ADDITIONAL SYSTEM INFORMATION :
OS: macOS 26.1

JDK Version Affected: OpenJDK Runtime Environment (build 21.0.9)

JDK Version Working: OpenJDK Runtime Environment (build 25.0.1)

GC Type Exhibiting Issue: ShenandoahGC (-XX:+UseShenandoahGC)

A DESCRIPTION OF THE PROBLEM :
When executing the attached test case (a modified version of dead11) with ShenandoahGC enabled (-XX:+UseShenandoahGC) on JDK 21.0.9, the program exhibits extreme performance degradation, failing to complete execution even after 30 minutes. The same test completes successfully within approximately 30-120 seconds when using other GCs (SerialGC, G1GC, ParallelGC, ZGC) on the same JDK version.

Key Observations:

GC-Specific: The issue is isolated to the ShenandoahGC implementation in JDK 21.

JDK Version Discrepancy: The identical test case and ShenandoahGC configuration run to completion in under 100 seconds on JDK 25.0.1, suggesting a regression fix or significant optimization between these releases.
Test Pattern: The test involves a combination of:

Infinite recursion via main() calling itself.

Allocation of multi-dimensional WeakReference arrays (WeakReference[16][128][32]).

Intentional throwing and catching of OutOfMemoryError (OOME).

Volatile field updates.

This behavior strongly indicates a pathological interaction within ShenandoahGC's concurrent or degenerated collection phases under the stress of this specific allocation/exception pattern, which appears to have been resolved by JDK 25.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the attached test.java file.

Run with the following command on JDK 21.0.9:

bash
java -XX:+UseShenandoahGC -Xms4g -Xmx4g test

Monitor the execution time.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The program should execute and terminate within a timeframe similar to other GCs (seconds to a few minutes)
ACTUAL -
The program does not terminate. It appears to hang or enter a state of severe performance degradation, consuming CPU but making no progress towards completion even after 30 minutes of runtime.

---------- BEGIN SOURCE ----------
import java.io.PrintStream;
import java.lang.ref.WeakReference;
import java.util.Random;

public class test {
    int SIZE = 30;
    private WeakReference[][][] field_1810 = new WeakReference[16][128][32];
    public static long TRAPCOUNT = 0L;
    protected static volatile ArrayIndexOutOfBoundsException[] field_1818;

    int f() {
        try {
        } catch (OutOfMemoryError var2) {
            ++TRAPCOUNT;
        }

        int[] var3 = new int[this.SIZE];
        
        for (int i = 0; i < this.SIZE; i++) {
            var3[i] = i;
        }

        new test();

        return var3[12];
    }

    int fopt() {
        try {
            throw new OutOfMemoryError();
        } catch (OutOfMemoryError var3) {
            ++TRAPCOUNT;
        }

        int[] var4 = new int[this.SIZE];
        var4[12] = 12;

        Random var1 = new Random(-3L);
        int var2 = var4.length;

        if (var2 > 0) {
            int randomIndex = var1.nextInt(var2);
            var4[randomIndex] = 1;
        }

        var4[12] = 12;
        return 12;
    }

    public static void main(String[] var0) {
        try {
        } catch (OutOfMemoryError var4) {
            TRAPCOUNT++;
            InheritableThreadLocal[] var5 =new InheritableThreadLocal[16];
        }

        test var6 = new test();
        PrintStream var1 = System.out;

        try {
            String[] var8 = new String[16];
            field_1818 = new ArrayIndexOutOfBoundsException[16];
            main(var8);

            if (var6 != null) {
                throw new OutOfMemoryError();
            }
        } catch (OutOfMemoryError var3) {
            ++TRAPCOUNT;
        }

        var1.println("f()=" + var6.f() + "; fopt()=" + var6.fopt());

        if (var6.f() == var6.fopt()) {
            PrintStream var9 = System.out;
            var6.field_1810 = new WeakReference[16][128][32];
            var9.println("Test ShenandoahGCTimeOut.dead11 Passed.");
        } else {
            throw new Error("Test ShenandoahGCTimeOut.dead11 Failed: f()=" + var6.f() + " != fopt()=" + var6.fopt());
        }
    }
}
---------- END SOURCE ----------