compare values with inline assembler

Question

I'm searching for a code example, for comparing values with inline assembly in gcc. I need to asign values to the asm code. I've tried some examples, they didn't worked correctly. There is always an error or i don't understand the results.

I need just to compare two values and return a result.

  movl my_val, %eax
  cmpl %eax,$0xfffffffa
  je   equal
  equal:
  movl $0xfffffffa,my_val

Answer 1

This program will compare value taken from argv[1] with $0x1 on my amd64, so you might have to fix it to work on your architecture:

#include <stdio.h>

int main(int argc, char* argv[]) {

  int value, result;
  value = atoi(argv[1]);
  result = 0;

  __asm__ ( "xor $0x1,%%eax;\n"
    "jnz end;\n"
    "movl $0xfffffffa,%%ebx;\n"
    "end:\n"
    :"=b"(result)
    :"a"(value)
  );

  if (result) {
    printf("Equals!\n");
  }

  return 0;
}

Now compile and run it:

$ gcc -o comp comp.c
$ ./comp 0
$ ./comp 1
Equals!

Reference:

• GCC Inline Assembly Howto