c++ - Unoptimized clang++ code generates unneeded "movl $0, -4(%rbp)" in a trivial main()

Question

I created a minimal C++ program:

int main() {
    return 1234;
}

and compiled it with clang++5.0 with optimization disabled (the default -O0). The resulting assembly code is:

  pushq %rbp
  movq %rsp, %rbp
  movl $1234, %eax # imm = 0x4D2
  movl $0, -4(%rbp)
  popq %rbp
  retq

I understand most of the lines, but I do not understand the "movl $0, -4(%rbp)". It seems the program initializes some local variable to 0. Why?

What compiler-internal detail leads to this store that doesn't correspond to anything in the source?

Answer 1

TL;DR : In unoptimized code your CLANG++ set aside 4 bytes for the return value of main and set it to zero as per the C++(including C++11) standards. It generated the code for a main function that didn't need it. This is a side effect of not being optimized. Often an unoptimized compiler will generate code it may need, then doesn't end up needing it, and nothing is done to clean it up.

---

Because you are compiling with -O0 there is a very minimum of optimizations done on code (-O0 may remove dead code etc). Trying to understand artifacts in unoptimized code is usually a wasted exercise. The results of unoptimized code are extra loads and stores and other artifacts of raw code generation.

In this case main is special because in C99/C11 and C++ the standards effectively say that when reaching the outer block of main the default return value is 0. The C11 standard says:

5.1.2.2.3 Program termination

1 If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument;11) reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

The C++11 standard says:

3.6.1 Main function

5) A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std::exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the effect is that of executing

 return 0;

In the version of CLANG++ you are using the unoptimized 64-bit code by default has the return value of 0 placed at dword ptr [rbp-4].

The problem is that your test code is a bit too trivial to see how this default return value comes in to play. Here is an example that should be a better demonstration:

int main() {
    int a = 3;
    if (a > 3) return 5678;
    else if (a == 3) return 42;
}

This code has two exit explicit exit points via return 5678 and return 42; but there isn't a final return at the end of the function. If } is reached the default is to return 0. If we examine the godbolt output we see this:

main:                                   # @main
        push    rbp
        mov     rbp, rsp
        mov     dword ptr [rbp - 4], 0        # Default return value of 0
        mov     dword ptr [rbp - 8], 3
        cmp     dword ptr [rbp - 8], 3        # Is a > 3
        jle     .LBB0_2
        mov     dword ptr [rbp - 4], 5678     # Set return value to 5678
        jmp     .LBB0_5                       # Go to common exit point .LBB0_5
.LBB0_2:
        cmp     dword ptr [rbp - 8], 3        # Is a == 3?
        jne     .LBB0_4
        mov     dword ptr [rbp - 4], 42       # Set return value to 42
        jmp     .LBB0_5                       # Go to common exit point .LBB0_5
.LBB0_4:
        jmp     .LBB0_5                       # Extraneous unoptimized jump artifact 
# This is common exit point of all the returns from `main`
.LBB0_5:
        mov     eax, dword ptr [rbp - 4]      # Use return value from memory
        pop     rbp
        ret

As one can see the compiler has generated a common exit point that sets the return value (EAX) from the stack address dword ptr [rbp-4]. At the beginning of the code dword ptr [rbp-4] is explicitly set to 0. In the simpler case, the unoptimized code still generates that instruction but goes unused.

If you build the code with the option -ffreestanding you should see the default return value for main no longer set to 0. This is because the requirement for a default return value of 0 from main applies to a hosted environment and not a freestanding one.