Allocate Array.Empty<> on a frozen segment (NonGC heap) (#85559)

Co-authored-by: Jakob Botsch Nielsen <[email protected]>

Author: EgorBo

docs/design/coreclr/botr/readytorun-format.md

@@ -798,6 +798,8 @@ enum ReadyToRunHelper
     READYTORUN_HELPER_GenericNonGcTlsBase       = 0x67,
     READYTORUN_HELPER_VirtualFuncPtr            = 0x68,
     READYTORUN_HELPER_IsInstanceOfException     = 0x69,
+    READYTORUN_HELPER_NewMaybeFrozenArray       = 0x6A,
+    READYTORUN_HELPER_NewMaybeFrozenObject      = 0x6B,
 
     // Long mul/div/shift ops
     READYTORUN_HELPER_LMul                      = 0xC0,

src/coreclr/inc/corinfo.h

@@ -404,6 +404,7 @@ enum CorInfoHelpFunc
        which is the right helper to use to allocate an object of a given type. */
 
     CORINFO_HELP_NEWFAST,
+    CORINFO_HELP_NEWFAST_MAYBEFROZEN, // allocator for objects that *might* allocate them on a frozen segment
     CORINFO_HELP_NEWSFAST,          // allocator for small, non-finalizer, non-array object
     CORINFO_HELP_NEWSFAST_FINALIZE, // allocator for small, finalizable, non-array object
     CORINFO_HELP_NEWSFAST_ALIGN8,   // allocator for small, non-finalizer, non-array object, 8 byte aligned
@@ -412,6 +413,7 @@ enum CorInfoHelpFunc
     CORINFO_HELP_NEW_MDARR,// multi-dim array helper for arrays Rank != 1 (with or without lower bounds - dimensions passed in as unmanaged array)
     CORINFO_HELP_NEW_MDARR_RARE,// rare multi-dim array helper (Rank == 1)
     CORINFO_HELP_NEWARR_1_DIRECT,   // helper for any one dimensional array creation
+    CORINFO_HELP_NEWARR_1_MAYBEFROZEN, // allocator for arrays that *might* allocate them on a frozen segment
     CORINFO_HELP_NEWARR_1_OBJ,      // optimized 1-D object arrays
     CORINFO_HELP_NEWARR_1_VC,       // optimized 1-D value class arrays
     CORINFO_HELP_NEWARR_1_ALIGN8,   // like VC, but aligns the array start
@@ -583,8 +585,6 @@ enum CorInfoHelpFunc
     CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE, // Convert from a TypeHandle (native structure pointer) to RuntimeTypeHandle at run-time
     CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE_MAYBENULL, // Convert from a TypeHandle (native structure pointer) to RuntimeTypeHandle at run-time, handle might point to a null type
 
-    CORINFO_HELP_ARE_TYPES_EQUIVALENT, // Check whether two TypeHandles (native structure pointers) are equivalent
-
     CORINFO_HELP_VIRTUAL_FUNC_PTR,      // look up a virtual method at run-time
 
     // Not a real helpers. Instead of taking handle arguments, these helpers point to a small stub that loads the handle argument and calls the static helper.
@@ -2619,12 +2619,6 @@ class ICorStaticInfo
             CORINFO_CLASS_HANDLE        parent  // base type
             ) = 0;
 
-    // TRUE if cls1 and cls2 are considered equivalent types.
-    virtual bool areTypesEquivalent(
-            CORINFO_CLASS_HANDLE        cls1,
-            CORINFO_CLASS_HANDLE        cls2
-            ) = 0;
-
     // See if a cast from fromClass to toClass will succeed, fail, or needs
     // to be resolved at runtime.
     virtual TypeCompareState compareTypesForCast(

src/coreclr/inc/corjitflags.h

@@ -55,7 +55,7 @@ class CORJIT_FLAGS
         CORJIT_FLAG_OSR                     = 13, // Generate alternate method for On Stack Replacement
 
