后增量和前增量运算符之间的性能差异? [英] Performance difference between post- and pre- increment operators?

问题描述

在Java中，使用后增量与前增量运算符会对性能产生影响吗? (在其他语言中，在某些情况下，预增量可能比后增量快.)

In Java, would there be any performance impact in using post increment vs pre increment operator? (In other languages, a pre-increment can be faster than a post-increment in certain contexts.)

例如，这些循环在性能上有区别吗?

For example, is there a performance difference in these loops?

for (int n = 0; idx < max; ++idx) { /* Note pre-increment */
  f(max);
}

VS.

for (int n = 0; idx < max; idx++) { /* Note post-increment */
  f(max);
}

推荐答案

只有在功能行为相同的情况下，性能问题才有意义(因为如果功能不同，则正确的行为要优于每分钟-更快)，所以我假设您所指的是不使用表达式值的情况?也就是说，表达式的唯一目的是增加i吗?在这种情况下，答案是否定的:没有性能差异，实际上没有任何差异.我刚刚编译了此类:

A performance question only makes sense in a context where the functional behavior is identical (since, if the functionality is different, a correct behavior is superior to a minutely-faster one), so I'm assuming that you're referring to a situation where the value of the expression is not used? That is, where the only purpose of the expression is to increment i? In such a situation, the answer is no: no performance difference, and in fact, no difference whatsoever. I just compiled this class:

public class Foo
{
    public static void main(final String args[])
    {
        int i = Integer.parseInt(args[0]);
        i++;
    }
}

并计算所得Foo.class的MD5校验和；并且，对于带有++i的版本也是如此.它们具有相同的校验和，表明这两个版本被编译为 exact 相同的字节码，因此它们在 literally 上的执行方式相同.

and computed the MD5 checksum of the resulting Foo.class; and, similarly for a version with ++i instead. They had the same checksum, indicating that the two versions were compiled into the exact same bytecode, and would therefore perform literally identically.

(自然，从理论上讲，这可能取决于编译器.即使在等效的情况下，其他编译器也可能会决定不同于++i来编译i++，但是我对此表示怀疑，它是真的，即使是这样也不值得担心.)

(Naturally, this could, in theory, depend on the compiler. A different compiler might decide to compile i++ differently from ++i even in a context where they're equivalent. But I doubt that, and it's really not worth worrying about even if it the case.)

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