Array -> Memory -> Span overly defensive?

[benaadams]

Creating a {ReadOnly}Memory from an array does lots of checks; which is correct
https://github.com/dotnet/coreclr/blob/9d1e7dd8eb744b14eb4a90b401e854d7a5f79fff/src/System.Private.CoreLib/shared/System/Memory.cs#L96-L113

Slicing a {ReadOnly}Memory also performs bounds checks based on the confirmed offset and lengths

However creating a Span from and array backed {ReadOnly}Memory uses the regular array constructor
https://github.com/dotnet/coreclr/blob/9d1e7dd8eb744b14eb4a90b401e854d7a5f79fff/src/System.Private.CoreLib/shared/System/Memory.cs#L286-L289

Which then rechecks all the parameters again
https://github.com/dotnet/coreclr/blob/9d1e7dd8eb744b14eb4a90b401e854d7a5f79fff/src/System.Private.CoreLib/shared/System/Span.Fast.cs#L73-L89

This makes an array backed Memory the most expensive kind of Span creator.

Does it need to recheck everything or can it use an internal Span .ctor that bypasses the additional checks?

/cc @jkotas @stephentoub @GrabYourPitchforks @davidfowl @ahsonkhan

[benaadams]

Issue with struct tearing on Memory<T> perhaps? Should still be able to skip the array type checks?

[jkotas]

Issue with struct tearing on Memory<T> perhaps?

Right.

[GrabYourPitchforks]

I think the JIT will automatically elide the null check in the Span<T> ctor, because it should be able to reason that the argument cannot be null due to the shape of the call site in Memory<T>.get_Span, and everything gets inlined. However I don't think it can automatically elide the typeof(T[]) check, even though within Memory<T>.get_Span we know this check will succeed. Maybe a potential micro-improvement opportunity?

[benaadams]

@GrabYourPitchforks this is fixed now?

[ahsonkhan]

However creating a Span from and array backed {ReadOnly}Memory uses the regular array constructor

Does it need to recheck everything or can it use an internal Span .ctor that bypasses the additional checks?

Yes, it no longer uses the regular (checked) span ctor:
https://github.com/dotnet/coreclr/blob/4e23b22ee5cce5db9f8aafb4c6e3b17dbfe07567/src/System.Private.CoreLib/shared/System/Memory.cs#L344

Fixed in dotnet/coreclr#20386