Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Max Moiseev via swift-evolution]

Hi Susan,

Was there any motivation for this proposal that I missed? If not then, can you 
please provide it in a few sentences? Otherwise it’s not clear to me what 
problem it is supposed to fix.

Thanks,
Max 


> On Jul 6, 2017, at 8:21 PM, Susan Cheng via swift-evolution 
>  wrote:
> 
> IMO, it has unclear representation when FixedWidthInteger working with 
> endianness specific type.
> 
> so I want to introduce the endianness specific wrapper:
> 
> public struct BEInteger : FixedWidthInteger {
> 
> public var bigEndian: BEInteger { get }
> 
> public var littleEndian: LEInteger { get }
> }
> 
> public struct LEInteger : FixedWidthInteger {
> 
> public var bigEndian: BEInteger { get }
> 
> public var littleEndian: LEInteger { get }
> }
> 
> also, we should change the FixedWidthInteger as follow:
> 
> public protocol FixedWidthInteger : BinaryInteger {
> 
> /// deprecated, we should use value.bigEndian instead
> init(bigEndian value: Self)
> 
> /// deprecated, we should use value.littleEndian instead
> init(littleEndian value: Self)
> 
> associatedtype EndianRepresentingValue : FixedWidthInteger
> 
> var bigEndian: BEInteger { get }
> 
> var littleEndian: LEInteger { get }
> }
> 
> =
> 
> this is my working alternative implementation:
> 
> 
> @_versioned
> protocol EndianInteger : FixedWidthInteger {
> 
> associatedtype BitPattern : FixedWidthInteger
> 
> associatedtype RepresentingValue : FixedWidthInteger
> 
> var bitPattern: BitPattern { get }
> 
> init(bitPattern: BitPattern)
> 
> var representingValue : RepresentingValue { get set }
> 
> init(representingValue: RepresentingValue)
> }
> 
> extension EndianInteger {
> 
> @_transparent
> public init(integerLiteral value: RepresentingValue.IntegerLiteralType) {
> self.init(representingValue: RepresentingValue(integerLiteral: value))
> }
> 
> @_transparent
> public init?(exactly source: T) where T : BinaryInteger {
> guard let value = RepresentingValue(exactly: source) else { return 
> nil }
> self.init(representingValue: value)
> }
> 
> @_transparent
> public init?(exactly source: T) where T : FloatingPoint {
> guard let value = RepresentingValue(exactly: source) else { return 
> nil }
> self.init(representingValue: value)
> }
> 
> @_transparent
> public init(_ value: RepresentingValue) {
> self.init(representingValue: value)
> }
> 
> @_transparent
> public init(_ source: T) where T : FloatingPoint {
> self.init(representingValue: RepresentingValue(source))
> }
> 
> @_transparent
> public init(_ source: T) where T : BinaryInteger {
> self.init(representingValue: RepresentingValue(source))
> }
> 
> @_transparent
> public init(extendingOrTruncating source: T) where T : BinaryInteger {
> self.init(representingValue: RepresentingValue(extendingOrTruncating: 
> source))
> }
> 
> @_transparent
> public init(clamping source: T) where T : BinaryInteger {
> self.init(representingValue: RepresentingValue(clamping: source))
> }
> 
> @_transparent
> public init(_truncatingBits bits: UInt) {
> self.init(representingValue: RepresentingValue(_truncatingBits: bits))
> }
> }
> 
> extension EndianInteger {
> 
> @_transparent
> public static var isSigned: Bool {
> return RepresentingValue.isSigned
> }
> 
> @_transparent
> public static var bitWidth: Int {
> return RepresentingValue.bitWidth
> }
> 
> @_transparent
> public static var max: Self {
> return Self(representingValue: RepresentingValue.max)
> }
> 
> @_transparent
> public static var min: Self {
> return Self(representingValue: RepresentingValue.min)
> }
> }
> 
> extension EndianInteger {
> 
> @_transparent
> public var hashValue: Int {
> return representingValue.hashValue
> }
> 
> @_transparent
> public var description: String {
> return representingValue.description
> }
> 
> @_transparent
> public var bitWidth: Int {
> return representingValue.bitWidth
> }
> 
> @_transparent
> public var magnitude: RepresentingValue.Magnitude {
> return representingValue.magnitude
> }
> 
> @_transparent
> public var trailingZeroBitCount: Int {
> return representingValue.trailingZeroBitCount
> }
> 
> @_transparent
> public var nonzeroBitCount: Int {
> return representingValue.nonzeroBitCount
> }
> 
> @_transparent
> public var leadingZeroBitCount: Int {
> return representingValue.leadingZeroBitCount
> }
> 
> @_transparent
> public var byteSwapped: Self {
> return Self(rep

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Susan Cheng via swift-evolution]

Here are two problems being fixed.

First, considering the example:

struct MyRawDataStruct {

  var size: UInt32
  var signature: UInt32
  var width: UInt32
  var height: UInt32
}

The type UInt32 doesn't tall us the endianness of the value. Also, if we read 
the value of it, the value is being byte-swapped when endianness is not 
matching with the system.

This causes us have to manual convert the value from/to correct endianness.

struct MyRawDataStruct {

  var size: BEInteger
  var signature: BEInteger
  var width: BEInteger
  var height: BEInteger
}

So, my proposal fix the problem. We can easily to get the value.

let header: MyRawDataStruct = data.withUnsafePointer { $0.pointee }

print(header.size)  // print the representing value

Second, it's misleading means of bigEndian and littleEndian from 
FixedWidthInteger

if we do this

let a = 1

print(a.bigEndian.bigEndian)

