Add typed self-counting WASM sections

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

Note

This error may be related to your runner configuration. You can now configure runners for Copilot code review separately from Copilot cloud agent by creating a copilot-code-review.yml file with your setup steps. Read the docs for details.

[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Rename WasmObjectWriter to WebCilObjectWriter #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

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[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

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

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

[Copilot]

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Note

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[adamperlin]

I just had two nits, but otherwise this looks good to me!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local section is never used; this produces an unused-local warning. You can keep the side effect (ensuring the section exists) without storing the return value.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:154

• WasmExternallyCountedSection.EmitToStream only asserts that the entry count was set. In non-Debug builds this can silently emit a zero count if SetEntryCount is missed, producing an invalid/incorrect module without a clear failure. Consider throwing in release builds as well.

 public override int EmitToStream(Stream outputFileStream)
{
Debug.Assert(_entryCountSet);
return base.EmitToStream(outputFileStream);
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (4)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:530

• SectionDataEmitter section is assigned but never used, which can trigger CS0219 (and may fail the build if warnings are treated as errors). The call is only needed to obtain the writer / ensure the section exists.

 {
SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);
writer.WriteULEB128(NumDataSegments); // number of data segments

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:234

• Avoid var here: the type is not apparent from the right-hand side (base.GetOrCreateSection(...)). This may violate the repo's C# style settings and is less readable than using the explicit SectionWriter type.

 private TSection GetOrCreateSection<TSection>(ObjectNodeSection section)
where TSection : SectionDataEmitter
{
var writer = base.GetOrCreateSection(section);
return _sections.GetSection<TSection>(writer.SectionIndex);
}

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:40

• SectionDataEmitter assumes stream is non-null (it is dereferenced via Length/Position later). Adding a guard here produces a clearer exception and avoids null silently flowing into later emission logic.

 public SectionDataEmitter(Stream stream, Utf8String name, int sectionIndex)
{
ContentReadStream = stream;
SectionName = name;
SectionIndex = sectionIndex;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:27

• GetSection<TSection> relies on an unchecked cast. If the section mapping is ever wrong, this will throw an InvalidCastException with little context. A Debug.Assert keeps release perf the same while making the failure much easier to diagnose in Debug builds.

 public TSection GetSection<TSection>(int sectionIndex)
where TSection : SectionDataEmitter
{
SectionDataEmitter section = _sections[sectionIndex];
return (TSection)section;
}

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.

Suppressed comments (5)

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs:529

• The local variable section is assigned but never used. This can trigger analyzer warnings and makes the intent less clear; the return value can be ignored.

 SectionDataEmitter section = GetOrCreateSection(WasmObjectNodeSection.DataCountSection, out SectionWriter writer);

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs:6

• using System; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs:6

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSegment.cs:7

• using System.Collections.Generic; is unused in this file and can trigger IDE0005 (remove unnecessary usings).

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmDataSection.cs:23

• The name parameter on WasmDataSection's constructor is no longer used after removing the WasmSection base class. This can trigger unused-parameter analyzers and is misleading for callers.

 public WasmDataSection(List<WasmDataSegment> segments, Utf8String name, int contentAlign = 1)
{
_segments = segments;
_contentAlign = contentAlign;
}

[jtschuster]

Could I get a review/re-review if anyone gets a chance?

[jtschuster]

/ba-g Failures are unrelated, WASM failure is #131924