私有方法内联 [英] Private method inlining

问题描述

在Scala 不可变矢量代码有一条评论说：

In the Scala immutable Vector code there is a comment that says:

原则上，大多数成员应该是私人的。但是，必须谨慎选择访问权限以防止方法内联

In principle, most members should be private. However, access privileges must be carefully chosen to not prevent method inlining

1. 私有对内联的影响是什么决定？


2. 这是否也适用于Java？

推荐答案

这通常不适用于Java。访问检查仅在决议过程。当Java方法进行JIT编译时，符号引用已经过解析和验证。实际上，内联不是在原始字节码上执行，而是在编译器特定的中间表示上执行。因此，访问修饰符通常不具有性能副作用。

This generally does not apply to Java. The access checks are performed only once during the Resolution process. When Java method is JIT-compiled, the symbolic references are already resolved and verified. In fact, the inlining is performed not on the original bytecode, but on the compiler-specific intermediate representation. So, the access modifiers does not usually have performace side-effects.

但是，我可以编写一个人工测试用例，其中 private / public 修饰符会显着影响性能：

However, I can write an artificial test case where a private/public modifier dramatically affects the performance:

public class Test {
    static final Inner inner = new Inner();

    static class Inner {
        int x = 1;

        int getX1() { return x; }
        int getX2() { return getX1(); }
        int getX3() { return getX2(); }
        int getX4() { return getX3(); }
        int getX5() { return getX4(); }
        int getX6() { return getX5(); }
        int getX7() { return getX6(); }
        int getX8() { return getX7(); }
        int getX9() { return getX8(); }

        private int getPrivate() { return getX9(); }
        public int getPublic() { return getX9(); }
    }

    @GenerateMicroBenchmark
    public int inlinePrivate() {
        return inner.getPrivate();
    }

    @GenerateMicroBenchmark
    public int inlinePublic() {
        return inner.getPublic();
    }
}

---

---

Benchmark                Mode Thr    Cnt  Sec         Mean   Mean error    Units
b.Test.inlinePrivate    thrpt   1      3    5   289480,928     2247,656 ops/msec
b.Test.inlinePublic     thrpt   1      3    5  1157970,245    18473,139 ops/msec

此效果用合成方法解释访问$ 000 生成 javac 以允许访问内部类的私有成员。在上面的测试用例中，这个额外的访问器阻止了内联，因为HotSpot中默认的最大内联级别是9（ -XX：MaxInlineLevel = 9 ）。由于 getPrivate（）无法直接从外部类调用，额外的访问$ 000（）方法使得10级调用，因此没有内联。

This effect is explained by a synthetic method access$000 which javac generates to allow access to a private member of the inner class. In the above test case this extra accessor prevents from inlining, because the default maximum level of inlining in HotSpot is 9 (-XX:MaxInlineLevel=9). Since getPrivate() cannot be called directly from the outer class, the extra access$000() method makes 10th level of invocation and therefore is not inlined.

这篇关于私有方法内联的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！