Why are function parameters pushed earlier on call stack than the return address?

Question

From http://en.wikipedia.org/wiki/Stack_pointer#Structure

I am wondering why the return address for a function is placed above the parameters for that function?

It makes more sense to have Return Address pushed onto the stack before the Parameters for Drawline because the parameters are not required any more when the Return Address is popped for returning back to the calling function.

What are the reasons for preferring the implementation shown in diagram above?

Answer 1

The reason is simple: The function arguments are pushed onto the stack by the calling function (which is the only one which can do it because only it has the necessary information; after all the whole point of doing so is to pass that information to the called function). The return address is pushed to the stack by the function call mechanism. The function is called after the calling function has set up the parameters, because after the call it's the called function which is executed, not the calling one.

OK, so now you could argue that the calling function could put the parameters beyond the currently used stack, and the called function could then just adjust the stack pointer accordingly. But that would not work out well because at any time there could be an interrupt or a signal, which would push the current state onto the stack in order to restore later (I wouldn't be surprised if a task switch did so, too). But if you set up the parameters beyond the current stack, those asynchronous events would overwrite it, and since you cannot predict when they will happen, you cannot avoid that (beyond disabling, which may have other drawbacks or even be impossible, in the case of task switch). Basically, everything beyond the current stack has to be considered volatile.

Also note that this is independent of the question of who cleans up the parameters. In principle, the called function could call call destructors of the arguments even if physically they lie in the caller's stack frame. Also, many processors (including the x86) have instructions which automatically pop extra space above the return address on return (for example, Pascal compilers usually did that because in Pascal you don't have any cleanup beyond returning memory, and at least fr the processors of the time, it was more efficient to clean up with that processor instruction (I have no idea if that is still true for modern processors). However C didn't use that mechanism due to variable-length argument lists: For those, the mechanism wasn't applicable because you'd need to know at compile time how much extra space to release, and K&R C did not require to forward-declare variadic functions (C89 does, but few if any compilers take advantage of that, due to compatibility with old code), so there was no way for the calling function to know whether to clean up the arguments unless it had to do that always.

Answer 2

The return address is usually pushed via the call machine command, [which in the native language's instruction set] while the parameters and variables are pushed with several machine commands - which the compiler creates.

Thus, the return address is the last thing pushed by the caller, and before anything [local variables] pushed by the callee.

The parameters are all pushed before the return address, because the jump to the actual function and the insertion of the return address to the stack is done in the same machine command.

Also, another reason is - the caller is the one allocating space on stack for the parameters - It [the caller] should also be the one who cleans it up.