JIT: Fix SysV first/second return register GC info mismatch when generating calls

src/coreclr/jit/codegenxarch.cpp

@@ -6293,6 +6293,18 @@ void CodeGen::genCallInstruction(GenTreeCall* call X86_ARG(target_ssize_t stackA
         {
             params.retSize       = emitTypeSize(retTypeDesc->GetReturnRegType(0));
             params.secondRetSize = emitTypeSize(retTypeDesc->GetReturnRegType(1));
+
+            if (retTypeDesc->GetABIReturnReg(1, call->GetUnmanagedCallConv()) == REG_INTRET)
+            {
+                // If the second return register is REG_INTRET, then the first
+                // return is expected to be in a SIMD register.
+                // The emitter has hardcoded belief that params.retSize corresponds to
+                // REG_INTRET and secondRetSize to REG_INTRET_1, so fix up the
+                // situation here.
+                assert(!EA_IS_GCREF_OR_BYREF(params.retSize));
+                params.retSize       = params.secondRetSize;
+                params.secondRetSize = EA_UNKNOWN;
+            }
         }
         else
         {

src/tests/JIT/Regression/JitBlue/Runtime_115815/Runtime_115815.cs

@@ -0,0 +1,42 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Threading;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_115815
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        var destination = new KeyValuePair<Container, double>[1_000_000];
+
+        // loop to make this method fully interruptible + to get into OSR version
+        for (int i = 0; i < destination.Length; i++)
+        {
+            destination[i] = default;
+        }
+
+        for (int i = 0; i < 5; i++)
+        {
+            for (int j = 0; j < destination.Length; j++)
+            {
+                destination[j] = GetValue(j);
+            }
+
+            Thread.Sleep(10);
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static KeyValuePair<Container, double> GetValue(int i)
+        => KeyValuePair.Create(new Container(i.ToString()), (double)i);
+
+    private struct Container
+    {
+        public string Name;
+        public Container(string name) { this.Name = name; }
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_115815/Runtime_115815.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>