[wasm] Emit webcil payloadSize/tableSize into boot config for streaming instantiation by pavelsavara · Pull Request #131658 · dotnet/runtime

pavelsavara · 2026-07-31T17:22:03Z

Split from #129634

Summary

The WASM loader previously instantiated each webcil-in-wasm assembly by buffering its bytes and then calling the module's getWebcilSize/getWebcilPayload exports to discover the payload size at runtime. This records the payload size — and, for ReadyToRun images, the indirect-call table size — in the boot config so the loader can stream-instantiate the module directly.

Changes

Boot config (BootJsonData.GeneralAsset): adds optional payloadSize (emitted for every webcil-in-wasm assembly) and tableSize (R2R images only). Both use EmitDefaultValue = false, so non-R2R assets are unchanged apart from payloadSize.
Build tasks: ConvertDllsToWebcil computes the sizes from the produced webcil's wasm data segment and exposes them through a new WebcilSizes output. GenerateWasmBootJson consumes that output, with a self-contained fallback that re-reads the sizes straight from the produced webcil when the incremental convert task was skipped (e.g. a boot-config property changed but no assembly did). BootJsonBuilderHelper maps the sizes onto the emitted assets, culture-qualified for satellites.
Loader / host (host/assets.ts, loader/assets.ts): instantiateWebcilModule takes payloadSize/tableSize and uses WebAssembly.instantiateStreaming when possible, with no byte buffering and no data-section parsing. For R2R images it wires the host ABI handshake (stack pointer, exception tag, indirect-call table + base, image base) and grows the table before instantiation.

Notes

payloadSize is emitted for all webcil-in-wasm assemblies; tableSize only appears once ReadyToRun webcil-in-wasm images are produced (separate follow-up work).
Public loader API surface adds an optional WebcilAsset type (payloadSize/tableSize) in public-api.ts.

Note

This pull request description was generated with the assistance of GitHub Copilot.

…ng instantiation The WASM loader previously instantiated each webcil-in-wasm assembly by buffering its bytes and then calling the module's getWebcilSize/getWebcilPayload exports to discover the payload size at runtime. This records the payload size (and, for ReadyToRun images, the indirect-call table size) in the boot config so the loader can stream-instantiate directly. - Boot config: GeneralAsset gains optional payloadSize (emitted for every webcil-in-wasm assembly) and tableSize (R2R images only). Both use EmitDefaultValue=false so non-R2R assets stay unchanged apart from payloadSize. - ConvertDllsToWebcil computes the sizes from the produced webcil's wasm data segment and exposes them via a new WebcilSizes output. GenerateWasmBootJson consumes that output, with a self-contained fallback that re-reads the sizes straight from the produced webcil when the incremental convert task was skipped (e.g. a boot-config property changed but no assembly did). - Loader/host: instantiateWebcilModule now takes payloadSize/tableSize and uses WebAssembly.instantiateStreaming when possible, without buffering the bytes or parsing the wasm data section. For R2R images it wires the host ABI handshake (stack pointer, exception tag, indirect-call table + base, image base) and grows the table before instantiation.

azure-pipelines · 2026-07-31T17:22:56Z

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.

dotnet-policy-service · 2026-07-31T17:23:32Z

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

Copilot

Pull request overview

This PR threads Webcil-in-wasm payload sizing (and future R2R table sizing) from the build pipeline into the generated boot config, then updates the JS loader/host to use those sizes to instantiate WebAssembly modules without buffering and data-section parsing.

Changes:

Extend boot config assets to optionally include payloadSize (all webcil-in-wasm) and tableSize (R2R-only), and plumb these values through boot config generation.
Teach the ConvertDllsToWebcil task to compute sizes by parsing the produced wasm and emit them as an MSBuild output; add a GenerateWasmBootJson fallback that re-reads sizes from disk when the convert task is skipped incrementally.
Update the JS host/loader to pass sizes into instantiateWebcilModule and prefer streaming instantiation when the response MIME type allows it.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/GenerateWasmBootJson.cs	Collects Webcil sizes (task output or fallback re-read) and passes them into asset emission.
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/ConvertDllsToWebCil.cs	Computes `payloadSize`/`tableSize` by parsing produced wasm and exposes them via a new MSBuild output.
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/BootJsonData.cs	Adds optional `payloadSize` / `tableSize` fields to the emitted asset shape.
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/BootJsonBuilderHelper.cs	Maps computed sizes onto emitted `GeneralAsset` entries (including culture-qualified satellites).
src/native/libs/System.Native.Browser/libSystem.Native.Browser.Utils.footer.js	Preserves additional wasm exports needed for the R2R host ABI handshake.
src/native/libs/Common/JavaScript/types/public-api.ts	Adds a public `WebcilAsset` type documenting `payloadSize`/`tableSize`.
src/native/libs/Common/JavaScript/types/internal.ts	Extends internal asset entries to carry `payloadSize`/`tableSize` through loader pipelines.
src/native/libs/Common/JavaScript/types/ems-ambient.ts	Extends ambient emscripten typings for additional preserved wasm exports.
src/native/libs/Common/JavaScript/loader/assets.ts	Passes `tableSize`/`payloadSize` from assets into the host instantiation call.
src/native/libs/Common/JavaScript/host/assets.ts	Implements size-aware (streaming when possible) Webcil module instantiation and R2R import wiring.
src/mono/sample/wasm/Directory.Build.targets	Includes `PublishReadyToRun` in nested sample build propagation.
src/mono/sample/wasm/Directory.Build.props	Flows nested `PublishReadyToRun` into the outer build property when unset.
src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets	Wires the new `WebcilSizes` task output into boot config generation for build and publish.

