly is unlikely to see any win from short-cutting). While the
gain from a correct shortcut could be really large on a synthetic
micro-benchmark, I still suspect the added test might be more costly in
aggregate on the fabled "real application".
With compact strings it's easy to construct strings with different coders but
the same `byte[]`, and string de-duplication could be implemented so that you
end up sharing the same array between two semantically different strings (I
suspect this is the case, but I might be wrong). This means there's a possible
correctness issue in some places where you're shortcutting before checking
either length or coder.
All things considered I don't think this is worthwhile.
src/java.base/share/classes/java/lang/String.java line 1859:
> 1857: byte v1[] = value;
> 1858: byte v2[] = sb.getValue();
> 1859: if (v1 == v2) {
This one is a bit tricky since it's only correct since we've asserted that `len
== sb.length()`. It's unlikely to consistently give much of a boost, though,
since `sb.getValue()` is very likely to have some unused padding at the end.
-------------
PR: https://git.openjdk.java.net/jdk/pull/5370
