Discussion on: 🚀 Demystifying memory management in modern programming languages

[Sergio Pedri]

Hey, no problem, glad I could help, and I'm always interested in sparking up a discussion where everyone can hopefully learn something new 😄

To reply to your points:

1. Stack and heap are just two segments in the virtual address space of a process. They only real difference is the range of addresses they use, but they're fundamentally exactly the same: a given area in the virtual address space. Hence, the access time is exactly the same: in both case you'd still be dereferencing a pointer. I think you might be confusing what you see in a high level language (like Java, or even C) with what is actually happening at the hardware level (I mean in assembly). Even if you have a local value type variable in a function in Java, when you read it the CPU will still use a pointer to read it from the stack. The only difference is that the address in this case is automatically calculated by adding a certain offset to the %esp register, which is a CPU register that always points to the top of the stack. Whereas with an object in the heap you'd be manually keeping track of the reference to it (either via a raw pointer in C#, or through a managed pointer in a managed language like Java). The GC doesn't have anything to do with this, nor does the possible fragmentation of the heap itself. The moment you're trying to read any value from your virtual address space you're always going to just load a pointer to your target location (be it in the heap or the stack) and then dereference it. Actually, in assembly it's even the same exact instruction (mov, in general) - the CPU doesn't really care what segment that address belong to at all. What's more, in many languages (eg. C, C++, C#) you can even get pointers to local variables on the stack, and pass those to functions. Once you're inside a function, that function can't possibly know (nor does it care either) whether that pointer actually points to a variable in the heap or the stack, but again, that doesn't matter. It's the same exact behavior, and the speed is the same too. As I've mentioned, the heap is generally considered slower because people associate the actual allocation cost to it, and the garbage collection cost. But just reading/writing data to it is equally as fast as doing so on the stack.

2. I agree that the data on the stack must be finite in size (though the same could be said for the heap as well, the limit is just bigger but it's still finite), my point was that you don't need to know at compile time the amount of data stored on the stack at all. You could very much have a function that takes an int and then allocates a buffer of int on the stack, with a length equal to that input int. In that case you'd have absolutely no way of knowing the size of that buffer at compile time, and yet the buffer is created on the stack just fine.

3. Sure thing, you can read up here for instance. The stack is generally referred to as LIFO since that's the way new values are pushed and popped to it, including stack frames during function calls, but as I said you can actually read/write at any position in the stack at all times. There's a CPU register called %esp (Stack Pointer) which keeps the address to the top of the stack, and you can index any position in the stack by just adding an offset to that address and dereferencing it. For instance, consider this function and the resulting assembly x86:
   

// C code
int sumFour(int a, int b, int c, int d)
{
    return a + b + c + d;
}

// asm x86
sumThree(Int32, Int32, Int32)
0000: add edx, ecx        // b += a
0002: mov eax, edx        // Move b in %eax
0004: add eax, [esp+0x8]  // %eax += c
0008: add eax, [esp+0x4]  // %eax += d
000c: ret

You can see that while the compiler is passing the a and b parameter through the ecx and edx registers to minimize memory access, the c and d parameters are passed through the stack, and they're being read with that [esp+0x4] and [esp+0x8] addresses, which are not the top of the stack. This happens all the time, eg. when you have many local parameters in a function that can't all fit in a register and need to be stored on the stack (which is much slower). Point is: you can read/write from any position within the stack with no problems, you're absolutely not limited to just the top value.

Hope this helps, let me know if there's anything else! 😄

[Deepu K Sasidharan]

Hey, thanks for the quick response. Actually I updated my comment above as I made a mistake there, right before you added this reply, could you take a look? I'll respond to this once you have seen the update to avoid any confusion.

[Sergio Pedri]

I've seen the updates, but what I said in my last comment still applies, so it's fine, you can go ahead and read/reply to that one if you want 😊

[Deepu K Sasidharan]

Thanks again. I did learn something new now.

I guess you are talking from a lower level, like assembly, point of view and I'm talking from a high-level language point of view.

1. Agree at a low level that it would be same, but since I'm talking about how languages handle memory, I think accessing Heap is still slower than accessing Stack at language level, might not be by much, but even if you count by instructions, it still has to do few more steps to read from heap compared to stack. Even a low-level language like Rust, for example, says the same. See this doc.rust-lang.org/1.30.0/book/firs...

The stack is very fast, and is where memory is allocated in Rust by default. But the allocation is local to a function call, and is limited in size. The heap, on the other hand, is slower, and is explicitly allocated by your program. But it’s effectively unlimited in size, and is globally accessible.

1. I'm talking about individual data(variables & constants) and it has to be finite at compile time for most languages(Java, Rust, Go, etc) to be on Stack and you cannot have buffers on the stack in these and since the language automatically manages these we have no control over this. I guess C/C++ is an exception when you manage memory yourself, but then again they are very low-level languages. So yes this is very much language-dependent I would say but holds for most languages IMO.

2. Yes, it makes absolute sense at a low level. At a higher level, its easier to talk of it as LIFO though as the inner process is not exposed to end users in most languages