比较内联 c 汇编中的数字

Question

我正在尝试感受一下 C 语言中的内联汇编，而我之前从未进行过任何汇编编程，因此我正在尝试将必须进行汇编的随机代码段转换。我在下面的代码中遇到了问题，我想让它做的是如果 a 在下方则返回 0x20 (32)0x20 (32) 或高于 0x7e (126)。 (那是不可打印的字符)但是它也会转换字符这些数字之间变成 0x20 (32)

#include <stdio.h>
int foo(int a) {
  asm (
    "mov $0x20, %%ebx;"
    "cmp %%eax, %%ebx;"
    "jl l1;"
    "mov $0x7e, %%ebx;"
    "cmp %%eax, %%ebx;"
    "jg l1;"
    "jmp l2;"
    "l1: movl $0x20, %%eax;"
    "l2:;"

    : "=a" (a)
    : "a" (a)
    : "%ebx"
  );
  return a;
}
int main(void) {
  int i;
  for (i = 0; i <= 255; i++) {
    printf("%i: %i\n", i, foo(i));
  }
}

这个输出
0: 32
1: 32
2: 32
...
31: 32
32: 32
33: 32
34: 32
35: 32
...
125: 32
126: 32
127: 32
128: 32
...

Answer 1

代码将返回 32，因为您没有指定 else:

  ...
  asm (
    "mov $0x20, %%ebx;"
    "cmp %%eax, %%ebx;" // the order should be ebx, eax
    "jl l1;"
    "mov $0x7e, %%ebx;"
    "cmp %%eax, %%ebx;" // the order should be ebx, eax
    "jg l1;"
                        // missing step here
    "jmp l2;"
    "l1: movl $0x20, %%eax;"
    "l2:;"
  ...

您无法告诉编译器变量a 将存储在哪里，您只是将a 传递给asm。您可以将其修复为:

#include <stdio.h>
int foo(int a) {
  asm (
    "cmp $0x20, %%eax;"
    "jl l1;"
    "cmp $0x7e, %%eax;"
    "jg l1;"
    "movl $0, %%eax;"   // what should a be otherwise? 0?
    "jmp l2;"
    "l1: movl $0x20, %%eax;"
    "l2:;"

    : "=a" (a)
    : "a" (a)
    : "%ebx"
  );
  return a;
}
int main(void) {
  int i;
  for (i = 0; i <= 255; i++) {
    printf("%i: %i\n", i, foo(i));
  }
  return 0;
}

或:

#include <stdio.h>
int foo(int a) {
  asm (
    "cmp  $0x20, %0;"  // %0 => a
    "jl   l1;"
    "cmp  $0x7e, %0;"  // %0 => a
    "jg   l1;"
    "movl $0, %1;"     // what should a be otherwise? 0?
    "jmp  l2;"
    "l1:"
    "movl $0x20, %0;"  // %0 = a = 0x20
    "l2:;"
    : "=r" (a) // use r(egister)
    : "r" (a)
    :
  );
  return a;
}
int main(void) {
  int i;
  for (i = 0; i <= 255; i++) {
    printf("%i: %i\n", i, foo(i));
  }
  return 0;
}