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[RyuJIT Wasm] Add Object Writer Imports Section + Constant Exprs to Encode R2R Payload Placement#123739

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adamperlin merged 10 commits intodotnet:mainfrom
adamperlin:adamperlin/wasm-object-writer-imports
Feb 3, 2026
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[RyuJIT Wasm] Add Object Writer Imports Section + Constant Exprs to Encode R2R Payload Placement#123739
adamperlin merged 10 commits intodotnet:mainfrom
adamperlin:adamperlin/wasm-object-writer-imports

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@adamperlin adamperlin commented Jan 28, 2026 •
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This PR adds support for generating an import section to the WasmObjectWriter. We currently import:

  • env.memory
  • __stack_pointer
  • __r2r_start
    Which are all expected to be provided by the runtime. Various data segments are then placed into __r2r_start + <segment_offset>. We use the constant expressions extension for these address calculations.

Notably this PR removes our export of the module memory, as we want to share memory with the runtime.

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Add basic import section to WasmObject writer, importing memory, stac…
bd0cac5 
…k pointer, and r2r start address by default
@adamperlin
Add addtional expression types for wasm instructions
da9edbb 
Lay out R2R data segments relative to imported __r2r_start global
@adamperlin adamperlin requested a review from kg January 28, 2026 23:53
@github-actions github-actions bot added the needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners label Jan 28, 2026
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kg commented Jan 28, 2026

I will try and update the test harness to be compatible with this

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@am11 am11 added arch-wasm WebAssembly architecture area-crossgen2-coreclr and removed needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners labels Jan 29, 2026
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Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
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Copilot AI review requested due to automatic review settings January 29, 2026 23:38
@adamperlin adamperlin marked this pull request as ready for review January 29, 2026 23:42
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adamperlin commented Jan 29, 2026

@kg I'd love a review on this when you have a chance!

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Pull request overview

Adds initial support for emitting a Wasm import section from WasmObjectWriter, and switches R2R data segment placement to use constant expressions based on an imported __r2r_start symbol.

Changes:

  • Introduces a new wasm.import object node section and emits it as Wasm section type Import.
  • Reworks combined data segment placement to use an instruction expression (global.get __r2r_start + i32.const offset + i32.add) instead of a fixed DataStartOffset.
  • Adds a small Wasm encoding model (imports, globals, memory type, expression/instruction encoding) and a helper to write UTF-8 strings with a length prefix.

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Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs Adds import section emission and uses __r2r_start + offset constant expressions for data segment placement.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs Adds encodable abstractions/types for Wasm expressions and import encodings (memory/global types, etc.).
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs Adds WriteUtf8WithLength helper for Wasm-style length-prefixed strings.

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

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kg commented Jan 30, 2026

This looks fairly good, all of Copilot's feedback seemed accurate though so we need to address it.
The Encode/EncodeSize split is a little awkward but I don't see an obvious way to get rid of it other than doing something weird with empty spans.

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adamperlin commented Jan 30, 2026

This looks fairly good, all of Copilot's feedback seemed accurate though so we need to address it. The Encode/EncodeSize split is a little awkward but I don't see an obvious way to get rid of it other than doing something weird with empty spans.

Agreed, I'd prefer to get rid of that split if we can find a clean way to do it. Copilot already caught a bug introduced because of this. Honestly, a solution using empty spans to calculate the exact length before writing might be preferrable for ensuring correctness.

Copilot AI review requested due to automatic review settings January 30, 2026 20:07
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Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Copilot AI review requested due to automatic review settings January 30, 2026 21:37
@adamperlin adamperlin force-pushed the adamperlin/wasm-object-writer-imports branch from e3da59a to 1d3f7fc Compare January 30, 2026 21:37
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Pull request overview

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

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

@adamperlin adamperlin requested a review from Copilot January 30, 2026 23:34
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adamperlin commented Jan 30, 2026 •
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This looks fairly good, all of Copilot's feedback seemed accurate though so we need to address it. The Encode/EncodeSize split is a little awkward but I don't see an obvious way to get rid of it other than doing something weird with empty spans.

Agreed, I'd prefer to get rid of that split if we can find a clean way to do it. Copilot already caught a bug introduced because of this. Honestly, a solution using empty spans to calculate the exact length before writing might be preferrable for ensuring correctness.

I think I'll address this as a refactor in a follow up. My thinking here is to create a lightweight wrapper type that can be consumed as a ref struct by all the Encode methods instead of consuming the spans directly, which is capable of either counting bytes that would be written or writing into a target span depending on if its internal span is empty or not.

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adamperlin commented Jan 30, 2026

@kg This should be ready for another look when you have a moment!

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Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

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adamperlin commented Feb 3, 2026

/ba-g unrelated failures

@adamperlin adamperlin merged commit dd87c10 into dotnet:main Feb 3, 2026
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@adamperlin adamperlin deleted the adamperlin/wasm-object-writer-imports branch February 3, 2026 22:54
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