RFR: 8338967: Improve performance for MemorySegment::fill [v4]

[Per Minborg]

On Mon, 26 Aug 2024 21:38:37 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Here is a benchmark that fills segments of various random sizes:
>> 
>> 
>> 
>> @BenchmarkMode(Mode.AverageTime)
>> @Warmup(iterations = 5, time = 500, timeUnit = TimeUnit.MILLISECONDS)
>> @Measurement(iterations = 10, time = 500, timeUnit = TimeUnit.MILLISECONDS)
>> @State(Scope.Thread)
>> @OutputTimeUnit(TimeUnit.NANOSECONDS)
>> @Fork(value = 3)
>> public class TestFill {
>> 
>>     private static final int SIZE = 16;
>>     private static final int[] INDICES = new Random(42).ints(0, 8)
>>             .limit(SIZE)
>>             .toArray();
>> 
>> 
>>     private MemorySegment[] segments;
>> 
>>     @Setup
>>     public void setup() {
>>         segments = IntStream.of(INDICES)
>>                 .mapToObj(i -> MemorySegment.ofArray(new byte[i]))
>>                 .toArray(MemorySegment[]::new);
>>     }
>> 
>>     @Benchmark
>>     public void heap_segment_fill() {
>>         for (int i = 0; i < SIZE; i++) {
>>             segments[i].fill((byte) 0);
>>         }
>>     }
>> 
>> }
>> 
>> 
>> This produces the following on my Mac M1:
>> 
>> 
>> Benchmark                   Mode  Cnt   Score   Error  Units
>> TestFill.heap_segment_fill  avgt   30  59.054 ? 3.723  ns/op
>> 
>> 
>> On average, an operation will take 59/16 = ~3 ns per operation (including looping).
>> 
>> A test with the same size for every benchmark looks like this on my machine:
>> 
>> 
>> Benchmark                   (ELEM_SIZE)  Mode  Cnt  Score   Error  Units
>> TestFill.heap_segment_fill            0  avgt   30  1.112 ? 0.027  ns/op
>> TestFill.heap_segment_fill            1  avgt   30  1.602 ? 0.060  ns/op
>> TestFill.heap_segment_fill            2  avgt   30  1.583 ? 0.004  ns/op
>> TestFill.heap_segment_fill            3  avgt   30  1.909 ? 0.055  ns/op
>> TestFill.heap_segment_fill            4  avgt   30  1.605 ? 0.059  ns/op
>> TestFill.heap_segment_fill            5  avgt   30  1.900 ? 0.064  ns/op
>> TestFill.heap_segment_fill            6  avgt   30  1.891 ? 0.038  ns/op
>> TestFill.heap_segment_fill            7  avgt   30  2.237 ? 0.091  ns/op
>
> As discussed offline, can't we use a stable array of functions or something like that which can be populated lazily? That way you can access the function you want in a single array access, and we could put all these helper methods somewhere else.

Unfortunately, a stable array of functions/MethodHandles didn't work from a performance perspective.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20712#discussion_r1732503991