[RyuJIT Wasm] WasmObjectWriter Webcil Envelope Support

[adamperlin]

This PR implements WebCIL envelope support for the WasmObjectWriter. It builds up the data segment and assigns RVAs/Raw data addresses to each Webcil section as a first pass. As a second pass, it emits the proper headers and sections, and and also implements relocation resolution for IMAGE_BASE* type relocs that may appear in Webcil data sections using the pre-assigned section addresses.

The WebcilDataSegment is emitted as two Wasm data segments according to the Webcil spec like so:

  (data "\0f\00\00\00") ;; data segment 0: payload size as a 4 byte LE uint32, 
                        ;; may have extra padding so that the next segment is properly aligned
  (data "payload\cc")  ;; data segment 1: webcil payload (start is 4 byte aligned)
      | webcil header
      __________________________
      | section header (1)
      | ...
      | section header (n)
      ___________________________
      | section 1 (align 16-byte)
      ___________________________
      | ...
      ___________________________
      | section n (align 16-byte)
      ___________________________

The PR also adds a simple validator program which uses Microsoft.NET.Webcil.Validator. I am fine if we don't want to include this or want this refactored, I just included it in case it would be useful to have as I used it for testing purposes.

[Copilot]

Pull request overview

Adds initial WebCIL “envelope” emission support to the CoreCLR AOT Wasm object writer so the runtime can locate PE/CLI header data and debug directory data inside a WebAssembly binary.

Changes:

• Introduces WebcilHeader / WebcilSectionHeader encoders and wires Webcil.cs into ILCompiler projects.
• Extends WasmObjectWriter to build a flat WebCIL segment, resolve PE-style relocations into it, and emit it as passive Wasm data segments (with DataCount + exported helpers/globals).
• Adjusts dependency tracking so Wasm methods get explicit signature nodes, and updates some Wasm-specific section placement logic.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/coreclr/tools/aot/ILCompiler.ReadyToRun/ILCompiler.ReadyToRun.csproj	Adds Webcil.cs to the ReadyToRun tool build.
src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/MethodWithGCInfo.cs	Adds a Wasm32-only dependency on WasmTypeNode for explicit signatures.
src/coreclr/tools/aot/ILCompiler.Compiler/ILCompiler.Compiler.csproj	Adds Webcil.cs to the compiler tool build.
src/coreclr/tools/Common/Compiler/ObjectWriter/Webcil.cs	New: encodes WebCIL header + section headers.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Major: builds/emits WebCIL payload as Wasm data segments; adds DataCount/Global handling and relocation resolution changes.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds WasmGlobal encoding support.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.cs	Adds instruction helpers needed for emitted Wasm stubs (locals + memory.init).
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.cs	Adds Wasm-related adjustments and several debug Console.WriteLine statements during emission/relocation.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmTypes.cs	Formatting tweak in WasmValueType.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Relocation.cs	Exposes WASM_PADDED_RELOC_SIZE_32 publicly for reuse.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/AssemblyStubNode.cs	Places AssemblyStubNode into read-only data for Wasm targets.

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 14 changed files in this pull request and generated 5 comments.

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 14 changed files in this pull request and generated 7 comments.

Comments suppressed due to low confidence (1)

src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.cs:389

• This change makes all ISymbolRangeNode handling unconditional (previously gated by LayoutMode == CodeDataLayout.Unified). If the original condition was important for non-Wasm backends or for separate-layout scenarios, this risks changing behavior beyond Wasm. Consider restricting this to the Wasm path (or otherwise documenting why it is now correct for all targets/layout modes).

                // TODO-WASM: emit symbol ranges properly when code and data are separated
                // Right now we still need to determine placements for some traditionally text-placed nodes,
                // such as DebugDirectoryEntryNode and AssemblyStubNode
                if (depNode is ISymbolRangeNode symbolRange)
                {
                    symbolRangeNodes.Add(symbolRange);
                    continue;
                }

---

You can also share your feedback on Copilot code review. Take the survey.

[pavelsavara]

LGTM.

I wonder if we can add smoke test.
Produce trivial IL-only webcil with the object writer and test it with coreCLR loader.
Even manual/unmerged test doing this would be nice.

[adamperlin]

LGTM.

I wonder if we can add smoke test. Produce trivial IL-only webcil with the object writer and test it with coreCLR loader. Even manual/unmerged test doing this would be nice.

I have validated that the basic structure is correct for a simple example using the existing Microsoft.NET.WebAssembly.WebcilReader (and a simple driver program that calls the reader), but I'd definitely like to have a smoke test! What would it take to use the CoreCLR loader at the moment?

[davidwrighton]

LGTM modulo comments.

[pavelsavara]

What would it take to use the CoreCLR loader at the moment?

Just produce valid WebCIL 1 IL-only assembly (no R2R). I think you are close to it, right ?

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/coreclr/tools/Common/Compiler/DependencyAnalysis/Relocation.cs:768

• ReadValue was updated to treat WASM_TABLE_INDEX_U32/U64 like the other “target-only” Wasm relocs, but WriteValue still has no cases for these relocation types. Any attempt to resolve/write these relocs (e.g., in WasmObjectWriter.ResolveRelocations) will currently fall through to the default Debug.Fail path and leave the placeholder unchanged. Add WriteValue support for WASM_TABLE_INDEX_U32/U64 (write 32/64-bit indices as appropriate) to keep Read/Write symmetry and enable correct resolution.

                case RelocType.WASM_FUNCTION_INDEX_LEB:
                case RelocType.WASM_TABLE_INDEX_SLEB:
                case RelocType.WASM_TABLE_INDEX_U32:
                case RelocType.WASM_TABLE_INDEX_U64:
                case RelocType.WASM_TYPE_INDEX_LEB:
                case RelocType.WASM_GLOBAL_INDEX_LEB:
                    // These wasm relocs do not have offsets, just targets
                    return 0;

[adamperlin]

/ba-g infra failures