JIT struct work planned for .NET 10

[jakobbotsch]

This issue captures the planned work items for .NET 10 with respect to struct improvements. This list is expected to change throughout the release cycle according to ongoing planning and discussions, with possible additions and subtractions to the scope.

The overall goal in .NET 10 is to expand the scope of physical promotion even further than we did in .NET 9. We have seen good improvements from switching classes of struct locals from old promotion to physical promotion and expect similar improvements for the remaining cases. A stretch goal is the full removal of old promotion.

ABI handling for physical promotion

The main limitation that stops physical promotion from replacing regular promotion is currently that it lacks the support around ABI boundaries that regular promotion has. This comes out of the fact that our multireg support is tied very directly into the existing promotion mechanism.
Thus, we expect to work on the following items in .NET 10:

• Support FIELD_LIST for returns in the backend
• Support producing FIELD_LIST in physical promotion for returned values, allowing physically promoted return values to stay in registers (JIT: More elaborate store forwarding patterns with physical promotion #86665)
• Add a new representation for register-passed struct parameters to the backend (see experiment in JIT: [experiment] Add an explicit IR representation for parameter definitions #92026 and conclusions)
• Allow struct fields of physically promoted parameters to stay in registers via a lowering transformation
• Add a new representation for register-returned struct values to the backend (in the style of GT_SWIFT_ERROR)
• Allow struct fields of physically promoted call returns to stay in registers via a lowering transformation

Missing handling for physical promotion

Old promotion handles cases that new promotion does not handle yet. There is work to be done to identify these cases and come up with replacements.

• JIT: Support decomposition into WithElement/GetElement in physical promotion #87554
• JIT: Physical promotion bails on signedness mismatch on small types #104898
• JIT: "Never-negative" handling is not implemented for Span length fields promoted by physical promotion #104573

Miscellaneous struct-related work items

• JIT: Propagate LCL_ADDRs into handlers #109182
• JIT: Propagate LCL_ADDRs into natural loops #109190

Stretch goals

• Remove old promotion

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