This document has some tips that might be useful when you work on MJIT.

Supported platforms

The following platforms are either tested on CI or assumed to work.

Not supported

The MJIT support for the following platforms is no longer maintained.



If you see an “MJIT bindgen” GitHub Actions failure, please commit the git diff shown on the failed job.

Refer to the following instructions for doing the same thing locally. Similar to make yjit-bindgen, make mjit-bindgen requires libclang. See also: mjit-bindgen.yml

macOS seems to have libclang by default, but I’m not sure how to deal with 32bit architectures. For now, you may generate c_64.rb with a 64bit binary, and then manually modify c_32.rb accordingly.


sudo apt install \
  build-essential \
  libssl-dev libyaml-dev libreadline6-dev \
  zlib1g-dev libncurses5-dev libffi-dev \
./configure --enable-yjit=dev_nodebug --disable-install-doc
make -j
make mjit-bindgen


sudo dpkg --add-architecture i386
sudo apt install \
  crossbuild-essential:i386 \
  libssl-dev:i386 libyaml-dev:i386 libreadline6-dev:i386 \
  zlib1g-dev:i386 libncurses5-dev:i386 libffi-dev:i386 \
./configure --disable-install-doc
make -j
make mjit-bindgen

Note that you cannot run x86_64 bindgen with an i686 binary, and vice versa. Also, when you install libclang1:i386, libclang1 will be uninstalled. You can have only either of these at a time.

Local development

Always run make install

Always run make install before running MJIT. It could easily cause a SEGV if you don’t. MJIT looks for the installed header for security reasons.

–mjit-debug vs –mjit-debug=-ggdb3

--mjit-debug=[flags] allows you to specify arbitrary flags while keeping other compiler flags like -O3, which is useful for profiling benchmarks.

--mjit-debug alone, on the other hand, disables -O3 and adds debug flags. If you’re debugging MJIT, what you need to use is not --mjit-debug=-ggdb3 but --mjit-debug.