WASM MozJS — AOT Compilation Pipeline

This package provides build-time AOT (ahead-of-time) compilation of the MozJS WASM component so it can be embedded directly into a binary as a pre-compiled artifact.

Why AOT?

The raw .wasm component takes ~40 s to JIT-compile with wasmtime at runtime. AOT compilation does this work once at build time, producing a serialized .cwasm that can be deserialized near-instantly via Component::deserialize().

Pipeline

 @mozjs_wasm//file              (mozjs_wasm_api.wasm, downloaded from S3)
       │
       ▼
 wasmtime compile
       │
       ▼
 mozjs_wasm_api.cwasm           (serialized pre-compiled component)
       │
       ▼
 objcopy  (embed_mozjs_wasm_obj genrule)
       │
       ▼
 embedded_mozjs_wasm.o          (ELF .o, .rodata section)
       │
       ▼
 linked into final binary       (symbols: _binary_mozjs_wasm_api_cwasm_{start,end})

The AOT tool must be built with the same wasmtime library version and engine configuration as the binary that will deserialize the .cwasm.

Bazel Targets

Target	Kind	Output
`:aot_compile_mozjs_wasm`	`aot_compile_wasm`	`mozjs_wasm_api.cwasm`
`:embed_mozjs_wasm_obj`	`genrule`	`embedded_mozjs_wasm.o` (linkable ELF object)

Linking into a Binary

To embed the pre-compiled WASM module into mongod/mongos or any other binary, add the embedded object as a linker input:

python

mongo_cc_binary(
    name = "mongod",
    # ... existing srcs/deps ...
    deps = [
        # ... existing deps ...
        "@crates//:wasmtime_c",
    ],
    additional_linker_inputs = [
        "//src/mongo/scripting/mozjs/wasm:embed_mozjs_wasm_obj",
    ],
    linkopts = [
        "$(location //src/mongo/scripting/mozjs/wasm:embed_mozjs_wasm_obj)",
    ],
)

At runtime, the embedded bytes are accessible via symbols produced by objcopy:

cpp

extern "C" {
extern const uint8_t _binary_mozjs_wasm_api_cwasm_start[];
extern const uint8_t _binary_mozjs_wasm_api_cwasm_end[];
}

// Load the pre-compiled component (near-instant).
std::vector<uint8_t> bytes(
    _binary_mozjs_wasm_api_cwasm_start,
    _binary_mozjs_wasm_api_cwasm_end);

wasmtime::Config config;
config.wasm_component_model(true);
wasmtime::Engine engine(std::move(config));

auto component = wasmtime::component::Component::deserialize(engine, bytes);

Platform Support

Linux only (target_compatible_with = ["@platforms//os:linux"]).
Supports both x86_64 and aarch64 (the objcopy genrule handles both).

Building

bash

# Build the full pipeline (AOT compile + embed)
bazel build //src/mongo/scripting/mozjs/wasm:embed_mozjs_wasm_obj

# Build just the AOT compile tool
bazel build //src/mongo/scripting/mozjs/wasm:wasm_aot_compile_tool