It's just swap bytes twice but not converting value to big-endian

My proposal solves the problem

let b = a.bigEndian   //BEInteger

b.bigEndian// remain big-endian of a

> Max Moiseev  於 2017年7月8日 上午1:48 寫道：
> 
> Hi Susan,
> 
> Was there any motivation for this proposal that I missed? If not then, can 
> you please provide it in a few sentences? Otherwise it’s not clear to me what 
> problem it is supposed to fix.
> 
> Thanks,
> Max 
> 
> 
>> On Jul 6, 2017, at 8:21 PM, Susan Cheng via swift-evolution 
>>  wrote:
>> 
>> IMO, it has unclear representation when FixedWidthInteger working with 
>> endianness specific type.
>> 
>> so I want to introduce the endianness specific wrapper:
>> 
>> public struct BEInteger : FixedWidthInteger {
>> 
>> public var bigEndian: BEInteger { get }
>> 
>> public var littleEndian: LEInteger { get }
>> }
>> 
>> public struct LEInteger : FixedWidthInteger {
>> 
>> public var bigEndian: BEInteger { get }
>> 
>> public var littleEndian: LEInteger { get }
>> }
>> 
>> also, we should change the FixedWidthInteger as follow:
>> 
>> public protocol FixedWidthInteger : BinaryInteger {
>> 
>> /// deprecated, we should use value.bigEndian instead
>> init(bigEndian value: Self)
>> 
>> /// deprecated, we should use value.littleEndian instead
>> init(littleEndian value: Self)
>> 
>> associatedtype EndianRepresentingValue : FixedWidthInteger
>> 
>> var bigEndian: BEInteger { get }
>> 
>> var littleEndian: LEInteger { get }
>> }
>> 
>> =
>> 
>> this is my working alternative implementation:
>> 
>> 
>> @_versioned
>> protocol EndianInteger : FixedWidthInteger {
>> 
>> associatedtype BitPattern : FixedWidthInteger
>> 
>> associatedtype RepresentingValue : FixedWidthInteger
>> 
>> var bitPattern: BitPattern { get }
>> 
>> init(bitPattern: BitPattern)
>> 
>> var representingValue : RepresentingValue { get set }
>> 
>> init(representingValue: RepresentingValue)
>> }
>> 
>> extension EndianInteger {
>> 
>> @_transparent
>> public init(integerLiteral value: RepresentingValue.IntegerLiteralType) {
>> self.init(representingValue: RepresentingValue(integerLiteral: 
>> value))
>> }
>> 
>> @_transparent
>> public init?(exactly source: T) where T : BinaryInteger {
>> guard let value = RepresentingValue(exactly: source) else { return 
>> nil }
>> self.init(representingValue: value)
>> }
>> 
>> @_transparent
>> public init?(exactly source: T) where T : FloatingPoint {
>> guard let value = RepresentingValue(exactly: source) else { return 
>> nil }
>> self.init(representingValue: value)
>> }
>> 
>> @_transparent
>> public init(_ value: RepresentingValue) {
>> self.init(representingValue: value)
>> }
>> 
>> @_transparent
>> public init(_ source: T) where T : FloatingPoint {
>> self.init(representingValue: RepresentingValue(source))
>> }
>> 
>> @_transparent
>> public init(_ source: T) where T : BinaryInteger {
>> self.init(representingValue: RepresentingValue(source))
>> }
>> 
>> @_transparent
>> public init(extendingOrTruncating source: T) where T : BinaryInteger {
>> self.init(representingValue: 
>> RepresentingValue(extendingOrTruncating: source))
>> }
>> 
>> @_transparent
>> public init(clamping source: T) where T : BinaryInteger {
>> self.init(representingValue: RepresentingValue(clamping: source))
>> }
>> 
>> @_transparent
>> public init(_truncatingBits bits: UInt) {
>> self.init(representingValue: RepresentingValue(_truncatingBits: 
>> bits))
>> }
>> }
>> 
>> extension EndianInteger {
>> 
>> @_transparent
>> public static var isSigned: Bool {
>> return RepresentingValue.isSigned
>> }
>> 
>> @_transparent
>> public static var bitWidth: Int {
>> return Re

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson via swift-evolution]

On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
swift-evolution@swift.org> wrote:

> Hi Susan,
>
> Swift does not currently specify a layout for Swift structs.  You
> shouldn’t be using them for memory mapped i/o or writing to a file, because
> their layout can change.  When ABI stability for fragile structs lands, you
> will be able to count on it, but until then something like this is probably
> a bad idea.
>
> -Chris
>

Does this imply that you should never use Swift structs to eg interact with
Metal?

This seems to be a very common practice. Here is a typical example (from a
Metal tutorial at raywenderlich.com):

struct Vertex {
  var x,y,z: Float // position data
  var r,g,b,a: Float   // color data

  func floatBuffer() -> [Float] {
return [x,y,z,r,g,b,a]
  }
}

(
https://www.raywenderlich.com/146416/metal-tutorial-swift-3-part-2-moving-3d
 )

Also, does it imply that we cannot use structs (of only primitive types)
like:

struct RgbaFloatsLinearGamma {
var r, g, b, a: Float
…
}
struct BgraBytesSrgbGamma {
var b, g, r, a: UInt8
}

for manipulating raster image data?

I vaguely remember a swift evo discussion where it was concluded that such
usage was considered OK provided the stored properties of the structs was
only primitive types, but I can't find it now.

Perhaps it could be considered OK at least when the intended platforms are
known to be only iOS devices?

/Jens
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Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Chris Lattner via swift-evolution]

> On Jul 9, 2017, at 12:23 AM, Jens Persson  wrote:
> 
> 
> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution 
> mailto:swift-evolution@swift.org>> wrote:
> Hi Susan,
> 
> Swift does not currently specify a layout for Swift structs.  You shouldn’t 
> be using them for memory mapped i/o or writing to a file, because their 
> layout can change.  When ABI stability for fragile structs lands, you will be 
> able to count on it, but until then something like this is probably a bad 
> idea.
> 
> -Chris
> 
> Does this imply that you should never use Swift structs to eg interact with 
> Metal?

No.

> This seems to be a very common practice. Here is a typical example (from a 
> Metal tutorial at raywenderlich.com ):
> 
> struct Vertex {
>   var x,y,z: Float // position data
>   var r,g,b,a: Float   // color data
> 
>   func floatBuffer() -> [Float] {
> return [x,y,z,r,g,b,a]
>   }
> }

This doesn’t appear to expose the layout of the struct.
> Also, does it imply that we cannot use structs (of only primitive types) like:
> 
> struct RgbaFloatsLinearGamma {
> var r, g, b, a: Float
> …
> }
> struct BgraBytesSrgbGamma {
> var b, g, r, a: UInt8
> }
> 
> for manipulating raster image data?

I don’t see why that would be a problem.

> I vaguely remember a swift evo discussion where it was concluded that such 
> usage was considered OK provided the stored properties of the structs was 
> only primitive types, but I can't find it now.
> 
> Perhaps it could be considered OK at least when the intended platforms are 
> known to be only iOS devices?

I think you’re misunderstanding what I’m saying.  It isn’t correct to take 
(e.g.) an unsafepointer to the beginning of a struct, and serialize that out to 
disk, and expect that the fields are emitted in some order with some specific 
padding between them.  None of the uses above try to do this.

