什么是内在函数？ [英] What are intrinsics?

问题描述

任何人都可以解释它们是什么，为什么我需要它们？如果需要使用内在函数，我应该构建什么样的应用程序？

Can anyone explain what they are and why I would need them? What kind of applications am I building if I need to use intrinsics?

推荐答案

通常， intrinsics是指所构建的函数-in-即编译器可以/将生成内联而不是在库中调用实际函数的大多数标准库函数。例如，对于x86，像这样的调用： memset（array1，10，0）可以像这样编译：

Normally, "intrinsics" refers to functions that are built-in -- i.e. most standard library functions that the compiler can/will generate inline instead of calling an actual function in the library. For example, a call like: memset(array1, 10, 0) could be compiled for an x86 as something like:

 mov ecx, 10
 xor eax, eax
 mov edi, offset FLAT:array1
 rep stosb

这样的内部特性纯粹是一种优化。 需要内部函数很可能是编译器支持内部函数的一种情况，这些内部函数可以让您生成编译器无法（或通常不会）直接生成的代码。举一个显而易见的例子，很多x86编译器都具有 MMX Intrinsics，可让您使用实际上只是MMX指令直接表示的函数。

Intrinsics like this are purely an optimization. "Needing" intrinsics would most likely be a situation where the compiler supports intrinsics that let you generate code that the compiler can't (or usually won't) generate directly. For an obvious example, quite a few compilers for x86 have "MMX Intrinsics" that let you use "functions" that are really just direct representations of MMX instructions.

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