GcInfo: Report Struct containing one GcPointer correctly

This change fixes a bug in the the GcInfo header,
where the ReturnKind is reported incorrectly for certain structs.

Structs that contain only one GCPointer or ByRef, can be returned
in a single register (Windows/Unix/OSX). The ReturnKind for these
struct-returns should be RT_Object/RT_ByRef, but they are currently
incorrectly reported as RT_Scalar. This change fixes the problem.

Fixes: dotnet/corefx#11260

src/jit/compiler.cpp

@@ -906,7 +906,7 @@ var_types Compiler::getArgTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
 //         that multiple registers are used to return this struct.
 //
 var_types Compiler::getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
-                                           structPassingKind*   wbReturnStruct,
+                                           structPassingKind*   wbReturnStruct /* = nullptr */,
                                            unsigned             structSize /* = 0 */)
 {
     var_types         useType           = TYP_UNKNOWN;

src/jit/compiler.h

@@ -3839,7 +3839,7 @@ class Compiler
     // If you have already retrieved the struct size then pass it as the optional third argument
     //
     var_types getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
-                                     structPassingKind*   wbPassStruct,
+                                     structPassingKind*   wbPassStruct = nullptr,
                                      unsigned             structSize = 0);
 
 #ifdef DEBUG

src/jit/gcencode.cpp

@@ -3541,6 +3541,7 @@ void GCInfo::gcInfoBlockHdrSave(GcInfoEncoder* gcInfoEncoder, unsigned methodSiz
     switch (compiler->info.compRetType)
     {
         case TYP_REF:
+        case TYP_ARRAY:
             returnKind = RT_Object;
             break;
         case TYP_BYREF:
@@ -3549,19 +3550,40 @@ void GCInfo::gcInfoBlockHdrSave(GcInfoEncoder* gcInfoEncoder, unsigned methodSiz
         case TYP_STRUCT:
         {
             CORINFO_CLASS_HANDLE structType = compiler->info.compMethodInfo->args.retTypeClass;
-            if (compiler->IsMultiRegReturnedType(structType) && !compiler->IsHfa(structType))
+            var_types retType = compiler->getReturnTypeForStruct(structType);
+
+            switch (retType)
             {
-                BYTE gcPtrs[2] = {TYPE_GC_NONE, TYPE_GC_NONE};
-                compiler->info.compCompHnd->getClassGClayout(structType, gcPtrs);
+            case TYP_REF:
+            case TYP_ARRAY:
+                returnKind = RT_Object;
+                break;
 
-                ReturnKind first  = GCTypeToReturnKind((CorInfoGCType)gcPtrs[0]);
-                ReturnKind second = GCTypeToReturnKind((CorInfoGCType)gcPtrs[1]);
+            case TYP_BYREF:
+                returnKind = RT_ByRef;
+                break;
 
-                returnKind = GetStructReturnKind(first, second);
-            }
-            else
-            {
+            case TYP_STRUCT:
+                if (compiler->IsHfa(structType))
+                {
+                    returnKind = RT_Scalar;
+                }
+                else
+                {
+                    // Multi-reg return
+                    BYTE gcPtrs[2] = { TYPE_GC_NONE, TYPE_GC_NONE };
+                    compiler->info.compCompHnd->getClassGClayout(structType, gcPtrs);
+
+                    ReturnKind first = GCTypeToReturnKind((CorInfoGCType)gcPtrs[0]);
+                    ReturnKind second = GCTypeToReturnKind((CorInfoGCType)gcPtrs[1]);
+
+                    returnKind = GetStructReturnKind(first, second);
+                }
+                break;
+
+            default:
                 returnKind = RT_Scalar;
+                break;
             }
             break;
         }