         CORJIT_FLAG_ALT_JIT                 = 14, // JIT should consider itself an ALT_JIT
-        CORJIT_FLAG_UNUSED8                 = 15,
+        CORJIT_FLAG_FROZEN_ALLOC_ALLOWED    = 15, // JIT is allowed to use *_MAYBEFROZEN allocators
         CORJIT_FLAG_UNUSED9                 = 16,
         CORJIT_FLAG_UNUSED10                = 17,
 

src/coreclr/inc/icorjitinfoimpl_generated.h

@@ -339,10 +339,6 @@ bool canCast(
           CORINFO_CLASS_HANDLE child,
           CORINFO_CLASS_HANDLE parent) override;
 
-bool areTypesEquivalent(
-          CORINFO_CLASS_HANDLE cls1,
-          CORINFO_CLASS_HANDLE cls2) override;
-
 TypeCompareState compareTypesForCast(
           CORINFO_CLASS_HANDLE fromClass,
           CORINFO_CLASS_HANDLE toClass) override;

src/coreclr/inc/jiteeversionguid.h

@@ -43,11 +43,11 @@ typedef const GUID *LPCGUID;
 #define GUID_DEFINED
 #endif // !GUID_DEFINED
 
-constexpr GUID JITEEVersionIdentifier = { /* 387bcec3-9a71-4422-a11c-e7ce3b73c592 */
-    0x387bcec3,
-    0x9a71,
-    0x4422,
-    {0xa1, 0x1c, 0xe7, 0xce, 0x3b, 0x73, 0xc5, 0x92}
+constexpr GUID JITEEVersionIdentifier = { /* 4e6355a0-3844-45e2-8cef-082c18217e14 */
+    0x4e6355a0,
+    0x3844,
+    0x45e2,
+    {0x8c, 0xef, 0x8, 0x2c, 0x18, 0x21, 0x7e, 0x14}
   };
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////////

src/coreclr/inc/jithelpers.h

@@ -70,6 +70,7 @@
 
     // Allocating a new object
     JITHELPER(CORINFO_HELP_NEWFAST,                     JIT_New,    CORINFO_HELP_SIG_REG_ONLY)
+    JITHELPER(CORINFO_HELP_NEWFAST_MAYBEFROZEN,         JIT_NewMaybeFrozen,CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_NEWSFAST,             JIT_New,    CORINFO_HELP_SIG_REG_ONLY)
     JITHELPER(CORINFO_HELP_NEWSFAST_FINALIZE,           NULL,       CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_NEWSFAST_ALIGN8,      JIT_New,    CORINFO_HELP_SIG_REG_ONLY)
@@ -78,6 +79,7 @@
     JITHELPER(CORINFO_HELP_NEW_MDARR,                   JIT_NewMDArr,CORINFO_HELP_SIG_4_STACK)
     JITHELPER(CORINFO_HELP_NEW_MDARR_RARE,              JIT_NewMDArr,CORINFO_HELP_SIG_4_STACK)
     JITHELPER(CORINFO_HELP_NEWARR_1_DIRECT,             JIT_NewArr1,CORINFO_HELP_SIG_REG_ONLY)
+    JITHELPER(CORINFO_HELP_NEWARR_1_MAYBEFROZEN,        JIT_NewArr1MaybeFrozen,CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_NEWARR_1_OBJ,         JIT_NewArr1,CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_NEWARR_1_VC,          JIT_NewArr1,CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_NEWARR_1_ALIGN8,      JIT_NewArr1,CORINFO_HELP_SIG_REG_ONLY)
@@ -252,8 +254,6 @@
     JITHELPER(CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE, JIT_GetRuntimeType, CORINFO_HELP_SIG_REG_ONLY)
     JITHELPER(CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE_MAYBENULL, JIT_GetRuntimeType_MaybeNull, CORINFO_HELP_SIG_REG_ONLY)
 
-    JITHELPER(CORINFO_HELP_ARE_TYPES_EQUIVALENT, NULL, CORINFO_HELP_SIG_REG_ONLY)
-
     JITHELPER(CORINFO_HELP_VIRTUAL_FUNC_PTR,    JIT_VirtualFunctionPointer, CORINFO_HELP_SIG_4_STACK)
 
