RuntimeHelpers.CreateSpan optimization for stackalloc

[svick]

This PR changes the IL generated for stackalloc in the following way:

• If an existing optimization applies (loading byte arrays directly from PE data; using initblk when all bytes are the same), there is no change.
• Otherwise, if it's converted to ReadOnlySpan<T> and all initializers are constant, the whole stackalloc is replaced by a call to CreateSpan.
• Otherwise (e.g. when converted to Span<T> or when only some initializers are constant), CreateSpan is used as a part of regular stackalloc codegen, together with cpblk.

The IL for array initializers is also changed:

• If an array initializer for a type larger than byte with all initializers constant is converted to ROS, the whole expression is replaced with CreateSpan.

There are still some questions I have and things that would need addressing to make this code production quality:

1. Is it okay to call GetPinnableReference (without any pinning) to get the backing data for the result of CreateSpan, for use in cpblk?
2. Alignment. As I understand it, the compiler currently does not align the metadata blocks containing initialization data in any way. If this is a requirement, that would need to change.
3. ENC. This optimization does not work with ENC and the code needs to know whether to apply this optimization at the lowering stage. But I don't know how to find out if ENC is used from inside of LocalRewriter. Is there some way to do that?

[RikkiGibson]

cool!

[cston]

Thanks @svick!

[alrz]

lowlevelhackathon

So this is not supposed to reach main?

[svick]

@alrz

So this is not supposed to reach main?

It is, at some point. As far as I can see, the required runtime code is ready (dotnet/runtime#60948 (comment)), so I think the next step would be to get answers to the questions I had above and then this could be probably merged into main.

I don't know if I'm going to be able to do that soon.

[jaredpar]

It's on the list of features we're trying to get into C# 11. This feature is mostly in the "just need to find some keyboard time" vs. "needs a lot of design". That is why we've pushed it further back in the schedule as we've prioritized items that need more design work.

The schedule is very tight right now though as we had a number of last second asks that we're dealing with.