From 379d0eca364b0d132851b0762280e290d0531c5e Mon Sep 17 00:00:00 2001
From: xtqqczze <45661989+xtqqczze@users.noreply.github.com>
Date: Sat, 6 Apr 2024 12:55:59 +0100
Subject: [PATCH] Optimize `MemoryMarshal.Cast` for same type

---
 .../System/Runtime/InteropServices/MemoryMarshal.cs    | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
index 0b0da448e4eba..6f3dd4c01b73c 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
@@ -113,7 +113,7 @@ public static Memory<T> AsMemory<T>(ReadOnlyMemory<T> memory) =>
         /// Thrown when <typeparamref name="TFrom"/> or <typeparamref name="TTo"/> contains pointers.
         /// </exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Span<TTo> Cast<TFrom, TTo>(Span<TFrom> span)
+        public static unsafe Span<TTo> Cast<TFrom, TTo>(Span<TFrom> span)
             where TFrom : struct
             where TTo : struct
         {
@@ -122,6 +122,9 @@ public static Span<TTo> Cast<TFrom, TTo>(Span<TFrom> span)
             if (RuntimeHelpers.IsReferenceOrContainsReferences<TTo>())
                 ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TTo));
 
+            if (typeof(TFrom) == typeof(TTo))
+                return *(Span<TTo>*)&span;
+
             // Use unsigned integers - unsigned division by constant (especially by power of 2)
             // and checked casts are faster and smaller.
             uint fromSize = (uint)Unsafe.SizeOf<TFrom>();
@@ -168,7 +171,7 @@ ref Unsafe.As<TFrom, TTo>(ref span._reference),
         /// Thrown when <typeparamref name="TFrom"/> or <typeparamref name="TTo"/> contains pointers.
         /// </exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span)
+        public static unsafe ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span)
             where TFrom : struct
             where TTo : struct
         {
@@ -177,6 +180,9 @@ public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span)
             if (RuntimeHelpers.IsReferenceOrContainsReferences<TTo>())
                 ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TTo));
 
+            if (typeof(TFrom) == typeof(TTo))
+                return *(ReadOnlySpan<TTo>*)&span;
+
             // Use unsigned integers - unsigned division by constant (especially by power of 2)
             // and checked casts are faster and smaller.
             uint fromSize = (uint)Unsafe.SizeOf<TFrom>();