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

[Jens Persson]

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 <clattner at nondot.org> wrote:

>
> On Jul 9, 2017, at 12:23 AM, Jens Persson <jens at bitcycle.com> wrote:
>
>
> On Sat, Jul 8, 2017 at 6:28 PM, Chris Lattner via swift-evolution <
> swift-evolution at 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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