JIT: Add support for SwiftSelf<T> in Swift calling convention

This adds support to allow SwiftSelf<T> with a frozen struct as T. Swift
allows enregistration of 'self' in these cases, but the 'self' must
still be passed in the dedicated context register when the frozen struct
is not enregistered, which makes this support necessary to handle as
part of the calling convention.

A few notes:
- If `T` is not a value class we `BADCODE`
- If the signature includes `SwiftSelf<T>`, then it must be the first
  argument of the signature, which matches how 'self' gets enregistered
  on the Swift side, otherwise we `BADCODE`
- There is not support for reverse pinvokes for `SwiftSelf<T>`. That's
  because the context passed to function pointers is always a pointer,
  so I do not see any use case for this in reverse pinvokes.
- Some care must be taken since `SwiftSelf<T>` is a generic struct whose
  layout generally is not going to match `T` without tail padding (until
  we get something like dotnet#100896).

src/coreclr/jit/importercalls.cpp

@@ -2027,6 +2027,7 @@ void Compiler::impPopArgsForSwiftCall(GenTreeCall* call, CORINFO_SIG_INFO* sig,
     unsigned short swiftErrorIndex = sig->numArgs;
     unsigned short swiftSelfIndex  = sig->numArgs;
 
+    CORINFO_CLASS_HANDLE selfType = NO_CLASS_HANDLE;
     // We are importing an unmanaged Swift call, which might require special parameter handling
     bool checkEntireStack = false;
 
@@ -2090,6 +2091,34 @@ void Compiler::impPopArgsForSwiftCall(GenTreeCall* call, CORINFO_SIG_INFO* sig,
                 swiftSelfIndex = argIndex;
                 // Fall through to make sure the struct value becomes a local.
             }
+            else if ((strcmp(className, "SwiftSelf`1") == 0) &&
+                     (strcmp(namespaceName, "System.Runtime.InteropServices.Swift") == 0))
+            {
+                // We expect a SwiftSelf struct to be passed, not a pointer/reference
+                if (argIsByrefOrPtr)
+                {
+                    BADCODE("Expected SwiftSelf<T> struct, got pointer/reference");
+                }
+
+                if (swiftSelfIndex != sig->numArgs)
+                {
+                    BADCODE("Duplicate SwiftSelf parameter");
+                }
+
+                if (argIndex != 0)
+                {
+                    BADCODE("SwiftSelf<T> must be the first argument in the signature");
+                }
+
+                selfType                = info.compCompHnd->getTypeInstantiationArgument(argClass, 0);
+                CorInfoType selfCorType = info.compCompHnd->asCorInfoType(selfType);
+                if (selfCorType != CORINFO_TYPE_VALUECLASS)
+                {
+                    BADCODE("SwiftSelf<T> expects T to be a value class");
+                }
+
+                swiftSelfIndex = argIndex;
+            }
             // TODO: Handle SwiftAsync
         }
 
@@ -2157,7 +2186,7 @@ void Compiler::impPopArgsForSwiftCall(GenTreeCall* call, CORINFO_SIG_INFO* sig,
         // For the self arg, change it from the SwiftSelf struct to a
         // TYP_I_IMPL primitive directly. It must also be marked as a well
         // known arg because it has a non-standard calling convention.
-        if (argIndex == swiftSelfIndex)
+        if ((argIndex == swiftSelfIndex) && (selfType == NO_CLASS_HANDLE))
         {
             assert(arg->GetNode()->OperIsLocalRead());
             GenTree*   primitiveSelf = gtNewLclFldNode(structVal->GetLclNum(), TYP_I_IMPL, structVal->GetLclOffs());
@@ -2166,7 +2195,8 @@ void Compiler::impPopArgsForSwiftCall(GenTreeCall* call, CORINFO_SIG_INFO* sig,
         }
         else
         {
-            const CORINFO_SWIFT_LOWERING* lowering = GetSwiftLowering(arg->GetSignatureClassHandle());
+            CORINFO_CLASS_HANDLE argClass = argIndex == swiftSelfIndex ? selfType : arg->GetSignatureClassHandle();
+            const CORINFO_SWIFT_LOWERING* lowering = GetSwiftLowering(argClass);
             if (lowering->byReference)
             {
                 JITDUMP("  Argument %d of type %s must be passed by reference\n", argIndex,
@@ -2188,7 +2218,13 @@ void Compiler::impPopArgsForSwiftCall(GenTreeCall* call, CORINFO_SIG_INFO* sig,
                 GenTree* addrNode = gtNewLclAddrNode(structVal->GetLclNum(), structVal->GetLclOffs());
                 JITDUMP("    Passing by reference\n");
 
