JIT: Fold string fields in structs stored in static readonly fields

src/coreclr/jit/valuenum.cpp

@@ -5426,6 +5426,41 @@ void Compiler::fgValueNumberFieldLoad(GenTree* loadTree, GenTree* baseAddr, Fiel
 {
     noway_assert(fieldSeq != nullptr);
 
+    // Check if the load represents a frozen gc object located in a struct which is stored in a static readonly field,
+    // e.g.:
+    //
+    //   struct MyStruct { string Name; }
+    //   static readonly MyStruct MyStr = new() { Name = "Hey!" };
+    //
+    //   string GetName() => MyStr.Name; // <- loadTree
+    //
+    if ((baseAddr == nullptr) && loadTree->TypeIs(TYP_REF) &&
+        // it should be a static field
+        (fieldSeq->GetKind() == FieldSeq::FieldKind::SimpleStatic))
+    {
+        uint8_t buffer[TARGET_POINTER_SIZE] = {0};
+        if (((UINT)offset < INT_MAX) &&
+            info.compCompHnd->getReadonlyStaticFieldValue(fieldSeq->GetFieldHandle(), buffer, TARGET_POINTER_SIZE,
+                                                          (int)offset))
+        {
+            // In case of 64bit jit emitting 32bit codegen this handle will be 64bit
+            // value holding 32bit handle with upper half zeroed (hence, "= NULL").
+            // It's done to match the current crossgen/ILC behavior.
+            ssize_t objHandle = 0;
+            memcpy(&objHandle, buffer, TARGET_POINTER_SIZE);
+            if (objHandle == 0)
+            {
+                loadTree->gtVNPair.SetBoth(vnStore->VNForNull());
+            }
+            else
+            {
+                loadTree->gtVNPair.SetBoth(vnStore->VNForHandle(objHandle, GTF_ICON_OBJ_HDL));
+                setMethodHasFrozenObjects();
+            }
+            return;
+        }
+    }
+
     // Two cases:
     //
     //  1) Instance field / "complex" static: heap[field][baseAddr][offset + load size].

src/coreclr/vm/jitinterface.cpp

@@ -11688,8 +11688,60 @@ bool CEEInfo::getReadonlyStaticFieldValue(CORINFO_FIELD_HANDLE fieldHnd, uint8_t
 
             if (size >= (UINT)bufferSize && valueOffset >= 0 && (UINT)valueOffset <= size - (UINT)bufferSize)
             {
-                memcpy(buffer, (uint8_t*)baseAddr + valueOffset, bufferSize);
-                result = true;
+                // For structs containing GC pointers we want to make sure those GC pointers belong to FOH
+                // so we expect valueOffset to be a real field offset (same for bufferSize)
+                if (!field->IsRVA() && field->GetFieldType() == ELEMENT_TYPE_VALUETYPE)
+                {
+                    PTR_MethodTable structType = field->GetFieldTypeHandleThrowing().AsMethodTable();
+                    if (structType->ContainsPointers())
+                    {
+                        ApproxFieldDescIterator fieldIterator(structType, ApproxFieldDescIterator::INSTANCE_FIELDS);
+                        for (FieldDesc* subField = fieldIterator.Next(); subField != NULL; subField = fieldIterator.Next())
+                        {
+                            // TODO: If subField is also a struct we might want to inspect its fields too
+                            if (subField->GetOffset() == (DWORD)valueOffset && subField->GetSize() == (UINT)bufferSize &&
+                                subField->IsObjRef())
+                            {
+                                GCX_COOP();
+
+                                // Read field's value
+                                Object* subFieldValue = nullptr;
+                                memcpy(&subFieldValue, (uint8_t*)baseAddr + valueOffset, bufferSize);
+
+                                if (subFieldValue == nullptr)
+                                {
+                                    // Report null
+                                    memset(buffer, 0, bufferSize);
+                                    result = true;
+                                }
+                                else if (GCHeapUtilities::GetGCHeap()->IsInFrozenSegment(subFieldValue))
+                                {
+                                    CORINFO_OBJECT_HANDLE handle = getJitHandleForObject(
+                                        ObjectToOBJECTREF(subFieldValue), /*knownFrozen*/ true);
+
+                                    // GC handle is either from FOH or null
+                                    memcpy(buffer, &handle, bufferSize);
+                                    result = true;
+                                }
+
+                                // We're done with this struct
+                                break;
+                            }
+                        }
+                    }
+                    else
+                    {
+                        // No gc pointers in the struct
+                        memcpy(buffer, (uint8_t*)baseAddr + valueOffset, bufferSize);
+                        result = true;
+                    }
+                }
+                else
+                {
+                    // Primitive or RVA
+                    memcpy(buffer, (uint8_t*)baseAddr + valueOffset, bufferSize);
+                    result = true;
+                }
             }
         }
     }

src/tests/JIT/opt/ValueNumbering/StaticReadonlyStructWithGC.cs

@@ -0,0 +1,71 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading;
+
+class StaticReadonlyStructWithGC
+{
+    static int Main()
+    {
+        // Pre-initialize host type
+        RuntimeHelpers.RunClassConstructor(typeof(StaticReadonlyStructWithGC).TypeHandle);
+
+        if (!Test1()) throw new Exception("Test1 failed");
+        if (!Test2()) throw new Exception("Test2 failed");
+        if (!Test3()) throw new Exception("Test3 failed");
+        if (!Test4()) throw new Exception("Test4 failed");
+        if (!Test5()) throw new Exception("Test5 failed");
+        if (!Test6()) throw new Exception("Test6 failed");
+        if (!Test7()) throw new Exception("Test7 failed");
+        if (!Test8()) throw new Exception("Test8 failed");
+        if (!Test9()) throw new Exception("Test9 failed");
+        return 100;
+    }
+
+    static readonly MyStruct MyStructFld = new()
+    {
+        A = "A",
+        B = 111111.ToString(), // non-literal
+        C = new MyStruct2 { A = "AA" },
+        D = typeof(int),
+        E = () => 42,
+        F = new MyStruct3 { A = typeof(double), B = typeof(string) },
+        G = new int[0],
+        H = null
+    };
+
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test1() => MyStructFld.A == "A";
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test2() => MyStructFld.B == "111111";
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test3() => MyStructFld.C.A == "AA";
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test4() => MyStructFld.D == typeof(int);
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test5() => MyStructFld.E() == 42;
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test6() => MyStructFld.F.A == typeof(double);
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test7() => MyStructFld.F.B == typeof(string);
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test8() => MyStructFld.G.Length == 0;
+    [MethodImpl(MethodImplOptions.NoInlining)] static bool Test9() => MyStructFld.H == null;
+
+    struct MyStruct
+    {
+        public string A;
+        public string B;
+        public MyStruct2 C;
+        public Type D;
+        public Func<int> E;
+        public MyStruct3 F;
+        public int[] G;
+        public object H;
+    }
+
+    struct MyStruct2
+    {
+        public string A;
+    }
+
+    struct MyStruct3
+    {
+        public Type A;
+        public Type B;
+    }
+}

src/tests/JIT/opt/ValueNumbering/StaticReadonlyStructWithGC.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>