RFR: 8374582: [REDO] Move input validation checks to Java for java.lang.StringCoding intrinsics

[Volkan Yazici]

On Mon, 12 Jan 2026 10:29:39 GMT, Damon Fenacci <dfenacci at openjdk.org> wrote:

> ## Issue
> 
> This is a redo of [JDK-8361842](https://bugs.openjdk.org/browse/JDK-8361842) which was backed out by [JDK-8374210](https://bugs.openjdk.org/browse/JDK-8374210) due to C2-related regressions. The original change moved input validation checks for java.lang.StringCoding from the intrinsic to Java code (leaving the intrinsic check only with the `VerifyIntrinsicChecks` flag). Refer to the [original PR](https://github.com/openjdk/jdk/pull/25998) for details.
> 
> This additional issue happens because, in some cases, for instance when the Java checking code is not inlined and we give an out-of-range constant as input, we fold the data path but not the control path and we crash in the backend.
> 
> ## Causes
> 
> The cause of this is that the out-of-range constant (e.g. -1) floats into the intrinsic and there (assuming the input is valid) we add a constraint to its type to positive integers (e.g. to compute the array address) which makes it top.
> 
> ## Fix
> 
> A possible fix is to introduce an opaque node (OpaqueGuardNode) similar to what we do in `must_be_not_null` for values that we know cannot be null:
> https://github.com/openjdk/jdk/blob/ce721665cd61d9a319c667d50d9917c359d6c104/src/hotspot/share/opto/graphKit.cpp#L1484
> This will temporarily add the range check to ensure that C2 figures that out-of-range values cannot reach the intrinsic. Then, during macro expansion, we replace the opaque node with the corresponding constant (true/false) in product builds such that the actually unneeded guards are folded and do not end up in the emitted code.
> 
> # Testing
> 
> * Tier 1-3+
> * 2 JTReg tests added
>   * `TestRangeCheck.java` as regression test for the reported issue
>   * `TestOpaqueGuardNodes.java` to check that opaque guard nodes are added when parsing and removed at macro expansion

I'd like to provide some help for reviewers:

1. [JDK-8361842] (integrated in 655dc516c22) implemented changes for `java.lang.StringCoding`
2. [JDK-8374210] (integrated in 7e18de137c3) reported regressions against JDK-8361842, and used as the BACKOUT issue.
3. [JDK-8374582] (this PR) is the REDO of JDK-8361842, plus the fix for regressions reported in JDK-8374210

That is, this PR starts with 3c466d372b7 (i.e, the revert of 7e18de137c3), and continues with the fix, which is **the interesting part, and that can be viewed by diff'ing 3c466d372b7...ff22857609d**. (ff22857609d is the last commit as of date.)

[JDK-8361842]: https://bugs.openjdk.org/browse/JDK-8361842
[JDK-8374210]: https://bugs.openjdk.org/browse/JDK-8374210
[JDK-8374582]: https://bugs.openjdk.org/browse/JDK-8374582

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PR Comment: https://git.openjdk.org/jdk/pull/29164#issuecomment-3790314570