Bug ID: JDK-8365978 [lworld] C2: assert(vk->maybe_flat_in_array()) when using compareAndSetFlatValue with -XX:-UseArrayFlattening

Type: Bug
Component: hotspot
Sub-Component: compiler
Affected Version: repo-valhalla

Priority: P4
Status: In Progress
Resolution: Unresolved

Submitted: 2025-08-22
Updated: 2025-09-05

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repo-valhallaUnresolved

Attached reproducer, to run with

javac --enable-preview --source 25 --add-exports java.base/jdk.internal.misc=ALL-UNNAMED PutFlatValueCrash.java

java -ea --enable-preview --add-exports java.base/jdk.internal.misc=ALL-UNNAMED -XX:-UseArrayFlattening -Xcomp -XX:CompileCommand=compileonly,"PutFlatValueCrash::test" -XX:-TieredCompilation PutFlatValueCrash


This is because compareAndSetFlatValue uses compareAndSetFlatValueAsBytes which uses putFlatValue on a flat array (provided by newSpecialArray), which is intrinsified and crashes when UseArrayFlattening is disabled.

Note that newSpecialArray would raise if called with -XX:-UseArrayFlattening, this is fine. But putFlatValue shouldn't crash (or be rejected) compile time because it might just be not executed at runtime.