Selfie

Selfie is a project of the Computational Systems Group at the Department of Computer Sciences of the University of Salzburg in Austria and the Faculty of Information Technology of the Czech Technical University in Prague (CTU) in the Czech Republic. The tools rotor and bitme were co-developed at the University of Salzburg and at CTU, where they were developed under the project Robotics and Advanced Industrial Production (Roboprox, reg. no. CZ.02.01.01/00/22_008/0004590). For details on these tools, see software.md.

The Selfie Project provides an educational platform for teaching undergraduate and graduate students the design and implementation of programming languages and runtime systems. The focus is on the construction of compilers, libraries, operating systems, and virtual machine monitors. The common theme is to identify and resolve self-reference in systems code which is seen as the key challenge when teaching systems engineering, hence the name.

Selfie is a self-contained 64-bit, 12KLOC C implementation of:

1. a self-compiling compiler called starc that compiles a tiny but still fast subset of C called C Star (C*) to a tiny and easy-to-teach subset of RISC-V called RISC-U,
2. a self-executing emulator called mipster that executes RISC-U code including itself when compiled with starc,
3. a self-hosting hypervisor called hypster that provides RISC-U virtual machines that can host all of selfie, that is, starc, mipster, and hypster itself, and
4. a tiny C* library called libcstar utilized by selfie.

Selfie is implemented in a single (!) file and kept minimal for simplicity. There is also a simple in-memory linker, a RISC-U disassembler, a garbage collector, L1 instruction and data caches, a profiler, and a debugger with replay as well as minimal operating system support in the form of RISC-V system calls built into the emulator and hypervisor. The garbage collector is conservative and even self-collecting. It may operate as library in the same address space as the mutator and/or as part of the emulator in the address space of the kernel.

Selfie generates ELF binaries that run on real RISC-V hardware as well as on QEMU and are compatible with the official RISC-V toolchain, in particular the spike emulator and the pk kernel.

Selfie is designed as 64-bit system and requires as such a 64-bit system to run (LP64 data model). However, selfie also compiles on systems that support compiling and executing 32-bit binaries (ILP32 data model). In that case, selfie becomes a 32-bit system that generates and executes 32-bit binaries out-of-the-box. This is possible because the implementation of selfie carefully avoids 32-bit overflows throughout the system.

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