Optimize Array.Clear using SpanHelpers (#18101)

Reimplement most of Array.Clear in managed code using Span Clear helpers.

Fixes dotnet/corefx#29848

Author: jkotas

src/classlibnative/bcltype/arraynative.cpp

@@ -1070,48 +1070,25 @@ FCIMPL6(void, ArrayNative::ArrayCopy, ArrayBase* m_pSrc, INT32 m_iSrcIndex, Arra
 FCIMPLEND
 
 
-FCIMPL3(void, ArrayNative::ArrayClear, ArrayBase* pArrayUNSAFE, INT32 iIndex, INT32 iLength)
+FCIMPL5(void*, ArrayNative::GetRawArrayGeometry, ArrayBase* pArray, UINT32* pNumComponents, UINT32* pElementSize, INT32* pLowerBound, CLR_BOOL* pContainsGCPointers)
 {
-    FCALL_CONTRACT;
-
-    BASEARRAYREF pArray = (BASEARRAYREF)pArrayUNSAFE;
-
-    HELPER_METHOD_FRAME_BEGIN_1(pArray);
-
-    // cannot pass null for array
-    if (pArray == NULL)
-        COMPlusThrowArgumentNull(W("array"), W("ArgumentNull_Array"));
-
-    // array must be an array
-    _ASSERTE(pArray->GetMethodTable()->IsArray());
-
-    // array bounds checking
-    int lb = pArray->GetLowerBoundsPtr()[0];
-    if (iIndex < lb || (iIndex - lb) < 0 || iLength < 0)
-        COMPlusThrow(kIndexOutOfRangeException);
+   VALIDATEOBJECT(pArray);
 
-    if ((iIndex - lb) > (int)pArray->GetNumComponents() - iLength)
-        COMPlusThrow(kIndexOutOfRangeException);
+   _ASSERTE(pArray != NULL);
 
-    if (iLength > 0)
-    {
-        char* array = (char*)pArray->GetDataPtr();
-
-        SIZE_T size = pArray->GetComponentSize();
-        _ASSERTE(size >= 1);
+    MethodTable *pMT = pArray->GetMethodTable();
 
-        ZeroMemoryInGCHeap(array + (iIndex - lb) * size, iLength * size);
-    }
+    *pNumComponents = pArray->GetNumComponents();
+    *pElementSize = pMT->RawGetComponentSize();
+    *pLowerBound = pArray->GetLowerBoundsPtr()[0];
+    *pContainsGCPointers = !!pMT->ContainsPointers();
 
-    HELPER_METHOD_FRAME_END();
+    return (BYTE*)pArray + ArrayBase::GetDataPtrOffset(pMT);
 }
 FCIMPLEND
 
 
 
-
-
-
 // Check we're allowed to create an array with the given element type.
 void ArrayNative::CheckElementType(TypeHandle elementType)
 {

src/classlibnative/bcltype/arraynative.h

@@ -35,7 +35,8 @@ class ArrayNative
     static FCDECL1(void, Initialize, ArrayBase* pArray);
 
     static FCDECL6(void, ArrayCopy, ArrayBase* m_pSrc, INT32 m_iSrcIndex, ArrayBase* m_pDst, INT32 m_iDstIndex, INT32 m_iLength, CLR_BOOL reliable);
-    static FCDECL3(void, ArrayClear, ArrayBase* pArrayUNSAFE, INT32 iIndex, INT32 iLength);
+
+    static FCDECL5(void*, GetRawArrayGeometry, ArrayBase* pArray, UINT32* pNumComponents, UINT32* pElementSize, INT32* pLowerBound, CLR_BOOL* pContainsGCPointers);
 
     // This method will create a new array of type type, with zero lower
     // bounds and rank.

src/mscorlib/src/System/Array.cs

@@ -21,6 +21,12 @@
 using System.Security;
 using Internal.Runtime.CompilerServices;
 
+#if BIT64
+using nuint = System.UInt64;
+#else
+using nuint = System.UInt32;
+#endif
+
 namespace System
 {
     // Note that we make a T[] (single-dimensional w/ zero as the lower bound) implement both 
@@ -265,8 +271,29 @@ public static void Copy(Array sourceArray, long sourceIndex, Array destinationAr
         // Sets length elements in array to 0 (or null for Object arrays), starting
         // at index.
         //
-        [MethodImplAttribute(MethodImplOptions.InternalCall)]
-        public static extern void Clear(Array array, int index, int length);
+        public static unsafe void Clear(Array array, int index, int length)
+        {
+            if (array == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+
+            ref byte p = ref GetRawArrayGeometry(array, out uint numComponents, out uint elementSize, out int lowerBound, out bool containsGCPointers);
+
+            int offset = index - lowerBound;
+
+            if (index < lowerBound || offset < 0 || length < 0 || (uint)(offset + length) > numComponents)
+                ThrowHelper.ThrowIndexOutOfRangeException();
+
+            ref byte ptr = ref Unsafe.AddByteOffset(ref p, (uint)offset * (nuint)elementSize);
+            nuint byteLength = (uint)length * (nuint)elementSize;
+
+            if (containsGCPointers)
+                SpanHelpers.ClearWithReferences(ref Unsafe.As<byte, IntPtr>(ref ptr), byteLength / (uint)sizeof(IntPtr));
+            else
+                SpanHelpers.ClearWithoutReferences(ref ptr, byteLength);
+        }
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern ref byte GetRawArrayGeometry(Array array, out uint numComponents, out uint elementSize, out int lowerBound, out bool containsGCPointers);
 
         // The various Get values...
         public unsafe Object GetValue(params int[] indices)

src/vm/ecalllist.h

@@ -810,7 +810,7 @@ FCFuncStart(gArrayFuncs)
     FCFuncElement("GetDataPtrOffsetInternal", ArrayNative::GetDataPtrOffsetInternal)
     FCFuncElement("Initialize", ArrayNative::Initialize)
     FCFuncElement("Copy", ArrayNative::ArrayCopy)
-    FCFuncElement("Clear", ArrayNative::ArrayClear)
+    FCFuncElement("GetRawArrayGeometry", ArrayNative::GetRawArrayGeometry)
     FCFuncElement("InternalCreate", ArrayNative::CreateInstance)
     FCFuncElement("InternalGetReference", ArrayNative::GetReference)
     FCFuncElement("InternalSetValue", ArrayNative::SetValue)