[SwiftBindings] Describe projecting generic structs

[jkurdek]

Follow up to #2771. Proposes projections of generic structs.

[stephen-hawley]

Minor adjustments to make and notes

[stephen-hawley]

Agreed. Let's open an issue to design the caching mechanism.
Starting point:

public static class TypeMetadataCache {
    TypeMetadata MetadataOf(Type t, Func<Type, TypeMetadata> generator) {
          // make thread safe:
          if (cache.TryGet(t, out var result)) return result;
          var md = generator(t);
          cache.Add(t, md);
          return md;
}

Then you can rewrite this as:

static TypeMetadata typeMetadata = TypeMetadataCache.MetadataOf(typeof(Pair<T, U>), (t) => {
    var typeMetadataT = TypeMetadataCache.MetadataOf (typeof (T), (tT) => TypeMetadata.GetTypeMetadataOrThrow(tT));
    var typeMetadataU = TypeMetadataCache.MetadataOf (typeof(U), (tU) => TypeMetadata.GetTypeMetadataOrThrow(tU));
    return PairPInvokes.PInvokeMetadata (typeMetadataRequest.Complete, tT, TU);
});
public static TypeMetadata Metadata => typeMetadata;

Again, this is a start. The notion is that there is a cache which has a dictionary of Type onto TypeMetadata and takes a type to look up and if it's not present, takes a generator to call to retrieve that type.

[stephen-hawley]

Seeing that you didn't check the return tells me that we should absolutely have a GetOrThrow accessor that will throw an exception on fail.

[stephen-hawley]

This should use the InitializeWithCopy from the value witness table or else you aren't abiding by swift's memory model.

[stephen-hawley]

Like the way you're breaking this out into the possible cases.

[matouskozak]

Is the @frozen generic struct with non-frozen parameters a supported (or recommended) scenario in Swift? To me it seems like @frozen on the generic struct wouldn't help much with ABI stability then...

[jkurdek]

I cannot speak about whether it is recommended. It is definitely supported. There are some frozen generics in SwiftUI. I do not think it is possible to enforce using frozen parameters (maybe I am wrong), so it only up to the user of such generic struct to be kind enough to instantiate it with frozen types.

[matouskozak]

You're right, more info here https://github.com/swiftlang/swift-evolution/blob/main/proposals/0260-library-evolution.md

It does not necessarily have a known size or alignment. A generic frozen struct's layout might depend on the generic argument provided at run time.

Even concrete instantiations may not have a known size or alignment. A frozen struct with a field that's a *non-*frozen, has a size that may not be known until run time.

[kotlarmilos]

Does this mean that the number of fields is fixed and won't change?

[jkurdek]

I believe so. Both number of fields and their order

[kotlarmilos]

Let's treat representation and runtime lowering separately for a moment.

If a frozen struct doesn't require a copy constructor when moved in memory, I would still consider it a value type (struct). Additionally, as we discussed offline, the projection will flip if a generic used becomes frozen. From the user’s perspective, this can occur without their control, which is my primary concern when introducing this rule.

I understand that it doesn't match the lowering on the ABI level. However, I see this as implementation detail that belongs to the runtime. @amanasifkhalid I would like to hear your perspective.

[matouskozak]

I think the problem is that user-facing C# Pair is a class. In the specific case of Pair<FrozenStruct, FrozenStruct>, it could be a struct (and that's the wrapper introduced later) but we would need to have 2x C# Pair (class and a struct) to handle the case of NonFrozenStruct as well. @jkurdek please correct me if I'm wrong here.

[jkurdek]

If a frozen struct doesn't require a copy constructor when moved in memory, I would still consider it a value type (struct).

So for generics strictly. I think we cannot tell at compile time whether a generic will have or wont have to use a copy constructor as we have no idea what the generic parameter will be. Some instances of the generic, Pair<SomethingThatRequiresCopyConstructor, int> will require it. Some like Pair< FrozenStruct, FrozenStruct> will not.

@matouskozak I think you got it right. Pair<FrozenStruct, FrozenStruct> would ideally be represented as struct.

Additionally, as we discussed offline, the projection will flip if a generic used becomes frozen. From the user’s perspective, this can occur without their control, which is my primary concern when introducing this rule.

