转换后的代码不执行给定的操作 [英] The code after conversion does not execute a given action

问题描述

我承担了根据AT& T语法将特定代码从C ++转换为ASM的任务，因此我从简单的示例开始，遇到了第一个问题.

I was given a task to convert a certain code from C++ to ASM based on AT&T syntax, so I started with simple examples and encountered the first problem.

我从中开始练习的代码

void func() {
  int num = 1;
  std::cout << "before: " << num << std::endl;
  num = num << 3;
  std::cout << "after: " << num << std::endl;
}

给出结果:

before: 1
after: 8

我的翻译变量num是第一个局部变量，因此应位于地址-4(％ebp)

my translation variable num is first local variable so it should be in address -4(%ebp)

void func() {
  int num = 1;
  std::cout << "before: " << num << std::endl;
  asm (
    "mov -4(%ebp), %eax         \n"
    "sall $3, %eax              \n"
    "mov %eax, -4(%ebp)         \n"
  );
  std::cout << "after: " << num << std::endl;
}

给出结果:

before: 1
after: 1

为什么此代码对num var没有影响?

why this code has no effect on num var ?

推荐答案

您正在编写的代码是特定于实现的.在您的情况下，该代码可能无法正常工作，因为您正在使用32位寻址寄存器的 ebp ，而在使用 rbp 的64位计算机上运行时代码>.

The code you are writing is very implementation specific. In your case the code likely doesn't work because you are using ebp which is a 32-bit addressing register, while you are running on a 64-bit machine, which uses rbp.

但是，您使用的方法不正确.您可以编写纯汇编程序，也可以使用正确的(扩展的)C内联汇编程序来正确地与局部变量进行接口.否则，一旦您经历了一些更改，代码就会中断.

HOWEVER you are approaching this incorrectly. Either you write pure assembly, or you use the correct (extended) C inline assembly, that correctly interfaces with local variables. Else the code will break as soon as you change something, as you have experienced yourself.

根据此答案内联汇编程序应如下所示:

according to this answer the inline asm should look like:

asm ( "assembly code"
    : output operands                  /* optional */
    : input operands                   /* optional */
    : list of clobbered registers      /* optional */
);

所以您的代码看起来像

asm (
    "mov %0, %%eax;"
    "sal $3, %%eax;"
    "mov %%eax, %1;"
    :"=r" (num)
    :"r" (num)
    :"%eax"
);

但是，您无需指定和使用 eax ，因此可以将代码简化(并阐明)为

However, you don't need to specify and use eax, thus the code could be simplified (and clarified) to

asm (
    "sal $3, %[num];"
    :[num] "+r" (num)
);

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