[swift-users] Passing value types or members of value types?

[Daniel Dunbar]

> On May 6, 2017, at 10:27 PM, Nate Birkholz via swift-users <swift-users at swift.org> wrote:
> 
> Classes and structs are different things. Classes are passed by reference, structs are passed by value with copy on write. 

This is not correct. Array, and other types, implement copy-on-write themselves, but this is not true of arbitrary structs.

To answer Kelvin's question, yes, the optimizer will be able to see through that code _assuming_ it can see the definition in a way it can optimize (generally speaking, this means not something which will be dynamically dispatched, and where the implementation is available (i.e. the same file, or the same module with whole module optimization enabled)).

You can verify this by simply writing a small sample program and spot checking the assembly matches what you want, for example:
--
$ cat z.swift 
struct Vertex {
    let x: Double, y: Double
    var r:Int, g:Int, b:Int
    let s:Double, t:Double
    var selected:Bool
}

private func taxicab_distance(_ v1:Vertex, _ v2:Vertex) -> Double {
    return v2.x - v1.x + v2.y - v1.y
}

func f0(a: Vertex, b: Vertex) -> Double {
  return taxicab_distance(a, b)
}

$ swiftc -Ounchecked -S -o - z.swift | grep 'main2f0' -A8
	.private_extern	__T04main2f0SdAA6VertexV1a_AD1btF
	.globl	__T04main2f0SdAA6VertexV1a_AD1btF
	.p2align	4, 0x90
__T04main2f0SdAA6VertexV1a_AD1btF:
	pushq	%rbp
	movq	%rsp, %rbp
	movsd	(%rsi), %xmm0
	subsd	(%rdi), %xmm0
	addsd	8(%rsi), %xmm0
	subsd	8(%rdi), %xmm0
	popq	%rbp
	retq
--

 - Daniel


> 
> Strings and Arrays are structs, not classes.
> 
> On Sat, May 6, 2017 at 10:24 PM, Kelvin Ma <kelvin13ma at gmail.com <mailto:kelvin13ma at gmail.com>> wrote:
> I hear that a lot but why do all the official sources sound like copy-on-write is something you have to manually implement with a class and isUniquelyReferenced if it’s not an array or a string?
> 
> On May 7, 2017, at 12:02 AM, Nate Birkholz <nbirkholz at gmail.com <mailto:nbirkholz at gmail.com>> wrote:
> 
>> Let me try one more time, my stupid phone keeps changing words. 
>> 
>> ACTUALLY, the struct is a pointer until it's written to, and is only copied on write.
>> 
>> Sent from my iPhone, please excuse brevity and errors
>> 
>> On May 6, 2017, at 9:59 PM, Nate Birkholz <nbirkholz at gmail.com <mailto:nbirkholz at gmail.com>> wrote:
>> 
>>> Oddly, until it's *written* you will be working with a pointer.
>>> 
>>> Sent from my iPhone, please excuse brevity and errors
>>> 
>>> On May 6, 2017, at 9:33 PM, Kelvin Ma via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>>> If I have a “large” struct like
>>>> 
>>>> struct Vertex
>>>> {
>>>>     let x:Double, y:Double
>>>>     var r:Int, g:Int, b:Int
>>>>     let s:Double, t:Double
>>>>     var selected:Bool
>>>> }
>>>> 
>>>> and I want to pass it to a function without modifying it like
>>>> 
>>>> func taxicab_distance(_ v1:Vertex, _ v2:Vertex) -> Double
>>>> {
>>>>     return v2.x - v1.x + v2.y - v1.y
>>>> }
>>>> 
>>>> Will the entire struct Vertex get copied and pushed onto the stack, or will only the relevant members be passed to the function?
>>>> 
>>>> In other words, does the compiler know to optimize this call down to
>>>> 
>>>> func taxicab_distance(v2x:Double, v1x:Double, v2y:Double, v1y:Double) -> Double
>>>> {
>>>>     return v2x - v1x + v2y - v1y
>>>> }
>>>> 
>>>> ?
>>>> _______________________________________________
>>>> swift-users mailing list
>>>> swift-users at swift.org <mailto:swift-users at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
> 
> 
> 
> -- 
> Nate Birkholz
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