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

adamperlin · 2026-01-28T23:52:53Z

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.

…k pointer, and r2r start address by default

Lay out R2R data segments relative to imported __r2r_start global

kg · 2026-01-28T23:58:44Z

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

dotnet-policy-service · 2026-01-29T17:21:31Z

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

…o WasmImport

adamperlin · 2026-01-29T23:43:22Z

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

Copilot

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.

Reviewed changes

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.

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

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

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src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

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

kg · 2026-01-30T08:06:51Z

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.

adamperlin · 2026-01-30T19:45:04Z

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.

…classes in a separate file and namespace

Copilot

Pull request overview

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

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

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

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

Copilot

Pull request overview

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

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

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adamperlin · 2026-01-30T23:42:39Z

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.

adamperlin · 2026-01-30T23:44:03Z

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

Copilot

Pull request overview

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

Copilot · 2026-01-30T23:44:50Z

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.cs

+// This namespace implements encodings for certain Wasm expressions (instructions)
+// which are used in the object writer.  
+// For now, these instructions are only used for constructing constant expressions
+// to calculate placements for data segments based on imported globals. 


There appears to be trailing whitespace in the comment lines here (e.g., after the period). The repo enforces trim_trailing_whitespace = true (.editorconfig:13), so please remove trailing spaces to avoid formatting-only diffs and potential CI/style checks.

Copilot · 2026-01-30T23:44:51Z

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

+        private void WriteImports()
+        {
+            // Calculate the minimum required memory size based on the combined data section size
+            ulong contentSize = (ulong)SectionByName(WasmObjectNodeSection.CombinedDataSection.Name).ContentSize;
+            uint dataPages = checked((uint)((contentSize + (1<<16) - 1) >> 16));
+            uint numPages = Math.Max(dataPages, 1); // Ensure at least one page is allocated for the minimum
+
+            _defaultImports[0] = new WasmImport("env", "memory", WasmExternalKind.Memory,
+                new WasmMemoryType(WasmLimitType.HasMin, numPages)); // memory limits: flags (0 = only minimum)
+


The PR description says the new import section currently imports __stack_pointer and __r2r_start, but the implementation also changes the module to import env.memory (and removes the previously emitted memory section/export). Please update the PR description (or the code) so reviewers/consumers understand this behavioral/interface change.

Copilot · 2026-01-30T23:44:51Z

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

+        }
+
+        private void WriteExports()
+        {


WriteExports() no longer exports the module's linear memory (previously exported as "memory"). If any downstream expects to access the instance memory via an export (common for JS tooling/debugging), this is a breaking interface change. Consider exporting the imported memory as well, or document the change and ensure consumers are updated.

Suggested change

{

{

WriteMemoryExport("memory", 0);

Copilot · 2026-01-30T23:44:51Z

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

+        WasmValueType ValueType;
+        WasmMutabilityType Mutability;
+


These are private fields but don't follow the repo naming rule for private/internal fields (_camelCase per .editorconfig:76-83). Please rename to _valueType / _mutability (and similarly for other new private fields below) to match existing conventions.

Copilot · 2026-01-30T23:44:51Z

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

+        WasmLimitType LimitType;
+        uint Min;
+        uint? Max;
+


Private fields LimitType / Min / Max don't follow the repo's _camelCase convention for private/internal fields (.editorconfig:76-83). Renaming to _limitType / _min / _max would keep this consistent with the rest of the file (e.g., _params, _returns).

Copilot · 2026-01-30T23:44:52Z

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs

+
+#nullable disable


#nullable disable is placed inside WasmImport and (as written) disables nullability for the remainder of the file without an obvious reason. This can hide nullability warnings in future edits; consider removing it or scoping it more explicitly (e.g., disable/restore around the specific code that needs it).

Suggested change

#nullable disable

adamperlin added 2 commits January 27, 2026 17:00

Add basic import section to WasmObject writer, importing memory, stac…

bd0cac5

…k pointer, and r2r start address by default

Add addtional expression types for wasm instructions

da9edbb

Lay out R2R data segments relative to imported __r2r_start global

adamperlin requested a review from kg January 28, 2026 23:53

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

dotnet-policy-service bot assigned adamperlin Jan 28, 2026

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

Slight refactoring: Add WasmExpr dsl and move some encoding logic int…

6232eef

…o WasmImport

Copilot AI review requested due to automatic review settings January 29, 2026 23:38

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adamperlin marked this pull request as ready for review January 29, 2026 23:42

adamperlin requested a review from MichalStrehovsky as a code owner January 29, 2026 23:42

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This was referenced Jan 30, 2026

iOS.Device test WorkItemExecutions #122874

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[Wasm] The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing #123572

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kg reviewed Jan 30, 2026

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Address copilot feedback

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Implement review feedback: Place Wasm DSL and associated instruction …

4684843

…classes in a separate file and namespace

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Copilot AI review requested due to automatic review settings January 30, 2026 21:37

adamperlin force-pushed the adamperlin/wasm-object-writer-imports branch from e3da59a to 1d3f7fc Compare January 30, 2026 21:37

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System.IO.Tests.Directory_Create_Tests.FileSystemWatcher_Directory_Create_InNestedDirectory fails with "Created event did not occur as expected" #103630

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XHarness package install failure on iOS due to devicectl NSPOSIXErrorDomain error 49 #123796

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

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

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