boostrapping a (Linux x86-64) C compiler

Compilers nowadays are written in languages like C, which themselves need to be compiled. But then, you need a C compiler to compile your C compiler! Of course, the very first C compiler was not written in C (because how would it be compiled?). Instead, it was slowly built up, starting from a very basic assembler, eventually reacing a full-scale compiler. This process is known as bootstrapping. In this repository, we'll explore how that's done. Each directory represents a new "stage" in the process. The first one, 00, is a hand-written executable, and the last one will be a C compiler. Each directory has its own README explaining what's going on.

You can run bootstrap.sh to run through and test every stage. To get HTML versions of all README pages, run make.

the basics

In this series, I want to explain everything that's going on. I'm going to need to assume some passing knowledge, so here's a quick overview of what you'll want to know before starting. I can't explain everything so you may need to do your own research. You don't need to understand each of these in full, just get a general idea at least:

• what an operating system is
• what memory is
• what a programming language is
• what a compiler is
• what an executable file is
• number bases -- if a number is preceded by 0x, 0o, or 0b in this series, that means hexadecimal/octal/binary respectively. So 0xff = FF hexadecimal = 255 decimal.
• what a CPU is
• what a CPU architecture is
• what a CPU register is
• bits, bytes, kilobytes, etc.
• bitwise operations (not, or, and, xor, left shift, right shift)
• 2's complement
• null-terminated strings
• how pointers work
• how floating-point numbers work
• maybe some basic Intel-style x86-64 assembly (you can probably pick it up on the way though)

It will help you a lot to know how to program (with any programming language), but it's not strictly necessary.

instruction set

x86-64 has a gigantic instruction set. The manual for it is over 2,000 pages long! So, it makes sense to select only a small subset of it to use for all the stages of our compiler. The set I've chosen can be found in instructions.txt. I think it achieves a pretty good balance between having few enough instructions to be manageable and having enough instructions to be useable. To be clear, you don't need to read that file to understand the series, at least not right away.

principles

• as simple as possible

Bootstrapping a compiler is not an easy task, so we're trying to make it as easy as possible. We don't even necessarily need a standard-compliant C compiler, we only need enough to compile someone else's C compiler, specifically we'll be using TCC since it's written in standard C89.

• efficiency is not a concern

We will create big and slow executables, and that's okay. It doesn't really matter if compiling TCC takes 8 as opposed to 0.01 seconds; once we compile TCC with itself, we'll get the same executable either way.

reflections on trusting trust

In 1984, Ken Thompson wrote the well-known article Reflections on Trusting Trust. This is one of the things that inspired me to start this project. To summarize the article: it is possible to create a malicious C compiler which will replicate its own malicious functionalities (e.g. detecting password-checking routines to make them also accept another password the attacker knows) when used to compile other C compilers. For all we know, such a compiler was used to compile GCC, say, and so all programs around today could be compromised. Of course, this is practically definitely not the case, but it's still an interesting experiment to try to create a fully trustable compiler. This project can't necessarily even do that though, because the Linux kernel, which we depend on, is compiled from C, so we can't fully trust it. To truly create a fully trustable compiler, you'd need to manually write to a USB with a circuit, create an operating system from nothing (without even a text editor), and then follow this series, or maybe you don't even trust your CPU vendor... I'll leave that to someone else

license

This project is in the public domain. Any copyright protections from any law
for this project are forfeited by the author(s). No warranty is provided for
this project, and the author(s) shall not be held liable in connection with it.

contributing

If you notice a mistake/want to clarify something, you can submit a pull request via GitHub, or email pommicket at pommicket.com. Translations are welcome!