内联汇编语言是否比本机 C++ 代码慢?

You can always produce an example where handmade assembly code is better than compiled code but usually it's a fictional example or a single routine not a true program of 500.000+ lines of C++ code). I think compilers will produce better assembly code 95% times and sometimes, only some rare times, you may need to write assembly code for few, short, highly used, performance critical routines or when you have to access features your favorite high-level language does not expose. Do you want a touch of this complexity? Read this awesome answer here on SO.

为什么会这样?

首先，因为编译器可以进行我们甚至无法想象的优化(请参阅这个短列表)，他们会在秒内完成(当我们可能需要几天时间时).

First of all because compilers can do optimizations that we can't even imagine (see this short list) and they will do them in seconds (when we may need days).

当您在汇编中编码时，您必须使用明确定义的调用接口创建明确定义的函数.但是他们可以考虑整个程序优化和过程间优化如注册分配、常量传播、常见子表达式消除、指令调度和其他复杂的、不明显的优化(Polytope 模型，例如).在 RISC 架构上，人们多年前就不再担心这个问题了(例如，指令调度非常困难)手动调谐)和现代CISC CPU 有很长的管道也是.

When you code in assembly you have to make well-defined functions with a well-defined call interface. However they can take in account whole-program optimization and inter-procedural optimization such as register allocation, constant propagation, common subexpression elimination, instruction scheduling and other complex, not obvious optimizations (Polytope model, for example). On RISC architecture guys stopped worrying about this many years ago (instruction scheduling, for example, is very hard to tune by hand) and modern CISC CPUs have very long pipelines too.

对于一些复杂的微控制器，甚至系统库都是用 C 语言编写的，而不是用汇编语言编写的，因为它们的编译器会生成更好(且易于维护)的最终代码.

For some complex microcontrollers even system libraries are written in C instead of assembly because their compilers produce a better (and easy to maintain) final code.

编译器有时可以自行自动使用一些 MMX/SIMDx 指令，如果您不要使用它们你根本无法比较(其他答案已经很好地审查了你的汇编代码).仅用于循环，这是一个循环优化的简短列表常见 由编译器检查(当 C# 程序的日程安排已经确定后，你认为你可以自己做吗?)如果你用汇编写一些东西，我认为你至少必须考虑一些 简单优化.数组的教科书示例是展开循环(其大小在编译时已知).这样做并再次运行您的测试.

Compilers sometimes can automatically use some MMX/SIMDx instructions by themselves, and if you don't use them you simply can't compare (other answers already reviewed your assembly code very well). Just for loops this is a short list of loop optimizations of what is commonly checked for by a compiler (do you think you could do it by yourself when your schedule has been decided for a C# program?) If you write something in assembly, I think you have to consider at least some simple optimizations. The school-book example for arrays is to unroll the cycle (its size is known at compile time). Do it and run your test again.

如今，由于另一个原因需要使用汇编语言也非常罕见:过多的不同CPU.你想支持他们吗?每个都有一个特定的微架构和一些特定指令集.它们具有不同数量的功能单元，应安排汇编指令以保持它们忙.如果您用 C 编写，您可以使用 PGO 但在汇编中，您将需要丰富的知识特定架构(以及为另一个架构重新思考和重做一切).对于小任务，编译器通常做得更好，而对于复杂任务通常，工作没有得到回报(并且 编译器可能做得更好.

These days it's also really uncommon to need to use assembly language for another reason: the plethora of different CPUs. Do you want to support them all? Each has a specific microarchitecture and some specific instruction sets. They have different number of functional units and assembly instructions should be arranged to keep them all busy. If you write in C you may use PGO but in assembly you will then need a great knowledge of that specific architecture (and rethink and redo everything for another architecture). For small tasks the compiler usually does it better, and for complex tasks usually the work isn't repaid (and compiler may do better anyway).

如果你坐下来看看你的代码，你可能会发现重新设计算法比转换为汇编会获得更多(阅读这篇这里是SO的好帖子)，您可以在之前有效地应用高级优化(和编译器提示)你需要求助于汇编语言.可能值得一提的是，经常使用内在函数可以获得您正在寻找的性能提升，并且编译器仍然能够执行大部分优化.

If you sit down and you take a look at your code probably you'll see that you'll gain more to redesign your algorithm than to translate to assembly (read this great post here on SO), there are high-level optimizations (and hints to compiler) you can effectively apply before you need to resort to assembly language. It's probably worth to mention that often using intrinsics you will have performance gain your're looking for and compiler will still be able to perform most of its optimizations.

综上所述，即使您可以生成快 5 到 10 倍的汇编代码，您也应该询问您的客户他们是否愿意支付一周您的时间或购买速度快 50 美元的 CPU.我们大多数人通常不需要极端优化(尤其是在 LOB 应用程序中).

All this said, even when you can produce a 5~10 times faster assembly code, you should ask your customers if they prefer to pay one week of your time or to buy a 50$ faster CPU. Extreme optimization more often than not (and especially in LOB applications) is simply not required from most of us.

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