     JITHELPER(CORINFO_HELP_READYTORUN_NEW,                 NULL,   CORINFO_HELP_SIG_NO_ALIGN_STUB)

src/coreclr/inc/readytorun.h

@@ -18,7 +18,7 @@
 //  src/coreclr/tools/Common/Internal/Runtime/ModuleHeaders.cs
 //  src/coreclr/nativeaot/Runtime/inc/ModuleHeaders.h
 #define READYTORUN_MAJOR_VERSION 0x0009
-#define READYTORUN_MINOR_VERSION 0x0000
+#define READYTORUN_MINOR_VERSION 0x0001
 
 #define MINIMUM_READYTORUN_MAJOR_VERSION 0x009
 
@@ -30,6 +30,7 @@
 //     R2R 6.0 is not backward compatible with 5.x or earlier.
 // R2R Version 8.0 Changes the alignment of the Int128 type
 // R2R Version 9.0 adds support for the Vector512 type
+// R2R Version 9.1 adds new helpers to allocate objects on frozen segments
 
 struct READYTORUN_CORE_HEADER
 {
@@ -335,6 +336,8 @@ enum ReadyToRunHelper
     READYTORUN_HELPER_GenericNonGcTlsBase       = 0x67,
     READYTORUN_HELPER_VirtualFuncPtr            = 0x68,
     READYTORUN_HELPER_IsInstanceOfException     = 0x69,
+    READYTORUN_HELPER_NewMaybeFrozenArray       = 0x6A,
+    READYTORUN_HELPER_NewMaybeFrozenObject      = 0x6B,
 
     // Long mul/div/shift ops
     READYTORUN_HELPER_LMul                      = 0xC0,

src/coreclr/inc/readytorunhelpers.h

@@ -55,6 +55,8 @@ HELPER(READYTORUN_HELPER_GenericNonGcTlsBase,       CORINFO_HELP_GETGENERICS_NON
 
 HELPER(READYTORUN_HELPER_VirtualFuncPtr,            CORINFO_HELP_VIRTUAL_FUNC_PTR,                  )
 HELPER(READYTORUN_HELPER_IsInstanceOfException,     CORINFO_HELP_ISINSTANCEOF_EXCEPTION,            )
+HELPER(READYTORUN_HELPER_NewMaybeFrozenArray,       CORINFO_HELP_NEWARR_1_MAYBEFROZEN,              )
+HELPER(READYTORUN_HELPER_NewMaybeFrozenObject,      CORINFO_HELP_NEWFAST_MAYBEFROZEN,               )
 
 HELPER(READYTORUN_HELPER_LMul,                      CORINFO_HELP_LMUL,                              )
 HELPER(READYTORUN_HELPER_LMulOfv,                   CORINFO_HELP_LMUL_OVF,                          )

src/coreclr/jit/ICorJitInfo_names_generated.h

@@ -84,7 +84,6 @@ DEF_CLR_API(getBuiltinClass)
 DEF_CLR_API(getTypeForPrimitiveValueClass)
 DEF_CLR_API(getTypeForPrimitiveNumericClass)
 DEF_CLR_API(canCast)
-DEF_CLR_API(areTypesEquivalent)
 DEF_CLR_API(compareTypesForCast)
 DEF_CLR_API(compareTypesForEquality)
 DEF_CLR_API(mergeClasses)

src/coreclr/jit/ICorJitInfo_wrapper_generated.hpp

@@ -793,16 +793,6 @@ bool WrapICorJitInfo::canCast(
     return temp;
 }
 
-bool WrapICorJitInfo::areTypesEquivalent(
-          CORINFO_CLASS_HANDLE cls1,
-          CORINFO_CLASS_HANDLE cls2)
-{
-    API_ENTER(areTypesEquivalent);
-    bool temp = wrapHnd->areTypesEquivalent(cls1, cls2);
-    API_LEAVE(areTypesEquivalent);
-    return temp;
-}
-
 TypeCompareState WrapICorJitInfo::compareTypesForCast(
           CORINFO_CLASS_HANDLE fromClass,
           CORINFO_CLASS_HANDLE toClass)

