jvm 会在每次执行时评估一个明显的 return 值吗？

Question

相信几乎每个人都熟悉most downvoted question(java tagged) on SO。复制粘贴代码段以确保完整性：

k = (j = (i = 0) + 2) + 1;
return i|= j|= k|= (j+= i) - - (k+++k) - - (i =+j);

以上代码段总是 returns 11 无论如何。所以我的问题是：jvm 会在每次调用时评估 this/similar 疯狂吗？

Answer 1

不知道这个算不算答案，不过好像JVM可以证明不需要评价：

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Warmup(iterations = 20, time = 20)
@Measurement(iterations = 20, time = 20)
public class MostDownVoted {

    public static void main(String[] args) throws Exception {
        Options opt = new OptionsBuilder()
            .include(MostDownVoted.class.getSimpleName())
            .build();

        new Runner(opt).run();
    }

    @State(Scope.Benchmark)
    public static class Holder {

        int k;
        int i;
        int j;

        @Setup(Level.Iteration)
        public void setUp() {
            k = ThreadLocalRandom.current().nextInt();
            i = ThreadLocalRandom.current().nextInt();
            j = ThreadLocalRandom.current().nextInt();
        }

    }

    @Fork(1)
    @Benchmark
    public int test1(Holder h) {
        h.k = (h.j = (h.i = 0) + 2) + 1;
        return h.i |= h.j |= h.k |= (h.j += h.i) - -(h.k++ + h.k) - -(h.i = +h.j);
    }

    @Fork(1)
    @Benchmark
    public int test2(Holder h) {
        return 11;
    }

    @Benchmark
    @Fork(value = 1, jvmArgsAppend = "-XX:TieredStopAtLevel=1")
    public int test3(Holder h) {
        h.k = (h.j = (h.i = 0) + 2) + 1;
        return h.i |= h.j |= h.k |= (h.j += h.i) - -(h.k++ + h.k) - -(h.i = +h.j);
    }

}

结果表明，一旦 C2 编译器启动，return 11 的结果与您拥有的结果相当：

MostDownVoted.test1  avgt   20  2.816 ± 0.003  ns/op
MostDownVoted.test2  avgt   20  2.122 ± 0.016  ns/op
MostDownVoted.test3  avgt   20  3.979 ± 0.758  ns/op

jvm 会在每次执行时评估一个明显的 return 值吗？

Will jvm evaluate a obvious return value on each execution?

java

jvm

jit

jvm-hotspot

jvm-bytecode