Drop generic type constraints from Unsafe.BitCast

src/coreclr/jit/importercalls.cpp

@@ -4717,6 +4717,13 @@ GenTree* Compiler::impSRCSUnsafeIntrinsic(NamedIntrinsic          intrinsic,
             ClassLayout*         toLayout  = nullptr;
             var_types            toType    = TypeHandleToVarType(toTypeHnd, &toLayout);
 
+            if (fromType == TYP_REF || info.compCompHnd->getBoxHelper(fromTypeHnd) == CORINFO_HELP_BOX_NULLABLE ||
+                toType == TYP_REF || info.compCompHnd->getBoxHelper(toTypeHnd) == CORINFO_HELP_BOX_NULLABLE)
+            {
+                // Fallback to the software implementation to throw when the types don't fit "where T : struct"
+                return nullptr;
+            }
+
             unsigned fromSize = fromLayout != nullptr ? fromLayout->GetSize() : genTypeSize(fromType);
             unsigned toSize   = toLayout != nullptr ? toLayout->GetSize() : genTypeSize(toType);
 
@@ -4729,8 +4736,6 @@ GenTree* Compiler::impSRCSUnsafeIntrinsic(NamedIntrinsic          intrinsic,
                 return nullptr;
             }
 
-            assert((fromType != TYP_REF) && (toType != TYP_REF));
-
             GenTree* op1 = impPopStack().val;
 
             op1 = impImplicitR4orR8Cast(op1, fromType);

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.cs

@@ -129,6 +129,9 @@ internal static bool IsPrimitiveType(this CorElementType et)
 
         [Intrinsic]
         internal static bool IsKnownConstant(char t) => false;
+
+        [Intrinsic]
+        internal static bool IsKnownConstant<T>(T t) where T : struct => false;
 #pragma warning restore IDE0060
     }
 }

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/Unsafe.cs

@@ -242,15 +242,14 @@ public static bool AreSame<T>([AllowNull] ref readonly T left, [AllowNull] ref r
         /// <summary>
         /// Reinterprets the given value of type <typeparamref name="TFrom" /> as a value of type <typeparamref name="TTo" />.
         /// </summary>
-        /// <exception cref="NotSupportedException">The size of <typeparamref name="TFrom" /> and <typeparamref name="TTo" /> are not the same.</exception>
+        /// <exception cref="NotSupportedException">The sizes of <typeparamref name="TFrom" /> and <typeparamref name="TTo" /> are not the same
+        /// or the type parameters are not <see langword="struct"/>s.</exception>
         [Intrinsic]
         [NonVersionable]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static TTo BitCast<TFrom, TTo>(TFrom source)
-            where TFrom : struct
-            where TTo : struct
         {
-            if (sizeof(TFrom) != sizeof(TTo))
+            if (sizeof(TFrom) != sizeof(TTo) || default(TFrom) is null || default(TTo) is null)
             {
                 ThrowHelper.ThrowNotSupportedException();
             }

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Packed.cs

@@ -27,7 +27,7 @@ public static unsafe bool CanUsePackedIndexOf<T>(T value)
             Debug.Assert(RuntimeHelpers.IsBitwiseEquatable<T>());
             Debug.Assert(sizeof(T) == sizeof(ushort));
 
-            return *(ushort*)&value - 1u < 254u;
+            return Unsafe.BitCast<T, ushort>(value) - 1u < 254u;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs

@@ -5,7 +5,6 @@
 using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.CompilerServices;
-using System.Runtime.InteropServices;
 using System.Runtime.Intrinsics;
 
 #pragma warning disable 8500 // sizeof of managed types
@@ -225,7 +224,7 @@ public static int IndexOf<T>(ref T searchSpace, int searchSpaceLength, ref T val
         }
 
         // Adapted from IndexOf(...)
-        public static unsafe bool Contains<T>(ref T searchSpace, T value, int length) where T : IEquatable<T>?
+        public static bool Contains<T>(ref T searchSpace, T value, int length) where T : IEquatable<T>?
         {
             Debug.Assert(length >= 0);
 
@@ -297,7 +296,7 @@ public static unsafe bool Contains<T>(ref T searchSpace, T value, int length) wh
             return true;
         }
 
