Optimize Memory<T> to ReadOnlyMemory<T> cast (dotnet/coreclr#14309)

They have the same layout; we can just use Unsafe.As.  In a microbenchmark that just repeatedly reads from a `Memory<byte>` field and writes it to a `ReadOnlyMemory<byte>` field, this doubles throughput.

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: stephentoub

src/Common/src/CoreLib/System/Memory.cs

@@ -16,6 +16,9 @@ namespace System
     [DebuggerTypeProxy(typeof(MemoryDebugView<>))]
     public struct Memory<T>
     {
+        // NOTE: With the current implementation, Memory<T> and ReadOnlyMemory<T> must have the same layout,
+        // as code uses Unsafe.As to cast between them.
+
         // The highest order bit of _index is used to discern whether _arrayOrOwnedMemory is an array or an owned memory
         // if (_index >> 31) == 1, object _arrayOrOwnedMemory is an OwnedMemory<T>
         // else, object _arrayOrOwnedMemory is a T[]
@@ -100,13 +103,8 @@ internal Memory(OwnedMemory<T> owner, int index, int length)
         /// <summary>
         /// Defines an implicit conversion of a <see cref="Memory{T}"/> to a <see cref="ReadOnlyMemory{T}"/>
         /// </summary>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static implicit operator ReadOnlyMemory<T>(Memory<T> memory)
-        {
-            if (memory._index < 0)
-                return new ReadOnlyMemory<T>((OwnedMemory<T>)memory._arrayOrOwnedMemory, memory._index & RemoveOwnedFlagBitMask, memory._length);
-            return new ReadOnlyMemory<T>((T[])memory._arrayOrOwnedMemory, memory._index, memory._length);
-        }
+        public static implicit operator ReadOnlyMemory<T>(Memory<T> memory) =>
+            Unsafe.As<Memory<T>, ReadOnlyMemory<T>>(ref memory);
 
         //Debugger Display = {T[length]}
         private string DebuggerDisplay => string.Format("{{{0}[{1}]}}", typeof(T).Name, _length);

src/Common/src/CoreLib/System/ReadOnlyMemory.cs

@@ -16,6 +16,9 @@ namespace System
     [DebuggerTypeProxy(typeof(MemoryDebugView<>))]
     public struct ReadOnlyMemory<T>
     {
+        // NOTE: With the current implementation, Memory<T> and ReadOnlyMemory<T> must have the same layout,
+        // as code uses Unsafe.As to cast between them.
+
         // The highest order bit of _index is used to discern whether _arrayOrOwnedMemory is an array or an owned memory
         // if (_index >> 31) == 1, object _arrayOrOwnedMemory is an OwnedMemory<T>
         // else, object _arrayOrOwnedMemory is a T[]
@@ -273,4 +276,4 @@ private static int CombineHashCodes(int h1, int h2, int h3)
         }
 
     }
-}
+}