Building SpiderMonkey

Before you begin, make sure you have the right build tools for your computer:

This guide shows you how to build SpiderMonkey using mach, which is Mozilla’s multipurpose build tool. For builds using configure && make, and translations into other languages see these instructions on MDN.

These instructions assume you have a clone of mozilla-central and are interested in building the JS shell.

Developer (debug) build

For developing and debugging SpiderMonkey itself, it is best to have both a debug build (for everyday debugging) and an optimized build (for performance testing), in separate build directories. We’ll start by covering how to create a debug build.

Setting up a MOZCONFIG

First, we will create a MOZCONFIG file. This file describes the characteristics of the build you’d like mach to create. Since it is likely you will have a couple of MOZCONFIGs, a directory like $HOME/mozconfigs is a useful thing to have.

A basic MOZCONFIG file for doing a debug build, put into $HOME/mozconfigs/debug looks like this

# Build only the JS shell
ac_add_options --enable-application=js

# Disable Optimization, for the most accurate debugging experience
ac_add_options --disable-optimize
# Enable the debugging tools: Assertions, debug only code etc.
ac_add_options --enable-debug

To activate a particular MOZCONFIG, set the environment variable:

export MOZCONFIG=$HOME/mozconfigs/debug


Once you have activated a MOZCONFIG by setting the environment variable you can then ask mach, located in the top directory of your checkout, to do your build:

$ cd <path to mozilla-central>
$ ./mach build


Note: If you are on Mac and baldrdash fails to compile with something similar to

/usr/local/Cellar/llvm/7.0.1/lib/clang/7.0.1/include/inttypes.h:30:15: fatal error: 'inttypes.h' file not found

This is because, starting from Mojave, headers are no longer installed in /usr/include. Refer the release notes  under Command Line Tools -> New Features

The release notes also states that this compatibility package will no longer be provided in the near future, so the build system on macOS will have to be adapted to look for headers in the SDK

Until then, the following should help,

open /Library/Developer/CommandLineTools/Packages/

Once you have successfully built the shell, you can run it using mach run.


Once built, you can then use mach to run the jit-tests:

$ ./mach jit-test

Optimized Builds

To switch to an optimized build, one need only have an optimized build MOZCONFIG, and then activate it. An example $HOME/mozconfigs/optimized MOZCONFIG looks like this:

# Build only the JS shell
ac_add_options --enable-application=js

# Enable optimization for speed
ac_add_options --enable-optimize
# Enable the debugging tools: Assertions, debug only code etc.
# For performance testing you would probably want to change this
# to --disable-debug.
ac_add_options --enable-debug

# Use a separate objdir for optimized builds to allow easy
# switching between optimized and debug builds while developing.
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-opt-@CONFIG_GUESS@

SpiderMonkey on Android aarch64

Building SpiderMonkey on Android

  • First, run mach bootstrap and answer GeckoView/Firefox for Android when asked which project you want to build. This will download a recent Android NDK, make sure all the build dependencies required to compile on Android are present, etc.

  • Make sure that $MOZBUILD_DIR/android-sdk-linux/platform-tools is present in your PATH environment. You can do this by running the following line in a shell, or adding it to a shell profile init file:

$ export PATH="$PATH:~/.mozbuild/android-sdk-linux/platform-tools"
  • Create a typical mozconfig file for compiling SpiderMonkey, as outlined in the Setting up a MOZCONFIG documentation, and include the following line:

ac_add_options --target=aarch64-linux-android
  • Then compile as usual with mach compile with this MOZCONFIG file.

Running jit-tests on Android

  • Plug your Android device to the machine which compiled the shell for aarch64 as described above, or make sure it is on the same subnetwork as the host. It should appear in the list of devices seen by adb:

adb devices

This command should show you a device ID with the name of the device. If it doesn’t, make sure that you have enabled Developer options on your device, as well as enabled USB debugging on the device.

  • Run mach jit-test –remote {JIT_TEST_ARGS} with the android-aarch64 MOZCONFIG file. This will upload the JS shell and its dependencies to the Android device, in a temporary directory (/data/local/tmp/test_root/bin as of 2020-09-02). Then it will start running the jit-test suite.

Debugging jit-tests on Android

Debugging on Android uses the GDB remote debugging protocol, so we’ll set up a GDB server on the Android device, that is going to be controlled remotely by the host machine.

