在我的工作中，有三个原因让我了解和使用组装。按重要性排序:

Debugging - I often get library code that has bugs or incomplete documentation. I figure out what it's doing by stepping in at the assembly level. I have to do this about once a week. I also use it as a tool to debug problems in which my eyes don't spot the idiomatic error in C/C++/C#. Looking at the assembly gets past that. Optimizing - the compiler does fairly well in optimizing, but I play in a different ballpark than most. I write image processing code that usually starts with code that looks like this: for (int y=0; y < imageHeight; y++) { for (int x=0; x < imageWidth; x++) { // do something } } the "do something part" typically happens on the order of several million times (ie, between 3 and 30). By scraping cycles in that "do something" phase, the performance gains are hugely magnified. I don't usually start there - I usually start by writing the code to work first, then do my best to refactor the C to be naturally better (better algorithm, less load in the loop etc). I usually need to read assembly to see what's going on and rarely need to write it. I do this maybe every two or three months. doing something the language won't let me. These include - getting the processor architecture and specific processor features, accessing flags not in the CPU (man, I really wish C gave you access to the carry flag), etc. I do this maybe once a year or two years.

尽管C语言“接近”于对8位、16位、32位和64位数据的低级操作，但仍有一些C语言不支持的数学操作通常可以在某些汇编指令集中优雅地执行:

Fixed-point multiplication: The product of two 16-bit numbers is a 32-bit number. But the rules in C says that the product of two 16-bit numbers is a 16-bit number, and the product of two 32-bit numbers is a 32-bit number -- the bottom half in both cases. If you want the top half of a 16x16 multiply or a 32x32 multiply, you have to play games with the compiler. The general method is to cast to a larger-than-necessary bit width, multiply, shift down, and cast back: int16_t x, y; // int16_t is a typedef for "short" // set x and y to something int16_t prod = (int16_t)(((int32_t)x*y)>>16);` In this case the compiler may be smart enough to know that you're really just trying to get the top half of a 16x16 multiply and do the right thing with the machine's native 16x16multiply. Or it may be stupid and require a library call to do the 32x32 multiply that's way overkill because you only need 16 bits of the product -- but the C standard doesn't give you any way to express yourself. Certain bitshifting operations (rotation/carries): // 256-bit array shifted right in its entirety: uint8_t x[32]; for (int i = 32; --i > 0; ) { x[i] = (x[i] >> 1) | (x[i-1] << 7); } x[0] >>= 1; This is not too inelegant in C, but again, unless the compiler is smart enough to realize what you are doing, it's going to do a lot of "unnecessary" work. Many assembly instruction sets allow you to rotate or shift left/right with the result in the carry register, so you could accomplish the above in 34 instructions: load a pointer to the beginning of the array, clear the carry, and perform 32 8-bit right-shifts, using auto-increment on the pointer. For another example, there are linear feedback shift registers (LFSR) that are elegantly performed in assembly: Take a chunk of N bits (8, 16, 32, 64, 128, etc), shift the whole thing right by 1 (see above algorithm), then if the resulting carry is 1 then you XOR in a bit pattern that represents the polynomial.

尽管如此，除非有严重的性能限制，否则我不会求助于这些技术。正如其他人所说，汇编代码比C代码更难记录/调试/测试/维护:性能的提高伴随着一些严重的代价。

编辑:3。溢出检测在汇编中是可能的(在C中不能真正做到)，这使得一些算法更容易。

下面是一个真实的例子:固定点在旧编译器上进行乘法运算。

这些不仅在没有浮点数的设备上很方便，在精度方面也很出色，因为它们可以提供32位精度和可预测的错误(浮点数只有23位，很难预测精度损失)。即在整个范围内均匀的绝对精度，而不是接近均匀的相对精度(浮点数)。

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现代编译器很好地优化了这个定点示例，因此对于仍然需要特定于编译器的代码的更现代的示例，请参见

获得64位整数乘法的高部分:使用uint64_t for 32x32 => 64位乘法的便携版本在64位CPU上无法优化，因此你需要intrinsic或__int128来在64位系统上实现高效的代码。 Windows 32位上的_umul128: MSVC在将32位整数转换为64时并不总是做得很好，因此intrinsic有很大帮助。

