How do C compilers implement functions that return large structures?

abi, c, calling-convention, compiler-optimization, compiler-theory

Solution

None; no copies are done.

The address of the caller's Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller's stack frame.

This is known as the named return value optimisation. Also see the c++ faq on this topic.

commercial-grade C++ compilers implement return-by-value in a way that lets them eliminate the overhead, at least in simple cases

...

When yourCode() calls rbv(), the compiler secretly passes a pointer to the location where rbv() is supposed to construct the "returned" object.

You can demonstrate that this has been done by adding a destructor with a printf to your struct. The destructor should only be called once if this return-by-value optimisation is in operation, otherwise twice.

Also you can check the assembly to see that this happens:

Data createData() 
{
    Data data;
    // initialize data values...
    data.values[5] = 6;
    return data;
}

here's the assembly:

__Z10createDatav:
LFB2:
        pushl   %ebp
LCFI0:
        movl    %esp, %ebp
LCFI1:
        subl    $1032, %esp
LCFI2:
        movl    8(%ebp), %eax
        movl    $6, 20(%eax)
        leave
        ret     $4
LFE2:

Curiously, it allocated enough space on the stack for the data item `subl $1032, %esp`, but note that it takes the first argument on the stack `8(%ebp)` as the base address of the object, and then initialises element 6 of that item. Since we didn't specify any arguments to createData, this is curious until you realise this is the secret hidden pointer to the parent's version of Data.

Problem

The return value of a function is usually stored on the stack or in a register. But for a large structure, it has to be on the stack. How much copying has to happen in a real compiler for this code? Or is it optimized away? For example: ``` struct Data { unsigned values[256]; }; Data createData() { Data data; // initialize data values... return data; } ``` (Assuming the function cannot be inlined..)

Original source