C++固定大小数组与相同类型的多个对象

发布于 2024-10-14 21:16:27 字数 321 浏览 8 评论 0原文

我想知道（除了明显的语法差异之外）包含对象（同一类型）的多个实例的类或该类型的固定大小的对象数组之间是否存在任何效率差异。

在代码中：

struct A {
  double x;
  double y;
  double z;
};

struct B {
  double xvec[3];
};

实际上，我会使用 boost::arrays，它是 C 风格数组的更好的 C++ 替代品。

我主要关心构造/销毁和读/写这样的双精度数，因为这些类通常被构造只是为了调用它们的成员函数之一。

感谢您的帮助/建议。

原文

I was wondering whether (apart from the obvious syntax differences) there would be any efficiency difference between having a class containing multiple instances of an object (of the same type) or a fixed size array of objects of that type.

In code:

struct A {
  double x;
  double y;
  double z;
};

struct B {
  double xvec[3];
};

In reality I would be using boost::arrays which are a better C++ alternative to C-style arrays.

I am mainly concerned with construction/destruction and reading/writing such doubles, because these classes will often be constructed just to invoke one of their member functions once.

Thank you for your help/suggestions.

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作死小能手 2024-10-21 21:16:27

通常，这两个结构的表示形式完全相同。但是，如果您为您的用例选择了错误的选项，则可能会导致性能不佳。

例如，如果您需要使用数组访问循环中的每个元素，您可以执行以下操作：

for (int i = 0; i < 3; i++)
    dosomething(xvec[i]);

但是，如果没有数组，您要么需要重复代码：

dosomething(x);
dosomething(y);
dosomething(z);

这意味着代码重复 - 可以采用任何一种方式。一方面循环代码更少；另一方面，非常紧密的循环在现代处理器上可能会非常快，并且代码重复可能会破坏 I-cache。

另一个选项是开关：

for (int i = 0; i < 3; i++) {
    int *r;
    switch(i) {
        case 0: r = &x; break;
        case 1: r = &y; break;
        case 1: r = &z; break;
    }
    dosomething(*r); // assume this is some big inlined code
}

这可以避免可能较大的 i-cache 占用空间，但会对性能产生巨大的负面影响。不要这样做。

另一方面，原则上，如果您的编译器不是很聪明，数组访问可能会更慢：

xvec[0] = xvec[1] + 1;
dosomething(xvec[1]);

由于 xvec[0] 和 xvec[1] 是不同的，原则上，编译器应该能够将 xvec[1] 的值保存在寄存器中，因此不必在下一行重新加载该值。但是，某些编译器可能不够聪明，无法注意到 xvec[0] 和 xvec[1] 没有别名。在这种情况下，使用单独的字段可能会快一点。

简而言之，这并不是说其中之一在所有情况下都要快。这是关于将表示形式与您使用它的方式相匹配。

就我个人而言，我建议使用任何使 xvec 上运行的代码最自然的代码。不值得花费大量的时间去担心那些最多可能只会产生很小的性能差异的事情，以至于你只能在微基准测试中发现它。

Typically the representation of those two structs would be exactly the same. It is, however, possible to have poor performance if you pick the wrong one for your use case.

For example, if you need to access each element in a loop, with an array you could do:

for (int i = 0; i < 3; i++)
    dosomething(xvec[i]);

However, without an array, you'd either need to duplicate code:

dosomething(x);
dosomething(y);
dosomething(z);

This means code duplication - which can go either way. On the one hand there's less loop code; on the other hand very tight loops can be quite fast on modern processors, and code duplication can blow away the I-cache.

The other option is a switch:

for (int i = 0; i < 3; i++) {
    int *r;
    switch(i) {
        case 0: r = &x; break;
        case 1: r = &y; break;
        case 1: r = &z; break;
    }
    dosomething(*r); // assume this is some big inlined code
}

This avoids the possibly-large i-cache footprint, but has a huge negative performance impact. Don't do this.

On the other hand, it is, in principle, possible for array accesses to be slower, if your compiler isn't very smart:

xvec[0] = xvec[1] + 1;
dosomething(xvec[1]);

Since xvec[0] and xvec[1] are distinct, in principle, the compiler ought to be able to keep the value of xvec[1] in a register, so it doesn't have to reload the value at the next line. However, it's possible some compilers might not be smart enough to notice that xvec[0] and xvec[1] don't alias. In this case, using seperate fields might be a very tiny bit faster.

In short, it's not about one or the other being fast in all cases. It's about matching the representation to how you use it.

Personally, I would suggest going with whatever makes the code working on xvec most natural. It's not worth spending a lot of human time worrying about something that, at best, will probably only produce such a small performance difference that you'll only catch it in micro-benchmarks.

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月野兔 2024-10-21 21:16:27

MVC++ 2010 生成了与您的示例中的两个 POD 结构完全相同的读/写代码。由于读/写的偏移量在编译时是可计算的，因此这并不奇怪。建造和破坏也是如此。

至于实际性能，一般规则适用：如果重要则对其进行分析，如果不重要 - 为什么要关心？

对于结构体的用户来说，对数组成员进行索引可能需要更多工作，但话又说回来，他可以更轻松地迭代元素。

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酸甜透明夹心 2024-10-21 21:16:27

如果您无法决定并希望保持选择开放，您可以使用匿名联合：

struct Foo
{
    union
    {
        struct
        {
            double x;
            double y;
            double z;
        } xyz;
        double arr[3];
    };
};

int main()
{
    Foo a;
    a.xyz.x = 42;
    std::cout << a.arr[0] << std::endl;
}

某些编译器还支持匿名结构，在这种情况下您可以保留 xyz 部分。

In case you can't decide and want to keep your options open, you can use an anonymous union:

struct Foo
{
    union
    {
        struct
        {
            double x;
            double y;
            double z;
        } xyz;
        double arr[3];
    };
};

int main()
{
    Foo a;
    a.xyz.x = 42;
    std::cout << a.arr[0] << std::endl;
}

Some compilers also support anonymous structs, in that case you can leave the xyz part out.

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