Move Buffer constants to platform specific files

src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems

@@ -1037,6 +1037,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Internal\IO\File.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Microsoft\Win32\SafeHandles\SafeFileHandle.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Microsoft\Win32\SafeHandles\SafeFindHandle.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffer.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Environment.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\DebugProvider.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\CalendarData.Windows.cs" />
@@ -1189,6 +1190,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Interop\Unix\System.Native\Interop.Write.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Internal\IO\File.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Microsoft\Win32\SafeHandles\SafeFileHandle.Unix.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffer.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DateTime.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\DebugProvider.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Environment.Unix.cs" />

src/System.Private.CoreLib/shared/System/Buffer.Unix.cs

@@ -0,0 +1,20 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System
+{
+    public static partial class Buffer
+    {
+#if ARM64
+            // Managed code is currently faster than glibc unoptimized memmove
+            // TODO-ARM64-UNIX-OPT revisit when glibc optimized memmove is in Linux distros
+            // https://github.com/dotnet/coreclr/issues/13844
+            const nuint MemmoveNativeThreshold = ulong.MaxValue;
+#elif ARM
+            const nuint MemmoveNativeThreshold = 512;
+#else
+            const nuint MemmoveNativeThreshold = 2048;
+#endif
+    }
+}

src/System.Private.CoreLib/shared/System/Buffer.Windows.cs

@@ -0,0 +1,17 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System
+{
+    public static partial class Buffer
+    {
+#if ARM64 || ARM
+        // Determine optimal value for Windows.
+        // https://github.com/dotnet/coreclr/issues/13843
+        const nuint MemmoveNativeThreshold = ulong.MaxValue;
+#else
+        const nuint MemmoveNativeThreshold = 2048;
+#endif
+    }
+}

src/System.Private.CoreLib/shared/System/Buffer.cs

@@ -142,23 +142,6 @@ internal static unsafe void Memcpy(byte* pDest, int destIndex, byte[] src, int s
         // This method has different signature for x64 and other platforms and is done for performance reasons.
         internal static unsafe void Memmove(byte* dest, byte* src, nuint len)
         {
-#if AMD64 || (BIT32 && !ARM)
-            const nuint CopyThreshold = 2048;
-#elif ARM64
-#if PLATFORM_WINDOWS
-            // Determined optimal value for Windows.
-            // https://github.com/dotnet/coreclr/issues/13843
-            const nuint CopyThreshold = ulong.MaxValue;
-#else // PLATFORM_WINDOWS
-            // Managed code is currently faster than glibc unoptimized memmove
-            // TODO-ARM64-UNIX-OPT revisit when glibc optimized memmove is in Linux distros
-            // https://github.com/dotnet/coreclr/issues/13844
-            const nuint CopyThreshold = ulong.MaxValue;
-#endif // PLATFORM_WINDOWS
-#else
-            const nuint CopyThreshold = 512;
-#endif // AMD64 || (BIT32 && !ARM)
-
             // P/Invoke into the native version when the buffers are overlapping.
             if (((nuint)dest - (nuint)src < len) || ((nuint)src - (nuint)dest < len))
             {
@@ -260,14 +243,14 @@ internal static unsafe void Memmove(byte* dest, byte* src, nuint len)
 
             MCPY05:
             // PInvoke to the native version when the copy length exceeds the threshold.
-            if (len > CopyThreshold)
+            if (len > MemmoveNativeThreshold)
             {
                 goto PInvoke;
             }
 
             // Copy 64-bytes at a time until the remainder is less than 64.
             // If remainder is greater than 16 bytes, then jump to MCPY00. Otherwise, unconditionally copy the last 16 bytes and return.
-            Debug.Assert(len > 64 && len <= CopyThreshold);
+            Debug.Assert(len > 64 && len <= MemmoveNativeThreshold);
             nuint n = len >> 6;
 
         MCPY06:
@@ -356,23 +339,6 @@ ref Unsafe.As<T, byte>(ref source),
         // This method has different signature for x64 and other platforms and is done for performance reasons.
         private static void Memmove(ref byte dest, ref byte src, nuint len)
         {
-#if AMD64 || (BIT32 && !ARM)
-            const nuint CopyThreshold = 2048;
-#elif ARM64
-#if PLATFORM_WINDOWS
-            // Determined optimal value for Windows.
-            // https://github.com/dotnet/coreclr/issues/13843
-            const nuint CopyThreshold = ulong.MaxValue;
-#else // PLATFORM_WINDOWS
-            // Managed code is currently faster than glibc unoptimized memmove
-            // TODO-ARM64-UNIX-OPT revisit when glibc optimized memmove is in Linux distros
-            // https://github.com/dotnet/coreclr/issues/13844
-            const nuint CopyThreshold = ulong.MaxValue;
-#endif // PLATFORM_WINDOWS
-#else
-            const nuint CopyThreshold = 512;
-#endif // AMD64 || (BIT32 && !ARM)
-
             // P/Invoke into the native version when the buffers are overlapping.
             if (((nuint)Unsafe.ByteOffset(ref src, ref dest) < len) || ((nuint)Unsafe.ByteOffset(ref dest, ref src) < len))
             {
@@ -484,14 +450,14 @@ private static void Memmove(ref byte dest, ref byte src, nuint len)
 
         MCPY05:
             // PInvoke to the native version when the copy length exceeds the threshold.
-            if (len > CopyThreshold)
+            if (len > MemmoveNativeThreshold)
             {
                 goto PInvoke;
             }
 
             // Copy 64-bytes at a time until the remainder is less than 64.
             // If remainder is greater than 16 bytes, then jump to MCPY00. Otherwise, unconditionally copy the last 16 bytes and return.
-            Debug.Assert(len > 64 && len <= CopyThreshold);
+            Debug.Assert(len > 64 && len <= MemmoveNativeThreshold);
             nuint n = len >> 6;
 
         MCPY06: