[wasm][coreCLR] R2R: VM-side load of a flat webcil composite image

[lewing]

What

Minimal, OS-neutral VM plumbing so the runtime can load a flat webcil-in-wasm composite R2R image. crossgen2 emits the wasm startup R2R composite as a flat webcil image, but the R2R/PE-decoder load path assumes a mapped PE — a named RTR_HEADER export and IsMapped() RVA addressing — so today the VM can't find the R2R header or address sections in a flat webcil composite. Four small touch-points close that gap:

File	Change
vm/nativeimage.cpp	webcil composite exposes no named RTR_HEADER export → fall back to the decoder's R2R header
vm/readytoruninfo.cpp	treat flat webcil as RVA-addressable directly from base (IsWebcilFormat() alongside IsMapped())
vm/peimagelayout.inl	route IsComponentAssembly() through the webcil decoder (DECODER_DISPATCH)
vm/peimagelayout.cpp	guard against re-applying base relocations to a host-probed shared buffer

Guarded on TARGET_WASM / FEATURE_PORTABLE_ENTRYPOINTS, so the same code serves browser and wasi from one path.

Why

Part of #130524. Addresses the composite-loadability open question in #130519 ("whether the composite container is fully browser-loadable today or needs work"). This is the VM-consumer half — the producer/packaging side stays with #130519.

Feedback wanted — the relocation guard

The one design question in this PR is the peimagelayout.cpp guard. Host-probed webcil R2R images are backed by a single shared in-memory buffer, not a copy-on-write file mapping. The assembly binder can open such an image more than once (identity probe + load) as independent PEImage/layout objects over the same buffer, and each open would re-run ApplyBaseRelocations on that memory. WASM table-index relocations are additive (index += tableBase), so a second application doubles tableBase and pushes indirect-call indices out of the function table.

This PR guards it with a VM-local set of already-relocated base addresses (SetSHash<TADDR>). Because the duplicate opens are distinct objects sharing one buffer, a per-PEImageLayout/per-PEImage flag wouldn't dedup them — the state has to key on the shared resource. It is intentionally not synchronized (relies on the current single-threaded wasm runtime) and is marked INTERIM in-code.

The cleaner production direction is probably to relocate the host-probed buffer once at the point it's handed to the runtime and skip webcil in ApplyBaseRelocations entirely — but that reaches into the host/probe boundary, which is out of scope for this VM-only load change. Guidance on the preferred layer here is the main thing I'm looking for.

Scope / out of scope

Out: composite production/packaging (#130519); the CoreLib-R2R execution gates — QCall/PInvoke, class-init, etc. (#129850); SIMD instruction-set reporting; the host-side merge/probe mechanism.

Testing

• Compiles under ./build.sh clr -os wasi -c Release (the TARGET_WASM branches are only exercised by a wasm build).
• Validated on the WASI R2R prototype: a composite System.Private.CoreLib R2R image (~49k R2R functions) loads and dispatches through exactly these four changes.
• Cannot run standalone yet — a loaded CoreLib R2R image needs the execution gates tracked in Running wasm with R2R'd System.Private.CoreLib #129850 to get past startup. This PR is the load prerequisite for that work, landing incrementally behind the wasm guards like the rest of the wasm R2R bring-up.

Note

This PR (code and description) was drafted with GitHub Copilot assistance.

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[Copilot]

Pull request overview

This PR adds CoreCLR VM support for loading a flat Webcil-in-Wasm composite ReadyToRun (R2R) image by adjusting a few PE/R2R assumptions (exported RTR header, RVA addressing, component detection routing) and adding a WASM-specific safeguard against double-applying relocations over a shared host-provided buffer.

