By accident, I discovered that PopulateIndex does not work very well with L/F/D. I added these lines to
/oracle-work/jdk-fork2/open/test/hotspot/jtreg/compiler/vectorization/runner/ArrayIndexFillTest.java

The culprit is that we split the ConvI2L/D/F through the AddI, at least in some cases of the pack, and that confuses the packing logic. If init/limit are variable, the split cannot happen, and it vectorizes.

Not sure if this can be fixed, or is worth fixing.

    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, "=0"})
    // The ConvI2L can be split through the AddI, creating a mix of
    // ConvI2L(AddI) and AddL(ConvI2L) cases, which do not vectorize.
    public long[] fillLongArray() {
        long[] res = new long[SIZE];
        for (int i = 0; i < SIZE; i++) {
            res[i] = i;
        }
        return res;
    }
                                                                                                                                                                                              
    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, ">0"})
    // The variable init/limit has the consequence that we do not split
    // the ConvI2L through the AddI.
    public long[] fillLongArray2() {
        long[] res = new long[SIZE];
        for (int i = init; i < limit; i++) {
            res[i] = i;
        }
        return res;
    }
                                                                                                                                                                                              
    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, "=0"})
    public float[] fillFloatArray() {
        float[] res = new float[SIZE];
        for (int i = 0; i < SIZE; i++) {
            res[i] = i;
        }
        return res;
    }
                                                                                                                                                                                              
    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, ">0"})
    public float[] fillFloatArray2() {
        float[] res = new float[SIZE];
        for (int i = init; i < limit; i++) {
            res[i] = i;
        }
        return res;
    }
                                                                                                                                                                                              
    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, "=0"})
    public double[] fillDoubleArray() {
        double[] res = new double[SIZE];
        for (int i = 0; i < SIZE; i++) {
            res[i] = i;
        }
        return res;
    }
                                                                                                                                                                                              
    @Test
    @IR(applyIfCPUFeatureOr = {"sve", "true", "avx2", "true"},
        counts = {IRNode.POPULATE_INDEX, ">0"})
    public double[] fillDoubleArray2() {
        double[] res = new double[SIZE];
        for (int i = init; i < limit; i++) {
            res[i] = i;
        }
        return res;
    }