[wasm] Export cDAC contract descriptor getter on non-emscripten wasm (wasi-sdk)

[lewing]

Summary

On non-emscripten wasm (wasi-sdk), the cDAC contract descriptor getter GetDotNetRuntimeContractDescriptor is not exported, so --gc-sections strips the descriptor from the final module. This prevents a native host / the cDAC reader from discovering the runtime data-contract descriptor on WASI.

The emscripten path already handles this via EMSCRIPTEN_KEEPALIVE, which both exports the getter and keeps the descriptor alive. There was no equivalent for the wasi-sdk (__wasm__, non-emscripten) toolchain.

Fix

Add an #elif defined(EXPORT_CONTRACT) && defined(__wasm__) branch that emits the getter with __attribute__((export_name("Get" #CONTRACT_NAME))). The explicit wasm export both makes the symbol reachable by a native host and roots it so --gc-sections does not remove the descriptor — mirroring what EMSCRIPTEN_KEEPALIVE does on the emscripten path.

The GETTER_NAME token-paste macros are hoisted above the #if (shared by both branches) and a GETTER_NAME_STR stringize pair is added to produce the literal export name "GetDotNetRuntimeContractDescriptor".

Validation

• WASI CoreCLR build (clr -os wasi -c Release): compiles clean (0W/0E).
• Confirmed the compiled object exports GetDotNetRuntimeContractDescriptor with the no_strip flag, so it survives --gc-sections.

This unblocks live cDAC discovery on WASI (non-emscripten) — the runtime-side counterpart to the cDAC-wasm validation work (#131161). Fixture-based cDAC tests do not depend on it.

Note

This PR was authored with the assistance of GitHub Copilot.

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[Copilot]

Pull request overview

This PR updates CoreCLR’s shared cDAC contract-descriptor generator to ensure the contract descriptor getter is exported (and thus retained by the linker) when building for non-Emscripten WebAssembly toolchains (e.g., WASI via wasi-sdk).

Changes:

• Hoists the getter-name token-paste macros so they can be shared across export implementations.
• Adds a non-Emscripten WebAssembly export path using __attribute__((export_name(...))) to export the getter and keep the descriptor reachable under --gc-sections.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[jkoritzinsky]

Is this something we can unify between Emscripten and WASI? Or does Emscripten not support the WASI mechanism?

[lewing]

Is this something we can unify between Emscripten and WASI? Or does Emscripten not support the WASI mechanism?

possibly, the emscripten version wires up some things automatically. I will check how it is consumed.

[lewing]

They could share the export_name path at the wasm level — emscripten's clang supports the attribute — but I kept them split here on purpose, because the two toolchains surface the export differently and only the WASI side is validated in this PR:

• WASI / wasi-sdk: consumers reach the descriptor via the raw wasm export. Validated object-level — the getter is emitted with no_strip and survives --gc-sections.
• emscripten / browser: the descriptor is reached through emscripten's runtime glue — the cDAC discovery path calls getDotnetRuntime(0).INTERNAL.GetDotNetRuntimeContractDescriptor(), i.e. the getter surfaced into INTERNAL by emscripten's Module/JS generation (which EMSCRIPTEN_KEEPALIVE participates in, and which per the existing comment may also need EXPORTED_FUNCTIONS). Whether a bare export_name still lands in that INTERNAL bridge the same way is unvalidated, and there's reason to doubt it (raw wasm-ld export vs. emscripten's export convention; _Name naming asymmetry).

So unifying now would risk the one proven path (browser discovery) for no functional gain in a WASI-scoped change. I'd rather unify as a follow-up gated on an explicit browser cDAC re-validation: build an emscripten CoreCLR with the emscripten branch switched to bare export_name, run browser discovery, and confirm INTERNAL.GetDotNetRuntimeContractDescriptor() still returns the descriptor pointer. If it does → unify; if not → the split is load-bearing.

Note

This reply was drafted with the assistance of GitHub Copilot.

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Comments suppressed due to low confidence (1)

src/coreclr/debug/datadescriptor-shared/contract-descriptor.c.in:49

• Consider including the build-system WASM define in addition to the compiler-provided __wasm__. The repo generally prefers HOST_*/TARGET_* defines over compiler macros (see .github/instructions/review-native.instructions.md), and other CoreCLR code already uses defined(HOST_WASM) || defined(__wasm__) to cover both cases (e.g. src/coreclr/gc/init.cpp:1321). Using both here would keep the “works for all wasm toolchains” intent while matching established patterns.

#elif defined(EXPORT_CONTRACT) && defined(__wasm__)
// Non-emscripten wasm (e.g. wasi-sdk): export the getter with an explicit export name so the
// descriptor symbol is both reachable by a native host and, critically, kept alive by the linker.
// Without an export rooting it, --gc-sections removes the descriptor from the final module (the
// emscripten path above relies on EMSCRIPTEN_KEEPALIVE for the same reason).