-        public static unsafe int IndexOf<T>(ref T searchSpace, T value, int length) where T : IEquatable<T>?
+        public static int IndexOf<T>(ref T searchSpace, T value, int length) where T : IEquatable<T>?
         {
             Debug.Assert(length >= 0);
 
@@ -1304,11 +1303,11 @@ public static int SequenceCompareTo<T>(ref T first, int firstLength, ref T secon
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        internal static unsafe bool ContainsValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
+        internal static bool ContainsValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
         {
             if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(T) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value))
             {
-                return PackedSpanHelpers.Contains(ref Unsafe.As<T, short>(ref searchSpace), *(short*)&value, length);
+                return PackedSpanHelpers.Contains(ref Unsafe.As<T, short>(ref searchSpace), Unsafe.BitCast<T, short>(value), length);
             }
 
             return NonPackedContainsValueType(ref searchSpace, value, length);
@@ -1478,15 +1477,15 @@ internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value, int
             => IndexOfValueType<T, Negate<T>>(ref searchSpace, value, length);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static unsafe int IndexOfValueType<TValue, TNegator>(ref TValue searchSpace, TValue value, int length)
+        private static int IndexOfValueType<TValue, TNegator>(ref TValue searchSpace, TValue value, int length)
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
             if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value))
             {
                 return typeof(TNegator) == typeof(DontNegate<short>)
-                    ? PackedSpanHelpers.IndexOf(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value, length)
-                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value, length);
+                    ? PackedSpanHelpers.IndexOf(ref Unsafe.As<TValue, char>(ref searchSpace), Unsafe.BitCast<TValue, char>(value), length)
+                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), Unsafe.BitCast<TValue, char>(value), length);
             }
 
             return NonPackedIndexOfValueType<TValue, TNegator>(ref searchSpace, value, length);
@@ -1665,24 +1664,33 @@ internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T
             => IndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, length);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static unsafe int IndexOfAnyValueType<TValue, TNegator>(ref TValue searchSpace, TValue value0, TValue value1, int length)
+        private static int IndexOfAnyValueType<TValue, TNegator>(ref TValue searchSpace, TValue value0, TValue value1, int length)
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
             if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1))
             {
-                if ((*(char*)&value0 ^ *(char*)&value1) == 0x20)
+                char char0 = Unsafe.BitCast<TValue, char>(value0);
+                char char1 = Unsafe.BitCast<TValue, char>(value1);
+
+                if (RuntimeHelpers.IsKnownConstant(value0) && RuntimeHelpers.IsKnownConstant(value1))
                 {
-                    char lowerCase = (char)Math.Max(*(char*)&value0, *(char*)&value1);
+                    // If the values differ only in the 0x20 bit, we can optimize the search by reducing the number of comparisons.
+                    // This optimization only applies to a small subset of values and the throughput difference is not too significant.
+                    // We avoid introducing per-call overhead for non-constant values by guarding this optimization behind RuntimeHelpers.IsKnownConstant.
+                    if ((char0 ^ char1) == 0x20)
+                    {
+                        char lowerCase = (char)Math.Max(char0, char1);
 
-                    return typeof(TNegator) == typeof(DontNegate<short>)
-                        ? PackedSpanHelpers.IndexOfAnyIgnoreCase(ref Unsafe.As<TValue, char>(ref searchSpace), lowerCase, length)
-                        : PackedSpanHelpers.IndexOfAnyExceptIgnoreCase(ref Unsafe.As<TValue, char>(ref searchSpace), lowerCase, length);
+                        return typeof(TNegator) == typeof(DontNegate<short>)
+                            ? PackedSpanHelpers.IndexOfAnyIgnoreCase(ref Unsafe.As<TValue, char>(ref searchSpace), lowerCase, length)
+                            : PackedSpanHelpers.IndexOfAnyExceptIgnoreCase(ref Unsafe.As<TValue, char>(ref searchSpace), lowerCase, length);
+                    }
                 }
 
                 return typeof(TNegator) == typeof(DontNegate<short>)
-                    ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, length)
-                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, length);
+                    ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), char0, char1, length)
+                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), char0, char1, length);
             }
 