src/coreclr/jit/compiler.h

@@ -3038,10 +3038,6 @@ class Compiler
     GenTree* gtFoldBoxNullable(GenTree* tree);
     GenTree* gtFoldExprCompare(GenTree* tree);
     GenTree* gtFoldExprConditional(GenTree* tree);
-    GenTree* gtCreateHandleCompare(genTreeOps             oper,
-                                   GenTree*               op1,
-                                   GenTree*               op2,
-                                   CorInfoInlineTypeCheck typeCheckInliningResult);
     GenTree* gtFoldExprCall(GenTreeCall* call);
     GenTree* gtFoldTypeCompare(GenTree* tree);
     GenTree* gtFoldTypeEqualityCall(bool isEq, GenTree* op1, GenTree* op2);

src/coreclr/jit/gentree.cpp

@@ -2158,30 +2158,17 @@ GenTree* Compiler::getArrayLengthFromAllocation(GenTree* tree DEBUGARG(BasicBloc
 
         if (call->gtCallType == CT_HELPER)
         {
-            switch (eeGetHelperNum(call->gtCallMethHnd))
+            CorInfoHelpFunc helper = eeGetHelperNum(call->gtCallMethHnd);
+            switch (helper)
             {
+                case CORINFO_HELP_NEWARR_1_MAYBEFROZEN:
                 case CORINFO_HELP_NEWARR_1_DIRECT:
                 case CORINFO_HELP_NEWARR_1_OBJ:
                 case CORINFO_HELP_NEWARR_1_VC:
                 case CORINFO_HELP_NEWARR_1_ALIGN8:
                 {
                     // This is an array allocation site. Grab the array length node.
-                    arrayLength = call->gtArgs.GetArgByIndex(1)->GetNode();
-                    break;
-                }
-
-                case CORINFO_HELP_READYTORUN_NEWARR_1:
-                {
-                    // On arm when compiling on certain platforms for ready to
-                    // run, a handle will be inserted before the length. To
-                    // handle this case, we will grab the last argument as
-                    // that's always the length. See
-                    // CallArgs::AddFinalArgsAndDetermineABIInfo for where the
-                    // handle is inserted.
-                    for (CallArg& arg : call->gtArgs.Args())
-                    {
-                        arrayLength = arg.GetNode();
-                    }
+                    arrayLength = call->gtArgs.GetUserArgByIndex(1)->GetNode();
                     break;
                 }
 
@@ -13614,47 +13601,6 @@ GenTree* Compiler::gtFoldExprConditional(GenTree* tree)
     return replacement;
 }
 
-//------------------------------------------------------------------------
-// gtCreateHandleCompare: generate a type handle comparison
-//
-// Arguments:
-//    oper -- comparison operation (equal/not equal)
-//    op1 -- first operand
-//    op2 -- second operand
-//    typeCheckInliningResult -- indicates how the comparison should happen
-//
-// Returns:
-//    Type comparison tree
-//
-
-GenTree* Compiler::gtCreateHandleCompare(genTreeOps             oper,
-                                         GenTree*               op1,
-                                         GenTree*               op2,
-                                         CorInfoInlineTypeCheck typeCheckInliningResult)
-{
-    // If we can compare pointers directly, just emit the binary operation
-    if (typeCheckInliningResult == CORINFO_INLINE_TYPECHECK_PASS)
-    {
-        return gtNewOperNode(oper, TYP_INT, op1, op2);
-    }
-
-    assert(typeCheckInliningResult == CORINFO_INLINE_TYPECHECK_USE_HELPER);
-
-    // Emit a call to a runtime helper
-    GenTree* ret = gtNewHelperCallNode(CORINFO_HELP_ARE_TYPES_EQUIVALENT, TYP_INT, op1, op2);
-    if (oper == GT_EQ)
-    {
-        ret = gtNewOperNode(GT_NE, TYP_INT, ret, gtNewIconNode(0, TYP_INT));
-    }
-    else
-    {
-        assert(oper == GT_NE);
-        ret = gtNewOperNode(GT_EQ, TYP_INT, ret, gtNewIconNode(0, TYP_INT));
-    }
-
-    return ret;
-}
-
 //------------------------------------------------------------------------
 // gtFoldTypeCompare: see if a type comparison can be further simplified
 //
@@ -13767,9 +13713,9 @@ GenTree* Compiler::gtFoldTypeCompare(GenTree* tree)
             inliningKind = info.compCompHnd->canInlineTypeCheck(cls2Hnd, CORINFO_INLINE_TYPECHECK_SOURCE_TOKEN);
         }
 
