Ensure the layout of StackAllocatedBox<T> matches boxed layouts (#1…

…07050)

The boxed layout of a struct always has its data at +8, as evidenced by
`Object::UnBox`. This means that `StackAllocatedBox<T>` should have
`Pack = 1`, otherwise this may not be the case. In the test failure we
had a `StackAllocatedBox<Int128>` which had its `_value` field at offset
16. After object stack allocation this meant that we were saving data in
padding of the structure, which promotion does not guarantee to
preserve.

Fix #106947

src/coreclr/jit/valuenum.cpp

@@ -10299,7 +10299,9 @@ void ValueNumStore::vnDumpSimdType(Compiler* comp, VNFuncApp* simdType)
     int         simdSize    = ConstantValue<int>(simdType->m_args[0]);
     CorInfoType baseJitType = (CorInfoType)ConstantValue<int>(simdType->m_args[1]);
 
-    printf("%s(simd%d, %s)", VNFuncName(simdType->m_func), simdSize, varTypeName(JitType2PreciseVarType(baseJitType)));
+    printf("%s(simd%d, %s)", VNFuncName(simdType->m_func), simdSize,
+           baseJitType == CORINFO_TYPE_UNDEF ? varTypeName(TYP_UNDEF)
+                                             : varTypeName(JitType2PreciseVarType(baseJitType)));
 }
 #endif // FEATURE_SIMD
 

src/coreclr/vm/corelib.h

@@ -1104,6 +1104,7 @@ DEFINE_FIELD_U(_condition,          ContractExceptionObject,    _Condition)
 DEFINE_CLASS(MODULEBASE,        Reflection,         Module)
 
 DEFINE_CLASS(STACKALLOCATEDBOX,   CompilerServices,     StackAllocatedBox`1)
+DEFINE_FIELD(STACKALLOCATEDBOX,   VALUE,                _value)
 
 DEFINE_CLASS(UTF8STRINGMARSHALLER, Marshalling, Utf8StringMarshaller)
 DEFINE_METHOD(UTF8STRINGMARSHALLER, CONVERT_TO_MANAGED, ConvertToManaged, SM_PtrByte_RetStr)

src/coreclr/vm/jitinterface.cpp

@@ -6168,6 +6168,12 @@ CORINFO_CLASS_HANDLE  CEEInfo::getTypeForBoxOnStack(CORINFO_CLASS_HANDLE cls)
         TypeHandle stackAllocatedBox = CoreLibBinder::GetClass(CLASS__STACKALLOCATEDBOX);
         TypeHandle stackAllocatedBoxInst = stackAllocatedBox.Instantiate(boxedFieldsInst);
         result = static_cast<CORINFO_CLASS_HANDLE>(stackAllocatedBoxInst.AsPtr());
+
+#ifdef _DEBUG
+        FieldDesc* pValueFD = CoreLibBinder::GetField(FIELD__STACKALLOCATEDBOX__VALUE);
+        DWORD index = pValueFD->GetApproxEnclosingMethodTable()->GetIndexForFieldDesc(pValueFD);
+        _ASSERTE(stackAllocatedBoxInst.GetMethodTable()->GetFieldDescByIndex(index)->GetOffset() == TARGET_POINTER_SIZE);
+#endif
     }
 
     EE_TO_JIT_TRANSITION();

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs

@@ -7,7 +7,7 @@
 namespace System.Runtime.CompilerServices
 {
     [NonVersionable]
-    [StructLayout(LayoutKind.Sequential)]
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
     internal unsafe struct StackAllocatedBox<T>
     {
         // These fields are only accessed from jitted code