What causes "failed to register unwind information" in wasmerio/wasmer?

Regenerate the compilation artifacts for the exact target platform (don't reuse cross-compiled caches)

failed to register unwind information

Error message

failed to register unwind information

What it means

During copy_function, after copying a compiled function body into code memory, unwind information (e.g. Windows X64 unwind info) is registered with the unwind registry; the code .expect()s success because missing unwind info breaks stack unwinding/profiling. A panic here means the registry rejected the registration.

Source

Thrown at lib/compiler/src/engine/code_memory.rs:220

        body.copy_from_slice(func.body());
        let vmfunc = Self::view_as_mut_vmfunc_slice(body);

        let unwind_info = func.unwind_info().map(|o| o.get());
        if let Some(CompiledFunctionUnwindInfoReference::WindowsX64(info)) = unwind_info {
            // Windows unwind information is written following the function body
            // Keep unwind information 32-bit aligned (round up to the nearest 4 byte boundary)
            let unwind_start = func_len.next_multiple_of(4);
            let unwind_size = info.len();
            let padding = unwind_start - func_len;
            assert!((func_len + padding).is_multiple_of(4));
            let slice = remainder.split_at_mut(padding + unwind_size).0;
            slice[padding..].copy_from_slice(info);
        }

        if let Some(ref info) = unwind_info {
            registry
                .register(vmfunc.as_ptr() as usize, 0, func_len as u32, info)
                .expect("failed to register unwind information");
        }

        vmfunc
    }

    /// Convert mut a slice from u8 to VMFunctionBody.
    fn view_as_mut_vmfunc_slice(slice: &mut [u8]) -> &mut [VMFunctionBody] {
        let byte_ptr: *mut [u8] = slice;
        let body_ptr = byte_ptr as *mut [VMFunctionBody];
        unsafe { &mut *body_ptr }
    }

    /// Register the frame info, so it's free when the memory gets freed
    pub fn register_frame_info(&mut self, frame_info: GlobalFrameInfoRegistration) {
        self.frame_info_registration = Some(frame_info);
    }
}

View on GitHub (pinned to 8c4b9ee9d3)

Solutions

Regenerate the compilation artifacts for the exact target platform (don't reuse cross-compiled caches)
Upgrade wasmer; unwind registration bugs on specific targets have been fixed in patches
Disable unwind info generation if your target allows (compile without unwind info) and verify stack walking is not needed
Report with target triple and module if the failure persists — indicates an internal unwind-info bug

Defensive patterns

Strategy: try-catch

Try / catch

std::panic::catch_unwind(|| engine.publish_artifact(funcs, unwind))
    .map_err(|_| anyhow::anyhow!("unwind registration failed on this target"))?

Prevention

Always compile artifacts for the exact target triple that will load them
Delete stale compilation caches when upgrading OS or wasmer versions
On Windows x64, verify unwind info generation settings in the toolchain
Test module loading on the real target platform in CI

When it happens

Trigger: Calling copy_function (triggered when publishing compiled module code) when registry.register fails — typically on Windows x64 with malformed or misaligned unwind info, or duplicate/conflicting registration ranges.

Common situations: Windows x64 targets with RtlAddFunctionTable-style failures; compiled functions whose func_len or pointer ranges are misaligned; cross-target artifacts compiled for a different architecture.

Related errors

AI-assisted analysis of wasmerio/wasmer@8c4b9ee9d3 (2026-09-01). Data as JSON: /api/errors/453b60cfb14771d5. Report an issue: GitHub.