-        assert(inliningKind == CORINFO_INLINE_TYPECHECK_PASS || inliningKind == CORINFO_INLINE_TYPECHECK_USE_HELPER);
+        assert(inliningKind == CORINFO_INLINE_TYPECHECK_PASS);
 
-        GenTree* compare = gtCreateHandleCompare(oper, op1ClassFromHandle, op2ClassFromHandle, inliningKind);
+        GenTree* compare = gtNewOperNode(oper, TYP_INT, op1ClassFromHandle, op2ClassFromHandle);
 
         // Drop any now-irrelevant flags
         compare->gtFlags |= tree->gtFlags & (GTF_RELOP_JMP_USED | GTF_DONT_CSE);
@@ -13805,11 +13751,10 @@ GenTree* Compiler::gtFoldTypeCompare(GenTree* tree)
 
         arg2 = gtNewMethodTableLookup(arg2);
 
-        CorInfoInlineTypeCheck inliningKind =
-            info.compCompHnd->canInlineTypeCheck(nullptr, CORINFO_INLINE_TYPECHECK_SOURCE_VTABLE);
-        assert(inliningKind == CORINFO_INLINE_TYPECHECK_PASS || inliningKind == CORINFO_INLINE_TYPECHECK_USE_HELPER);
+        assert(info.compCompHnd->canInlineTypeCheck(nullptr, CORINFO_INLINE_TYPECHECK_SOURCE_VTABLE) ==
+               CORINFO_INLINE_TYPECHECK_PASS);
 
-        GenTree* compare = gtCreateHandleCompare(oper, arg1, arg2, inliningKind);
+        GenTree* compare = gtNewOperNode(oper, TYP_INT, arg1, arg2);
 
         // Drop any now-irrelevant flags
         compare->gtFlags |= tree->gtFlags & (GTF_RELOP_JMP_USED | GTF_DONT_CSE);
@@ -13907,7 +13852,8 @@ GenTree* Compiler::gtFoldTypeCompare(GenTree* tree)
     GenTree* const objMT = gtNewMethodTableLookup(objOp);
 
     // Compare the two method tables
-    GenTree* const compare = gtCreateHandleCompare(oper, objMT, knownMT, typeCheckInliningResult);
+    assert(typeCheckInliningResult == CORINFO_INLINE_TYPECHECK_PASS);
+    GenTree* const compare = gtNewOperNode(oper, TYP_INT, objMT, knownMT);
 
     // Drop any now irrelevant flags
     compare->gtFlags |= tree->gtFlags & (GTF_RELOP_JMP_USED | GTF_DONT_CSE);
@@ -18576,6 +18522,7 @@ CORINFO_CLASS_HANDLE Compiler::gtGetHelperCallClassHandle(GenTreeCall* call, boo
         }
 
         case CORINFO_HELP_NEWARR_1_DIRECT:
+        case CORINFO_HELP_NEWARR_1_MAYBEFROZEN:
         case CORINFO_HELP_NEWARR_1_OBJ:
         case CORINFO_HELP_NEWARR_1_VC:
         case CORINFO_HELP_NEWARR_1_ALIGN8:

src/coreclr/jit/importer.cpp

@@ -9734,11 +9734,57 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                 // So if we have an int, explicitly extend it to be a native int.
                 op2 = impImplicitIorI4Cast(op2, TYP_I_IMPL);
 
