How to return a complex return value?

Question

Currently I am writing some assembly language procedures. As some convention says, when I want to return some value to the caller, say an integer, I should return it in the EAX register. Now I am wondering what if I want to return a float, a double, an enum, or even a complex struct. How to return these type of values?

I can think of returning an address in the EAX which points to the real value in memory. But is it the standard way?

Many thanks~~~

Answer 1

It is all up to you, if the caller is your code. If the caller is not under your control, you have to either follow their existing convention or develop your own convention together.

For example, on x86 platform when floating-point arithmetic is processed by FPU instructions, the result of a function is returned as the top value on the FPU register stack. (If you know, x86 FPU registers are organized into a "circular stack" of sorts). At that moment it is neither float nor double, it is a value stored with internal FPU precision (which could be higher than float or double) and it is the caller's responsibility to retrieve that value from the top of FPU stack and convert it to whatever type it desires. In fact, that is how a typical FPU instruction works: it takes its arguments from the top of FPU stack and pushes the result back onto FPU stack. By implementing your function in the same way you essentially emulate a "complex" FPU instruction with your function - a rather natural way to do it.

When floating-point arithmetic is processed by SSE instructions, you can choose some SSE register for the same purpose (use xmm0 just like you use EAX for integers).

For complex structures (i.e. ones that are larger than a register or a pair of registers), the caller would normally pass a pointer to a reserved buffer to the function. And the function would put the result into the buffer. In other words, under the hood, functions never really "return" large objects, but rather construct them in a caller-provided memory buffer.

Of course, you can use this "memory buffer" method for returning values of any type, but with smaller values, i.e. values of scalar type, it is much more efficient to use registers than a memory location. This applies, BTW, to small structures as well.

Enums are usually just a conceptual wrapper over some integer type. So, there's no difference between returning a enum or an integer.

Answer 2

A double should be returned as the 1st item in the stack.

Here is a C++ code example (x86):

double sqrt(double n)
{
    _asm fld n
    _asm fsqrt
}  

If you prefer to manage the stack manually (saving some CPU cycles):

double inline __declspec (naked) __fastcall sqrt(double n)
{
    _asm fld qword ptr [esp+4]
    _asm fsqrt
    _asm ret 8
}

For complex types, you should pass a pointer, or return a pointer.

Answer 3

When you have questions about calling conventions or assembly language, write a simple function in high level language (in a separate file). Next, have your compiler generate an assembly language listing or have your debugger display "interleaved assembly".

Not only will the listing tell you how the compiler implements code, but also show you the calling conventions. A lot easier than posting to S.O. and usually faster. ;-)