-                insertAfter = call->gtArgs.InsertAfter(this, insertAfter, NewCallArg::Primitive(addrNode, TYP_I_IMPL));
+                NewCallArg newArg = NewCallArg::Primitive(addrNode, TYP_I_IMPL);
+                if (argIndex == swiftSelfIndex)
+                {
+                    newArg = newArg.WellKnown(WellKnownArg::SwiftSelf);
+                }
+
+                insertAfter = call->gtArgs.InsertAfter(this, insertAfter, newArg);
             }
             else
             {

src/tests/Interop/Swift/SwiftSelfContext/CMakeLists.txt

@@ -11,7 +11,7 @@ if (NOT SWIFT_COMPILER_TARGET AND CLR_CMAKE_TARGET_OSX)
 endif()
 
 add_custom_target(${SOURCE} ALL
-    COMMAND xcrun swiftc -target ${SWIFT_COMPILER_TARGET} -emit-library ${CMAKE_CURRENT_SOURCE_DIR}/${SOURCE}.swift -o ${CMAKE_CURRENT_BINARY_DIR}/lib${SOURCE}.dylib
+    COMMAND xcrun swiftc -target ${SWIFT_COMPILER_TARGET} -enable-library-evolution -emit-library ${CMAKE_CURRENT_SOURCE_DIR}/${SOURCE}.swift -o ${CMAKE_CURRENT_BINARY_DIR}/lib${SOURCE}.dylib
     DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${SOURCE}.swift
     COMMENT "Generating ${SOURCE} library"
 )

src/tests/Interop/Swift/SwiftSelfContext/SwiftSelfContext.cs

@@ -15,11 +15,11 @@ public class SelfContextTests
     public unsafe static extern void* getInstance();
 
     [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
-    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext0B7LibraryC14getMagicNumberSiyF")]
+    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext0B7LibraryC14getMagicNumberSiyFTj")]
     public static extern nint getMagicNumber(SwiftSelf self);
 
     [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
-    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext0B7LibraryC14getMagicNumberSiyF")]
+    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext0B7LibraryC14getMagicNumberSiyFTj")]
     public static extern nint getMagicNumberOnStack(int dummy0, int dummy1, int dummy2, int dummy3, int dummy4, int dummy5, int dummy6, int dummy7, int dummy8, int dummy9, SwiftSelf self);
 
     [Fact]
@@ -44,4 +44,43 @@ public unsafe static void TestSwiftSelfContextOnStack()
         int result = (int)getMagicNumberOnStack(i, i + 1, i + 2, i + 3, i + 4, i + 5, i + 6, i + 7, i + 8, i + 9, self);
         Assert.True(result == 42, "The result from Swift does not match the expected value.");
     }
+
+    public struct FrozenEnregisteredStruct
+    {
+        public long A;
+        public long B;
+    }
+
+    public struct FrozenNonEnregisteredStruct
+    {
+        public long A;
+        public long B;
+        public long C;
+        public long D;
+        public long E;
+    }
+
+    [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
+    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext24FrozenEnregisteredStructV3Sums5Int64VyF")]
+    public static extern long SumFrozenEnregisteredStruct(SwiftSelf<FrozenEnregisteredStruct> self);
+
+    [UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvSwift) })]
+    [DllImport(SwiftLib, EntryPoint = "$s16SwiftSelfContext27FrozenNonEnregisteredStructV3Sums5Int64VyF")]
+    public static extern long SumFrozenNonEnregisteredStruct(SwiftSelf<FrozenNonEnregisteredStruct> self);
+
+    [Fact]
+    [SkipOnMono("SwiftSelf<T> is not supported on Mono")]
+    public unsafe static void TestSelfIsFrozenEnregisteredStruct()
+    {
+        long sum = SumFrozenEnregisteredStruct(new SwiftSelf<FrozenEnregisteredStruct>(new FrozenEnregisteredStruct { A = 10, B = 20 }));
+        Assert.Equal(30, sum);
+    }
+
+    [Fact]
+    [SkipOnMono("SwiftSelf<T> is not supported on Mono")]
+    public unsafe static void TestSelfIsFrozenNonEnregisteredStruct()
+    {
+        long sum = SumFrozenNonEnregisteredStruct(new SwiftSelf<FrozenNonEnregisteredStruct>(new FrozenNonEnregisteredStruct { A = 10, B = 20, C = 30, D = 40, E = 50 }));
+        Assert.Equal(150, sum);
+    }
 }

src/tests/Interop/Swift/SwiftSelfContext/SwiftSelfContext.swift

@@ -19,3 +19,27 @@ public class SelfLibrary {
         return pointer
     }
 }
+
+@frozen
+public struct FrozenEnregisteredStruct
+{
+    let a : Int64;
+    let b : Int64;
+
+    public func Sum() -> Int64 {
+        return a + b
+    }
+}
+
+@frozen
+public struct FrozenNonEnregisteredStruct {
+    let a : Int64;
+    let b : Int64;
+    let c : Int64;
+    let d : Int64;
+    let e : Int64;
+
+    public func Sum() -> Int64 {
+        return a + b + c + d + e
+    }
+}