Local variables: are they always on the stack?

Question

In the following procedure, will the array be allocated on the stack?

procedure One:
var
  arr: array[0..1023] of byte;
begin
end;

What is the largest item that can go on the stack?

Is there a speed difference between accessing variable on the stack and on the heap?

Answer 1

In the following procedure, will the array be allocated on the stack?

Yes, provided that the local variable is not captured by an anonymous method. Such local variables reside on the heap.

What is the largest item that can go on the stack?

It depends on how large the stack is, and how much of the stack has already been used, and how much of the stack is used by calls made by the function itself. The stack is a fixed size, determined when the thread is created. The stack overflows if it grows beyond that size. On Windows at least, the default stack size is 1MB, so I would not expect you to encounter problems with a 1KB array as can be seen here.

Is there a speed difference between accessing variable on the stack and on the heap?

By and large no, but again this depends. Variables on the stack are probably more likely to be accessed frequently, and so probably easier to be cached. But for a decently sized object, like the 1KB array we can see here, I would not expect there to be any difference in access time. In terms of the underlying memory architecture, there's no difference between stack and heap, it's all just memory.

Now, where there is a difference in performance is in allocation. Heap allocation is more expensive than stack allocation. And especially if you have a multi-threaded application, heap allocation can be a bottleneck. In particular, the default Delphi memory manager does not scale well in multi-threaded use.