-Chris


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Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson via swift-evolution]

I don't think I'm misunderstanding you, but I might be, so I'll add more
detail:

If you look at the Metal article, you'll see that the (Swift) struct
"Vertex" is used to specify the data that is sent to Metal for creating a
buffer (using MTLDevice.makeBuffer). The result that the GPU will produce
surely depends on the fields of the Vertex struct (x, y, z, r, g, b, a)
being in the specified order (ie swapping the red channel with the x
coordinate would produce an unexpected result).

And regarding the second example, pixel structs used for manipulating
raster image data. Manipulating raster image data presumably includes stuff
like displaying to screen, loading and saving raster images.
I currently use this way of doing this right now without any problems, but
if the order of the fields (eg a, r, g, b) should change in the future,
then my code would break (the colors of the images would at least not come
out as expected).

/Jens






On Sun, Jul 9, 2017 at 5:53 PM, Chris Lattner  wrote:

>
> On Jul 9, 2017, at 12:23 AM, Jens Persson  wrote:
>
>
> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> Hi Susan,
>>
>> Swift does not currently specify a layout for Swift structs.  You
>> shouldn’t be using them for memory mapped i/o or writing to a file, because
>> their layout can change.  When ABI stability for fragile structs lands, you
>> will be able to count on it, but until then something like this is probably
>> a bad idea.
>>
>> -Chris
>>
>
> Does this imply that you should never use Swift structs to eg interact
> with Metal?
>
>
> No.
>
> This seems to be a very common practice. Here is a typical example (from a
> Metal tutorial at raywenderlich.com):
>
> struct Vertex {
>   var x,y,z: Float // position data
>   var r,g,b,a: Float   // color data
>
>   func floatBuffer() -> [Float] {
> return [x,y,z,r,g,b,a]
>   }
> }
>
>
> This doesn’t appear to expose the layout of the struct.
>
> Also, does it imply that we cannot use structs (of only primitive types)
> like:
>
> struct RgbaFloatsLinearGamma {
> var r, g, b, a: Float
> …
> }
> struct BgraBytesSrgbGamma {
> var b, g, r, a: UInt8
> }
>
> for manipulating raster image data?
>
>
> I don’t see why that would be a problem.
>
> I vaguely remember a swift evo discussion where it was concluded that such
> usage was considered OK provided the stored properties of the structs was
> only primitive types, but I can't find it now.
>
> Perhaps it could be considered OK at least when the intended platforms are
> known to be only iOS devices?
>
>
> I think you’re misunderstanding what I’m saying.  It isn’t correct to take
> (e.g.) an unsafepointer to the beginning of a struct, and serialize that
> out to disk, and expect that the fields are emitted in some order with some
> specific padding between them.  None of the uses above try to do this.
>
> -Chris
>
>
>
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Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson via swift-evolution]

I should perhaps add that in my image processing code, I use code like this:

func withVImageBuffer(for table: Table, body:
(vImage_Buffer) -> R) -> R
where
Data.Coordinate.Index == VectorIndex2
{
let vib = vImage_Buffer(
data: table.baseAddress,
height: vImagePixelCount(table.size.e1),
width: vImagePixelCount(table.size.e0),
rowBytes: table.stride.e1
)
return withExtendedLifetime(table) { body(vib) }
}

Here, Table is the raster image. Data.Coordinate == VectorIndex2
makes it a 2D table, and a Table's Data also has a type parameter
Data.Value which can be eg one of the "pixel"-struct I showed before.
This works without any problems (I've tested and used the some variant of
this type of code for years) but it would surely break if the memory layout
of simple structs changed.

I can't see how this usage is much different from the one in the Metal
tutorial. It too uses pointers to point into a data created using the
(Swift) struct "Vertex", and the GPU hardware has its expectations on the
memory layout of that data, so the code would break if the memory layout of
the Vertex struct changed.

/Jens


On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson  wrote:

> I don't think I'm misunderstanding you, but I might be, so I'll add more
> detail:
>
> If you look at the Metal article, you'll see that the (Swift) struct
> "Vertex" is used to specify the data that is sent to Metal for creating a
> buffer (using MTLDevice.makeBuffer). The result that the GPU will produce
> surely depends on the fields of the Vertex struct (x, y, z, r, g, b, a)
> being in the specified order (ie swapping the red channel with the x
> coordinate would produce an unexpected result).
>
> And regarding the second example, pixel structs used for manipulating
> raster image data. Manipulating raster image data presumably includes stuff
> like displaying to screen, loading and saving raster images.
> I currently use this way of doing this right now without any problems, but
> if the order of the fields (eg a, r, g, b) should change in the future,
> then my code would break (the colors of the images would at least not come
> out as expected).
>
> /Jens
>
>
>
>
>
>
> On Sun, Jul 9, 2017 at 5:53 PM, Chris Lattner  wrote:
>
>>
>> On Jul 9, 2017, at 12:23 AM, Jens Persson  wrote:
>>
>>
>> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>>> Hi Susan,
>>>
>>> Swift does not currently specify a layout for Swift structs.  You
>>> shouldn’t be using them for memory mapped i/o or writing to a file, because
>>> their layout can change.  When ABI stability for fragile structs lands, you
>>> will be able to count on it, but until then something like this is probably
>>> a bad idea.
>>>
>>> -Chris
>>>
>>
>> Does this imply that you should never use Swift structs to eg interact
>> with Metal?
>>
>>
>> No.
>>
>> This seems to be a very common practice. Here is a typical example (from
>> a Metal tutorial at raywenderlich.com):
>>
>> struct Vertex {
>>   var x,y,z: Float // position data
>>   var r,g,b,a: Float   // color data
>>
>>   func floatBuffer() -> [Float] {
>> return [x,y,z,r,g,b,a]
>>   }
>> }
>>
>>
>> This doesn’t appear to expose the layout of the struct.
>>
>> Also, does it imply that we cannot use structs (of only primitive types)
>> like:
>>
>> struct RgbaFloatsLinearGamma {
>> var r, g, b, a: Float
>> …
>> }
>> struct BgraBytesSrgbGamma {
>> var b, g, r, a: UInt8
>> }
>>
>> for manipulating raster image data?
>>
>>
>> I don’t see why that would be a problem.
>>
>> I vaguely remember a swift evo discussion where it was concluded that
>> such usage was considered OK provided the stored properties of the structs
>> was only primitive types, but I can't find it now.
>>
>> Perhaps it could be considered OK at least when the intended platforms
>> are known to be only iOS devices?
>>
>>
>> I think you’re misunderstanding what I’m saying.  It isn’t correct to
>> take (e.g.) an unsafepointer to the beginning of a struct, and serialize
>> that out to disk, and expect that the fields are emitted in some order with
>> some specific padding between them.  None of the uses above try to do this.
>>
>> -Chris
>>
>>
>>
>
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Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson via swift-evolution]

Sorry for making so much off topic noise in this thread, but I made a
mistake regarding the Metal tutorial:
Looking more carefully I see now that they rebuild a vertedData: [Float]
from their vertices: [Vertex] using the floatBuffer() method of the Vertex
struct, which returns an Array with the stored properties of Vertex in
correct order.

