In computer science, bootstrapping is the process of writing a compiler (or assembler) in the source programming language that it intends to compile. Applying this technique leads to a self-hosting compiler. An initial minimal core version of the compiler is generated in a different language; from that point, successive expanded versions of the compiler are run using the minimal core of the language.
Many compilers for many programming languages are bootstrapped, including compilers for BASIC, ALGOL, C, D, Pascal, PL/I, Factor, Haskell, Modula-2, Oberon, OCaml, Common Lisp, Scheme, Go, Java, Rust, Python, Scala, Nim, Eiffel, and more.
- it is a non-trivial test of the language being compiled, and as such is a form of dogfooding.
- compiler developers only need to know the language being compiled.
- compiler development can be done in the higher level language being compiled.
- improvements to the compiler's back-end improve not only general purpose programs but also the compiler itself.
- it is a comprehensive consistency check as it should be able to reproduce its own object code.
The chicken and egg problem
If one needs to obtain a compiler for language X (which is written in language X), there is the issue of how the first compiler can be written. The different methods that are used in practice to solving this chicken or the egg problem include:
- Implementing an interpreter or compiler for language X in language Y. Niklaus Wirth reported that he wrote the first Pascal compiler in Fortran.
- Another interpreter or compiler for X has already been written in another language Y; this is how Scheme is often bootstrapped.
- Earlier versions of the compiler were written in a subset of X for which there existed some other compiler; this is how some supersets of Java, Haskell, and the initial Free Pascal compiler are bootstrapped.
- A compiler supporting non-standard language extensions or optional language features can be written without using those extensions and features, to enable it being compiled with another compiler supporting the same base language but a different set of extensions and features. The main parts of the C++ compiler clang were written in a subset of C++ that can be compiled by both g++ and Microsoft Visual C++. Advanced features are written with some GCC extensions.
- The compiler for X is cross compiled from another architecture where there exists a compiler for X; this is how compilers for C are usually ported to other platforms. Also this is the method used for Free Pascal after the initial bootstrap.
- Writing the compiler in X; then hand-compiling it from source (most likely in a non-optimized way) and running that on the code to get an optimized compiler. Donald Knuth used this for his WEB literate programming system.
Methods for distributing compilers in source code include providing a portable bytecode version of the compiler, so as to bootstrap the process of compiling the compiler with itself. Such methods are also one way of detecting or avoiding (or both) the potential problem pointed out in Reflections on Trusting Trust. The T-diagram is a notation used to explain these compiler bootstrap techniques. In some cases, the most convenient way to get a complicated compiler running on a system that has little or no software on it involves a series of ever more sophisticated assemblers and compilers.
Assemblers were the first language tools to bootstrap themselves.
Hart and Levin wrote a Lisp compiler in Lisp at MIT in 1962, testing it inside an existing Lisp interpreter. Once they had improved the compiler to the point where it could compile its own source code, it was self-hosting.
The compiler as it exists on the standard compiler tape is a machine language program that was obtained by having the S-expression definition of the compiler work on itself through the interpreter.— AI Memo 39
This technique is only possible when an interpreter already exists for the very same language that is to be compiled. It borrows directly from the notion of running a program on itself as input, which is also used in various proofs in theoretical computer science, such as the proof that the halting problem is undecidable.
List of languages having self-hosting compilers
The following programming languages have self-hosting compilers:
- Burroughs Algol
- C++ (compilers: Visual C++, clang, probably others)
- C# and Visual Basic .NET via Microsoft Roslyn
- Common Lisp
- Free Pascal
- Perl 6 (compilers: Rakudo Perl & Niecza Perl 6 are both self-hosting)
- Compilers and Compiler Generators: An Introduction With C++. Patrick D. Terry 1997. International Thomson Computer Press. ISBN 1-85032-298-8
- "Compiler Construction and Bootstrapping" by P.D.Terry 2000. HTML. PDF.
- "Bootstrapping a simple compiler from nothing" by Edmund GRIMLEY EVANS 2001
- Tim Hart and Mike Levin. "AI Memo 39-The new compiler" (PDF). Retrieved 2008-05-23.