Crossgen Wasm Object Writer V0

[adamperlin]

This is the first prototype of a Wasm object writer for crossgen that only writes out method bodies. It builds the minimum module structure which is needed to declare and export the module bodies so that the resulting module can be loaded and method bodies can be called by an external loader. I have tested the output with a simple JavaScript loader that instantiates the module and calls one of the exports.

What IS handled

Emitting simple method bodies from an assembly that have no relocations or external dependencies of any kind.

What is NOT handled

Relocations and any metadata attached to compiled methods.

R2R format envelope (the plan is to embed this in the data section in a future PR)

R2R metadata is currently unused.

Signature generation to meet ABI requirements

Right now, there is a single i32 -> i32 signature which is used as a placeholder for any method. Later work will need to build support for generating and storing wasm-level type signatures for JIT compiled methods.

JIT Integration

Currently, the JIT is not called (I believe it will hit asserts on the Wasm target until #121973 is in). Instead, a simple (i32.const 0) (return) stub is emitted for any JIT calls.

[Copilot]

Pull request overview

This PR introduces the first prototype of a WebAssembly object writer for crossgen2, enabling the compilation toolchain to output Wasm modules. The implementation focuses on emitting method bodies with minimal module structure required for loading and calling exported functions, while deferring relocations, R2R metadata, and proper ABI signature handling to future work.

Key Changes

• Added support for Wasm as a valid target architecture and container format in crossgen2
• Implemented core Wasm module writing infrastructure including section builders for Type, Function, Export, and Code sections
• Created a temporary stub mechanism to bypass JIT compilation for Wasm32 architecture until proper codegen support is available

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 15 comments.

Show a summary per file

File	Description
src/coreclr/tools/aot/crossgen2/Program.cs	Added Wasm to allowed non-composite container formats
src/coreclr/tools/aot/crossgen2/Crossgen2RootCommand.cs	Added "wasm" architecture and "wasi" OS to valid options, plus "wasm" output format
src/coreclr/tools/aot/ILCompiler.ReadyToRun/ILCompiler.ReadyToRun.csproj	Included new Wasm object writer files (contains developer-specific path that should be removed)
src/coreclr/tools/aot/ILCompiler.ReadyToRun/CodeGen/ReadyToRunObjectWriter.cs	Added factory method to create WasmObjectWriter with ABI context
src/coreclr/tools/aot/ILCompiler.ReadyToRun/CodeGen/ReadyToRunContainerFormat.cs	Added Wasm enum value to container format options
src/coreclr/tools/aot/ILCompiler.ReadyToRun/ClassDiagram1.cd	Added class diagram file (appears unrelated to Wasm implementation)
src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs	Implemented CompileWasmStub to emit placeholder bytecode for Wasm methods
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Core Wasm module writer implementation handling section generation and export
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Wasm type system definitions and encoding utilities
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs	Added WriteUtf8StringNoNull helper method for Wasm string encoding
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.cs	Changed visibility modifiers to enable WasmObjectWriter inheritance

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[adamperlin]

@SingleAccretion thank you for your comments! I think I've addressed them, please feel free to take another look!

[jkoritzinsky]

LGTM for a v0

[SingleAccretion]

LGTM with some comments.