While searching the internet about this I saw Joe Groff mentioning on
Twitter that:
"We plan to sort fields in padding order to minimize size, and may also
automatically pack bools and enums in bitfields."

So AFAICS my current image processing code is making the possibly invalid
assumption that eg
struct S {
var a, b, c, d: Float
}
will have a memory layout of 4*4=16 bytes (stride and size == 16) and an
alignment of 4, and most importantly that a, b, c, d will be in that order.

It looks like I should be defining my structs (the ones for which memory
layout is important) in C and import them.

Although I would be surprised if a Swift-struct containing only same-sized
fields (all of the same primitive type) would be reordered, and such
changes to the language would probably include some per-struct way to
express some sort of layout control (since being able to define structs to
be used for low level data manipulation is important in a systems language).

/Jens


On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution <
swift-evolution@swift.org> wrote:

> I should perhaps add that in my image processing code, I use code like
> this:
>
> func withVImageBuffer(for table: Table, body:
> (vImage_Buffer) -> R) -> R
> where
> Data.Coordinate.Index == VectorIndex2
> {
> let vib = vImage_Buffer(
> data: table.baseAddress,
> height: vImagePixelCount(table.size.e1),
> width: vImagePixelCount(table.size.e0),
> rowBytes: table.stride.e1
> )
> return withExtendedLifetime(table) { body(vib) }
> }
>
> Here, Table is the raster image. Data.Coordinate == VectorIndex2
> makes it a 2D table, and a Table's Data also has a type parameter
> Data.Value which can be eg one of the "pixel"-struct I showed before.
> This works without any problems (I've tested and used the some variant of
> this type of code for years) but it would surely break if the memory layout
> of simple structs changed.
>
> I can't see how this usage is much different from the one in the Metal
> tutorial. It too uses pointers to point into a data created using the
> (Swift) struct "Vertex", and the GPU hardware has its expectations on the
> memory layout of that data, so the code would break if the memory layout of
> the Vertex struct changed.
>
> /Jens
>
>
> On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson  wrote:
>
>> I don't think I'm misunderstanding you, but I might be, so I'll add more
>> detail:
>>
>> If you look at the Metal article, you'll see that the (Swift) struct
>> "Vertex" is used to specify the data that is sent to Metal for creating a
>> buffer (using MTLDevice.makeBuffer). The result that the GPU will
>> produce surely depends on the fields of the Vertex struct (x, y, z, r, g,
>> b, a) being in the specified order (ie swapping the red channel with the x
>> coordinate would produce an unexpected result).
>>
>> And regarding the second example, pixel structs used for manipulating
>> raster image data. Manipulating raster image data presumably includes stuff
>> like displaying to screen, loading and saving raster images.
>> I currently use this way of doing this right now without any problems,
>> but if the order of the fields (eg a, r, g, b) should change in the future,
>> then my code would break (the colors of the images would at least not come
>> out as expected).
>>
>> /Jens
>>
>>
>>
>>
>>
>>
>> On Sun, Jul 9, 2017 at 5:53 PM, Chris Lattner 
>> wrote:
>>
>>>
>>> On Jul 9, 2017, at 12:23 AM, Jens Persson  wrote:
>>>
>>>
>>> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
 Hi Susan,

 Swift does not currently specify a layout for Swift structs.  You
 shouldn’t be using them for memory mapped i/o or writing to a file, because
 their layout can change.  When ABI stability for fragile structs lands, you
 will be able to count on it, but until then something like this is probably
 a bad idea.

 -Chris

>>>
>>> Does this imply that you should never use Swift structs to eg interact
>>> with Metal?
>>>
>>>
>>> No.
>>>
>>> This seems to be a very common practice. Here is a typical example (from
>>> a Metal tutorial at raywenderlich.com):
>>>
>>> struct Vertex {
>>>   var x,y,z: Float // position data
>>>   var r,g,b,a: Float   // color data
>>>
>>>   func floatBuffer() -> [Float] {
>>> return [x,y,z,r,g,b,a]
>>>   }
>>> }
>>>
>>>
>>> This doesn’t appear to expose the layout of the struct.
>>>
>>> Also, does it imply that we cannot use structs (of only primitive types)
>>> like:
>>>
>>> struct RgbaFloatsLinearGamma {
>>> var r, g, b, a: Float

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[John McCall via swift-evolution]

> On Jul 9, 2017, at 4:49 PM, Jens Persson via swift-evolution 
>  wrote:
> 
> Sorry for making so much off topic noise in this thread, but I made a mistake 
> regarding the Metal tutorial:
> Looking more carefully I see now that they rebuild a vertedData: [Float] from 
> their vertices: [Vertex] using the floatBuffer() method of the Vertex struct, 
> which returns an Array with the stored properties of Vertex in correct order.
> 
> While searching the internet about this I saw Joe Groff mentioning on Twitter 
> that:
> "We plan to sort fields in padding order to minimize size, and may also 
> automatically pack bools and enums in bitfields."
> 
> So AFAICS my current image processing code is making the possibly invalid 
> assumption that eg
> struct S {
> var a, b, c, d: Float
> }
> will have a memory layout of 4*4=16 bytes (stride and size == 16) and an 
> alignment of 4, and most importantly that a, b, c, d will be in that order.

This is currently true, but may not always be.  We want to reserve the right to 
re-order the fields even if it doesn't improve packing — for example, if two 
fields are frequently accessed together, field reordering could yield 
substantial locality benefits.  We've also discussed reordering fields to put 
them in a canonical order for resilience, since it's a little counter-intuitive 
that reordering the fields of a struct should be ABI-breaking.  (There are 
arguments against doing that as well, of course — for example, the programmer 
may have chosen the current order for their own locality optimizations.)

> It looks like I should be defining my structs (the ones for which memory 
> layout is important) in C and import them.

This should always work, yes.

> Although I would be surprised if a Swift-struct containing only same-sized 
> fields (all of the same primitive type) would be reordered, and such changes 
> to the language would probably include some per-struct way to express some 
> sort of layout control (since being able to define structs to be used for low 
> level data manipulation is important in a systems language).

Exactly.  In the long term, Swift will have some explicit tools for layout 
control.

