什么是GCC __attribute __（（模式（XX））实际上呢？ [英] What does GCC __attribute__((mode(XX)) actually do?

问题描述

这从一个问题，今天早些时候出现在BIGNUM库和gcc黑客特定的主题，以C语言。具体而言，使用这两个声明：

 的typedef unsigned int类型dword_t __attribute __（（模式（DI）））;
 

在32位系统以及

 的typedef unsigned int类型dword_t __attribute __（（模式（TI）））;
 

在64位系统上。

我想给这是一个扩展的C语言不存在任何方式来实现不管它实现了在电流（C99）的标准。所以我的问题很简单：就是假设是正确的？而做这些语句做底层的内存？我认为结果是我有 2xsizeof（uint32_t的）为 DWORD 在32位系统和 2 * sizeof的（uint64_t中） 64位系统，我是正确的？

解决方案

这允许你显式地指定一个大小为一类，而不依赖于编译器或机器的语义，如长或INT。<大小/ p>

他们被描述在这个页面相当好。

我从该网页引述如下：

  

齐：一个整数，是一样宽的
  最小可寻址单元，通常是8
  位。

  
  

HI：一个整数，两倍宽
  具有益气模式整数，通常是16位。

  
  

SI：一个整数，四倍宽为QI
  模式整数，通常为32位。

  
  

DI：一个
  整，八倍宽为QI
  模式整数，通常是64位。

  
  

SF：一
  浮点值，宽为SI
  模式整数，通常为32位。

  
  

DF：一
  浮点值，宽为DI
  模式整数，通常是64位。

所以 DI 本质上是的sizeof（字符）* 8 。

进一步说明，包括 TI 模式下，可以发现的此处（可能比第一个环节更好，但两者供参考）。

所以 TI 本质上是的sizeof（char）的* 16 （128位）。

This arose from a question earlier today on the subject of bignum libraries and gcc specific hacks to the C language. Specifically, these two declarations were used:

typedef unsigned int dword_t __attribute__((mode(DI)));

On 32 bit systems and

typedef unsigned int dword_t __attribute__((mode(TI)));

On 64-bit systems.

I assume given this is an extension to the C language that there exists no way to achieve whatever it achieves in current (C99) standards. So my questions are simple: is that assumption correct? And what do these statements do to the underlying memory? I think the result is I have 2xsizeof(uint32_t) for a dword in 32-bit systems and 2*sizeof(uint64_t) for 64-bit systems, am I correct?

解决方案

These allow you to explicitly specify a size for a type without depending on compiler or machine semantics, such as the size of 'long' or 'int'.

They are described fairly well on this page.

I quote from that page:

QI: An integer that is as wide as the smallest addressable unit, usually 8 bits.

HI: An integer, twice as wide as a QI mode integer, usually 16 bits.

SI: An integer, four times as wide as a QI mode integer, usually 32 bits.

DI: An integer, eight times as wide as a QI mode integer, usually 64 bits.

SF: A floating point value, as wide as a SI mode integer, usually 32 bits.

DF: A floating point value, as wide as a DI mode integer, usually 64 bits.

So DI is essentially sizeof(char) * 8.

Further explanation, including TI mode, can be found here (possibly better than the first link, but both provided for reference).

So TI is essentially sizeof(char) * 16 (128 bits).

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