value type - Does swift copy on write for all structs?

Question

I know that swift will optimize to copy on write for arrays but will it do this for all structs? For example:

struct Point {
   var x:Float = 0
}

var p1 = Point()
var p2 = p1 //p1 and p2 share the same data under the hood
p2.x += 1 //p2 now has its own copy of the data

Answer 1

Array is implemented with copy-on-write behaviour – you'll get it regardless of any compiler optimisations (although of course, optimisations can decrease the number of cases where a copy needs to happen).

At a basic level, Array is just a structure that holds a reference to a heap-allocated buffer containing the elements – therefore multiple Array instances can reference the same buffer. When you come to mutate a given array instance, the implementation will check if the buffer is uniquely referenced, and if so, mutate it directly. Otherwise, the array will perform a copy of the underlying buffer in order to preserve value semantics.

However, with your Point structure – you're not implementing copy-on-write at a language level. Of course, as @Alexander says, this doesn't stop the compiler from performing all sorts of optimisations to minimise the cost of copying whole structures about. These optimisations needn't follow the exact behaviour of copy-on-write though – the compiler is simply free to do whatever it wishes, as long as the program runs according to the language specification.

In your specific example, both p1 and p2 are global, therefore the compiler needs to make them distinct instances, as other .swift files in the same module have access to them (although this could potentially be optimised away with whole-module optimisation). However, the compiler still doesn't need to copy the instances – it can just evaluate the floating-point addition at compile-time and initialise one of the globals with 0.0, and the other with 1.0.

And if they were local variables in a function, for example:

struct Point {
    var x: Float = 0
}

func foo() {
    var p1 = Point()
    var p2 = p1
    p2.x += 1
    print(p2.x)
}

foo()

The compiler doesn't even have to create two Point instances to begin with – it can just create a single floating-point local variable initialised to 1.0, and print that.

Regarding passing value types as function arguments, for large enough types and (in the case of structures) functions that utilise enough of their properties, the compiler can pass them by reference rather than copying. The callee can then make a copy of them only if needed, such as when needing to work with a mutable copy.

In other cases where structures are passed by value, it's also possible for the compiler to specialise functions in order to only copy across the properties that the function needs.

For the following code:

struct Point {
    var x: Float = 0
    var y: Float = 1
}

func foo(p: Point) {
    print(p.x)
}

var p1 = Point()
foo(p: p1)

Assuming foo(p:) isn't inlined by the compiler (it will in this example, but once its implementation reaches a certain size, the compiler won't think it worth it) – the compiler can specialise the function as:

func foo(px: Float) {
    print(px)
}

foo(px: 0)

It only passes the value of Point's x property into the function, thereby saving the cost of copying the y property.

So the compiler will do whatever it can in order to reduce the copying of value types. But with so many various optimisations in different circumstances, you cannot simply boil the optimised behaviour of arbitrary value types down to just copy-on-write.