Making something non-frozen frozen is an ABI breaking change. I do not think we can do much about it. One way of solving this would be representing every struct as a class on C# side and abstracting the implementation details

[kotlarmilos]

One way of solving this would be representing every struct as a class on C# side and abstracting the implementation details

Do you mean every generic struct? In this case, the size and alignment of an object of opaque layout, as well as whether it is trivial or bitwise movable, is determined by querying its value witness table. If contains frozen structs, the helper can be used.

[jkurdek]

I meant all structs. Its more of stretch idea thing than actual proposal. I feel that projecting some as structs and some as classes creates a weird UX for developers who are not Swift professionals. I would prefer to have uniformity - e.g. all structs to be classes. I do understand however that this would require a lot custom runtime work and would bring its own set of problems. Anyway it is a different discussion.

[kotlarmilos]

I meant all structs. Its more of stretch idea thing than actual proposal. I feel that projecting some as structs and some as classes creates a weird UX for developers who are not Swift professionals.

It seems we need this layer of abstraction for structs with opaque layouts (those not known until runtime). In other cases, this abstraction isn’t necessary. We should prioritize simplicity, ensuring that users reading Swift docs know what to expect. If a struct can be projected directly, we should do so.

I do understand however that this would require a lot custom runtime work and would bring its own set of problems. Anyway it is a different discussion.

Let's do cost estimate for this.

/cc: @jkoritzinsky

[jkurdek]

Lets move this to the #2797 issue

[kotlarmilos]

What is closed generic type? Is it constrained by a protocol?

[jkurdek]

Assuming we have generic Foo<T>, by closed generic type I meant Foo<ConcreteType> what would be a better way to phrase?

[matouskozak]

Good job Jeremi!

[jkurdek]

While working on those projections I have encountered interesting combinations of frozen and non-frozen structs. I think they might be relevant to this discussion #2797

[kotlarmilos]

Why this is not part of the class?

[jkurdek]

I think it can be part of the class, probably should.

[jkurdek]

I'm not sure whether the explicit size can be obtained at compile time for a generic instance, or it can be only done at runtime. We should be able to calculate it though in the worst scenario.

I also wonder whether we need to even specify the explicit size at all (cc @amanasifkhalid @kotlarmilos )

[kotlarmilos]

@matouskozak Do we use the size during lowering?

[matouskozak]

Essentialy the lowering algorithm can be done just with the knowledge about the fields.

Potentially, we could hit some issues where we assume the size is specified exactly but I don't think it would cause too much problems and we should be able to work around it. I would have to try it to verify.

[kotlarmilos]

Potentially, we could hit some issues where we assume the size is specified exactly but I don't think it would cause too much problems and we should be able to work around it. I would have to try it to verify.

Do you have any example of that?

[jkurdek]

My understanding is that we would need to specify size if the Swift type had some extra padding which is not visible on the c# side, dont know whether this is something we should worry about

[matouskozak]

I think particularly nested struct would cause a problem because of added padding to the inner struct on C# side. For example,

    [StructLayout(LayoutKind.Sequential, Size = 12)]
    struct F3_S0_S0
    {
        public nint F0;
        public uint F1;

        public F3_S0_S0(nint f0, uint f1)
        {
            F0 = f0;
            F1 = f1;
        }
    }

    [StructLayout(LayoutKind.Sequential, Size = 24)]
    struct F3_S0
    {
        public sbyte F0; // Offset 0
        public F3_S0_S0 F1; // Offset 8
        public uint F2; // Offset 20

        public F3_S0(sbyte f0, F3_S0_S0 f1, uint f2)
        {
            F0 = f0;
            F1 = f1;
            F2 = f2;
        }
    }

The F3_S0 should get lowered as: i8, i64, i64.

Without specifying the size of F3_S0_S0, the public uint F2 would be at offset 24 (because F3_S0_S0 size would be padded to 16), causing the current implementation of lowering algorithm to create a different lowered sequence: i8, i64, i32, i32 (because of the padding between the F1 field from F3_S0_S0 and F2 from F3_S0, preventing merging the two uint fields into single i64).

Note: I described the process based on Mono implementation, CoreCLR might differ, but I suspect the outcome to be similar.

While we should be able to work around this by using exact sizes of individual fields, that would still require us to have knowledge about (ALL) fields that are going to be present in the struct. I wonder if we would be able to define some kind of meta lowering for the generic struct that would get adjusted on the go based on different specializations.

[matouskozak]

However, I forgot that this discussion is only about frozen closed generic type (which are known at compile time #2796 (comment)), we should be able to get the sizes ahead of time, no? Or does the closed generic type refer only to the main struct and not recursively?