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

[Christian Hagedorn]

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

Overall, the fix idea with `Opaque` nodes looks good to me!

src/hotspot/share/opto/library_call.hpp line 161:

> 159:   Node* generate_negative_guard(Node* index, RegionNode* region,
> 160:                                 // resulting CastII of index:
> 161:                                 Node* *pos_index = nullptr,

Suggestion:

                                Node** pos_index = nullptr,

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

PR Review: https://git.openjdk.org/jdk/pull/29164#pullrequestreview-3710064929
PR Review Comment: https://git.openjdk.org/jdk/pull/29164#discussion_r2731064422