“%register"与“%register"之间的区别和“(％注册)"在 x86 汇编 AT&T 语法中? [英] Difference between "%register" and "(%register)" in x86 assembly AT&T syntax?

问题描述

到目前为止，我目前的理解是:movq %rdi, %rax 会将值从寄存器 %rdi 移动到寄存器 %rax

So far my current understanding is something along the lines of: movq %rdi, %rax will move the value from the register %rdi to the register %rax

和 movq (%rdi), %rax 会将值从内存中的 (%rdi) 移动到寄存器 %rax

and movq (%rdi), %rax will move the value from memory at (%rdi) to the register %rax

但是，我无法理解这在功能上的实际含义.在什么情况下，这两条装配线会以不同的结果结束?

However, I'm having trouble understanding what this actually means functionally. In what instance will these two assembly lines end with a different result?

推荐答案

每次地址 (%rdi) 处的内存不包含自己的地址时，都会产生不同的结果.换句话说，几乎总是.如果不是，这只是巧合或非常不寻常的代码的结果.

It will yield a different result every time the memory at adress (%rdi) does not contain its own adress. In other words, almost always. And when not, it's just a coincidence or a consequence of very unusual code.

一些 C 来演示等效问题两个 printf 语句何时会打印相同的结果?"

Some C to demonstrate the equivalent question "When will the two printf statements print the same result?"

#include <stdio.h>

int main()
{
  int a,*p;
  int b=5;
  p=&b;

  a=*p;
  printf("%d\n", a);
  a=(int)p;
  printf("%d\n", a);
}

而且，是的，这会产生警告 cast from pointer to integer of different size 但这不是重点.

And, yes, this yields the warning cast from pointer to integer of different size but that's beside the point.

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