John.

> 
> /Jens
> 
> 
> On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution 
> mailto:swift-evolution@swift.org>> wrote:
> I should perhaps add that in my image processing code, I use code like this:
> 
> func withVImageBuffer(for table: Table, body: (vImage_Buffer) 
> -> R) -> R
> where
> Data.Coordinate.Index == VectorIndex2
> {
> let vib = vImage_Buffer(
> data: table.baseAddress,
> height: vImagePixelCount(table.size.e1),
> width: vImagePixelCount(table.size.e0),
> rowBytes: table.stride.e1
> )
> return withExtendedLifetime(table) { body(vib) }
> }
> 
> Here, Table is the raster image. Data.Coordinate == VectorIndex2 makes 
> it a 2D table, and a Table's Data also has a type parameter Data.Value which 
> can be eg one of the "pixel"-struct I showed before.
> This works without any problems (I've tested and used the some variant of 
> this type of code for years) but it would surely break if the memory layout 
> of simple structs changed.
> 
> I can't see how this usage is much different from the one in the Metal 
> tutorial. It too uses pointers to point into a data created using the (Swift) 
> struct "Vertex", and the GPU hardware has its expectations on the memory 
> layout of that data, so the code would break if the memory layout of the 
> Vertex struct changed.
> 
> /Jens
> 
> 
> On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson  > wrote:
> I don't think I'm misunderstanding you, but I might be, so I'll add more 
> detail:
> 
> If you look at the Metal article, you'll see that the (Swift) struct "Vertex" 
> is used to specify the data that is sent to Metal for creating a buffer 
> (using MTLDevice.makeBuffer). The result that the GPU will produce surely 
> depends on the fields of the Vertex struct (x, y, z, r, g, b, a) being in the 
> specified order (ie swapping the red channel with the x coordinate would 
> produce an unexpected result).
> 
> And regarding the second example, pixel structs used for manipulating raster 
> image data. Manipulating raster image data presumably includes stuff like 
> displaying to screen, loading and saving raster images.
> I currently use this way of doing this right now without any problems, but if 
> the order of the fields (eg a, r, g, b) should change in the future, then my 
> code would break (the colors of the images would at least not come out as 
> expected).
> 
> /Jens
> 
> 
> 
> 
> 
> 
> On Sun, Jul 9, 2017 at 5:53 PM, Chris Lattner  > wrote:
> 
>> On Jul 9, 2017, at 12:23 AM, Jens Persson > > wrote:
>> 
>> 
>> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution 
>> mailto:swift-evolution@swift.org>> wrote:
>> Hi Susan,
>> 
>> Swift does not currently 

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[John McCall via swift-evolution]

> On Jul 9, 2017, at 6:14 PM, Jens Persson  wrote:
> 
> Thanks for that clarification John McCall.
> My code is using a lot of generic structs (in which memory layout is 
> important) though, an example would be:
> struct Vector4 : Vector {
> typealias Index = VectorIndex4
> typealias Element = E
> var e0, e1, e2, e3: Element
> …
> }
> And I guess I'm out of luck trying to represent those as C structs?
> So AFAICS it looks like it is currently impossible to write generic low level 
> code in Swift, unless I just keep doing what I've been doing (It does 
> currently work after all) knowing that it will probably break in some future 
> versions of Swift. But in that possible future version of Swift, I could 
> probably find a way to make it work again (using some possible explicit tools 
> for layout control present in that version of Swift).
> Correct?

Correct.

John.

> /Jens
> 
> 
> On Sun, Jul 9, 2017 at 11:41 PM, John McCall  > wrote:
> 
>> On Jul 9, 2017, at 4:49 PM, Jens Persson via swift-evolution 
>> mailto:swift-evolution@swift.org>> wrote:
>> 
>> Sorry for making so much off topic noise in this thread, but I made a 
>> mistake regarding the Metal tutorial:
>> Looking more carefully I see now that they rebuild a vertedData: [Float] 
>> from their vertices: [Vertex] using the floatBuffer() method of the Vertex 
>> struct, which returns an Array with the stored properties of Vertex in 
>> correct order.
>> 
>> While searching the internet about this I saw Joe Groff mentioning on 
>> Twitter that:
>> "We plan to sort fields in padding order to minimize size, and may also 
>> automatically pack bools and enums in bitfields."
>> 
>> So AFAICS my current image processing code is making the possibly invalid 
>> assumption that eg
>> struct S {
>> var a, b, c, d: Float
>> }
>> will have a memory layout of 4*4=16 bytes (stride and size == 16) and an 
>> alignment of 4, and most importantly that a, b, c, d will be in that order.
> 
> This is currently true, but may not always be.  We want to reserve the right 
> to re-order the fields even if it doesn't improve packing — for example, if 
> two fields are frequently accessed together, field reordering could yield 
> substantial locality benefits.  We've also discussed reordering fields to put 
> them in a canonical order for resilience, since it's a little 
> counter-intuitive that reordering the fields of a struct should be 
> ABI-breaking.  (There are arguments against doing that as well, of course — 
> for example, the programmer may have chosen the current order for their own 
> locality optimizations.)
> 
>> It looks like I should be defining my structs (the ones for which memory 
>> layout is important) in C and import them.
> 
> This should always work, yes.
> 
>> Although I would be surprised if a Swift-struct containing only same-sized 
>> fields (all of the same primitive type) would be reordered, and such changes 
>> to the language would probably include some per-struct way to express some 
>> sort of layout control (since being able to define structs to be used for 
>> low level data manipulation is important in a systems language).
> 
> Exactly.  In the long term, Swift will have some explicit tools for layout 
> control.
> 
> John.
> 
>> 
>> /Jens
>> 
>> 
>> On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution 
>> mailto:swift-evolution@swift.org>> wrote:
>> I should perhaps add that in my image processing code, I use code like this:
>> 
>> func withVImageBuffer(for table: Table, body: (vImage_Buffer) 
>> -> R) -> R
>> where
>> Data.Coordinate.Index == VectorIndex2
>> {
>> let vib = vImage_Buffer(
>> data: table.baseAddress,
>> height: vImagePixelCount(table.size.e1),
>> width: vImagePixelCount(table.size.e0),
>> rowBytes: table.stride.e1
>> )
>> return withExtendedLifetime(table) { body(vib) }
>> }
>> 
>> Here, Table is the raster image. Data.Coordinate == VectorIndex2 makes 
>> it a 2D table, and a Table's Data also has a type parameter Data.Value which 
>> can be eg one of the "pixel"-struct I showed before.
>> This works without any problems (I've tested and used the some variant of 
>> this type of code for years) but it would surely break if the memory layout 
>> of simple structs changed.
>> 
>> I can't see how this usage is much different from the one in the Metal 
>> tutorial. It too uses pointers to point into a data created using the 
>> (Swift) struct "Vertex", and the GPU hardware has its expectations on the 
>> memory layout of that data, so the code would break if the memory layout of 
>> the Vertex struct changed.
>> 
>> /Jens
>> 
>> 
>> On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson > > wrote:
>> I don't think I'm misunderstanding you, but I might be, so I'll add more 
>> detail:
>> 
>> If you look at the Metal article, you'll see that the (Swift) struct 
>> "Vertex" is used to specify the dat

