Why is 0 moved to stack when using return value?

Question

I\'m experimenting disassembling clang binaries of simple C programs (compiled with -O0), and I\'m confused about a certain instruction that gets g

Answer 1

The purpose of that area is revealed by the following code

int main(int argc, char** argv)
{
    if (rand() == 42)
      return 1;

    printf("Helo World!\n");
    return 0;
}

At the start it does

movl    $0, -4(%rbp)

then the early return looks as follows

callq   rand
cmpl    $42, %eax
jne .LBB0_2
movl    $1, -4(%rbp)
jmp .LBB0_3

and then at the end it does

.LBB0_3:
movl    -4(%rbp), %eax
addq    $32, %rsp
popq    %rbp
retq

So, this area is indeed reserved to store the function return value. It doesn't appear to be terribly necessary and it is not used in optimized code, but in -O0 mode that's the way it works.

Answer 2

movl   $0x0,-0x4(%rbp)

This instruction stores 0 at %rbp - 4. It seems that clang allocates a hidden local variable for an implicit return value from main.

From the clang mailing list:

Yes. We allocate an implicit local variable to hold the return value; return statements then just initialize the return slot and jump to the epilogue, where the slot is loaded and returned. We don't use a phi because the control flow for getting to the epilogue is not necessarily as simple as a simple branch, due to cleanups in local scopes (like C++ destructors).

Implicit return values like main's are handled with an implicit store in the prologue.

Source: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-February/019767.html

Answer 3

clang is making space on the stack for the arguments (registers edi and rsi) and puts the value 0 on the stack, too, for some reason. I assume that clang compiles your code to an SSA-representation like this:

int main(int argc, char** argv)
{
    int a;

    a = 0;
    return a;
}

This would explain why a stack slot is allocated. If clang does constant propagation, too, this would explain why eax is zeroed out instead of being loaded from -4(%rbp). In general, don't think too much about dubious constructs in unoptimized assembly. After all, you forbade the compiler from removing useless code.

Answer 4

According the the standard (for hosted environments), 5.1.2.2.1, main is required to returrn an int result. So do not expect defined behavior if violating this.

Furthermore, main is actually _not required to explicitly return 0; this is implicitly returned if it reaches the end of the function. (Note this is only for main, which also does not have a prototype.