Is my conversion from assembly to c code correct?

Question

I have a problem that asks to convert assembly code to C code.

I've made an attempt to convert the code and I think I got it mostly right but I am confused by the leaq instruction.

looper:
   movl  $o, %eax
   movl  $o, %edx
   jmp   .L2

.L4:
   movq  (%rsi, %rdx, 8), %rcx
   cmpq  %rcx, %rax
   jl    .L3
   movq  %rax, %rcx

.L3:
   leaq  1(%rcx), %rax
   addq  $1, %rdx

.L2:
   cmpq  %rdi, %rdx
   jl    .L4
   rep ret

Here is the C code that I got:

long looper(long n, long *a) {
  long i;
  long x = 0;

  for (i = 0; i < n; i++) {
    if (x < a[i]) {
      x = a[i] + 1;
    }
    x = a[i];
  }

  return x;
}

Answer 1

First of all, there seems to be an error in your assembly: looks like $o should be $0 (zero).

Your C code is almost correct, there are some errors:

1. The order of the branches generated by the jl .L3 instruction is quite significant, in fact, if %rax < %rcx the branch is taken, and the instruction movq %rax, %rcx is ignored. On the other hand, if the branch is NOT taken, then that move is executed before stepping into .L3. So you basically swapped the two branches in your C code.

2. The value of a[i] is not used directly every time, but it is saved in the %rcx register before being used. Both movq %rax, %rcx and movq (%rsi, %rdx, 8), %rcx assign to %rcx, then the value is passed from %rcx to %rax, so %rcx should be treated as a different variable. This means that writing x = a[i] + 1; is wrong. It should be:

   tmp = a[i];
   /* ... */
   x = tmp + 1; 
   

The resulting C code should be something like this:

int64_t looper(int64_t n, int64_t *arr) {
    int64_t result; // rax
    int64_t tmp;    // rcx
    int64_t i;      // rdx

    result = 0;

    for (i = 0; i < n; i++) {
        tmp = arr[i];

        if (result >= tmp)
            tmp = result;

        result = tmp + 1;
    }

    return result;
}

As an addition, compiling your binary using as -o prog prog.s and then disassembling with Radare2 gives this rather simple control flow graph:

    looper ();
        0x08000040      mov eax, 0
        0x08000045      mov edx, 0
    ,=< 0x0800004a      jmp 0x8000060
    |
    |   ; JMP XREF from 0x08000063 (looper)
  .---> 0x0800004c      mov rcx, qword [rsi + rdx*8]
  | |   0x08000050      cmp rax, rcx
,=====< 0x08000053      jl 0x8000058
| | |   0x08000055      mov rcx, rax
| | |
| | |   ; JMP XREF from 0x08000053 (looper)
`-----> 0x08000058      lea rax, qword [rcx + 1]
  | |   0x0800005c      add rdx, 1
  | |
  | |   ; JMP XREF from 0x0800004a (looper)
  | `-> 0x08000060      cmp rdx, rdi
  `===< 0x08000063      jl 0x800004c
        0x08000065      ret