RFR: JDK-8262994: Refactor String.split to help method inlining

[Ismael Juma]

On Tue, 27 Dec 2022 20:12:51 GMT, Christian Wimmer <cwimmer at openjdk.org> wrote:

> The method `String.split` contains a fast-path when the regular expression parameter is not really a regular expression, but just a single split character.
> This fast path vs. slow path check can be constant folded when the regular expression parameter is a literal constant - a quite frequent pattern (for example, all JDK usages of `String.split` have a constant expression parameter). But method inlining in JIT and AOT compilers can usually not inline `String.split` because the method body is too large. Factoring out the actual fast-path splitting logic into a separate method solves this problem: the JIT or AOT compiler can inline `String.split`, constant-fold the fast/slow path check, and then only the invoke of either the fast path or the slow path remains.

src/java.base/share/classes/java/lang/String.java line 3133:

> 3131:         {
> 3132:             // All the checks above can potentially be constant folded by
> 3133:             // a JIT/AOT compiler when the regex is a constant string.

Probably worth mentioning explicitly that the private `split` was extracted to help with inlining (to reduce the probability of future regressions).

-------------

PR: https://git.openjdk.org/jdk/pull/11791