Cleanup LowLevelList usage in GCLayout (#116316)

* Replace with bool[] in GC layout

* Cleanup empty layout input

* Cleanup _isReferenceTypeGCLayout

* Use Array.Empty

Author: huoyaoyuan

src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/EETypeCreator.cs

@@ -399,27 +399,20 @@ private static void CreateInstanceGCDesc(TypeBuilderState state, MethodTable* pT
                     pEEType->ContainsGCPointers = false;
                 }
             }
-            else if (gcBitfield != null)
+            else
             {
-                if (cbGCDesc != 0)
+                Debug.Assert(gcBitfield == null);
+
+                if (pTemplateEEType != null)
                 {
-                    pEEType->ContainsGCPointers = true;
-                    CreateGCDesc(gcBitfield, baseSize, isValueType, false, ((void**)pEEType) - 1);
+                    Buffer.MemoryCopy((byte*)pTemplateEEType - cbGCDesc, (byte*)pEEType - cbGCDesc, cbGCDesc, cbGCDesc);
+                    pEEType->ContainsGCPointers = pTemplateEEType->ContainsGCPointers;
                 }
                 else
                 {
                     pEEType->ContainsGCPointers = false;
                 }
             }
-            else if (pTemplateEEType != null)
-            {
-                Buffer.MemoryCopy((byte*)pTemplateEEType - cbGCDesc, (byte*)pEEType - cbGCDesc, cbGCDesc, cbGCDesc);
-                pEEType->ContainsGCPointers = pTemplateEEType->ContainsGCPointers;
-            }
-            else
-            {
-                pEEType->ContainsGCPointers = false;
-            }
         }
 
         private static unsafe int GetInstanceGCDescSize(TypeBuilderState state, MethodTable* pTemplateEEType, bool isValueType, bool isArray)
@@ -442,24 +435,24 @@ private static unsafe int GetInstanceGCDescSize(TypeBuilderState state, MethodTa
                     return series > 0 ? (series + 2) * IntPtr.Size : 0;
                 }
             }
-            else if (gcBitfield != null)
-            {
-                int series = CreateGCDesc(gcBitfield, 0, isValueType, false, null);
-                return series > 0 ? (series * 2 + 1) * IntPtr.Size : 0;
-            }
-            else if (pTemplateEEType != null)
-            {
-                return RuntimeAugments.GetGCDescSize(pTemplateEEType->ToRuntimeTypeHandle());
-            }
             else
             {
-                return 0;
+                Debug.Assert(gcBitfield == null);
+
+                if (pTemplateEEType != null)
+                {
+                    return RuntimeAugments.GetGCDescSize(pTemplateEEType->ToRuntimeTypeHandle());
+                }
+                else
+                {
+                    return 0;
+                }
             }
         }
 
-        private static bool IsAllGCPointers(LowLevelList<bool> bitfield)
+        private static bool IsAllGCPointers(bool[] bitfield)
         {
-            int count = bitfield.Count;
+            int count = bitfield.Length;
             Debug.Assert(count > 0);
 
             for (int i = 0; i < count; i++)
@@ -471,7 +464,7 @@ private static bool IsAllGCPointers(LowLevelList<bool> bitfield)
             return true;
         }
 
-        private static unsafe int CreateArrayGCDesc(LowLevelList<bool> bitfield, int rank, bool isSzArray, void* gcdesc)
+        private static unsafe int CreateArrayGCDesc(bool[] bitfield, int rank, bool isSzArray, void* gcdesc)
         {
             if (bitfield == null)
                 return 0;
@@ -495,7 +488,7 @@ private static unsafe int CreateArrayGCDesc(LowLevelList<bool> bitfield, int ran
             int first = -1;
             int last = 0;
             short numPtrs = 0;
-            while (i < bitfield.Count)
+            while (i < bitfield.Length)
             {
                 if (bitfield[i])
                 {
@@ -513,7 +506,7 @@ private static unsafe int CreateArrayGCDesc(LowLevelList<bool> bitfield, int ran
                     numSeries++;
                     numPtrs = 0;
 
-                    while ((i < bitfield.Count) && (bitfield[i]))
+                    while ((i < bitfield.Length) && (bitfield[i]))
                     {
                         numPtrs++;
                         i++;
@@ -531,7 +524,7 @@ private static unsafe int CreateArrayGCDesc(LowLevelList<bool> bitfield, int ran
             {
                 if (numSeries > 0)
                 {
-                    *ptr-- = (short)((first + bitfield.Count - last) * IntPtr.Size);
+                    *ptr-- = (short)((first + bitfield.Length - last) * IntPtr.Size);
                     *ptr-- = numPtrs;
 
