[Wasm RyuJIT] Generate element section and populate function pointer table

[kg]

• Rename r2r_start to image_base
• Add image_function_pointer_base import, the offset to populate the function pointer table at
• Fix us allocating the function pointer table with an initial size of 0
• Generate an element section in our modules containing a single element that creates a function pointer for each of our functions in sequential order, i.e.

(import "env" "__image_function_pointer_base" (global (;2;) i32))
...
(elem (;0;) (offset global.get 2 i32.const 0 i32.add) func 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22)

[kg]

cc @adamperlin

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[Copilot]

Pull request overview

This PR updates the Wasm object writer used by CoreCLR tooling to support function-pointer table initialization by emitting a WebAssembly element section, and aligns the imported globals used as bases for data and function-pointer placement.

Changes:

• Rename the imported module base global from __r2r_start to __image_base and update data segment placement to use it.
• Add a new imported global __image_pointer_base and use it as the offset expression for an emitted element segment.
• Emit an element section that initializes the table with function indices in sequential order, and prepend the element-segment count during emission.

---

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[kg]

With these changes applied, callVoidFunc works by accident given this module:

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public static unsafe class Program {
    [MethodImpl(MethodImplOptions.NoInlining)]
    static void voidFunc () {
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void callVoidFunc () {
        voidFunc();
    }
}

Because voidFunc gets allocated funcindex 0 and function pointer index 0, callVoidFunc can dereference the PEP 0 to get the function pointer index 0 and then perform an indirect call.

[Copilot]

Pull request overview

This PR updates the Wasm object writer to emit a Wasm Element section that initializes the function pointer table, and adjusts related global imports/offset handling to use __image_base/__image_pointer_base.

Changes:

• Added a Wasm Element section and logic to write an element segment that initializes table entries for all generated functions.
• Renamed __r2r_start usage to __image_base and added a new __image_pointer_base global import for table population offsets.
• Fixed table section limits so the function pointer table is allocated with a non-zero initial size.

---

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[adamperlin]

I think this looks good! Thanks for doing this work. Now that I'm reading it, we might want to rename __image_pointer_base to _image_table_base or something similar, just so we're very clear to distinguish between data pointers and function/table indices. Not crucial though.

[kg]

I think this looks good! Thanks for doing this work. Now that I'm reading it, we might want to rename __image_pointer_base to _image_table_base or something similar, just so we're very clear to distinguish between data pointers and function/table indices. Not crucial though.

image_func_pointer_base maybe?

[adamperlin]

I think this looks good! Thanks for doing this work. Now that I'm reading it, we might want to rename __image_pointer_base to _image_table_base or something similar, just so we're very clear to distinguish between data pointers and function/table indices. Not crucial though.

image_func_pointer_base maybe?

I think that works!

[Copilot]

Pull request overview

This PR updates the CoreCLR Wasm object writer to support initializing the function pointer table via an emitted element section, while also renaming the module base import and introducing a new imported global intended to represent the function-pointer-table base offset.

Changes:

• Add a new wasm.element section and emit a single active element segment that initializes the table with all generated functions.
• Rename the imported module base global from __r2r_start to __image_base and add a new imported global for the function pointer table base (__image_function_pointer_base).
• Fix the table limits encoding so the table’s minimum/maximum size are no longer emitted with an initial size of 0.

---

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[kg]

/ba-g iOS deadletter and known flakes for configurations unaffected by this wasm-only change