JIT: update impFixupStructReturnType to handle GT_RET_EXPR

src/coreclr/jit/importer.cpp

@@ -4626,8 +4626,13 @@ bool Compiler::impIsImplicitTailCallCandidate(
 // In the multi-reg case, we we force IR to be one of the following:
 // GT_RETURN(LCL_VAR) or GT_RETURN(CALL). If op is anything other than
 // a lclvar or call, it is assigned to a temp, which is then returned.
-// In the non-multireg case, the two special helpers with "fake" return
-// buffers are handled ("GETFIELDSTRUCT" and "UNBOX_NULLABLE").
+// In the non-multireg case, calls that return via a return buffer are
+// handled. This covers the two special helpers with "fake" return buffers
+// ("GETFIELDSTRUCT" and "UNBOX_NULLABLE"), as well as inline candidates
+// whose ABI requires a real return buffer. The latter reach here when an
+// intrinsic such as "Unsafe.BitCast" forwards the candidate's GT_RET_EXPR
+// as the return value of a method whose own return type does not use a
+// return buffer.
 //
 // Arguments:
 //    op - the return value
@@ -4643,12 +4648,32 @@ GenTree* Compiler::impFixupStructReturnType(GenTree* op)
     JITDUMP("\nimpFixupStructReturnType: retyping\n");
     DISPTREE(op);
 
-    if (op->IsCall() && op->AsCall()->ShouldHaveRetBufArg())
+    GenTreeCall* retBufCall = nullptr;
+
+    if (op->IsCall())
+    {
+        retBufCall = op->AsCall();
+    }
+    else if (op->OperIs(GT_RET_EXPR))
     {
-        // This must be one of those 'special' helpers that don't really have a return buffer, but instead
-        // use it as a way to keep the trees cleaner with fewer address-taken temps. Well now we have to
+        // The placeholder was created during this importation, so it cannot have been
+        // substituted yet; only "gtSubstExpr" can form chains of GT_RET_EXPRs, and those
+        // are walked later by "UpdateInlineReturnExpressionPlaceHolder".
+        assert(op->AsRetExpr()->gtSubstExpr == nullptr);
+        retBufCall = op->AsRetExpr()->gtInlineCandidate;
+    }
+
+    if ((retBufCall != nullptr) && retBufCall->ShouldHaveRetBufArg())
+    {
+        // This is either one of those 'special' helpers that don't really have a return buffer,
+        // but instead use it as a way to keep the trees cleaner with fewer address-taken temps,
+        // or a call that genuinely returns via a return buffer. Either way we now have to
         // materialize the return buffer as an address-taken temp. Then we can return the temp.
         //
+        // Note we store "op" itself rather than "retBufCall": for a GT_RET_EXPR this lets
+        // "impStoreStruct" insert the return buffer argument into the candidate and prune the
+        // placeholder, which is what establishes the argument the inliner later expects.
+        //
         unsigned tmpNum = lvaGrabTemp(true DEBUGARG("pseudo return buffer"));
 
         // No need to spill anything as we're about to return.

src/tests/JIT/Regression/JitBlue/Runtime_130135/Runtime_130135.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Runtime_130135;
+
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
+using Xunit;
+
+public static class Runtime_130135
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        V2<int> result = Unbox(GetV2());
+
+        Assert.Equal(1, result.X);
+        Assert.Equal(2, result.Y);
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static V2<int> GetV2() =>
+        Unsafe.BitCast<Vector64<int>, V2<int>>(CreateVector());
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static Vector64<int> CreateVector() => Vector64.Create(1, 2);
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static V2<int> Unbox(object value) => (V2<int>)value;
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct V2<T>(T x, T y)
+    {
+        public T X = x;
+        public T Y = y;
+    }
+}

src/tests/JIT/Regression/Regression_ro_2.csproj

@@ -112,6 +112,7 @@
     <Compile Include="JitBlue\Runtime_129970\Runtime_129970.cs" />
     <Compile Include="JitBlue\Runtime_129971\Runtime_129971.cs" />
     <Compile Include="JitBlue\Runtime_129972\Runtime_129972.cs" />
+    <Compile Include="JitBlue\Runtime_130135\Runtime_130135.cs" />
     <Compile Include="JitBlue\Runtime_130189\Runtime_130189.cs" />
     <Compile Include="JitBlue\Runtime_130216\Runtime_130216.cs" />
     <Compile Include="JitBlue\Runtime_130248\Runtime_130248.cs" />