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Jens Persson via swift-evolution]

Thanks for that clarification John McCall.
My code is using a lot of generic structs (in which memory layout is
important) though, an example would be:
struct Vector4 : Vector {
typealias Index = VectorIndex4
typealias Element = E
var e0, e1, e2, e3: Element
…
}
And I guess I'm out of luck trying to represent those as C structs?
So AFAICS it looks like it is currently impossible to write generic low
level code in Swift, unless I just keep doing what I've been doing (It does
currently work after all) knowing that it will probably break in some
future versions of Swift. But in that possible future version of Swift, I
could probably find a way to make it work again (using some possible
explicit tools for layout control present in that version of Swift).
Correct?
/Jens


On Sun, Jul 9, 2017 at 11:41 PM, John McCall  wrote:

>
> On Jul 9, 2017, at 4:49 PM, Jens Persson via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> Sorry for making so much off topic noise in this thread, but I made a
> mistake regarding the Metal tutorial:
> Looking more carefully I see now that they rebuild a vertedData: [Float]
> from their vertices: [Vertex] using the floatBuffer() method of the Vertex
> struct, which returns an Array with the stored properties of Vertex in
> correct order.
>
> While searching the internet about this I saw Joe Groff mentioning on
> Twitter that:
> "We plan to sort fields in padding order to minimize size, and may also
> automatically pack bools and enums in bitfields."
>
> So AFAICS my current image processing code is making the possibly invalid
> assumption that eg
> struct S {
> var a, b, c, d: Float
> }
> will have a memory layout of 4*4=16 bytes (stride and size == 16) and an
> alignment of 4, and most importantly that a, b, c, d will be in that order.
>
>
> This is currently true, but may not always be.  We want to reserve the
> right to re-order the fields even if it doesn't improve packing — for
> example, if two fields are frequently accessed together, field reordering
> could yield substantial locality benefits.  We've also discussed reordering
> fields to put them in a canonical order for resilience, since it's a little
> counter-intuitive that reordering the fields of a struct should be
> ABI-breaking.  (There are arguments against doing that as well, of course —
> for example, the programmer may have chosen the current order for their own
> locality optimizations.)
>
> It looks like I should be defining my structs (the ones for which memory
> layout is important) in C and import them.
>
>
> This should always work, yes.
>
> Although I would be surprised if a Swift-struct containing only same-sized
> fields (all of the same primitive type) would be reordered, and such
> changes to the language would probably include some per-struct way to
> express some sort of layout control (since being able to define structs to
> be used for low level data manipulation is important in a systems language).
>
>
> Exactly.  In the long term, Swift will have some explicit tools for layout
> control.
>
> John.
>
>
> /Jens
>
>
> On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> I should perhaps add that in my image processing code, I use code like
>> this:
>>
>> func withVImageBuffer(for table: Table, body:
>> (vImage_Buffer) -> R) -> R
>> where
>> Data.Coordinate.Index == VectorIndex2
>> {
>> let vib = vImage_Buffer(
>> data: table.baseAddress,
>> height: vImagePixelCount(table.size.e1),
>> width: vImagePixelCount(table.size.e0),
>> rowBytes: table.stride.e1
>> )
>> return withExtendedLifetime(table) { body(vib) }
>> }
>>
>> Here, Table is the raster image. Data.Coordinate == VectorIndex2
>> makes it a 2D table, and a Table's Data also has a type parameter
>> Data.Value which can be eg one of the "pixel"-struct I showed before.
>> This works without any problems (I've tested and used the some variant of
>> this type of code for years) but it would surely break if the memory layout
>> of simple structs changed.
>>
>> I can't see how this usage is much different from the one in the Metal
>> tutorial. It too uses pointers to point into a data created using the
>> (Swift) struct "Vertex", and the GPU hardware has its expectations on the
>> memory layout of that data, so the code would break if the memory layout of
>> the Vertex struct changed.
>>
>> /Jens
>>
>>
>> On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson  wrote:
>>
>>> I don't think I'm misunderstanding you, but I might be, so I'll add more
>>> detail:
>>>
>>> If you look at the Metal article, you'll see that the (Swift) struct
>>> "Vertex" is used to specify the data that is sent to Metal for creating a
>>> buffer (using MTLDevice.makeBuffer). The result that the GPU will
>>> produce surely depends on the fields of the Vertex struct (x, y, z, r, g,
>>> b, a) being in the specified order (ie swapping the red channel with th

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Robert Bennett via swift-evolution]

Just a question: how would/does allowing the reordering of fields affect the 
correctness and performance of the (de)serialization API added in Swift 4?

