答案很简单……一个非常了解汇编的人(也就是他身边有参考资料，并利用每一个小处理器缓存和管道特性等)保证能够产生比任何编译器更快的代码。

然而，如今在典型的应用程序中，这种差异并不重要。

我需要对192位或256位的每次中断进行移位操作，每50微秒发生一次。

它通过一个固定的映射(硬件限制)实现。使用C语言，制作它只需要大约10微秒。当我把它翻译到Assembler时，考虑到这个映射的特定特性，特定的寄存器缓存，并使用面向位的操作;它只花了不到3.5微秒的时间。

一个更著名的组装片段来自Michael Abrash的纹理映射循环(在这里详细解释):

add edx,[DeltaVFrac] ; add in dVFrac
sbb ebp,ebp ; store carry
mov [edi],al ; write pixel n
mov al,[esi] ; fetch pixel n+1
add ecx,ebx ; add in dUFrac
adc esi,[4*ebp + UVStepVCarry]; add in steps

现在，大多数编译器将高级CPU特定指令表示为intrinsic，即编译为实际指令的函数。MS Visual c++支持MMX、SSE、SSE2、SSE3和SSE4的intrinsic，因此您不必太过担心使用特定于平台的指令来进行汇编。Visual c++还可以通过适当的/ARCH设置来利用您所针对的实际体系结构。

我已经阅读了所有的答案(超过30个)，并没有找到一个简单的原因:如果你读过并练习过Intel®64和IA-32架构优化参考手册，汇编程序比C更快，所以汇编程序可能更慢的原因是编写这种慢汇编程序的人没有阅读优化手册。

In the good old days of Intel 80286, each instruction was executed at a fixed count of CPU cycles. Still, since Pentium Pro, released in 1995, Intel processors became superscalar, utilizing Complex Pipelining: Out-of-Order Execution & Register Renaming. Before that, on Pentium, produced in 1993, there were U and V pipelines. Therefore, Pentium introduced dual pipelines that could execute two simple instructions at one clock cycle if they didn't depend on one another. However, this was nothing compared with the Out-of-Order Execution & Register Renaming that appeared in Pentium Pro. This approach introduced in Pentium Pro is practically the same nowadays on most recent Intel processors.

Let me explain the Out-of-Order Execution in a few words. The fastest code is where instructions do not depend on previous results, e.g., you should always clear whole registers (by movzx) to remove dependency from previous values of the registers you are working with, so they may be renamed internally by the CPU to allow instruction execute in parallel or in a different order. Or, on some processors, false dependency may exist that may also slow things down, like false dependency on Pentium 4 for inc/dec, so you may wish to use add eax, 1 instead or inc eax to remove dependency on the previous state of the flags.

如果时间允许，您可以阅读更多无序执行和注册重命名。因特网上有大量的信息。

There are also many other essential issues like branch prediction, number of load and store units, number of gates that execute micro-ops, memory cache coherence protocols, etc., but the crucial thing to consider is the Out-of-Order Execution. Most people are simply not aware of the Out-of-Order Execution. Therefore, they write their assembly programs like for 80286, expecting their instructions will take a fixed time to execute regardless of the context. At the same time, C compilers are aware of the Out-of-Order Execution and generate the code correctly. That's why the code of such uninformed people is slower, but if you become knowledgeable, your code will be faster.

除了乱序执行之外，还有很多优化技巧和技巧。请阅读上面提到的优化手册:-)

However, assembly language has its own drawbacks when it comes to optimization. According to Peter Cordes (see the comment below), some of the optimizations compilers do would be unmaintainable for large code-bases in hand-written assembly. For example, suppose you write in assembly. In that case, you need to completely change an inline function (an assembly macro) when it inlines into a function that calls it with some arguments being constants. At the same time, a C compiler makes its job a lot simpler—and inlining the same code in different ways into different call sites. There is a limit to what you can do with assembly macros. So to get the same benefit, you'd have to manually optimize the same logic in each place to match the constants and available registers you have.

只有在使用编译器不支持的特殊用途指令集时。

为了最大限度地利用具有多个管道和预测分支的现代CPU的计算能力，您需要以这样一种方式来构造汇编程序:a)人类几乎不可能编写b)甚至更不可能维护。

此外，更好的算法、数据结构和内存管理将为您提供至少一个数量级的性能，而不是在汇编中进行的微观优化。

第一点不是答案。 即使你从来没有用它编程，我发现至少知道一个汇编指令集是有用的。这是程序员永无止境的追求的一部分，他们想知道得更多，从而变得更好。当你进入一个没有源代码的框架时，它也很有用，至少对正在发生的事情有一个粗略的了解。它还可以帮助您理解JavaByteCode和. net IL，因为它们都类似于汇编程序。

To answer the question when you have a small amount of code or a large amount of time. Most useful for use in embedded chips, where low chip complexity and poor competition in compilers targeting these chips can tip the balance in favour of humans. Also for restricted devices you are often trading off code size/memory size/performance in a way that would be hard to instruct a compiler to do. e.g. I know this user action is not called often so I will have small code size and poor performance, but this other function that look similar is used every second so I will have a larger code size and faster performance. That is the sort of trade off a skilled assembly programmer can use.

我还想补充一点，这里有很多中间地带，您可以用C编译代码并检查生成的程序集，然后更改C代码或调整并作为程序集进行维护。

我的朋友从事微控制器的工作，目前是用于控制小型电动机的芯片。他在低级c和汇编的组合中工作。他曾经告诉我，有一天他在工作中把主循环从48条指令减少到43条。他还面临着各种选择，比如代码已经增长到填满256k芯片，业务需要一个新功能，你呢

删除现有功能 减少部分或全部现有特性的大小，可能会以性能为代价。 提倡改用成本更高、功耗更高、外形更大的更大芯片。

我想补充一点，作为一个商业开发人员，我有很多的投资组合或语言、平台、应用程序类型，我从来没有觉得有必要深入编写程序集。我一直都很感激我所学到的知识。有时会被调试进去。

我知道我已经回答了“为什么我要学习汇编器”这个问题，但我觉得这是一个更重要的问题，而不是什么时候更快。

所以让我们再试一次 你应该考虑组装

致力于底层操作系统功能 在编译器上工作。 工作在一个极其有限的芯片，嵌入式系统等

记住比较你的程序集和生成的编译器，看看哪个更快/更小/更好。

大卫。