  • Upload the gdbserver precompiled binary from the NDK from the host machine to the Android device, using this command on the host:

adb push \
    ~/.mozbuild/android-ndk-r20/prebuilt/android-arm64/gdbserver/gdbserver \
  • Make sure that the ncurses5 library is installed on the host. On Debian-like distros, this can be done with sudo apt install -y libncurses5.

  • Set up port forwarding for the GDB port, from the Android device to the host, so we can connect to a local port from the host, without needing to find what the IP address of the Android device is:

adb forward tcp:5039 tcp:5039
  • Start gdbserver on the phone, passing the JS shell command line arguments to gdbserver:

adb shell export LD_LIBRARY_PATH=/data/local/tmp/test_root/bin '&&' /data/local/tmp/test_root/bin/gdbserver :5039 /data/local/tmp/test_root/bin/js /path/to/test.js


Note this will make the gdbserver listen on the 5039 port on all the network interfaces. In particular, the gdbserver will be reachable from every other devices on the same networks as your phone. Since the gdbserver protocol is unsafe, it is strongly recommended to double-check that the gdbserver process has properly terminated when exiting the shell, and to not run it more than needed.


You can find the full command line that the script is using by giving it the -s parameter, and copy/paste it as the final argument to the gdbserver invocation above.

  • On the host, start the precompiled NDK version of GDB that matches your host architecture, passing it the path to the shell compiled with mach above:

~/.mozbuild/android-ndk-r20/prebuilt/linux-x86_64/bin/gdb /path/to/objdir-aarch64-linux-android/dist/bin/js
  • Then connect remotely to the GDB server that’s listening on the Android device:

(gdb) target remote :5039
(gdb) continue


It is possible to cross-compile a SpiderMonkey shell binary for another architecture. For example, one can develop and compile on an x86 host while building a js binary for AArch64 (ARM64).

Unlike the rest of this document, this section will use the old-style configure script.

To do this, first you must install the appropriate cross-compiler and system libraries for the desired target. This is system- and distribution-specific. Look for a package such as (using AArch64 as an example) aarch64-linux-gnu-gcc. This document will assume that you have the appropriate compiler and libraries; you can test this by compiling a C or C++ hello-world program.

You will also need the appropriate Rust compiler target support installed. For example:

$ rustup target add aarch64-unknown-linux-gnu

Once you have these prerequisites installed, you simply need to set a few environment variables and configure the build appropriately:

$ cd js/src/
$ export CC=aarch64-linux-gnu-gcc  # adjust for target as appropriate.
$ export CXX=aarch64-linux-gnu-g++
$ export AR=aarch64-linux-gnu-ar
$ export BINDGEN_CFLAGS="--sysroot /usr/aarch64-linux-gnu/sys-root"
$ ../configure --target=aarch64-unknown-linux-gnu
$ make

This will produce a binary that is appropriate for the target architecture. Note that you will not be able to run this binary natively on your host system; to do so, keep reading to set up Qemu-based user-space emulation.

Cross-Architecture Testing using Qemu

It is sometimes desirable to test a cross-compiled binary directly. Unlike the target-ISA emulators that SpiderMonkey also supports, testing a cross-compiled binary ensures that the actual binary, running as it would on the target system, works appropriately. As far as the JS shell is concerned, it is running on the target ISA.

This is possible using the Qemu emulator. Qemu supports a mode called “user-space emulation”, where an individual process executes a binary that targets a non-native ISA, and system calls are translated as appropriate to the host system. This allows transparent execution of cross-compiled binaries.

To set this up, you will need Qemu (check your system package manager) and shared libraries for the target system. You will likely have the necessary shared libraries already if you cross-compiled as described above.

Then, write a small wrapper script that invokes the JS shell under Qemu. For example:


# This is the binary compiled in the previous section.
CROSS_BIN=`dirname $0`/BUILD_AARCH64.OBJ/dist/bin/js

# Adjust the library path as needed; this is prefixed to paths such as
# `/lib64/`, and so should contain `lib` (and perhaps `lib64`)
# subdirectories.
exec qemu-aarch64 -L /usr/aarch64-linux-gnu/sys-root/ $CROSS_BIN "$@"

You can then invoke this wrapper as if it were a normal JS shell, and use it with to run tests:

$ jit-test/ ./js-cross-wrapper