> On Jul 9, 2017, at 6:21 PM, Jens Persson via swift-evolution 
>  wrote:
> 
> Thanks for that clarification John McCall.
> My code is using a lot of generic structs (in which memory layout is 
> important) though, an example would be:
> struct Vector4 : Vector {
> typealias Index = VectorIndex4
> typealias Element = E
> var e0, e1, e2, e3: Element
> …
> }
> And I guess I'm out of luck trying to represent those as C structs?
> So AFAICS it looks like it is currently impossible to write generic low level 
> code in Swift, unless I just keep doing what I've been doing (It does 
> currently work after all) knowing that it will probably break in some future 
> versions of Swift. But in that possible future version of Swift, I could 
> probably find a way to make it work again (using some possible explicit tools 
> for layout control present in that version of Swift).
> Correct?
> /Jens
> 
> 
>> On Sun, Jul 9, 2017 at 11:41 PM, John McCall  wrote:
>> 
>>> On Jul 9, 2017, at 4:49 PM, Jens Persson via swift-evolution 
>>>  wrote:
>>> 
>>> Sorry for making so much off topic noise in this thread, but I made a 
>>> mistake regarding the Metal tutorial:
>>> Looking more carefully I see now that they rebuild a vertedData: [Float] 
>>> from their vertices: [Vertex] using the floatBuffer() method of the Vertex 
>>> struct, which returns an Array with the stored properties of Vertex in 
>>> correct order.
>>> 
>>> While searching the internet about this I saw Joe Groff mentioning on 
>>> Twitter that:
>>> "We plan to sort fields in padding order to minimize size, and may also 
>>> automatically pack bools and enums in bitfields."
>>> 
>>> So AFAICS my current image processing code is making the possibly invalid 
>>> assumption that eg
>>> struct S {
>>> var a, b, c, d: Float
>>> }
>>> will have a memory layout of 4*4=16 bytes (stride and size == 16) and an 
>>> alignment of 4, and most importantly that a, b, c, d will be in that order.
>> 
>> This is currently true, but may not always be.  We want to reserve the right 
>> to re-order the fields even if it doesn't improve packing — for example, if 
>> two fields are frequently accessed together, field reordering could yield 
>> substantial locality benefits.  We've also discussed reordering fields to 
>> put them in a canonical order for resilience, since it's a little 
>> counter-intuitive that reordering the fields of a struct should be 
>> ABI-breaking.  (There are arguments against doing that as well, of course — 
>> for example, the programmer may have chosen the current order for their own 
>> locality optimizations.)
>> 
>>> It looks like I should be defining my structs (the ones for which memory 
>>> layout is important) in C and import them.
>> 
>> This should always work, yes.
>> 
>>> Although I would be surprised if a Swift-struct containing only same-sized 
>>> fields (all of the same primitive type) would be reordered, and such 
>>> changes to the language would probably include some per-struct way to 
>>> express some sort of layout control (since being able to define structs to 
>>> be used for low level data manipulation is important in a systems language).
>> 
>> Exactly.  In the long term, Swift will have some explicit tools for layout 
>> control.
>> 
>> John.
>> 
>>> 
>>> /Jens
>>> 
>>> 
 On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution 
  wrote:
 I should perhaps add that in my image processing code, I use code like 
 this:
 
 func withVImageBuffer(for table: Table, body: 
 (vImage_Buffer) -> R) -> R
 where
 Data.Coordinate.Index == VectorIndex2
 {
 let vib = vImage_Buffer(
 data: table.baseAddress,
 height: vImagePixelCount(table.size.e1),
 width: vImagePixelCount(table.size.e0),
 rowBytes: table.stride.e1
 )
 return withExtendedLifetime(table) { body(vib) }
 }
 
 Here, Table is the raster image. Data.Coordinate == VectorIndex2 
 makes it a 2D table, and a Table's Data also has a type parameter 
 Data.Value which can be eg one of the "pixel"-struct I showed before.
 This works without any problems (I've tested and used the some variant of 
 this type of code for years) but it would surely break if the memory 
 layout of simple structs changed.
 
 I can't see how this usage is much different from the one in the Metal 
 tutorial. It too uses pointers to point into a data created using the 
 (Swift) struct "Vertex", and the GPU hardware has its expectations on the 
 memory layout of that data, so the code would break if the memory layout 
 of the Vertex struct changed.
 
 /Jens
 
 
> On Sun, Jul 9, 2017 at 6:35 PM, Jens Persson  wrote:
 
> I don't think I'm misunders

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[John McCall via swift-evolution]

> On Jul 9, 2017, at 6:57 PM, Robert Bennett  wrote:
> Just a question: how would/does allowing the reordering of fields affect the 
> correctness and performance of the (de)serialization API added in Swift 4?

The design of that is definitely proof against changes to the in-memory layout 
of the type.  I believe it's also proof against reordering but I'm not entirely 
certain about that.

John.

> 
> On Jul 9, 2017, at 6:21 PM, Jens Persson via swift-evolution 
> mailto:swift-evolution@swift.org>> wrote:
> 
>> Thanks for that clarification John McCall.
>> My code is using a lot of generic structs (in which memory layout is 
>> important) though, an example would be:
>> struct Vector4 : Vector {
>> typealias Index = VectorIndex4
>> typealias Element = E
>> var e0, e1, e2, e3: Element
>> …
>> }
>> And I guess I'm out of luck trying to represent those as C structs?
>> So AFAICS it looks like it is currently impossible to write generic low 
>> level code in Swift, unless I just keep doing what I've been doing (It does 
>> currently work after all) knowing that it will probably break in some future 
>> versions of Swift. But in that possible future version of Swift, I could 
>> probably find a way to make it work again (using some possible explicit 
>> tools for layout control present in that version of Swift).
>> Correct?
>> /Jens
>> 
>> 
>> On Sun, Jul 9, 2017 at 11:41 PM, John McCall > > wrote:
>> 
>>> On Jul 9, 2017, at 4:49 PM, Jens Persson via swift-evolution 
>>> mailto:swift-evolution@swift.org>> wrote:
>>> 
>>> Sorry for making so much off topic noise in this thread, but I made a 
>>> mistake regarding the Metal tutorial:
>>> Looking more carefully I see now that they rebuild a vertedData: [Float] 
>>> from their vertices: [Vertex] using the floatBuffer() method of the Vertex 
>>> struct, which returns an Array with the stored properties of Vertex in 
>>> correct order.
>>> 
>>> While searching the internet about this I saw Joe Groff mentioning on 
>>> Twitter that:
>>> "We plan to sort fields in padding order to minimize size, and may also 
>>> automatically pack bools and enums in bitfields."
>>> 
>>> So AFAICS my current image processing code is making the possibly invalid 
>>> assumption that eg
>>> struct S {
>>> var a, b, c, d: Float
>>> }
>>> will have a memory layout of 4*4=16 bytes (stride and size == 16) and an 
>>> alignment of 4, and most importantly that a, b, c, d will be in that order.
>> 
>> This is currently true, but may not always be.  We want to reserve the right 
>> to re-order the fields even if it doesn't improve packing — for example, if 
>> two fields are frequently accessed together, field reordering could yield 
>> substantial locality benefits.  We've also discussed reordering fields to 
>> put them in a canonical order for resilience, since it's a little 
>> counter-intuitive that reordering the fields of a struct should be 
>> ABI-breaking.  (There are arguments against doing that as well, of course — 
>> for example, the programmer may have chosen the current order for their own 
>> locality optimizations.)
>> 
>>> It looks like I should be defining my structs (the ones for which memory 
>>> layout is important) in C and import them.
>> 
>> This should always work, yes.
>> 
>>> Although I would be surprised if a Swift-struct containing only same-sized 
>>> fields (all of the same primitive type) would be reordered, and such 
>>> changes to the language would probably include some per-struct way to 
>>> express some sort of layout control (since being able to define structs to 
>>> be used for low level data manipulation is important in a systems language).
>> 
>> Exactly.  In the long term, Swift will have some explicit tools for layout 
>> control.
>> 
>> John.
>> 
>>> 
>>> /Jens
>>> 
>>> 
>>> On Sun, Jul 9, 2017 at 7:01 PM, Jens Persson via swift-evolution 
>>> mailto:swift-evolution@swift.org>> wrote:
>>> I should perhaps add that in my image processing code, I use code like this:
>>> 
>>> func withVImageBuffer(for table: Table, body: 
>>> (vImage_Buffer) -> R) -> R
>>> where
>>> Data.Coordinate.Index == VectorIndex2
>>> {
>>> let vib = vImage_Buffer(
>>> data: table.baseAddress,
>>> height: vImagePixelCount(table.size.e1),
>>> width: vImagePixelCount(table.size.e0),
>>> rowBytes: table.stride.e1
>>> )
>>> return withExtendedLifetime(table) { body(vib) }
>>> }
>>> 
>>> Here, Table is the raster image. Data.Coordinate == VectorIndex2 
>>> makes it a 2D table, and a Table's Data also has a type parameter 
>>> Data.Value which can be eg one of the "pixel"-struct I showed before.
>>> This works without any problems (I've tested and used the some variant of 
>>> this type of code for years) but it would surely break if the memory layout 
>>> of simple structs changed.
>>> 
>>> I can't see how this usage is much different from the one in the Metal 
>>> tutorial. It too uses po

