Stage 4 builds a basic C-like compiler that is used to bootstrap the final C compiler in stage 5.
This stage contains multiple sub-stages where we bootstrap a compiler in assembly and gradually increase its complexity.
The lexer is written in assembly and used for both sub-stages. It handles C tokens and very basic preprocessor declarations. It does not support complex macros at this time.
bootstrap4 makes use of the assembly compiler and linker in bootstrap3, and does not emit assembly code directly. This allows us
to build on top of a rich runtime with simple data structions and useful runtime functionality. One .c file is compiled to a single .s
assembly output, then assembled with bootstrap3's one-shot assembler and linker.
The compiler0 sub-stage is a very basic C compiler written entirely in assembly language. It uses the lexer directly and emits
equivalent, highly-unoptimized assembly from C-like code. It is very limited and cannot handle complex statements, expressions, and
has serious limitations around arrays.
The compiler1 sub-stage is a much richer C compiler, written in C (though the dialect is limited to what compiler0 supports). This
sub-stage is powerful enough to compile the bootstrap5 full C compiler, which is written in a subset of C89 and is also compilable
by any standard C compiler.
Ideally the C portion of compiler1 should compile with a standard C compiler, but this is not a hard requirement.