是什么原因，Swift 应该比 Objective-C 快得多? [英] Whats the cause, Swift is supposed to be that much faster than Objective-C?

问题描述

正如 Craig 在 Swift 的公告中透露的那样，据说到目前为止它比 Objective-C 更快.

As Craig revealed within the announcement of Swift, it is said to be faster than Objective-C by far.

但我不太相信这种比较，因为正确实现的算法在 C 中应该是最快的，因为它可以被编译器高度优化并且本质上是硬件友好的.所以我猜 Swift 能得到的最好的性能，应该是 C 中相同算法的性能.

But i'm not that convinced by that comparison because a properly implemented algorithm should be fastest in C, because it can be highly optimized by the compiler and is hardware-friendly by nature. So I would guess the best performance Swift could get, would be the performance of the same algorithm in C.

既然 C 是 Objective-C 的一部分，为什么 Objective-C 和 Swift 的性能之间会有这么大的差距?

And as C is part of Objective-C, why is there a that big gap between Objective-C and Swift performance?

我能想象到的唯一原因是 Apple 使用 NSNumbers 等而不是 C 类型，但不应该作为 Obj-C - Swift 比较的基础，对吧?

The only reason I could imagine is Apple using NSNumbers and such instead of C types, but should not be the base for a Obj-C - Swift comparison, right?

推荐答案

这主要是推测(虽然是知情推测)，但我的两大理论是:

This is largely speculation (although informed speculation) but my two big theories are:

1) No 减少动态方法调度.Objective-C 中的每个方法调用都经过 objc_msgSend.在最快的情况下，这可以像 16 条指令一样快，但也可能慢很多.Swift 在比 Objective-C 更少的情况下会产生这种惩罚，例如:对 swift-only 协议方法的方法调用不会命中 objc_msgSend，但是如果协议是在 Obj-C 中声明的，或者如果swift 协议用 @objc 修饰(这样它也可以被 Objective-C 对象采用)，然后对 swift 对象采用的该协议中的方法的方法调用似乎是通过 objc_msgSend.

1) No Reduced dynamic method dispatch. Every method call in Objective-C goes through objc_msgSend. In the fastest case, this can be as quick as 16 instructions, but it can also be a lot slower. Swift will incur this penalty in fewer situations than Objective-C will, for instance: method calls to swift-only protocol methods do not hit objc_msgSend, but if the protocol is declared in Obj-C, or if the swift protocol is decorated with @objc (such that it can be adopted by Objective-C objects as well), then method calls to methods in that protocol adopted by swift objects appear to be dispatched via objc_msgSend.

2) 避免堆分配.在 Objective-C 中，(实际上)每个对象都是堆分配的.使用静态类型系统，编译器可以推断出更多关于对象生命周期的信息，并在堆栈上分配它，除非它必须跨越 Objective-C 边界(或者太大而无法在堆栈上分配).

2) Avoiding heap allocations. In Objective-C, (effectively) every object is heap allocated. With a static type system, the compiler can infer more about the lifecycle of an object and allocate it on the stack unless it has to cross the Objective-C boundary (or is too big to be allocated on the stack).

我怀疑 #2 是这两者中更大的一个，但两者都可能是重要的贡献者.我敢肯定这不仅仅是这个，但这两个很有可能的贡献者.

I suspect that #2 is the much bigger of these two, but both are likely significant contributors. I'm sure there's more to it than just this, but these are two very likely contributors.

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