Re: [swift-evolution] [Proposal] Introduces endianness specific type

[Susan Cheng via swift-evolution]

Thanks, but we can implement Codable for BEInteger and LEInteger types.


public struct BEInteger : FixedWidthInteger
{



public var bitPattern: BitPattern



public init(bitPattern: BitPattern)



public var bigEndian: BEInteger { get }



public var littleEndian: LEInteger { get }

}


public struct LEInteger : FixedWidthInteger
{



public var bitPattern: BitPattern



public init(bitPattern: BitPattern)



public var bigEndian: BEInteger { get }



public var littleEndian: LEInteger { get }

}


extension BEInteger : Encodable where BitPattern : Encodable {



public func encode(to encoder: Encoder) throws {

try self.bitPattern.encode(to: encoder)

}

}


extension BEInteger : Decodable where BitPattern : Decodable {



public init(from decoder: Decoder) throws {

self.init(bitPattern: try BitPattern(from: decoder))

}

}


extension LEInteger : Encodable where BitPattern : Encodable {



public func encode(to encoder: Encoder) throws {

try self.bitPattern.encode(to: encoder)

}

}


extension LEInteger : Decodable where BitPattern : Decodable {



public init(from decoder: Decoder) throws {

self.init(bitPattern: try BitPattern(from: decoder))

}

}


2017-07-09 0:27 GMT+08:00 Chris Lattner :

> Hi Susan,
>
> Swift does not currently specify a layout for Swift structs.  You
> shouldn’t be using them for memory mapped i/o or writing to a file, because
> their layout can change.  When ABI stability for fragile structs lands, you
> will be able to count on it, but until then something like this is probably
> a bad idea.
>
> -Chris
>
> On Jul 7, 2017, at 6:16 PM, Susan Cheng via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> Here are two problems being fixed.
>
> First, considering the example:
>
> struct MyRawDataStruct {
>
>   var size: UInt32
>   var signature: UInt32
>   var width: UInt32
>   var height: UInt32
> }
>
> The type UInt32 doesn't tall us the endianness of the value. Also, if we
> read the value of it, the value is being byte-swapped when endianness is
> not matching with the system.
>
> This causes us have to manual convert the value from/to correct endianness.
>
> struct MyRawDataStruct {
>
>   var size: BEInteger
>   var signature: BEInteger
>   var width: BEInteger
>   var height: BEInteger
> }
>
> So, my proposal fix the problem. We can easily to get the value.
>
> let header: MyRawDataStruct = data.withUnsafePointer { $0.pointee }
>
> print(header.size)  // print the representing value
>
> Second, it's misleading means of bigEndian and littleEndian from
> FixedWidthInteger
>
> if we do this
>
> let a = 1
>
> print(a.bigEndian.bigEndian)
>
> It's just swap bytes twice but not converting value to big-endian
>
> My proposal solves the problem
>
> let b = a.bigEndian   //BEInteger
>
> b.bigEndian// remain big-endian of a
>
> Max Moiseev  於 2017年7月8日 上午1:48 寫道：
>
> Hi Susan,
>
> Was there any motivation for this proposal that I missed? If not then, can
> you please provide it in a few sentences? Otherwise it’s not clear to me
> what problem it is supposed to fix.
>
> Thanks,
> Max
>
>
> On Jul 6, 2017, at 8:21 PM, Susan Cheng via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> IMO, it has unclear representation when FixedWidthInteger working with
> endianness specific type.
>
> so I want to introduce the endianness specific wrapper:
>
> public struct BEInteger : FixedWidthInteger {
>
> public var bigEndian: BEInteger { get }
>
> public var littleEndian: LEInteger { get }
> }
>
> public struct LEInteger : FixedWidthInteger {
>
> public var bigEndian: BEInteger { get }
>
> public var littleEndian: LEInteger { get }
> }
>
> also, we should change the FixedWidthInteger as follow:
>
> public protocol FixedWidthInteger : BinaryInteger {
>
> /// deprecated, we should use value.bigEndian instead
> init(bigEndian value: Self)
>
> /// deprecated, we should use value.littleEndian instead
> init(littleEndian value: Self)
>
> associatedtype EndianRepresentingValue : FixedWidthInteger
>
> var bigEndian: BEInteger { get }
>
> var littleEndian: LEInteger { get }
>
> }
>
> =
>
> this is my working alternative implementation:
>
>
> @_versioned
> protocol EndianInteger : FixedWidthInteger {
>
> associatedtype BitPattern : FixedWidthInteger
>
> associatedtype RepresentingValue : FixedWidthInteger
>
> var bitPattern: BitPattern { get }
>
> init(bitPattern: BitPattern)
>
> var representingValue : RepresentingValue { get set }
>
> init(representingValue: RepresentingValue)
> }
>
> extension EndianInteger {
>
> @_transparent
> public init(integerLiteral value: RepresentingValue.IntegerLiteralType)
> {
> self.init(representingValue: RepresentingValue(integerLiteral:
> value))
> }
>
> @_transparent
> public init?(exactly so