                     *(void**)gcdesc = (void*)-numSeries;
@@ -542,69 +535,6 @@ private static unsafe int CreateArrayGCDesc(LowLevelList<bool> bitfield, int ran
             return numSeries;
         }
 
-        private static unsafe int CreateGCDesc(LowLevelList<bool> bitfield, int size, bool isValueType, bool isStatic, void* gcdesc)
-        {
-            int offs = 0;
-            // if this type is a class we have to account for the gcdesc.
-            if (isValueType)
-                offs = IntPtr.Size;
-
-            if (bitfield == null)
-                return 0;
-
-            void** ptr = (void**)gcdesc - 1;
-
-            int* staticPtr = isStatic ? ((int*)gcdesc + 1) : null;
-
-            int numSeries = 0;
-            int i = 0;
-            while (i < bitfield.Count)
-            {
-                if (bitfield[i])
-                {
-                    numSeries++;
-                    int seriesOffset = i * IntPtr.Size + offs;
-                    int seriesSize = 0;
-
-                    while ((i < bitfield.Count) && (bitfield[i]))
-                    {
-                        seriesSize += IntPtr.Size;
-                        i++;
-                    }
-
-
-                    if (gcdesc != null)
-                    {
-                        if (staticPtr != null)
-                        {
-                            *staticPtr++ = seriesSize;
-                            *staticPtr++ = seriesOffset;
-                        }
-                        else
-                        {
-                            seriesSize -= size;
-                            *ptr-- = (void*)seriesOffset;
-                            *ptr-- = (void*)seriesSize;
-                        }
-                    }
-                }
-                else
-                {
-                    i++;
-                }
-            }
-
-            if (gcdesc != null)
-            {
-                if (staticPtr != null)
-                    *(int*)gcdesc = numSeries;
-                else
-                    *(void**)gcdesc = (void*)numSeries;
-            }
-
-            return numSeries;
-        }
-
         public static RuntimeTypeHandle CreateFunctionPointerEEType(uint hashCodeOfNewType, RuntimeTypeHandle returnTypeHandle, RuntimeTypeHandle[] parameterHandles, FunctionPointerType functionPointerType)
         {
             TypeBuilderState state = new TypeBuilderState(functionPointerType);

src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeBuilder.cs

@@ -13,18 +13,6 @@
 
 namespace Internal.Runtime.TypeLoader
 {
-    internal static class LowLevelListExtensions
-    {
-        public static void Expand<T>(this LowLevelList<T> list, int count)
-        {
-            if (list.Capacity < count)
-                list.Capacity = count;
-
-            while (list.Count < count)
-                list.Add(default(T));
-        }
-    }
-
     internal class TypeBuilder
     {
         public TypeBuilder()
@@ -447,24 +435,22 @@ internal void ParseNativeLayoutInfo(TypeBuilderState state, TypeDesc type)
         /// </summary>
         internal unsafe struct GCLayout
         {
-            private LowLevelList<bool> _bitfield;
+            private bool[] _bitfield;
             private unsafe void* _gcdesc;
             private int _size;
-            private bool _isReferenceTypeGCLayout;
 
             public static GCLayout None { get { return default(GCLayout); } }
-            public static GCLayout SingleReference { get; } = new GCLayout(new LowLevelList<bool>(new bool[1] { true }), false);
+            public static GCLayout SingleReference { get; } = new GCLayout([true]);
 
             public bool IsNone { get { return _bitfield == null && _gcdesc == null; } }
 
-            public GCLayout(LowLevelList<bool> bitfield, bool isReferenceTypeGCLayout)
+            public GCLayout(bool[] bitfield)
             {
                 Debug.Assert(bitfield != null);
 
                 _bitfield = bitfield;
                 _gcdesc = null;
                 _size = 0;
-                _isReferenceTypeGCLayout = isReferenceTypeGCLayout;
             }
 
             public GCLayout(RuntimeTypeHandle rtth)
@@ -473,37 +459,27 @@ public GCLayout(RuntimeTypeHandle rtth)
                 Debug.Assert(MethodTable != null);
 
                 _bitfield = null;
-                _isReferenceTypeGCLayout = false; // This field is only used for the LowLevelList<bool> path
                 _gcdesc = MethodTable->ContainsGCPointers ? (void**)MethodTable - 1 : null;
                 _size = (int)MethodTable->BaseSize;
             }
 
             /// <summary>
-            /// Writes this layout to the given bitfield.
+            /// Gets this layout in bitfield array.
             /// </summary>
-            /// <param name="bitfield">The bitfield to write a layout to (may be null, at which
-            /// point it will be created and assigned).</param>
-            /// <param name="offset">The offset at which we need to write the bitfield.</param>
-            public void WriteToBitfield(LowLevelList<bool> bitfield, int offset)
+            /// <returns>The layout in bitfield.</returns>
+            public bool[] AsBitfield()
             {
-                ArgumentNullException.ThrowIfNull(bitfield);
-
-                if (IsNone)
-                    return;
+                // This method should only be called when not none.
+                Debug.Assert(!IsNone);
 
