Add WASM PerfMap support: OS/arch constants and correct R2R offsets

[pavelsavara]

Summary

Enables the ReadyToRun perfmap (.ni.r2rmap) for browser-wasm / wasi-wasm targets. Two sequential crashes were blocking it when PublishReadyToRunPerfmapFormatVersion was set:

1. NotImplementedException in PerfMapWriter.TranslateTargetDetailsToPerfmapConstants — PerfMapOSToken had no Browser/Wasi members and PerfMapArchitectureToken had no Wasm member, so the switch expressions hit their throwing default arms.
2. ArgumentOutOfRangeException in OutputInfoBuilder.EnumerateMethods (_sections[node.SectionIndex]) — the wasm object writers never populated _outputSectionLayout, unlike COFF/ELF/Mach-O/PE, so method nodes carried a valid SectionIndex but there were no OutputSections to index.

Changes

PerfMap constants

• Add Browser = 8 and Wasi = 9 to PerfMapOSToken, and Wasm = 7 to PerfMapArchitectureToken (appended after the highest existing value to preserve the numeric contract consumed by symbol-upload / PerfView tooling).
• Map TargetOS.Browser/TargetOS.Wasi and TargetArchitecture.Wasm32 in the PerfMapWriter switches.
• Update docs/design/coreclr/botr/r2r-perfmap-format.md, also filling in the already-shipped OpenBSD/RiscV64/LoongArch64 rows.

Correct wasm R2R offsets

• Populate _outputSectionLayout in WebCilObjectWriter.EmitObjectFile (index-aligned with the wasm section table); the code section records the module-file offset where its function-body content lands.
• The code section is LEB128-shrunk during final emission (ResolveCodeRelocations), so each method's recorded (pre-shrink) offset no longer matches the final module. Build a pre→post entry-boundary offset map and remap every method node's offset and length to its post-shrink position, so a method's RVA + Length equals the next code entry's offset.
• Document the perfmap offset scheme in docs/design/mono/webcil.md.

Address space / consumers

For wasm, a method's perfmap RVA is the byte offset from the start of the .wasm module to its function entry in the Code section — a file offset into the module, not a virtual address or a Webcil payload RVA.

Note

The only meaningful consumer of these wasm file offsets is a native-wasm profiler that keys frames by a code offset within the module (e.g. a wasm sampling profiler / DevTools-style correlation). EventPipe- and dotnet-trace-based symbolication identifies managed methods by module MVID + metadata token via method-load events and does not rely on these perfmap offsets. The perfmap is opt-in diagnostics; nothing in the runtime reads it.

Validation

• ILCompiler.ReadyToRun builds clean (Release).
• Ran the rebuilt wasm crossgen2 on the IL System.Private.CoreLib.dll with --perfmap --perfmap-format-version:1 --obj-format:wasm --targetarch:wasm --targetos:browser: exit 0 (previously ArgumentOutOfRangeException), produced a .ni.r2rmap with 47,731 methods, header tokens OS=8 (Browser) / arch=7 (Wasm) / version 1, and contiguous, non-overlapping method RVAs (confirming both offset and length are post-shrink correct).

Note

This PR description and the code changes were generated with the assistance of GitHub Copilot.

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[Copilot]

Pull request overview

Adds missing PerfMap v1 target OS/architecture tokens and wiring so Crossgen2 ReadyToRun perfmap emission can support WebAssembly targets (Browser/WASI) without falling into the “unknown target” throwing paths. Also updates the design doc to reflect the expanded token sets.

Changes:

• Extend PerfMapOSToken with Browser/Wasi and PerfMapArchitectureToken with Wasm.
• Map TargetOS.Browser/TargetOS.Wasi and TargetArchitecture.Wasm32 to the new tokens in PerfMapWriter.
• Update r2r-perfmap-format.md to include the current token tables.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/tools/aot/ILCompiler.Diagnostics/ReadyToRunDiagnosticsConstants.cs	Adds new perfmap OS/arch enum values used as the on-disk perfmap contract.
src/coreclr/tools/aot/ILCompiler.Diagnostics/PerfMapWriter.cs	Extends target-to-token translation to cover Browser/WASI and Wasm32.
docs/design/coreclr/botr/r2r-perfmap-format.md	Documents the updated token values for perfmap v1 headers.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

[adamperlin]

Overall looks good to me, I just had a suggestion to clarify a comment.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/coreclr/tools/Common/Compiler/ObjectWriter/OutputInfoBuilder.cs:185

• RemapMethodNodeOffsets updates OutputNode.Offset/Length, but leaves OutputSymbol.Offset unchanged. This makes the node/symbol lists inconsistent, and MapFileBuilder’s FindSymbol(node, ...) binary search can stop finding the corresponding symbol for code-section nodes once node offsets are remapped (affecting --map / --mapcsv output for wasm). Also consider checked when recomputing node.Offset + node.Length / casting the new length back to int to avoid silent wrap/truncation on unexpectedly large sections.

 internal void RemapMethodNodeOffsets(int sectionIndex, IReadOnlyDictionary<ulong, ulong> offsetMap)
{
foreach (OutputNode node in _nodes)
{
if (node.SectionIndex == sectionIndex
&& offsetMap.TryGetValue(node.Offset, out ulong postStart)
&& offsetMap.TryGetValue(node.Offset + (ulong)node.Length, out ulong postEnd))
{