Use MemoryMarshal.Cast in a few places (dotnet#99835)

* Use MemoryMarshal.Cast in a few places

* Removed special-casing

* Fix build

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/TensorPrimitives.Helpers.cs

@@ -6,6 +6,8 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
+#pragma warning disable CS8500 // This takes the address of, gets the size of, or declares a pointer to a managed type
+
 namespace System.Numerics.Tensors
 {
     /// <summary>Performs primitive tensor operations over spans of memory.</summary>
@@ -23,7 +25,31 @@ private static void ValidateInputOutputSpanNonOverlapping<T>(ReadOnlySpan<T> inp
         }
 
         /// <summary>Throws an <see cref="OverflowException"/> for trying to negate the minimum value of a two-complement value.</summary>
-        internal static void ThrowNegateTwosCompOverflow() => throw new OverflowException(SR.Overflow_NegateTwosCompNum);
+        private static void ThrowNegateTwosCompOverflow() => throw new OverflowException(SR.Overflow_NegateTwosCompNum);
+
+        /// <summary>Creates a span of <typeparamref name="TTo"/> from a <typeparamref name="TFrom"/> when they're the same type.</summary>
+        /// <remarks>
+        /// This is the same as MemoryMarshal.Cast, except only to be used when TFrom and TTo are the same type or effectively
+        /// the same type (e.g. int and nint in a 32-bit process). MemoryMarshal.Cast can't currently be used as it's
+        /// TFrom/TTo are constrained to be value types.
+        /// </remarks>
+        private static unsafe Span<TTo> Rename<TFrom, TTo>(Span<TFrom> span)
+        {
+            Debug.Assert(sizeof(TFrom) == sizeof(TTo));
+            return *(Span<TTo>*)(&span);
+        }
+
+        /// <summary>Creates a span of <typeparamref name="TTo"/> from a <typeparamref name="TFrom"/> when they're the same type.</summary>
+        /// <remarks>
+        /// This is the same as MemoryMarshal.Cast, except only to be used when TFrom and TTo are the same type or effectively
+        /// the same type (e.g. int and nint in a 32-bit process). MemoryMarshal.Cast can't currently be used as it's
+        /// TFrom/TTo are constrained to be value types.
+        /// </remarks>
+        private static unsafe ReadOnlySpan<TTo> Rename<TFrom, TTo>(ReadOnlySpan<TFrom> span)
+        {
+            Debug.Assert(sizeof(TFrom) == sizeof(TTo));
+            return *(ReadOnlySpan<TTo>*)(&span);
+        }
 
         /// <summary>Mask used to handle alignment elements before vectorized handling of the input.</summary>
         /// <remarks>

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Common/TensorPrimitives.IUnaryOperator.cs

@@ -1247,12 +1247,5 @@ static void VectorizedSmall8(ref TInput xRef, ref TOutput dRef, nuint remainder)
                 }
             }
         }
-
-        /// <summary>Creates a span of <typeparamref name="TTo"/> from a <typeparamref name="TTo"/> when they're the same type.</summary>
-        private static unsafe Span<TTo> Rename<TFrom, TTo>(Span<TFrom> span)
-        {
-            Debug.Assert(sizeof(TFrom) == sizeof(TTo));
-            return MemoryMarshal.CreateSpan(ref Unsafe.As<TFrom, TTo>(ref MemoryMarshal.GetReference(span)), span.Length);
-        }
     }
 }

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.ConvertHelpers.cs

@@ -643,13 +643,6 @@ static Vector512<uint> SingleToHalfAsWidenedUInt32(Vector512<float> value)
             }
         }
 
-        /// <summary>Creates a span of <typeparamref name="TTo"/> from a <typeparamref name="TTo"/> when they're the same type.</summary>
-        private static unsafe ReadOnlySpan<TTo> Rename<TFrom, TTo>(ReadOnlySpan<TFrom> span)
-        {
-            Debug.Assert(sizeof(TFrom) == sizeof(TTo));
-            return MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TFrom, TTo>(ref MemoryMarshal.GetReference(span)), span.Length);
-        }
-
         /// <summary>Gets whether <typeparamref name="T"/> is <see cref="uint"/> or <see cref="nuint"/> if in a 32-bit process.</summary>
         private static bool IsUInt32Like<T>() => typeof(T) == typeof(uint) || (IntPtr.Size == 4 && typeof(T) == typeof(nuint));
 