                 // Ensure exactly one of these two are set.
                 Debug.Assert(_gcdesc != null ^ _bitfield != null);
 
-                if (_bitfield != null)
-                    MergeBitfields(bitfield, offset);
-                else
-                    WriteGCDescToBitfield(bitfield, offset);
+                return _bitfield ?? WriteGCDescToBitfield();
             }
 
-            private unsafe void WriteGCDescToBitfield(LowLevelList<bool> bitfield, int offset)
+            private unsafe bool[] WriteGCDescToBitfield()
             {
-                int startIndex = offset / IntPtr.Size;
-
                 void** ptr = (void**)_gcdesc;
                 Debug.Assert(_gcdesc != null);
 
@@ -512,8 +488,8 @@ private unsafe void WriteGCDescToBitfield(LowLevelList<bool> bitfield, int offse
                 Debug.Assert(count >= 0);
 
                 // Ensure capacity for the values we are about to write
-                int capacity = startIndex + _size / IntPtr.Size - 2;
-                bitfield.Expand(capacity);
+                int capacity = _size / IntPtr.Size - 2;
+                bool[] bitfield = new bool[capacity];
 
                 while (count-- >= 0)
                 {
@@ -524,35 +500,10 @@ private unsafe void WriteGCDescToBitfield(LowLevelList<bool> bitfield, int offse
                     Debug.Assert(offs >= 0);
 
                     for (int i = 0; i < len; i++)
-                        bitfield[startIndex + offs + i] = true;
+                        bitfield[offs + i] = true;
                 }
-            }
 
-            private void MergeBitfields(LowLevelList<bool> outputBitfield, int offset)
-            {
-                int startIndex = offset / IntPtr.Size;
-
-                // These routines represent the GC layout after the MethodTable pointer
-                // in an object, but the LowLevelList<bool> bitfield logically contains
-                // the EETypepointer if it is describing a reference type. So, skip the
-                // first value.
-                int itemsToSkip = _isReferenceTypeGCLayout ? 1 : 0;
-
-                // Assert that we only skip a non-reported pointer.
-                Debug.Assert(itemsToSkip == 0 || _bitfield[0] == false);
-
-                // Ensure capacity for the values we are about to write
-                int capacity = startIndex + _bitfield.Count - itemsToSkip;
-                outputBitfield.Expand(capacity);
-
-
-                for (int i = itemsToSkip; i < _bitfield.Count; i++)
-                {
-                    // We should never overwrite a TRUE value in the table.
-                    Debug.Assert(!outputBitfield[startIndex + i - itemsToSkip] || _bitfield[i]);
-
-                    outputBitfield[startIndex + i - itemsToSkip] = _bitfield[i];
-                }
+                return bitfield;
             }
         }
 

src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeBuilderState.cs

@@ -306,9 +306,9 @@ public ushort NumVTableSlots
 
         // Sentinel static to allow us to initialize _instanceLayout to something
         // and then detect that InstanceGCLayout should return null
-        private static LowLevelList<bool> s_emptyLayout = new LowLevelList<bool>();
+        private static readonly bool[] s_emptyLayout = [];
 
-        private LowLevelList<bool> _instanceGCLayout;
+        private bool[] _instanceGCLayout;
 
         /// <summary>
         /// The instance gc layout of a dynamically laid out type.
@@ -324,14 +324,12 @@ public ushort NumVTableSlots
         /// If the type is a valuetype array, this is the layout of the valuetype held in the array if the type has GC reference fields
         /// Otherwise, it is the layout of the fields in the type.
         /// </summary>
-        public LowLevelList<bool> InstanceGCLayout
+        public bool[] InstanceGCLayout
         {
             get
             {
                 if (_instanceGCLayout == null)
                 {
-                    LowLevelList<bool> instanceGCLayout;
-
                     if (TypeBeingBuilt is ArrayType)
                     {
                         if (!IsArrayOfReferenceTypes)
@@ -340,9 +338,7 @@ public LowLevelList<bool> InstanceGCLayout
                             TypeBuilder.GCLayout elementGcLayout = GetFieldGCLayout(arrayType.ElementType);
                             if (!elementGcLayout.IsNone)
                             {
-                                instanceGCLayout = new LowLevelList<bool>();
-                                elementGcLayout.WriteToBitfield(instanceGCLayout, 0);
-                                _instanceGCLayout = instanceGCLayout;
+                                _instanceGCLayout = elementGcLayout.AsBitfield();
                             }
                         }
                         else