Recently we find HotSpot jtreg test/hotspot/jtreg/compiler/vectorization/runner/LoopLiveOutNodesTest.java fails intermittently on Arm Neoverse-N2 (a kind of AArch64 CPU with 128-bit Scalable Vector Extension). After some investigation, we find it's caused by a bug in the experimental post loop vectorization with parallel loop induction variables. We create below test case to stably reproduce the issue.

public class Foo {
    private static final int SIZE = 500;

    private static int[] a = new int[SIZE];
    private static int[] b = new int[SIZE];

    private static int init = 33;
    private static int iters = 335;

    private static int testLoop(int start, int end) {
        int i = 0, j = 0;
        for (i = start; i < end; i++, j++) {
            b[i] = a[i];
        }
        return j;
    }

    public static void main(String[] args) {
        // Warmup
        for (int warmup = 0; warmup < 20000; warmup++) {
            testLoop(0, 500);
        }
        // Test
        int result = testLoop(init, init + iters);
        if (result != iters) {
            throw new RuntimeException("Incorrect result: expected = " +
                    iters + ", actual = " + result);
        }
        System.out.println("Passed");
    }
}

$ java Foo
Passed

$ java -XX:+UnlockExperimentalVMOptions -XX:+PostLoopMultiversioning Foo
Exception in thread "main" java.lang.RuntimeException: Incorrect result: expected = 335, actual = 328
	at Foo.main(Foo.java:26)