jvm会在每次执行时评估一个明显的返回值吗

Question

我相信几乎每个人都熟悉SO上投票最多的问题(java标记(。为了完整起见，复制粘贴片段：

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

不管怎样，上面的代码段总是返回11。所以我的问题是：jvm会在每次调用中评估这种/类似的疯狂吗？

Answer 1

我不知道这是否算是一个答案，但似乎JVM可以证明不需要评估：

@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