             return NonPackedIndexOfAnyValueType<TValue, TNegator>(ref searchSpace, value0, value1, length);
@@ -1882,15 +1890,15 @@ internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T
             => IndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, value2, length);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static unsafe int IndexOfAnyValueType<TValue, TNegator>(ref TValue searchSpace, TValue value0, TValue value1, TValue value2, int length)
+        private static int IndexOfAnyValueType<TValue, TNegator>(ref TValue searchSpace, TValue value0, TValue value1, TValue value2, int length)
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
             if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1) && PackedSpanHelpers.CanUsePackedIndexOf(value2))
             {
                 return typeof(TNegator) == typeof(DontNegate<short>)
-                    ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, *(char*)&value2, length)
-                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, *(char*)&value2, length);
+                    ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), Unsafe.BitCast<TValue, char>(value0), Unsafe.BitCast<TValue, char>(value1), Unsafe.BitCast<TValue, char>(value2), length)
+                    : PackedSpanHelpers.IndexOfAnyExcept(ref Unsafe.As<TValue, char>(ref searchSpace), Unsafe.BitCast<TValue, char>(value0), Unsafe.BitCast<TValue, char>(value1), Unsafe.BitCast<TValue, char>(value2), length);
             }
 
             return NonPackedIndexOfAnyValueType<TValue, TNegator>(ref searchSpace, value0, value1, value2, length);
@@ -3466,15 +3474,15 @@ internal static int IndexOfAnyExceptInRangeUnsignedNumber<T>(ref T searchSpace,
             IndexOfAnyInRangeUnsignedNumber<T, Negate<T>>(ref searchSpace, lowInclusive, highInclusive, length);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static unsafe int IndexOfAnyInRangeUnsignedNumber<T, TNegator>(ref T searchSpace, T lowInclusive, T highInclusive, int length)
+        private static int IndexOfAnyInRangeUnsignedNumber<T, TNegator>(ref T searchSpace, T lowInclusive, T highInclusive, int length)
             where T : struct, IUnsignedNumber<T>, IComparisonOperators<T, T, bool>
             where TNegator : struct, INegator<T>
         {
             if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(T) == typeof(ushort) && PackedSpanHelpers.CanUsePackedIndexOf(lowInclusive) && PackedSpanHelpers.CanUsePackedIndexOf(highInclusive) && highInclusive >= lowInclusive)
             {
                 ref char charSearchSpace = ref Unsafe.As<T, char>(ref searchSpace);
-                char charLowInclusive = *(char*)&lowInclusive;
-                char charRange = (char)(*(char*)&highInclusive - charLowInclusive);
+                char charLowInclusive = Unsafe.BitCast<T, char>(lowInclusive);
+                char charRange = (char)(Unsafe.BitCast<T, char>(highInclusive) - charLowInclusive);
 
                 return typeof(TNegator) == typeof(DontNegate<ushort>)
                     ? PackedSpanHelpers.IndexOfAnyInRange(ref charSearchSpace, charLowInclusive, charRange, length)

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -13300,7 +13300,7 @@ public static partial class Unsafe
         [return: System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute("o")]
         public static T? As<T>(object? o) where T : class? { throw null; }
         public static ref TTo As<TFrom, TTo>(ref TFrom source) { throw null; }
-        public static TTo BitCast<TFrom, TTo>(TFrom source) where TFrom : struct where TTo : struct { throw null; }
+        public static TTo BitCast<TFrom, TTo>(TFrom source) { throw null; }
         public static System.IntPtr ByteOffset<T>([System.Diagnostics.CodeAnalysis.AllowNull] ref readonly T origin, [System.Diagnostics.CodeAnalysis.AllowNull] ref readonly T target) { throw null; }
         [System.CLSCompliantAttribute(false)]
         public static void CopyBlock(ref byte destination, ref readonly byte source, uint byteCount) { }

src/libraries/System.Runtime/tests/System.Runtime.CompilerServices.Unsafe.Tests/UnsafeTests.cs

@@ -1166,6 +1166,18 @@ public static unsafe void BitCast()
             Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<int, long>(5));
             Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<long, int>(5));
 
+            // Conversion to/from a class should fail
+
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<string, long>(string.Empty));
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<long, string>(42));
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<string, string>(string.Empty));
+
+            // Conversion to/from nullable value types should fail
+
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<int?, long>(42));
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<long, int?>(42));
+            Assert.Throws<NotSupportedException>(() => Unsafe.BitCast<int?, int?>(42));
+
             // Conversion between floating-point and same sized integral should succeed
 
             Assert.Equal(0x8000_0000u, Unsafe.BitCast<float, uint>(-0.0f));