Improve low level struct performance

[jaredpar]

This is a "User Story" to track the proposed improvements to low level struct performance improvements.

C# Language Features

• Allow ref fields in ref struct types allow references to structs to be stored in fields of ref structs csharplang#1147
• Allow safe fixed sized buffers of any type Champion: fixed-sized buffers csharplang#1314

The feature proposal for both C# features is captured here

Related Runtime Requests:

• Allow ref fields in ref struct Allow ref struct to contain ref fields #32060
• FixedSizBufferAttribute proposal Expose an attribute to allow sizing a fixed sized buffer based on type and element count. #12320
• Mono support Mono C# low-level struct feature support #48113

[Dotnet-GitSync-Bot]

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[benaadams]

What about...?

Span<object> span = stackalloc object[5];

Which is sorta

struct Obj5
{
    object o0;
    object o1;
    object o2;
    object o3;
    object o4;
}

Obj5 objs = default;
Span<object> span = CreateSpan(ref objs.o0, 5);

[jaredpar]

The stackalloc primitive doesn't support reference types.

[benaadams]

localloc returning a byte* pointer is obv problematic for a reference type; however if it was limited to a Span<ref type> receiver and a fixed size rather than variable size then t could be translated to something closer to the safe fixed sized buffer proposal? e.g.

struct <Buffer>e__FixedBuffer_1024<T>
{
    private T _e0;
    private T _e1;
    // _e2 ... _e1023
    private T _e1024;

    public ref T this[int index] => ref (uint)index <= 1024u ?
                                         ref RefAdd<T>(ref _e0, index):
}

Not sure what the GC would make of referring only to an internal element of a stack struct (if it was sliced)

[jkotas]

I think #25423 is the proposal we should follow to improve stackalloc.

[mangod9]

@jaredpar this is unlikely for 6 due to statics in interfaces work. Ok to move out?

[mangod9]

Moving this to future, since this is moved out of .net 6.