+    const res = await checkWebcilResponse(webcilPromise, virtualPath);
+    const payloadPtr = allocWebcilPayload(payloadSize);
+    const imports: WebAssembly.Imports = { webcil: buildWebcilImports(memory, payloadPtr, tableEntries) };

-    const { instance } = await instantiateWasm(webcilPromise, imports);
-    const webcilVersion = (instance.exports.webcilVersion as WebAssembly.Global).value;
-    if (webcilVersion !== 0) {
-        throw new Error(`Unsupported Webcil version: ${webcilVersion}`);
+    let instance: WebAssembly.Instance;


+    // Reads payloadSize and tableSize from data segment 0 of a Webcil-in-wasm image without
+    // instantiating it. tableSize > 0 indicates a ReadyToRun image. The data section is the last
+    // wasm section, so for R2R images (large code sections) it can start well beyond the first few
+    // KB; this streams through the section headers, seeking past each body, instead of reading a
+    // fixed prefix. All multi-byte integers in the wasm binary format are little-endian and are read
+    // as such regardless of host endianness. See docs/design/mono/webcil.md.


+    internal static bool TryReadWebcilSizes(string path, out int payloadSize, out int tableSize, out string failureReason)
+    {
+        payloadSize = 0;
+        tableSize = 0;
+        failureReason = null;


maraf · 2026-08-01T18:10:53Z

       causing WasmApp.Common.props (Mono) to be imported even for CoreCLR builds. -->
  <PropertyGroup>
    <RuntimeFlavor Condition="'$(Nested_RuntimeFlavor)' != ''">$(Nested_RuntimeFlavor)</RuntimeFlavor>
+    <PublishReadyToRun Condition="'$(PublishReadyToRun)' == '' and '$(Nested_PublishReadyToRun)' != ''">$(Nested_PublishReadyToRun)</PublishReadyToRun>


Just to be sure, only properties that needs to be available very very early are required here.
Is this the case?

maraf · 2026-08-01T18:13:01Z

+    /// this; it is omitted for plain (non-R2R) webcil.
+    /// </summary>
+    [DataMember(EmitDefaultValue = false)]
+    public int? tableSize { get; set; }


Please, create a WebcilAsset and put these specific props there.

maraf · 2026-08-01T18:15:43Z

    public ITaskItem[] WebcilCandidates { get; set; }

+    [Output]
+    public ITaskItem[] WebcilSizes { get; set; }


Can we store the size on the candidate as metadata?

maraf · 2026-08-01T18:18:29Z

+        // A prebuilt R2R webcil-in-wasm image from the runtime pack replaces conversion of the .dll:
+        // stage (copy) it into the webcil output so it flows through the same downstream metadata as a
+        // converted assembly, but carries native code. The .dll is kept only as the metadata source.
+        string r2rWebcilPath = candidate.GetMetadata("R2RWebcilPath");


Where is this metadata produced?

maraf · 2026-08-01T18:19:28Z

+    // KB; this streams through the section headers, seeking past each body, instead of reading a
+    // fixed prefix. All multi-byte integers in the wasm binary format are little-endian and are read
+    // as such regardless of host endianness. See docs/design/mono/webcil.md.
+    internal static bool TryReadWebcilSizes(string path, out int payloadSize, out int tableSize, out string failureReason)


Shouldn't this belong to the WebcilReader.cs?

maraf · 2026-08-01T18:21:47Z

+                    // emitted for every webcil; tableSize only for R2R. Identify them by the produced
+                    // ".wasm" extension, excluding native wasm (dotnet.native.wasm) which is handled
+                    // separately and is not a webcil module.
+                    bool isWebcilInWasmAssembly = IsTargeting100OrLater()


Is this really .NET10+?

maraf · 2026-08-01T18:23:48Z

+
+                    if (isWebcilInWasmAssembly)
+                    {
+                        // Fast path: ConvertDllsToWebcil already computed the sizes. Fall back to


I would expect we make the computation an incremental step as well, rather than doing a fallback here

Copilot AI review requested due to automatic review settings July 31, 2026 17:22

pavelsavara requested review from akoeplinger and lewing as code owners July 31, 2026 17:22

github-actions Bot added the area-Build-mono label Jul 31, 2026

Copilot started reviewing on behalf of pavelsavara July 31, 2026 17:22 View session

dotnet-policy-service Bot assigned pavelsavara Jul 31, 2026

pavelsavara requested a review from maraf July 31, 2026 17:23

pavelsavara added arch-wasm WebAssembly architecture area-Host and removed area-Build-mono labels Jul 31, 2026

pavelsavara added this to the 11.0.0 milestone Jul 31, 2026

Copilot AI reviewed Jul 31, 2026

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build-analysis Bot mentioned this pull request Jul 31, 2026

Test assert failure in X509Certificates.Tests.RevocationTests.AiaTests.AiaAcceptsCertTypesAndIgnoresNonCertTypes #107364

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[wasm] Emit webcil payloadSize/tableSize into boot config for streaming instantiation - #131658

[wasm] Emit webcil payloadSize/tableSize into boot config for streaming instantiation#131658
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pavelsavara:browsehost_load_r2r_host

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pavelsavara commented Jul 31, 2026 •

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