对于32位整数，为什么不使用左移位位移，“<”和“32位整数工作正常，超过32次？ [英] Why doesn't left bit-shift, "<<", for 32-bit integers work as expected when used more than 32 times?

问题描述

当我编写以下程序并使用GNU C ++编译器时，输出是 1 ，我认为是由于编译器执行的旋转操作。

  #include< iostream> 
 
 int main（）
 {
 int a = 1; 
 std :: cout<< （a << 32）< std :: endl; 
 
 return 0; 
} 
  

但从逻辑上说，宽度，输出应为0.发生了什么？

代码在ideone上， http://ideone.com/VPTwj 。

解决方案

的C中的未定义行为以及为IA-32处理器生成的代码具有应用于移位计数的5位掩码的事实。这意味着在IA-32处理器上，移位计数的范围仅为 0-31 。 ¹

从 C编程语言 ²

如果右操作数为负数，或大于或等于左表达式类型中的位数，则未定义。

em> IA-32英特尔架构软件开发人员手册 ³

8086不会屏蔽移位计数。但是，所有其他IA-32处理器（从Intel 286处理器开始）会将移位计数屏蔽为5位，导致最大计数为31.这种屏蔽在所有操作模式（包括虚拟8086模式）下完成，减少指令的最长执行时间。

---

//codeyarns.com/2004/12/20/c-shift-operator-mayhem/

² A7.8 Shift操作符，附录A参考手册，C编程语言

³ SAL / SAR / SHL / SHR - 设置参考，IA-32英特尔体系结构软件开发人员手册

When I write the following program and use the GNU C++ compiler, the output is 1 which I think is due to the rotation operation performed by the compiler.

#include <iostream>

int main()
{
    int a = 1;
    std::cout << (a << 32) << std::endl;

    return 0;
}

But logically, as it's said that the bits are lost if they overflow the bit width, the output should be 0. What is happening?

The code is on ideone, http://ideone.com/VPTwj.

解决方案

This is caused due to a combination of an undefined behaviour in C and the fact that code generated for IA-32 processors has a 5 bit mask applied on the shift count. This means that on IA-32 processors, the range of a shift count is 0-31 only. ¹

From The C programming language ²

The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expression’s type.

From IA-32 Intel Architecture Software Developer’s Manual ³

The 8086 does not mask the shift count. However, all other IA-32 processors (starting with the Intel 286 processor) do mask the shift count to 5 bits, resulting in a maximum count of 31. This masking is done in all operating modes (including the virtual-8086 mode) to reduce the maximum execution time of the instructions.

---

¹ http://codeyarns.com/2004/12/20/c-shift-operator-mayhem/

² A7.8 Shift Operators, Appendix A. Reference Manual, The C Programming Language

³ SAL/SAR/SHL/SHR – Shift, Chapter 4. Instruction Set Reference, IA-32 Intel Architecture Software Developer’s Manual

这篇关于对于32位整数，为什么不使用左移位位移，“<”和“32位整数工作正常，超过32次？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！