Bootstrapping still requires outside support

I've heard of the idea of bootstrapping a language, that is, writing a compiler/interpreter for the language in itself. I was wondering how this could be accomplished and looked around a bit, and saw someone say that it could only be done by either

• writing an initial compiler in a different language.
• hand-coding an initial compiler in Assembly, which seems like a special case of the first

To me, neither of these seem to actually be bootstrapping a language in the sense that they both require outside support. Is there a way to actually write a compiler in its own language?

Is there a way to actually write a compiler in its own language?

You have to have some existing language to write your new compiler in. If you were writing a new, say, C++ compiler, you would just write it in C++ and compile it with an existing compiler first. On the other hand, if you were creating a compiler for a new language, let's call it Yazzleof, you would need to write the new compiler in another language first. Generally, this would be another programming language, but it doesn't have to be. It can be assembly, or if necessary, machine code.

If you were going to bootstrap a compiler for Yazzleof, you generally wouldn't write a compiler for the full language initially. Instead you would write a compiler for Yazzle-lite, the smallest possible subset of the Yazzleof (well, a pretty small subset at least). Then in Yazzle-lite, you would write a compiler for the full language. (Obviously this can occur iteratively instead of in one jump.) Because Yazzle-lite is a proper subset of Yazzleof, you now have a compiler which can compile itself.

There is a really good writeup about bootstrapping a compiler from the lowest possible level (which on a modern machine is basically a hex editor), titled Bootstrapping a simple compiler from nothing. It can be found at https://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html.

The explanation you've read is correct. There's a discussion of this in Compilers: Principles, Techniques, and Tools (the Dragon Book):

• Write a compiler C1 for language X in language Y
• Use the compiler C1 to write compiler C2 for language X in language X
• Now C2 is a fully self hosting environment.

A super interesting discussion of this is in Unix co-creator Ken Thompson's Turing Award lecture.

He starts off with:

What I am about to describe is one of many "chicken and egg" problems that arise when compilers are written in their own language. In this ease, I will use a specific example from the C compiler.

and proceeds to show how he wrote a version of the Unix C compiler that would always allow him to log in without a password, because the C compiler would recognize the login program and add in special code.

The second pattern is aimed at the C compiler. The replacement code is a Stage I self-reproducing program that inserts both Trojan horses into the compiler. This requires a learning phase as in the Stage II example. First we compile the modified source with the normal C compiler to produce a bugged binary. We install this binary as the official C. We can now remove the bugs from the source of the compiler and the new binary will reinsert the bugs whenever it is compiled. Of course, the login command will remain bugged with no trace in source anywhere.