+                bool isFrozenAllocator = false;
+                // If we're jitting a static constructor and detect the following code pattern:
+                //
+                //  newarr
+                //  stsfld
+                //  ret
+                //
+                // we emit a "frozen" allocator for newarr to, hopefully, allocate that array on a frozen segment.
+                // This is a very simple and conservative implementation targeting Array.Empty<T>()'s shape
+                // Ideally, we want to be able to use frozen allocators more broadly, but such an analysis is
+                // not trivial.
+                //
+                if (((info.compFlags & FLG_CCTOR) == FLG_CCTOR) &&
+                    // Does VM allow us to use frozen allocators?
+                    opts.jitFlags->IsSet(JitFlags::JIT_FLAG_FROZEN_ALLOC_ALLOWED))
+                {
+                    // Check next two opcodes (have to be STSFLD and RET)
+                    const BYTE* nextOpcode1 = codeAddr + sizeof(mdToken);
+                    const BYTE* nextOpcode2 = nextOpcode1 + sizeof(mdToken) + 1;
+                    if ((nextOpcode2 < codeEndp) && (getU1LittleEndian(nextOpcode1) == CEE_STSFLD))
+                    {
+                        if (getU1LittleEndian(nextOpcode2) == CEE_RET)
+                        {
+                            // Check that the field is "static readonly", we don't want to waste memory
+                            // for potentially mutable fields.
+                            CORINFO_RESOLVED_TOKEN fldToken;
+                            impResolveToken(nextOpcode1 + 1, &fldToken, CORINFO_TOKENKIND_Field);
+                            CORINFO_FIELD_INFO fi;
+                            eeGetFieldInfo(&fldToken, CORINFO_ACCESS_SET, &fi);
+                            unsigned flagsToCheck = CORINFO_FLG_FIELD_STATIC | CORINFO_FLG_FIELD_FINAL;
+                            if ((fi.fieldFlags & flagsToCheck) == flagsToCheck)
+                            {
 #ifdef FEATURE_READYTORUN
-                if (opts.IsReadyToRun())
+                                if (opts.IsReadyToRun())
+                                {
+                                    // Need to restore array classes before creating array objects on the heap
+                                    op1 = impTokenToHandle(&resolvedToken, nullptr, true /*mustRestoreHandle*/);
+                                }
+#endif
+                                op1 = gtNewHelperCallNode(CORINFO_HELP_NEWARR_1_MAYBEFROZEN, TYP_REF, op1, op2);
+                                isFrozenAllocator = true;
+                            }
+                        }
+                    }
+                }
+
+#ifdef FEATURE_READYTORUN
+                if (opts.IsReadyToRun() && !isFrozenAllocator)
                 {
-                    op1 = impReadyToRunHelperToTree(&resolvedToken, CORINFO_HELP_READYTORUN_NEWARR_1, TYP_REF, nullptr,
-                                                    op2);
+                    helper                = CORINFO_HELP_READYTORUN_NEWARR_1;
+                    op1                   = impReadyToRunHelperToTree(&resolvedToken, helper, TYP_REF, nullptr, op2);
                     usingReadyToRunHelper = (op1 != nullptr);
 
                     if (!usingReadyToRunHelper)
@@ -9750,7 +9796,6 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                         //      3) Allocate the new array
                         // Reason: performance (today, we'll always use the slow helper for the R2R generics case)
 
-                        // Need to restore array classes before creating array objects on the heap
                         op1 = impTokenToHandle(&resolvedToken, nullptr, true /*mustRestoreHandle*/);
                         if (op1 == nullptr)
                         { // compDonotInline()
@@ -9759,15 +9804,15 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     }
                 }
 
-                if (!usingReadyToRunHelper)
+                if (!usingReadyToRunHelper && !isFrozenAllocator)
 #endif
                 {
                     /* Create a call to 'new' */
+                    helper = info.compCompHnd->getNewArrHelper(resolvedToken.hClass);
 
                     // Note that this only works for shared generic code because the same helper is used for all
                     // reference array types
-                    op1 =
-                        gtNewHelperCallNode(info.compCompHnd->getNewArrHelper(resolvedToken.hClass), TYP_REF, op1, op2);
+                    op1 = gtNewHelperCallNode(helper, TYP_REF, op1, op2);
                 }
 
                 op1->AsCall()->compileTimeHelperArgumentHandle = (CORINFO_GENERIC_HANDLE)resolvedToken.hClass;