[jtschuster]

Adds support for resolving IMAGE_REL_BASED_HIGHLOW relocations in the NativeAOT WASM object writer. This is only supported when the target is in a data segment that corresponds directly to an ObjectNodeSection (which most, if not all, will).

Also adds emission of export entries and handling of symbol aliases.

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[Copilot]

🟡 Changes recommended

The new export emission currently exports all functions (risking invalid/unloadable output) and the new HIGHLOW relocation handling has unchecked narrowing/unsupported-path exceptions that can produce incorrect output or unhelpful failures.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR extends the WASM NativeAOT object writer to resolve additional relocation types during emission, and adds initial plumbing for symbol aliases and export emission.

Changes:

• Add alias support to the WASM symbol manager (lookup via alias map).
• Resolve IMAGE_REL_BASED_HIGHLOW relocations in the WASM relocatable object writer.
• Emit function exports from the WASM relocatable object writer and adjust section entry-count asserts for optional sections.

File summaries

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmSymbolManager.cs	Adds alias storage and alias-aware symbol resolution.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs	Adds IMAGE_REL_BASED_HIGHLOW relocation resolution and emits exports; forces subsections-via-symbols as a temporary workaround.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Makes section entry-count asserts conditional on section presence.
src/coreclr/jit/emitwasm.cpp	Writes the correct relocation target for direct calls by passing params.addr into the relocatable constant descriptor.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 4
• Review effort level: Lite

[pavelsavara]

How much does this increase size of the module ?

[jtschuster]

How much does this increase size of the module ?

We don't have a baseline size for a working NativeAOT module, so there's not really a good answer for that.

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:172

• This fallback is reachable for existing NativeAOT nodes: FatFunctionPointerNode emits IMAGE_REL_BASED_HIGHLOW for factory.MethodEntrypoint(...), and method bodies are placed in the WASM code section. Thus a valid fat function pointer causes object generation to throw here instead of resolving the relocation; other data nodes also emit pointer relocs to method entrypoints. Handle code/function targets with the appropriate WASM function/table relocation or keep them as link-time relocations.

                            else
                            {
                                throw new NotImplementedException();

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:205

• These new TODOs document deferred relocation/COMDAT work but do not link a tracking issue or use a searchable issue-tagged TODO form. Please attach the follow-up issue to the TODO so this temporary workaround and the missing linker support do not become untracked.

        // TODO: This is a temporary workaround for the fact that we don't yet emit a COMDAT section (or any reloc / linking sections)
        private protected override bool UsesSubsectionsViaSymbols => true;

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (1)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:173

• An unsupported HIGHLOW target reaches a bare NotImplementedException, so NativeAOT failures for code or other non-data symbols have no relocation type, symbol, or section context. This is especially difficult to diagnose because the default branch below already includes those details; throw a contextual NotSupportedException here as well.

[!NOTE] This review comment was created by GitHub Copilot.

                                throw new NotImplementedException();

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (1)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmSymbolManager.cs:85

• Aliases and imports use separate dictionaries, and aliases are registered before WriteImports adds the default imports. Thus an export alias such as memory, table, or stackPointer can coexist with an imported symbol of the same name; GetEntry then prefers _entries, so WriteExports resolves the alias as the imported non-function and emits a function export with the wrong index space (the type check is only a Debug assertion). Enforce a single namespace across imports, definitions, and aliases, or reject collisions before emitting exports.

        Debug.Assert(!_entries.ContainsKey(alias), "Alias symbol name must not already exist.");
        Debug.Assert(_entries.ContainsKey(target), "Target symbol name must exist before adding an alias.");
        Entry entry = _entries[target];
        _aliases.Add(alias, entry with { Name = alias });

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (2)

src/coreclr/jit/emitwasm.cpp:303

• Passing params.addr here also exposes external direct-call targets to the new function-index relocation path. GetHelperFtnUncached returns ExternFunctionSymbol for helpers such as RhpNewFast, and those symbols are not registered by RecordMethodDeclaration or WasmSymbolManager; ResolveRelocations therefore fails at _definedSymbols[reloc.SymbolName] (or reports the symbol unregistered) instead of leaving the call for the linker/import table. Keep external calls as unresolved link-time relocations, or register them as function imports before self-resolving only defined methods.

            id  = emitNewInstrSC(EA_HANDLE_CNS_RELOC, (cnsval_ssize_t)params.addr);

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:168

• This declaration is flush-left inside the switch case, unlike the surrounding C# code and the repository's indentation convention; format it at the same indentation level as the other statements in this block.

long targetAddress = segment.GetMemoryAddressOfOffset((int)(definedSymbol.Value + addend)) + reloc.Addend;

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

Get a fresh assessment by requesting another Copilot review.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

One or more issues must be addressed before approval.

Review details

Suppressed comments (3)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:173

• This new relocation path has no end-to-end NativeAOT/WASM coverage: the existing WASM assertions exercise WebCilObjectWriter, while NativeAOT selects WasmRelocatableObjectWriter (src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/ObjectWriter.Aot.cs:26-33). Add a WASM NativeAOT case that emits a data pointer relocation and verifies the patched linear-memory address, ideally also covering the new unmanaged alias/export, so regressions here cannot leave the existing suite green.

                        case RelocType.IMAGE_REL_BASED_HIGHLOW:
                        {
                            if (_sections[definedSymbol.SectionIndex] is WasmDataSegmentEmitter segment)
                            {
long targetAddress = segment.GetMemoryAddressOfOffset((int)(definedSymbol.Value + addend)) + reloc.Addend;

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:13

• System.Linq is not used anywhere in this file; please remove the new using to avoid carrying an unnecessary import.

using System.Linq;

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs:173

• addend already contains the relocation delta read from the emitted bytes, and reloc.Addend is the same original delta passed by ObjectDataBuilder.EmitPointerReloc (for example, TypePreinit uses a nonzero field offset). Adding both shifts the pointer twice, so relocations to fields inside a data symbol resolve to the wrong address. Use the symbol value plus the decoded addend once when computing the segment offset.

long targetAddress = segment.GetMemoryAddressOfOffset((int)(definedSymbol.Value + addend)) + reloc.Addend;

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Lite

[jtschuster]

/ba-g failures are #132950 and #133931