In this article, we will learn how to construct C code from x86 assembly for loops. The article is divided into two parts. In the first part, we will write C code for loops and analyze the corresponding x86 assembly code. In the second part of the article, we will analyze the x86 assembly code and try to construct the corresponding C code that uses loops.

Let’s first write a small piece of C code that uses a loop.

#include <stdio.h>
int main() {
        int i = 10;
        while(i > 0) {
                i = i - 1;
        }
        return 0;
}

Now, let’s compile the code and generate the object code using the GCC compiler. After that, we can use the objdump command to look into the corresponding assembly code.

$ gcc -c loops.c
$ objdump -M intel -d loops.o

Please note that the “-M intel” option will generate the assembly code in the Intel syntax.

The x86 64-bit or x64 assembly code corresponding to the above C code will look like the following:

0000000000000000 <main>:
   0:	push   rbp
   1:	mov    rbp,rsp
   4:	mov    DWORD PTR [rbp-0x4],0xa
   b:	jmp    11 <main+0x11>
   d:	sub    DWORD PTR [rbp-0x4],0x1
  11:	cmp    DWORD PTR [rbp-0x4],0x0
  15:	jg     d <main+0xd>
  17:	mov    eax,0x0
  1c:	pop    rbp
  1d:	ret    

Now, let’s analyze the assembly code.