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C语言没有完整的乘法运算符(由n位输入产生2n位)。在C语言中表达它的通常方法是将输入转换为更宽的类型，并希望编译器能够识别输入的上半部分是不有趣的:

// on a 32-bit machine, int can hold 32-bit fixed-point integers.
int inline FixedPointMul (int a, int b)
{
  long long a_long = a; // cast to 64 bit.

  long long product = a_long * b; // perform multiplication

  return (int) (product >> 16);  // shift by the fixed point bias
}

这段代码的问题在于，我们做了一些不能直接用c语言表达的事情。我们希望将两个32位的数字相乘，得到一个64位的结果，并返回中间的32位。然而，在C语言中这个乘法是不存在的。您所能做的就是将整数提升为64位，并执行64*64 = 64乘法。

x86(以及ARM、MIPS和其他)可以在一条指令中完成乘法运算。一些编译器过去常常忽略这一事实，并生成调用运行时库函数来进行相乘的代码。移位到16也经常由库例程完成(x86也可以做这样的移位)。

所以我们只剩下一两个乘法库调用。这造成了严重的后果。不仅移位速度较慢，而且在整个函数调用中必须保留寄存器，而且对内联和展开代码也没有帮助。

如果你在(内联)汇编器中重写相同的代码，你可以获得显著的速度提升。

除此之外:使用ASM并不是解决问题的最佳方法。大多数编译器允许你以内在的形式使用一些汇编指令，如果你不能用c语言表达它们。例如，VS.NET2008编译器将32*32=64位的mul公开为__emul，将64位的移位公开为__ll_rshift。

使用intrinsic，你可以以一种c编译器有机会理解发生了什么的方式重写函数。这允许代码内联，寄存器分配，公共子表达式消除和常量传播也可以完成。与手工编写的汇编程序代码相比，您将获得巨大的性能改进。

供参考:VS.NET编译器的定点mul的最终结果是:

int inline FixedPointMul (int a, int b)
{
    return (int) __ll_rshift(__emul(a,b),16);
}

定点除法的性能差异更大。通过编写几行asm代码，我对除法重的定点代码进行了10倍的改进。

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使用Visual c++ 2013为这两种方式提供了相同的汇编代码。

2007年的gcc4.1也很好地优化了纯C版本。(Godbolt编译器资源管理器没有安装任何早期版本的gcc，但即使是较旧的gcc版本也可以在没有intrinsic的情况下做到这一点。)

在Godbolt编译器资源管理器上查看用于x86(32位)和ARM的source + asm。(不幸的是，它没有任何旧到足以从简单的纯C版本生成糟糕代码的编译器。)

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现代cpu可以做一些C语言根本没有操作符的事情，比如popcnt或位扫描来查找第一个或最后一个设置位。POSIX有一个ffs()函数，但是它的语义不匹配x86 bsf / bsr。见https://en.wikipedia.org/wiki/Find_first_set)。

一些编译器有时可以识别一个计数整数中设置位数的循环，并将其编译为popcnt指令(如果在编译时启用)，但在GNU C中使用__builtin_popcnt要可靠得多，或者在x86上(如果你的目标硬件是SSE4.2: _mm_popcnt_u32 from < immintrinh >)。

或者在c++中，赋值给std::bitset<32>并使用.count()。(在这种情况下，该语言已经找到了一种方法，可以通过标准库可移植地公开popcount的优化实现，以一种总是编译为正确的方式，并且可以利用目标支持的任何东西。)参见https://en.wikipedia.org/wiki/Hamming_weight#Language_support。

类似地，ntohl可以在一些具有它的C实现上编译为bswap(用于端序转换的x86 32位字节交换)。

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intrinsic或手写asm的另一个主要领域是使用SIMD指令进行手工向量化。编译器对于dst[i] += src[i] * 10.0;这样的简单循环并不糟糕，但是当事情变得更复杂时，编译器通常做得很糟糕，或者根本不自动向量化。例如，你不太可能得到任何像如何实现atoi使用SIMD?由编译器从标量代码自动生成。

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.

http://cr.yp.to/qhasm.html有很多例子。

使用SIMD指令的矩阵操作可能比编译器生成的代码更快。