dotnet/runtime · critical · Error

__stack_pointer was not preserved by the linker or optimizer

Error message

__stack_pointer was not preserved by the linker or optimizer

What it means

Thrown by buildWebcilImports() (src/native/libs/Common/JavaScript/host/assets.ts:114) for an R2R webcil asset (tableSize > 0). The host ABI requires the runtime to export __stack_pointer so the R2R image can share the runtime stack; if _ems_.wasmExports.__stack_pointer is falsy, the import cannot be satisfied and instantiation would fail with a confusing link error, so the host throws early.

Source

Thrown at src/native/libs/Common/JavaScript/host/assets.ts:121

    }
}

// Builds the `webcil` import object. For R2R images (tableSize > 0) the module imports the runtime's
// stack pointer, exception tag, indirect-call table and base globals; this also grows the table.
// These import names and the webcilVersion/getWebcilPayload/fillWebcilTable handshake in
// finishWebcilInstance are the R2R Webcil-in-Wasm host ABI defined by crossgen's WasmObjectWriter
// (src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs, CreateDefaultGlobalImports/
// WriteExports). Keep in sync with the corerun host
// (src/coreclr/hosts/corerun/wasm/libCorerun.js, BrowserHost_ExternalAssemblyProbe). Unlike corerun,
// which parses data segment 0 for payloadSize/tableSize, this loader receives them from boot config.
function buildWebcilImports(memory: WebAssembly.Memory, payloadPtr: number, tableSize: number): Record<string, WebAssembly.ImportValue> {
    const webcilImports: Record<string, WebAssembly.ImportValue> = { memory };
    if (tableSize > 0) {
        const stackPointer = _ems_.wasmExports?.__stack_pointer;
        const rtlRestoreContextTag = _ems_.wasmExports?.__coreclr_wasm_rtlrestorecontext_tag;
        const asyncContinuation = _ems_.wasmExports?.__async_continuation;
        if (!stackPointer) {
            throw new Error("__stack_pointer was not preserved by the linker or optimizer");
        }
        if (!rtlRestoreContextTag) {
            throw new Error("__coreclr_wasm_rtlrestorecontext_tag was not preserved by the linker or optimizer");
        }
        if (!asyncContinuation) {
            throw new Error("__async_continuation was not preserved by the linker or optimizer");
        }
        const tableStartIndex = _ems_.wasmTable.length;
        _ems_.wasmTable.grow(tableSize);
        webcilImports.stackPointer = stackPointer;
        webcilImports.rtlRestoreContextTag = rtlRestoreContextTag as unknown as WebAssembly.ImportValue;
        webcilImports.asyncContinuation = asyncContinuation as unknown as WebAssembly.ImportValue;
        webcilImports.table = _ems_.wasmTable;
        webcilImports.tableBase = new WebAssembly.Global({ value: "i32", mutable: false }, tableStartIndex);
        webcilImports.imageBase = new WebAssembly.Global({ value: "i32", mutable: false }, payloadPtr);
    }
    return webcilImports;
}

View on GitHub (pinned to 60108ba66e)

Solutions

1. Rebuild the runtime with the standard linker flags that preserve __stack_pointer (the default emscripten/runtime build does this).
2. Ensure the runtime .wasm version matches the toolchain that produced the R2R webcil assembly.
3. If customizing the build, add __stack_pointer (and the sibling exports) to the linker --export list.

Example fix

# before: custom linker flags drop internal exports
emcc ... -s EXPORTED_FUNCTIONS=[]

# after: keep the host-ABI exports
emcc ... \
  -s EXPORTED_FUNCTIONS=[__stack_pointer,__coreclr_wasm_rtlrestorecontext_tag,__async_continuation]

function runtimeExportsStackPointer(): boolean {
  return !!((globalThis as any).Module?.wasmExports?.__stack_pointer);
}

function hasStackPointer(exp: any): exp is { __stack_pointer: WebAssembly.Global } {
  return exp && !!exp.__stack_pointer;
}

When it happens

Trigger: An R2R (ReadyToRun) webcil assembly is being instantiated, tableSize > 0, and the loaded runtime wasm module does not export __stack_pointer. This means the runtime was built/linked with an optimizer or linker that stripped or renamed the export.

Common situations: Building the runtime with a custom emscripten/linker config that drops --export __stack_pointer; using an R2R assembly produced by a newer toolchain against an older runtime that lacks the export; a misconfigured linker 'export-keep' list.

Related errors

• __coreclr_wasm_rtlrestorecontext_tag was not preserved by th
• __async_continuation was not preserved by the linker or opti
• Unsupported Webcil version: ${webcilVersion}
• Unsupported Webcil version: ${webcilVersion}
• cwrap ${name} not found or not a function

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/537f907573b9329b. Report an issue: GitHub.