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.Round.cs

@@ -106,13 +106,13 @@ public static void Round<T>(ReadOnlySpan<T> x, int digits, MidpointRounding mode
             if (typeof(T) == typeof(float))
             {
                 ReadOnlySpan<float> roundPower10Single = [1e0f, 1e1f, 1e2f, 1e3f, 1e4f, 1e5f, 1e6f];
-                roundPower10 = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<float, T>(ref MemoryMarshal.GetReference(roundPower10Single)), roundPower10Single.Length);
+                roundPower10 = Rename<float, T>(roundPower10Single);
             }
             else if (typeof(T) == typeof(double))
             {
                 Debug.Assert(typeof(T) == typeof(double));
                 ReadOnlySpan<double> roundPower10Double = [1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11, 1e12, 1e13, 1e14, 1e15];
-                roundPower10 = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<double, T>(ref MemoryMarshal.GetReference(roundPower10Double)), roundPower10Double.Length);
+                roundPower10 = Rename<double, T>(roundPower10Double);
             }
             else
             {

src/libraries/System.Private.CoreLib/src/System/Number.Parsing.cs

@@ -883,16 +883,16 @@ internal static bool SpanStartsWith<TChar>(ReadOnlySpan<TChar> span, ReadOnlySpa
         {
             if (typeof(TChar) == typeof(char))
             {
-                ReadOnlySpan<char> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, char>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<char> typedValue = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, char>(ref MemoryMarshal.GetReference(value)), value.Length);
+                ReadOnlySpan<char> typedSpan = MemoryMarshal.Cast<TChar, char>(span);
+                ReadOnlySpan<char> typedValue = MemoryMarshal.Cast<TChar, char>(value);
                 return typedSpan.StartsWith(typedValue, comparisonType);
             }
             else
             {
                 Debug.Assert(typeof(TChar) == typeof(byte));
 
-                ReadOnlySpan<byte> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, byte>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<byte> typedValue = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, byte>(ref MemoryMarshal.GetReference(value)), value.Length);
+                ReadOnlySpan<byte> typedSpan = MemoryMarshal.Cast<TChar, byte>(span);
+                ReadOnlySpan<byte> typedValue = MemoryMarshal.Cast<TChar, byte>(value);
                 return typedSpan.StartsWithUtf8(typedValue, comparisonType);
             }
         }
@@ -903,17 +903,13 @@ internal static ReadOnlySpan<TChar> SpanTrim<TChar>(ReadOnlySpan<TChar> span)
         {
             if (typeof(TChar) == typeof(char))
             {
-                ReadOnlySpan<char> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, char>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<char> result = typedSpan.Trim();
-                return MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<char, TChar>(ref MemoryMarshal.GetReference(result)), result.Length);
+                return MemoryMarshal.Cast<char, TChar>(MemoryMarshal.Cast<TChar, char>(span).Trim());
             }
             else
             {
                 Debug.Assert(typeof(TChar) == typeof(byte));
 
-                ReadOnlySpan<byte> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, byte>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<byte> result = typedSpan.TrimUtf8();
-                return MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<byte, TChar>(ref MemoryMarshal.GetReference(result)), result.Length);
+                return MemoryMarshal.Cast<byte, TChar>(MemoryMarshal.Cast<TChar, byte>(span).TrimUtf8());
             }
         }
 
@@ -923,16 +919,16 @@ internal static bool SpanEqualsOrdinalIgnoreCase<TChar>(ReadOnlySpan<TChar> span
         {
             if (typeof(TChar) == typeof(char))
             {
-                ReadOnlySpan<char> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, char>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<char> typedValue = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, char>(ref MemoryMarshal.GetReference(value)), value.Length);
+                ReadOnlySpan<char> typedSpan = MemoryMarshal.Cast<TChar, char>(span);
+                ReadOnlySpan<char> typedValue = MemoryMarshal.Cast<TChar, char>(value);
                 return typedSpan.EqualsOrdinalIgnoreCase(typedValue);
             }
             else
             {
                 Debug.Assert(typeof(TChar) == typeof(byte));
 
-                ReadOnlySpan<byte> typedSpan = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, byte>(ref MemoryMarshal.GetReference(span)), span.Length);
-                ReadOnlySpan<byte> typedValue = MemoryMarshal.CreateReadOnlySpan(ref Unsafe.As<TChar, byte>(ref MemoryMarshal.GetReference(value)), value.Length);
+                ReadOnlySpan<byte> typedSpan = MemoryMarshal.Cast<TChar, byte>(span);
+                ReadOnlySpan<byte> typedValue = MemoryMarshal.Cast<TChar, byte>(value);
                 return typedSpan.EqualsOrdinalIgnoreCaseUtf8(typedValue);
             }
         }