JIT: Always create FIELD_LIST for struct args in physical promotion

[jakobbotsch]

Today physical promotion will create FIELD_LIST for returned struct values. However, arguments did not get the same treatment and had constraints motivated by old limitations of the backend. Since those limitations are gone now we can just indiscriminately produce FIELD_LIST in physical promotion.

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[jakobbotsch]

Spot checked some regressions. Mostly they happen when we start partially promoting a parameter, leading to some more spilling now.

This is a point-in-time problem. It will be improved with #112740 and even more once we start using ABI information to make promotion decisions (e.g. a struct being passed in registers should result in promoting it as fields compatible with those registers).

[jakobbotsch]

/azp run Fuzzlyn

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[Copilot]

Pull Request Overview

This PR simplifies physical promotion logic for struct arguments by always creating FIELD_LIST nodes, matching the behavior already used for struct return values. Previously, struct arguments had additional constraints that are no longer necessary due to backend improvements.

Key changes:

• Removes complex logic that determined when struct arguments could be passed in registers
• Consolidates field list creation into a shared method for both arguments and returns
• Simplifies the costing model by removing argument-specific constraints

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/jit/promotion.h	Updates method signatures to accept use parameter and adds new shared method declaration
src/coreclr/jit/promotion.cpp	Removes complex argument handling logic, extracts shared field list creation method, and simplifies costing

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

Diffs, diffs with old promotion disabled. Still dependent on tests above to run.

Spot checked some regressions and they were because of the reason above.

[jakobbotsch]

Some jitstress failures in the "Stress gtSplitTree" phase. Looking at it...

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jakobbotsch]

jitstress is looking good now. @EgorBo can you take a look?