[Wasm RyuJit] Call Codegen Fixes for R2R

[adamperlin]

Initially cherry-picked commit from #126662;

This PR:

• Adds explicit handling in lowering for managed calls using the PEP calling convention to pass the additional PEP parameter.
• Removes some dead code around handling of WellKnownArg::R2RIndireictionCell from wasm code gen.
• Adds fixes for helper call codegen to account for the extra level of indirection needed to load a helper call target from a PEP (Portable Entrypoint).

Lowering Transformation Details

The lowering transformation introduces a temp local (call it pep) to hold the PEP, which we then add to a managed call as a final parameter with associated ABI info. We then rewrite the control expr of the call to be *pep. This ensures that, conceptually, any call with an entrypoint of access type PVALUE will be lowered to something roughly like the following, in C notation:

void* pep = *(void**)(<value>);
(*(void**)pep)(arg0, arg1, ..., argN, pep)

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[Copilot]

Pull request overview

This PR adjusts WebAssembly RyuJIT call lowering/emission for ReadyToRun (R2R) so call targets and PortableEntryPoints (PEPs) are loaded through the correct number of indirections when the host exposes symbols as indirection cells (indirection cell -> PEP -> target).

Changes:

• Add an extra indirection when materializing the R2R indirection-cell argument on WASM so the value passed reflects the PEP rather than the cell itself.
• Update WASM direct-call lowering for IAT_PVALUE to dereference through indirection cell -> PEP -> target.
• Update WASM helper-call emission to load the PEP from the indirection cell (for managed helpers) and to load the final call target via an additional dereference.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/morph.cpp	Adds a WASM-specific extra indirection when preparing the R2R indirection-cell argument so the runtime sees the expected PEP pointer.
src/coreclr/jit/lower.cpp	Ensures WASM IAT_PVALUE direct call targets are lowered with an additional dereference to reach the actual dispatch target.
src/coreclr/jit/codegenwasm.cpp	Fixes helper-call emission to load the PEP from the indirection cell and to load the indirect call target by dereferencing the PEP.

[adamperlin]

See if it's possible to use the new ADDR_REL reloc type for this to avoid the i32.const <address>; i32.load pattern.

[AndyAyersMS]

Maybe a picture would be useful here?

I imagine it something like this: We have an indirection cell that refers to a PEP which refers to a function index, which we then call indirectly.

[adamperlin]

Maybe a picture would be useful here?

I imagine it something like this: We have an indirection cell that refers to a PEP which refers to a function index, which we then call indirectly.

This is the right mental model! I am attaching a diagram that I created though of calling a function F from generated code:

[jkotas]

I am attaching a diagram that I created though of calling a function F from generated code

This picture is missing the part that passes portable entrypoint address (the value fetched from R2R import table) as the hidden argument that it somewhat important detail.

[adamperlin]

I am attaching a diagram that I created though of calling a function F from generated code

This picture is missing the part that passes portable entrypoint address (the value fetched from R2R import table) as the hidden argument that it somewhat important detail.

Ah I wasn't sure whether to include that bit, I've updated the diagram to include it!

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

[adamperlin]

Could I get another review when you have a moment @jkotas @jakobbotsch? I've updated the PR based on your feedback to explicitly handle the PEP calling convention transformation in lower. Thinking about this further exposed some dead case handling in Wasm codegen around the handling of the indirection cell arg which will now not be materialized on wasm. This also exposed a bug in the stackifier that I'm currently working through here: #127412
Also cc'ing @kg due to wasm call codegen changes!

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

[jakobbotsch]

LGTM, feel free to address the ifdef nit in a follow up