Changes:

• Teach composite-owner lookup to treat Webcil images as RVA-addressable (alongside mapped PE images).
• Route IsComponentAssembly() through the active decoder via DECODER_DISPATCH.
• Add a WASM-only guard to avoid reapplying base relocations to a shared Webcil buffer.
• Fall back to GetReadyToRunHeader() when RTR_HEADER export is absent (WASM/Webcil composite).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/vm/nativeimage.cpp	Adds a WASM/Webcil-specific fallback to obtain the R2R header via the decoder when no RTR_HEADER export exists.
src/coreclr/vm/readytoruninfo.cpp	Adjusts composite-owner string resolution to treat Webcil as “mapped-like” for RVA addressing.
src/coreclr/vm/peimagelayout.inl	Switches IsComponentAssembly() to decoder dispatch for PE/Webcil parity.
src/coreclr/vm/peimagelayout.cpp	Adds a WASM-only guard to prevent double relocation of a shared host-probed Webcil buffer.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/coreclr/vm/readytoruninfo.cpp:543

• For Webcil images, ReadyToRun section "VirtualAddress" is still an RVA that must be translated through the Webcil section table (like WebcilDecoder::GetRvaData does). Using GetBase()+virtualAddress assumes an identity RVA->file-offset mapping that WebcilDecoder does not guarantee, and will point into the Webcil header/incorrect data when PointerToRawData != VirtualAddress.

Suggested: keep the direct base+RVA path only for mapped PE images, and use GetRvaData for Webcil.

    LPCUTF8 ownerCompositeExecutableName = NULL;
    if (pLayout->IsMapped() || pLayout->IsWebcilFormat())
    {
        // Mapped PE images and (flat) webcil images are RVA-addressable directly from the base:
        // webcil is flat-mapped so the R2R section VirtualAddress maps 1:1 onto GetBase()+VA.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/coreclr/vm/peimagelayout.cpp:261

• The relocation de-dup set is accessed without synchronization, but the WASM build can be configured with multithreading (e.g., WasmEnableThreads => FEATURE_MULTITHREADING). In that configuration, concurrent loads of the same shared webcil buffer could race on SetSHash mutation, or even both pass the Contains() check and apply relocations twice. Consider taking a lightweight lock when IsWebcilFormat() is true and holding it for the duration of ApplyBaseRelocations so at most one thread can relocate+record a given base at a time.

    typedef SetSHash< TADDR,
                      NoRemoveSHashTraits< NonDacAwareSHashTraits< SetSHashTraits<TADDR> > > > RelocatedWebcilSet;
    static RelocatedWebcilSet s_relocatedWebcilBases;
    const bool isHostProbedWebcil = IsWebcilFormat();
    const TADDR webcilBase = isHostProbedWebcil ? (TADDR)GetBase() : (TADDR)0;

[lewing]

Added src/coreclr/inc/webcildecoder.h (new commit): implement WebcilDecoder::IsComponentAssembly() from the R2R COMPONENT flag.

The peimagelayout.inl change here routes PEImageLayout::IsComponentAssembly() through DECODER_DISPATCH to the webcil decoder, but that method was stubbed return FALSE — so the dispatch was inert (every webcil composite reported as non-component). readytoruninfo.cpp gates the composite R2R load on IsComponentAssembly(), so this is required for composite detection to actually work (matches the PE decoder path). Verified building under ./build.sh clr -os wasi -c Release.

Note

Comment drafted with GitHub Copilot assistance.

[Copilot]

Pull request overview

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

[davidwrighton]

Looks pretty good. just a few more things to fix.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/coreclr/vm/nativeimage.cpp:219

• On TARGET_WASM this path now uses the decoder header whenever HasReadyToRunHeader() is true, regardless of image format. That means a PE composite that is missing the expected RTR_HEADER export could be silently accepted on WASM (since HasReadyToRunHeader() also works for PE), weakening validation beyond the stated webcil-only intent. Gate the decoder-based header lookup on IsWebcilFormat() and keep the export path for PE layouts.

#ifdef TARGET_WASM
        // On WebAssembly the runtime only loads flat webcil composites, which do not expose a named
        // "RTR_HEADER" export the way PE R2R images do; obtain the R2R header from the decoder instead.
        // PE R2R images (which rely on the export) cannot be loaded on WASM, so the export path is never
        // taken here. A genuinely non-R2R image still fails validation below via the NULL header check.
        if (peLoadedImage->HasReadyToRunHeader())
            *header = peLoadedImage->GetReadyToRunHeader();
#else // TARGET_WASM
        *header = (READYTORUN_HEADER *)peLoadedImage->GetExport("RTR_HEADER");
#endif // TARGET_WASM

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[lewing]

/ba-g remaining jobs are stuck infra