Subzero is a JIT compiler used as a back-end for Reactor. It originates from Chrome's Portable Native Client project. Its authoritative repository is at https://chromium.googlesource.com/native_client/pnacl-subzero/.
SwiftShader contains a fork of the Subzero source code (at the time of writing they are in sync). It is an alternative JIT compiler back-end, with LLVM still being the default for CMake builds. To build SwiftShader with Subzero instead of LLVM, specify -DREACTOR_BACKEND=Subzero in your CMake command (or change LLVM to Subzero in the CMake GUI). For Chrome builds that use the BUILD.gn files, Subzero is the default as it produces significantly smaller binaries than with LLVM.
Development on Subzero itself requires setting up the NaCl environment on a Linux system to be able to run its unit tests: