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Optimize MemoryMarshal.Cast for same type #100750

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Optimize MemoryMarshal.Cast for same type #100750

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10 changes: 8 additions & 2 deletions src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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
{
Expand All @@ -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))
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This change would be a regression for shared generic code. TFrom and TTo may require generic dictionary lookups.

The identical types should be covered by if (fromSize == toSize) path below. Is there anything preventing the JIT from generating the optimal code for that path?

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>();
Expand Down Expand Up @@ -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
{
Expand All @@ -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>();
Expand Down
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