xcc700: Self-hosting mini C compiler for esp32 / Xtensa

Why look into this project?

• A compiler you can fully grasp and tweak, on a modern platform where small is still cool.
• Basic features, not too entrenched, easy to morph into your language of choice.
• Reusable ELF writer, and a basic Xtensa bytecodes emitter.
• Possibly useful for hotfixes, CI, quick test/debug turnaround on esp32.

Sample output

./xcc700 xcc700.c -o xcc700.elf 

[ xcc700 ] BUILD COMPLETED > OK
> IN  : 700 Lines / 7977 Tokens
> SYM : 69 Funcs / 91 Globals
> REL : 152 Literals / 1027 Patches
> MEM : 1041 B .rodata / 17120 B .bss
> OUT : 27735 B .text / 33300 B ELF
[ 40 ms ] >> 17500 Lines/sec <<

Note: that timing is from esp32-s3. Timings on Mac/POSIX will be reported 1000x slower than they are, as on esp32 ticks are millisecond, and on POSIX microsecond, but there is no adjustment here.

Video Demo

https://github.com/user-attachments/assets/2bbd4fd5-04f7-4890-abf0-5e7454345148

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How to run xcc700?

Several options:

A. Compile with gcc xcc700.c and run it on your computer as a cross-compiler. It is fairly portable, tested on Mac x86_64 and arm64.

B. Compile for esp32 using xtensa-gcc or xcc700 from the option A (yes it can compile and cross-compile itself). Or grab the gcc-compiled version here: xcc700.elf (16kB). Run with ESP-IDF elf_loader.

C. If you use my BreezyBox shell component, like in this demo project, this will download and install it into your bin folder:

eget valdanylchuk/xcc700

D. Adapt the source code and call it as a function in your firmware.

What is included?

• C features: minimum required to write something like this compiler. While loop, if/then/else, limited support for int/char/pointers/arrays, function calls and definitions, basic arithmetic and bitwise operators.
• Single source .c file as input, single REL ELF file as output.
• The output files can be run directly by the ESP-IDF elf_loader component, which links them on load via relocation table to anything you have exposed in your firmware: newlib libc, LVGL, your custom functions, anything you like. Just declare the functions you use.

What is missing?

• The rest of the C: for/do, include/define, long/float/double, struct/union/typedef, switch/case, array initializers, .data section, multi-line comments, too much to list.
• Many features are implemented only partially. E.g. you can have .bss globals but not global initializers; ++/-- are only supported in prefix position, assignment as statement not expression, types are mostly not checked, etc.
• Error handling and reporting. It is wildly optimistic, enforces nothing, has only a few error checks, and will crash in spectacular and unexpected ways on the most trivial errors.
• Optimization. It treats the Xtensa CPU as a stack machine, with no attempt at register allocation, and no benefit from the sliding window. It is a major sacrifice of performance for simplicity. GCC-compiled: 16kB, 17,500 lines/s; self-compiled: 33kB, 3,900 lines/s.
• Miss a feature? Just fork it! With a working foundation in only 700 lines, it is fairly easy to get started.

License

This is free software under MIT License, see LICENSE.

Contributing: Forks Welcome!

While I do not believe the world needs another C99 implementation, and do not intend to add features here, I am dead curious to see where the other creative minds can take a tiny self-hosting compiler on esp32.

If you organize hackathons, or assign coursework, or write tutorials, please consider xcc700 as a base to fork and extend! It can run on the available PCs, or on a $5 MCU if you want real cool hardware for the final test. Or you can port it to other systems, and use ld to link those ELF files.

Why did I write this?

I was making an esp32 "cyberdeck", and thought it cool to build some binaries directly on it. Esp32 is underrated in userland. It can do everything a 90s PC could do and more.

You can also take this as an artistic statement, and ask yourself:

• How many Watts do you need to do fun/useful stuff on a computer?
• Isn't it nice to have simple, tinker-friendly versions of common apps?
• Do we really need 300MB mouse drivers?

Have fun!